IADC/SPE 112616 Troll West Oilfield Development—How a Giant Gas Field Became the Largest Oil Field in the NCS through Innovative Field and Technology Development Richard Dyve Jones, StatoilHydro AS, Erland Saeverhagen, Arve K. Thorsen, and Sveinung Gard, SPE, INTEQ Copyright 2008, IADC/SPE Drilling Conference This paper was prepared for presentation at the 2008 IADC/SPE Drilling Conference held in Orlando, Florida, U.S.A., 4–6 March 2008. This paper was selected for presentation by an IADC/SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the International Association of Drilling Contractors or the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the International Association of Drilling Contractors or the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IADC/SPE copyright. Abstract The Troll West Oil Field has been, and still is, developed with more than 110 horizontal sub sea wells including 53 multi lateral wells (MLT). Several of the MLT wells have over time been designed and drilled with multiple open hole sidetracks to increase drainage area for each wellhead. The Troll West Oil Field has been developed from the first test wells drilled in 1984 and 1986, with oil production on stream in 1994 and continuous development still ongoing. The commercial oil reserves on the field have gone from 0 in 1986 to more than 1,400 million barrels today1. To be able to achieve this tremendous economical upside, the thin oil rim has been developed through sub sea development and extensive horizontal drilling enhancement. The latest development is through extensive use of multilateral drilling and wells containing up to 7 horizontal branches. The process of drilling the MLT wells and the benefit and risk evaluation for the MLT process is discussed and illustrated in this paper. The additional drainage area gained from open hole sidetracks are delivering additional production to each well head. The unique method used for the open hole sidetracks has proved to be a low risk strategy and highly economical way to get access to additional reserves thus reducing the need for additional sub-sea templates on the field. This paper shows how the field and technology development has evolved over the last two decades and plans going forward to continue strengthening the Troll West Oil Field production for another 15 years+ prior to the gas drainage. The upcoming technologies are incorporating both drilling and Logging-While-Drilling technologies to enhance the understanding of the mechanics behind the field development. Introduction The Troll Field2 is located offshore Norway (Figure 1) on the Norwegian Continental Shelf (NCS) in 300 m water depth. The Troll field was discovered in 1979 by A/S Norske Shell on the discovery wellbore 31/21. The Troll field covers an area of 750 square kilometers in North Sea blocks 31/2, 3, 5, and 6. The Plan for Development and Operation (PDO) was delivered in 1986, drilling of the production wells started in 1994 and the Troll Field was producing from September 1995. The oil production has to September 2007 been 188.3 million Sm3 of oil, and Figure 1: Troll Field overview 2 IADC/SPE 112616 all achieved from horizontal wells. Mid 2007 the total number of wells on the Troll Field has exceeded 150 and through multi lateral drilling the amount of horizontal reservoir sections has exceeded 250. In total the number of meters drilled is 900.000 including 560.000 meters of reservoir section. The Troll field is currently operated by StatoilHydro. Field description and challenges Reservoir The reservoirs3 in Troll Øst and Troll Vest are mainly shallow marine Upper Jurassic sandstones in the Sognefjord formation. Part of the reservoir is also belonging to the underlying Middle Jurassic Fensfjord formation. The field consists of three relatively large rotated fault blocks. The fault block to the east constitutes Troll Øst. Pressure communication between Troll Øst and Troll Vest has been proven. The oil column in Troll Øst is from 0-4 meters thick. Evaluation of drilling horizontal wells in the very thin oil column on Troll Øst is ongoing and test production is planned. The gas and oil is found mainly in the Sognefjord formation, which consists of shallow marine sandstones of Upper Jurassic age. Part of the reservoir is also in the underlying Fensfjord formation. The oil in the Troll Vest province is formed as a 22-26 meter thick oil column under a small gas cap. In the Troll Vest gas province there is an oil column of around 12-14 meters and a gas column of up to 200 meters. There is a potential large volume of residual oil below the oil column in Troll Vest. A small oil discovery was made in 2005 in the Brent Group, which lies deeper than the oil in the main reservoir. Troll Reservoir Sands The Troll reservoir sands are in general made up of two distinct types, although with large grading in between. They are: C sand — a clean and course sandstone with permeabilities in the range of 1 to above 10 Darcy (D) and occasionally up to 30 D. Good sorting and excellent porosity and permeability is present in the C-sand. M sand — A micaceous sand with a finer grain size than the C sand and in general having permeabilities in the range of 1–100 milliDarcies (mD). The sorting in this sand is poorer than in the C sand and therefore the permeability is reduced. Calcite Cementation in the Troll Field A third lithological component on the Troll field is calcite cemented sandstones (Figure 2). Calcite cement usually appears in isolated zones in the sandstone acting as permeability barriers. The calcite cemented sandstones may Figure 2: outcrop of sandstone with calcite cemented layers. occur as discrete layers, layers of stratabound concretions, Note the irregularity in the cementation. scattered concretions, and more rarely, patchy calcite. Calcite cemented layers typically have thicknesses from around 10 cm to a meter or two, and vary widely in lateral extent. Recovery strategy Oil production at Troll Vest takes place through long horizontal wells drilled right above the oil/water contact in the thin oil zone. The main recovery strategy is pressure depletion, but there will be simultaneous expansion of the gas cap above and the water zone below the oil. In the Troll Vest oil province, some of the gas produced has been injected back into the reservoir to optimize the oil production. One important aspect of the strategy is to recover the oil quickly, because less oil can be extracted when the pressure declines in Troll Øst. For this reason, limits have also been placed on gas extraction from Troll Øst. The gas in Troll Øst is recovered by pressure depletion. Main drilling and logging challenges In the initial part of the Troll Field the main concern was the coning of the gas from the enormous gas cap above the oil column. Several methods were initiated to ensure the possibility to drill horizontal wells within the acceptable tolerances. This is described by Jones et al in 19914. The development of 3D RSS systems and accurate and advanced logging while drilling (LWD) technologies together with enhanced understanding of the drilling environment has led to the tremendous production that is seen from the Troll field today. As the drilling challenge was to drill true horizontal wells the logging challenges has been to acquire high quality logs over several thousands of meters of logged reservoir in a harsh drilling environment. Through understanding of the drilling environment and sustaining engineering the goals have been achieved in obtaining high quality logs in this environment. IADC/SPE 112616 3 Drilling feasibility study and deployment As the Troll field was discovered in 1979, all efforts and focus was put on the fields enormous gas resources, and therefore at first, the oil resources in the field were not a part of the equation, and was considered impossible to produce. Large quantities of oil were in place in the field which was separated into the TWOP (Troll West Oil Province) and TWGP (Troll West Gas Province). Both oil reservoirs where located about 3,936 ft (1,200 m) below the seabed. The reservoir thickness of the TWOP was 72–85 ft (22–26 m) and much thinner in the TWGP of only 36–43 ft (12–14 m). Technology available in 1979, suggested that Troll oil was impossible or in best case, too costly to produce, and it was clear that research would have to be conducted to come up with new techniques and technology development beyond normal standards at the time. • In 1983 Norsk Hydro acquired the operator ship for Troll oil, which was designated as a separate development from the gas field and consequently was named Troll Phase II at first and later Troll Oil. • In 1984, the Norwegian Petroleum Directorate (NPD) requested that the entire Troll License should be evaluated for the potential of oil production, using horizontal drilling technology. • In 1989 the first test well, No. 31/2 -16S, was drilled on the TWOP utilizing a standard DTU (Double tilted universal housing) PDM (positive displacement motor) , using traditional MWD(measurement while drilling) tools. About 1,640 ft (500 m) of horizontal section was drilled to verify drillability in the thin 22 m tick oil column. Figure 3: Test well at TWOP in 1989 Figure 4: Positive Displacement Motor (PDM) with Double Tilted Universal housing (DTU) • Figure 5: Test well at TWGP in 1990 In 1990 a second test well, No. 31/5 – 4S, was drilled on the TWGP with DTU & AKO (Adjustable kickoff sub) PDM motors and traditional MWD. About 2,625 ft (800 m) of horizontal section was drilled to verify drillability with existing technology in the even thinner 12m tick oil column, and was successfully followed by a 5-month production test, performed from the production ship Petrojarl. Figure 6: PDM with Adjustable Kick-Off sub (AKO) • Figure 7: Test well with 3D curve above the reservoir. In late 1991 a third test well , No. 31/2-17S, was drilled as a test of the characteristic Troll corkscrew profile to enable positioning of the heel of the well, almost directly beneath the floating rig’s location in the desired direction. The drilling test was performed with the current available drilling and MWD equipment, to verify the equipments ability to drill and deliver the required highly continuously curved well profiles in the larger hole sizes, which at the time were at groundbreaking levels. 4 IADC/SPE 112616 Based on the results from these 3 test wells, Norsk Hydro entered into a contract for drilling on the TWOP, where a joint technology development project between Norsk Hydro and the service provider was a crucial part, to enable drilling on the TWOP with the required precision. The development goal was to design and manufacture an instrumented PDM with near bit inclination sensors, prior to startup of the first well on TWOP. This led to a very hectic R&D period and prototype instrumented motors where delivered for startup of drilling on TWOP. The commercial version of the instrumented PDM motor thereafter drilled many wells on the TWOP as well as on other fields in the North Sea area, and represented the start of a long lasting relationship between Norsk Hydro (StatoilHydro today) and the service provider. The contract and relationship was funded on common goals for continuous technology development to enable precise placement of wells in the Troll Field. Drilling, MWD and LWD service development on Troll Drilling Services and Technology From early 1990 the development of new technology for the Troll field was speeded up and resulted in many technological breakthroughs like the Instrumented motor, 3D Rotary closed Loop Steerable System (RCLS) and RCLS with integrated PDM motor. However, the most important achievement was the increased precision in well placement and horizontally drilled section lengths. Instrumented PDM Motor In 1992, prior to contract award for drilling on the Troll West Oil province, the customer decided to fund further development and building of the instrumented motor concept, adding several major improvements, as a part of the contract scope. This new Instrumented PDM Motor was contracted to be available for start-up of the first well on the TWOP. Several technical breakthroughs during the development of the tool where achieved including: • • • Highly precise “gun-barrel drilling” of the PDM stator housing was done over a length of 13 ft (4 m). A hole through the stator housing allowed electrical cabling to be run between the sensor sub and the rest of the MWD suite. A combined Multi-frequency resistivity/gamma ray tool was developed and was named the “RNT (Reservoir Navigation Tool) sub.” Multiple frequencies and increased depth of investigation, at this time enabled placement of the well bore in the desired distance from the water. A “near-bit inclination” sensor was also placed in the RNT sub, ca. 16 ft (5 m) from the bit, and merged with the “gun barrel drilled” motor section. The resulting system was a complete reservoir navigation tool. Figure 8: Instrumented PDM Motor Integration of the RNT sub and AKO PDM motor constituted the Instrumented PDM Motor, which made it possible almost overnight to drill horizontal sections within a 3.24-ft (1-m) TVD window, with all steering done in sliding mode. 3D- RCLS (Rotary Closed Loop Steerable System) or 3D-RSS (Rotary Steerable System) A quantum leap in Troll drilling was achieved when Norsk Hydro started to use a 3D rotary closed loop steerable (RCLS) system, after steering control by sliding the instrumented motor became almost impossible with a conventional drill string. Sinusoidal as well as helical buckling was many times imposed on the drill string, and drilling progress could be achieved by rotational drilling. The first generation 3D RCLS was available in Norway in 1998 as a pilot series tool. The customer at this time decided to fund an expanded manufacturing plan for the RCLS tools to ensure availability for Troll. In order to extend reservoir sections and further optimize well placement in the reservoir, the RCLS system was utilized to deliver the required extension of horizontal sections immediately, allowing for even more accurate placement of the sections in sands with the highest production. The addition of each new generation of 3D RSS technology permitted faster drilling of longer horizontal extensions and optimized well placement within the reservoirs. The 3D RSS system was in early 21st century manufactured in larger sizes, which replaced the AKO motor in the 17 ½-in. and 12 ¼-in. sections on the Troll Field, and today has become the standard. The larger 3D RSS systems drill sections with high continuous curvature and precision, as well as enhanced “cork screw profiles.” More information can be found in Jaggi et al.(20075) IADC/SPE 112616 5 Figure 9: Standard BHA setup on Troll field today Later improvements and enhancements to the 3D RSS system was adopted by the customer on a continuous basis, and today the tool and service are run with a new high power PDM motor integrated and placed immediately above the steering head. The motor employs innovative pre-contoured stator technology in extended lengths to produce unprecedented, “extreme” levels of torque and power. This solution allows dramatic reduction in surface string revolutions hence giving higher bit revolutions and makes the drill bit last longer, particularly in sections of calcites. The “gun barrel drilling” innovation was also used on this “extreme” PDM motor section to enable wiring for communication with the near-bit sensors and the rest of the MWD suite. The “extreme” PDM motors unique torque capability was ideally suited for drilling with PDC bits at Troll. More information can be found in Ronnau et al: (20056) Figure 10: relationship of RPM; WOB and wear Precision in well placement In a historical perspective the achievements in precision drilling experienced a quantum leap with the introduction of 3D RSS BHA’s. The TVD window was overnight reduced with 50% from +/- 3 ft (1m) to +/- 1,5 ft ( 0.5m), and then further reduced to +/- 0.3m (1ft) by introduction of 3D RSS BHA generation 3.0. The second quantum leap was the fact that no drilling in sliding mode was required, leading to significant increase in horizontal section length to be drilled. The friction from static sliding of the drill string was now reduced to 0 by full string rotation at all times. Figure 11 Historical overview of horizontal section length and accuracy in the period 1989 - 2006 Due to the 3D RSS/RCLS (“rotary closed loop steering”) BHA’s ability to automatically adjust its designated target course, it is today possible to drill and position the wells on Troll with ultimate precision, within a TVD window as narrow as +/- 1,0 ft (0,3m), over horizontal section lengths of more than 18000ft (5500m) . Measured historical data of the standard deviation from a 90° inclination baseline for 2 different generations of 3D RSS BHA’s showed significant improvements while changing from 1 to 2 inclinometers. 6 IADC/SPE 112616 • • Generation 1.5 - was equipped with a single near-bit inclination sensor Generation 3.0 - was equipped with dual inclinometers. A significant reduction in survey uncertainty was achieved by introducing dual inclinometers, and from a geometrical perspective, this technology development has contributed significantly to the precise well placement achieved while drilling the ultra-long horizontal sections at Troll. Figure 12: Precision in well placement on Troll with 2 generations 3D RSS BHA’s . MWD and LWD services Several technologies have been deployed on the Troll Field. The LWD technologies used has always comprised GammaResistivity-Density-Neutron Porosity. The development of these technologies has been paramount for the Troll development. Some of the issues have been related to the harsh drilling environment and the strain on the equipment when the drilling processes went from short horizontal wells with a lot of sliding to 3D RSS and continuous rotation of the equipment Gamma Measurements on Troll Due to the nature of the geology on the Troll Field with alternating C and M sands the gamma measurement is vital in identifying the sand that is actually being drilled. Several steps have been taken in utilizing the gamma measurements, at the early stage the single gamma measurements were utilized and the approach of different layers was not determined based on the gamma measurement. The analyses improved when the ability to transmit highside and low side gamma in real-time became available, although a time consuming procedure in the first year, it gave vital information when making flat turns of the well trajectory to optimize the well trajectory. Succeeding that the gamma measurement became fully azimuthal in real time and both image logs and sectored gamma measurements are available in real time. This takes the guess work out of the structural interpretation and allows for real time verification of the model and adjustment of the wellbore. The log illustrated in Figure 133 shows Gamma in track 1, to the left, depth track, resistivity track Figure 13: Standard Troll LWD log with gamma image (logarithmic scale). Density / neutron porosity track and gamma image track to the right. This log illustrates the development from one gamma curve to up and down gamma curves as seen in the left track and then how that measurement is taken across to gamma image as illustrated by the arrows A. The image clearly illustrates the ease of reading an image, where the bed is first seen above the well trajectory, and then the well trajectory is drilling through the bed as the top side of the wellbore is first departing from the bed and then the base of the well is departing from the bed. Resistivity measurements on Troll To be able to determine the wellbore placement at the desired location, several options were evaluated and one of the most important was resistivity (Jones et. al1). The key element in well placement was to drill the wells horizontally, at 90 degrees, for several thousands of meters. In the infancy of the development drilling on the Troll Field the survey accuracy was not at the level needed and hence other methods was needed to verify the wellbore placement. As the wells were to be drilled at a set distance above the OWC and in the best sands, a fixed resistivity value was determined to give the right position, and hence the steering on resistivity was born on the IADC/SPE 112616 7 Troll Field. Using the resistivity as the main steering criteria to obtain the desired level above the OWC requires high accuracy in the resistivity measurement. Once the resistivity is determined to be the main logging measurement to be used for evaluating the distance to the OWC the understanding of resistivity needs to be highly developed. In clean thick sandstone bodies with uniform properties the resistivity value will be stationary over some lateral distance. Once the property of the rock changes the evaluation of the logging responses will also have to be accommodated towards those changes. As seen in Figure 14 the resistivity in track 3 is highly affected by the cemented sandstone areas as seen from the density / neutron porosity log. In a horizontal well the resistivity is highly influenced by the surrounding areas and therefore artifacts are common, which is also the case on the Troll Field. Curve separation and differences in the multiple measurements are caused by polarization effects, bed boundary effects and anisotropic effects caused by variations in the water content within the varying amount of mica present. Figure 14: Standard Troll log with Gamma and Density Image The interpretation of resistivity is therefore most important to understand the distance to the OWC and to derive the true resistivity (Rt) of the area being drilled. High focus on the resistivity accuracy and resistivity interpretation is maintained on the Troll Field from the start of the drilling of the field. Standard Triple Combo and LWD Imaging The standard base MWD/LWD system is an integrated part of the RCLS and comprises of gamma ray, resistivity, annular pressure, vibration, stick slip and directional measurements. The compensated density service provides environmentally characterized formation density (ρb) and photoelectric cross section (Pe) measurements. The density tool includes three acoustic transducers located between the formation density and the neutron porosity sensors, which are used for stand-off measurements. When the drillstring is rotated, magnetometer packages in the LWD tools continuously measure the tool orientation. The Density and Gamma Ray (GR) measurements are azimuthally sectored and the near wellbore formation is described through density and GR images. The LWD density images are created from 16 azimuthal sectors while the GR images consist of eight sectors (Holden et al, 20067). Figure 14 describes the standard Troll log with Gamma- ResistivityDensity/Neutron Porosity and Images. Based on the input from the LWD measurements and the MWD enhanced surveys the well trajectory is optimized in two ways. Firstly the position of the wellbore is known and secondly the log interpretation allows for the well trajectory to follow and optimize around the OWC for optimum production. The density measurement was most vulnerable to continuous rotation in hard cemented sandstone stringers and a high quality and robust design was necessitated to obtain the desired distance drilled and the drilling hours for the different sections. Acoustic LWD Measurements The advanced acoustic LWD technology (Bøen et.al. 20078) enhances the signal-to-noise ratio through multiple acquisitions and stacking of waveforms. The added benefit is a minimization of tool decentralization effects with advanced signal processing techniques. Formation slowness values and associated quality control indicators are computed downhole while drilling and transmitted through mud pulse telemetry to the surface. Raw waveform data are stored in downhole memory for post-processing and analysis. The acoustic measurements were run at an early stage of the LWD acoustic measurements on Troll and is expected to represent a large value in field understanding going forward. The advanced acoustic tool was programmed to acquire monopole and quadrupole data. The quadrupole mode is uniquely accurate for determining true formation shear slowness. In addition to the multi-mode capabilities, this technology also has a multi-frequency option, both for monopole and quadrupole acquisition. This capability has enabled simultaneous high-data quality in fast and slow formations. Formation Pressure and Mobility Measurements Formation pressure testing while-drilling technology (FTWD) was deployed to acquire high-quality data in as short a time as possible. The FTWD tool provides optimized test sequences with three individual draw downs, each followed by a buildup period. The optimized test is performed at a single depth station with the pad pressed against the formation throughout the test. In the optimized test, drawdown rate and drawdown volume are varied during the repeat tests based on an in-situ mobility analysis of the preceding test (Meister et al., 20039). 8 IADC/SPE 112616 The FTWD tool incorporates features to improve data quality and sealing efficiency, and to shorten test time. Two of these main new features are 1) a smooth drawdown option for tight formations and highly unconsolidated formations and 2) an intelligent closed-loop control of the pad pressure that enables optimum sealing efficiency (Gravem et.al., 200610). Formation pressure on Troll increases the understanding of the production profile and gives understanding to the sealing effects of localized faults. Azimuthal Propagation Resistivity While GR and density LWD images have depths of investigations on the order of inches, the Azimuthal Propagation Resistivity (APR) measurements are able to detect resistivity boundary positions and their orientations up to 5 meters away under favorable conditions (good conductivity contrasts and appropriate resistivity level). This unmatched depth of detection is achieved by a combination of transmitter to receiver distance, frequency, signal output, and signal processing capability. Bell et. al. 200611 and Wang et al. (200612) describe the tool, which uses axial oriented transmitting antennas and transverse receiving antennas. This configuration removes or greatly suppresses all major environmental effects (borehole, tool eccentricity, tool bending, temperature, etc.). The transmitter layout leaves the measurement maximally sensitive to remote bed boundaries. Using transverse or fully-tilted antennas removes the guesswork associated with partially tilted antennas, as there is no direct coupling between the transmitter and receiver. Consequently, unscrambling is not required to extract the azimuthally sensitive information from the gross measurement. Because the APR signal response is zero when the formation is uniform, it needs to be combined with other resistivity information to provide more information on the formations being drilled. When combined with standard LWD resistivity measurements, APR measurements can resolve the azimuth of an approaching bed from any angle around the borehole. The imaging algorithm described in Bell et al. 2006 removes this ambiguity and presents the data more intuitively, see figure 6 – Final Post-well RNS Model. The pseudo resistivity image is interpreted in the same way as conventional borehole imaging logs except that the former does not directly provide the bed dip angle. This is due to the fact that the “electrical diameter” of the measurement varies from zero to 5 meters or more. On the Troll field the azimuthal resistivity is used for both fluid and geological information. Integrated working operations During the process of enhancing the technology on the Troll Field the working processes has been developed simultaneously to achieve the best possible work process to utilize new technology. One key element when assessing Integrated and real time operations from other locations than the rig site is to develop attractive working conditions and give responsibility and demanding work tasks to the onshore personnel. The main reason for developing integrated services on Troll is to increase the qualities of the wellbore delivered and develop more efficient and higher quality service delivery methods (Dagestad et al. 200613). The success seen through integrated operations on Troll has taken the process further and increased focus have been levered towards more complex and demanding integrated operations going forward (Dagestad et al 200714). Drilling optimization development The drilling environment on the Troll field has been and still is a constant challenge. Landing in the reservoir at small targets are necessary for optimal placement of the multilateral junctions. The reservoir formations, usually drilled in the landing section with 12 ¼” hole size and the reservoir section in 8 ½”/ 9 ½” hole sizes, are largely spanned in terms of drillability. These formations are easily drilled, loose and almost unconsolidated sandstones on one side to the very hard calcite cemented stringers. The harder zones are scattered on the field and sometimes very hard to map. Drilling in such a formations can go from a 100 m/hr potential to <1m/hr instantaneously, and lead to significant challenges in terms of drill bit life, well placement and damage to BHA and drillstring. To control these challenges, several initiatives have been taken during the years. Figure 15: Teamwork between the different parties IADC/SPE 112616 9 Taskforce I & II, TPG & Total System Approach A few years after utilization of the new 3D RSS drilling systems had started, two task force work groups called Troll Task Force I and Troll Task Force II (TTFI and TTFII) where initiated. TTF I was set to determine and identify the drilling challenges in the reservoir sections with respect to this new method of drilling. The TTF II group gathered and analyzed the results and lessons learned from the drilling so far with respect to TTF I. In this second phase, the BHA system design as well as drill bit design and usage where studied in detail, and appropriate drilling practices were discussed and established. The operational focus was strong within all parties during the TTF I & II. Shortly after the TTF groups were dissolved, a decline in performance was experienced, possibly due to change in operational focus. The members of the TTF groups decided that the service provider should take on the total drilling system performance ownership and responsibility. This was done through a new work process, implementing multi-disciplinary personnel across the service provider divisions in the Troll Performance Group (TPG) late in 2002. Simultaneously, a downhole drilling dynamics and diagnosis tool had been actively introduced to the Troll field. This tool and service gave the drillers, directional drillers, drilling supervisors and optimization engineers a more active role in the drilling process through a better understanding of the complex drilling environment. Challenges could actively be sorted out through answers while drilling diagnosis of the downhole conditions. Through the TPG, a new work process called “The Total System Approach” evolved. Close cooperation between the service provider personnel and Norsk Hydro’s personnel in the Troll Petroleum Technology (PeTek) group and drilling departments were crucial in this process to bring Troll drilling to higher levels. Close collaboration in this process, lead to better common understanding of the Troll fields drilling challenges by all parties, and lead to design and utilization of application specific BHA’s, as well as application specific drilling practices and procedures. The Total System Approach is described in further detail in Stavland et al (200615). Figure 16: Performance improvement from Total System Approach Figure 17; work Process 10 IADC/SPE 112616 Drill bit development During the years on Troll, drill bit development have been extensive and gone through many steps, particularly in the challenging reservoir sections. In the very beginning roller cone bits were primarily used, which at the time had an acceptable life time of 15-20 hours on bottom. The PDC bits that were run proved these designs to be less steerable with stability challenges which could be detrimental for the BHA’s, and ROP did not increase to the expected level. As late as in 2001 the Troll Task Force II reinstated the conclusion that roller cone bits with TCI (Tungsten Carbide Inserts) were the only bit type that could deliver the required performance in Troll reservoir sections. With the TPG and Total Systems Approach initiatives, the PDC bits had their renaissance from year 2002 and onwards and again became the future on Troll. Through continuous research and development during the years before and after, a total of 130 Troll specific designs have been developed, leading to the high performance bits daily being used on the Troll Field. Formation drillability software was developed especially for the Troll reservoir sands, which delivered a systematic evaluation of the sections prior to and after a bit run. Through the Total System Approach process, there is a common understanding and realization today that drill bit optimization and selection is a crucial and integral part of any BHA’s performance. The end result is in fact that the PDC bit technology has been brought back as the primary choice of bit technology to be used on the Troll Field. More info can be found in Jaggi et al. (200716) Figure 18: Formation Drillability software allows drillers to optimize the bottom hole assembly design based on the anticipated degree of calcite cementation. Figure 19: Bit utilization in Troll reservoir Well Completions Through a total of ca. 40,000 man-hours in research and development between operator and service provider, a number of technological breakthroughs was achieved at Troll, that also are being used in difficult oil formations by many operators today. Natural gas lift was the preferred solution for increased oil recovery on the Troll field, and to prevent formation sand production in the start, sand screens was installed in the middle completion, followed by a gravel-packed annulus. Gravel pack of the annulus was abandoned after completion of the Figure 20: One –Trip completion design for Troll first 4 wells to simplify the completions. A one-trip gas cap completion with gas lift valve was introduced in 2002 with great success, and the single-trip completion was designed to isolate the gas zone, set the production packer, perforate the gas zone with side mounted guns, followed by IADC/SPE 112616 11 installation of the upper completion, ( Figure 20) Control lines for remotely operated inflow control valves, at each reservoir segment, is also included in this completion design. The laterals can now be individually choked, and as a result, both natural gas lift from the gas cap and adjustable oil inflow contributes to increased oil recovery. Sand screens In the first wells on Troll, the reservoir sections were equipped with dual wire-wrapped, pre-packed screens which at the time where considered the best way to set a mechanical guard against formation sand incursion. An internal wash-pipe was installed inside all screen completions, to allow for mud cleanup in the reservoir section. As the horizontal sections increased in length and subsequently became more difficult to complete, a new screen type was a necessity for future completions. Through a joint project development process, a stand-alone screen selection guide was presented in 1999, incorporating • • • Testing for mud flow back and sand retention, Testing of burst, collapse and tensile strengths Metallurgical measures. Based on this thorough evaluation and qualification, the project team introduced a new shrouded, coarse-weave premium screen in 2000, named EXCLUDER 2000™, which has a low-friction shroud protecting the mesh screen from mechanical damage during installation. This shrouded sand screen allows large amounts of mud to be flowed back during well cleanout without plugging the mesh, and therefore eliminates the need for a separate cleanout run or a pre-installed wash string in the screens. A total of 248 miles (400 km) of screens have been installed in the Troll oil reservoir since 1994. Figure 21: Equalizer premium screen with inflow control device Inflow control device The predominant reservoir drive mechanism in the Troll field is the gas drive. An extended well test on the TWGP demonstrated that due to the narrow oil layer, gas breakthrough would occur almost immediately in a conventionally completed horizontal well (Figure 23). As a part of the development project, an inflow control device (ICD) as an addition to the original sand control screen completion was introduced to the Troll completions in 1998. A helical channel along The ICD helps balance the inflow pressure and distributes the draw down along the entire horizontal section, which allows a balanced inflow profile to be made, where the gas cone develops evenly, as shown in Figure 22. The result is improved drainage efficiency compared to conventional horizontal completions. Today a combination of the Excluder2000™ sand control screen and the ICD has been made, which is named EQUALIZER premium screen. Current completion design includes EQUALIZER screens along the full length of the reservoir section. Comparison testing and 4-D seismic has proven that the EQUALIZER completion yields near-zero-velocity annular flow. Radioactive tracer technology proved that the wells with the inflow control feature are cleaned up more efficiently. To date, no sand production has been observed and no sand control failures. Figure 23: Gas-Oil contact without equalizers showing a rapid gas breakthrough Figure 22: Gas-oil Contact with equalizers showing a stable drawdown along the horizontal section 12 IADC/SPE 112616 Drilling Fluids During the years several initiatives have been taken to match the drilling fluids with the increased wellbore complexity. Drilling fluids will always be a very important parameter for successful drilling and completion of extreme well paths. A couple of examples of important steps will be discussed bellow. One parameter of significant importance for the Troll Field, is to drill the wells with lightweight fluids. Up to 2003 the reservoir sections had been drilled with water based mud with MW of approximately 1.25sg. This MW posed a potential problem as the lengths of the reservoir sections increased and thus well hydraulic problems gradually became an issue. In 2003 a new mud was introduced with a much lighter composition. The introduction of these lighter drilling fluids was vital for the possibilities to stretch the horizontal sections to new extreme lengths, up to the current longest 8 ½” reservoir section of 5500m. Today the wells are usually drilled with 1.12sg mud. The mud has been further refined over the last few years to facilitate for drilling and maximize production in the less permeable M-sands on the field. Results from these pure M-sands showed better production then expected, this lead to the KCl brine based mud to be utilized both on C- and M-sand wells. The particle size in this system is based on reservoir characteristics to ensure good bridging. Further enhancements in the drill-in and completion fluids include already newly developed systems based on recent experience and research by the service provider and Norsk Hydro on Troll, and will reduce mud density even more to enable even longer sections in this depleted reservoir. Another important step to increased section lengths in the reservoir was the introduction of friction reducing lubricants added in the waterbased mud used on Troll. The effect of the lubricants has been to reduce the friction co-efficient in the wellbore to provide a reserve of torque to facilitate for longer sections. Torque spikes that earlier, in some instances even in simpler and shorter wells, were a huge concern, have been reduced both in magnitude and frequency. For the completion side, the reduced drag in the hole has made it possible for completion strings to be run for further reach. Figur 24: Comparison of Delta Torque with lubrication additives Also the top sections on Troll will in the future pose higher level challenges. These transport sections are getting longer and more complex, and the conventional waterbased mud these sections currently are being drilled with are starting to become a limiting factor. Based on this the service provider and Norsk Hydro have recently evaluated a high-performance WBM system for 17 ½” and 12 ¼” sections. The results so far have been promising giving the following performance improvements; improved ROP on bottom, no bit/BHA balling, larger and drier cuttings, improved borehole conditions, good stability of reactive clays. Figur 25: With the addition of lubrication additives to the mud system, the large torque variations were virtually eliminated and the peak frequencies dropped to the low end of the scale (< 6 kNm). MLT To further understand why Troll is really going from a Gas Giant to being the biggest Oil Producer on the Norwegian Continental Shelf, the development and use of the advanced multilateral technology have to be discussed. In the beginning the wells being drilled where single wellbores giving limited drainage of this huge reservoir. In 1997 the first multilateral well was drilled on Troll. This was the first step in a dramatic increase of reservoir area being explored and accessed by the multiple multilateral wellbores on Troll. Soon all wells being drilled were multilateral, and the number of legs on each well increased. IADC/SPE 112616 13 Usually the first leg is drilled out of the 9 5/8” shoe, then the next lateral leg is drilled from pre-milled windows in the 9 5/8” x 10 ¾” liner. The first ML wells drilled where single junction wells, with one main bore and one sidetrack through the pre-milled window. Drilling and completion of these wells never induced any big problems, and soon the ML solution was further developed, introducing up to as much as 3 predrilled windows. This solution has been run several times and gave a total of 4 completed lateral legs connected back to the liner area. 0 500 1000 1500 5547 m 2000 5839 m 30 00 25 00 5047 m 1500 10 50 4913 m 500 0 20 00 00 0 0 50 50 0 10 10 00 15 00 2 15 7703 m 0 00 20 0 00 00 00 It is natural to believe that drilling and completion of several reservoir sections from one main wellbore would increase the total risk of these projects, and during the initial planning phase of the first multilaterals, the risk was weighted to ca. 50% chance of success. However, the high level of risk was evaluated against the enormous additional Figure 26: Schematic of longest well on Troll, including several open production potential by successfully completing a ML hole sidetracks well and the decision was made to go ahead. A later study has shown that so far on the Troll field, no single well or wellbore has ever been lost with the ML solutions. With the high number of ML-wells installed successfully on the Troll field, it is fair to say that this is a very solid system for these types of field developments. ML-wells is considered a proven concept and are running flawlessly on a daily basis on the Troll field. These ML wells therefore have boosted the production significantly, giving by far more value than any risk associated with these installations up till now. The use of open hole sidetracks for additional reservoir exposure in the laterals, is discussed below. 0 25 25 0 30 00 30 3 00 00 0 Near bit inclination in Y1H and Y1H T2 93,00 92,00 91,00 NB I Open hole sidetracks The introduction of the open hole sidetracking methodology took place in 2003, and has since then increased the drainage area even more on the Troll field. Open hole sidetracks was in the beginning performed utilizing a predrilled “ramp”, which is a “kink” upwards, made when the mother-well was drilled. This was later used as support for sidetracking downwards. Using the ramp as support, a groove was reamed lowside and by “time-drilling”, the well was kicked off. The whole process usually took 8-16 hours depending on the drillability of the formation in the sidetrack area. 03 50 5 0 NBI Y1H 90,00 NBI Y1H T2 89,00 88,00 87,00 4900,00 4920,00 4940,00 4960,00 4980,00 5000,00 5020,00 Depth Figure 27: illustrates the open hole sidetracks wellbore geometry, the blue line illustrating the nearbit inclination of the “old” wellbore ramp and the red line illustrating the nearbit inclination of the new wellbore kick off. True Vertical Depth (m) 0 500 30 inch 18 5/8 inch 9 5/8 inch 4 100 0 150 0 13 3/8 inch 200 0 250 0 1 0 200 0 5 500 150 st ( m) 0 100 6 2 0 Ea 150 7 3 0 100 0 500 0 000 2 rth No (m) 0 500 250 Figure 28: Schematic of first 7 legged well on Troll, 5 legs where drilled as open hole sidetracks. At a later stage the ramps have been discontinued. Due to continuous geosteering, the ramp was often not placed at the optimal spot, and geosteering often lead to movements up/down regardless; this means one can pick other spots considered more optimal. The open hole sidetracking process has also been further refined through several revisions of the procedures, and sidetracking flat horizontal spots or even downhill is considered fully possible today with the use of these procedures. The wellbores being sidetracked from are usually left open with no completion/screens installed. So far none of the laterals seem to have collapsed, and seismic has later shown the drainage to function properly from these open laterals. Due to these experiences, and the simplicity and low cost of these open hole sidetracks, this method is now a standard part of the well-planning on Troll. So far total of 41 14 IADC/SPE 112616 open hole sidetracks have been performed flawlessly on Troll, and the method is also widely used on several other fields for StatoilHydro. Future technology needs to meet the development challenges On the Troll Field there are large amounts of un-recovered oil reserves that are to be developed going forward. To be able to achieve these goals, further technology development is necessitated to achieve the drainage potential in an economically viable manner. Through the last two decades, new technologies and approaches have been tried and implemented to develop the Troll oil field. Working there is a dynamic process and further enhancements and developments are being implemented currently or planned for in the near future. There has been a continuous development of new technologies and solutions to benefit the Troll development. For StatoilHydro’s operations at Troll, there will be a constant commitment to develop new and better ways to place extended-reach wells more accurately to access and drain even more oil. The future plans to achieve these goals can be solved through delivering: • Real-time network and competence utilization; true enhanced integrated operations deliveries. • More and better proactive well placement. • Integrated work processes. • Drilling and completion efficiency. • New technology development. • Global competence utilization. • Improved seismic imaging. • Direct Hydrocarbon Indicator (DHI) technologies. • Confident lithology and fluid prediction. A thorough understanding of the goals for the Troll development is needed to be able to tailor the technology development towards the direct needs. Summary The Troll field development has through the last 10 years developed several key industry leading technologies for drilling horizontal wells, logging while drilling, drilling fluids and completion strategies for horizontal wells. Some of the technologies are rotary steerable systems, although in cooperation with Eni, PDC drill bit technology, and low solids water based mud systems and horizontal well completion through several innovative technologies. One key element to drive the drilling of the horizontal wells is in the understanding of the drilling environment. Downhole measurements of tortional, axial and lateral vibrations are significant in the understanding in development of new technology in addition to best drilling practices. Through commitment and mutual challenges between the oil company and the service provider the development of the Troll Field has become the success that it is today. The understanding of the geology and reservoir properties as seen on Troll are increased over time and as that understanding increases the wells are becoming more challenging in order to drain as much as possible from one well head. This mutual growth has gained the experience and understanding of horizontal drilling for the entire Troll team. To further enhance the reliability and quality of the wellbore delivery the working operations have been developed simultaneously. The working procedures need to follow the technical development to fully utilize the potential within the new technology development. To obtain safe and fast progress the operational procedures needs to be developed and followed by the entire team involved in the process. The development of several technologies simultaneously and the ability to merge the development into a total system approach where all elements are evaluated is vital in this process. The way forward will develop new technologies and new and safe working processes to obtain as much from the Troll field as possible Acknowledgements The authors would like to thank the Troll owners StatoilHydro, Petoro AS, ConocoPhillips Skandinavia AS, Total E&P Norge AS and A/S Norske Shell for permission to publish this paper. The opinions expressed are those of INTEQ and StatoilHydro authors and may not represent the views of other Troll partners. The authors would also like to thank the contributors to the PennWell Supplement “Troll gives up its oil” Anders Nesheim, Jan-Ove Dagestad, Oddbjorn Sola, John Evans, Erland Saeverhagen, Jan Frederik Namtvedt, Arve Thorsen, Alan Reid,; Trond Gravem and Svein Egil Steen. IADC/SPE 112616 15 References 1 2 http://www.statoilhydro.com/en/NewsAndMedia/News/2006/Pages/BigPotentialForRemainingTrollOil.aspx, published 2006 PennWell Supplement: “Troll Gives up its oil” October 2007 3 The NPD's Fact-pages, www.npd.no 4 Jones, Alvestad, Kjøsnes, ”TVD Control using high accuracy pressure and deep resistivity measurements” SPE/IADC 21986, prepared for presentation at the1991 IADC/SPE Drilling Conference held in Amsterdam, The Netherlands,11 - 14 March 1991 5 Jaggi et al: “Application Of Novel Technology Improves Drilling Performance In Multi Lateral Field Development Offshore West India Reducing Risk And Increasing Production”. SPE/IADC 104623 presented at the 2007 SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 20 –22 February 2007. 6 Ronnau et al: “Integration of a Performance Drilling Motor and a Rotary Steerable System Combines Benefits of Both Drilling Methods and Extends Drilling Envelopes” – IADC/SPE Paper 91810 – prepared for presentation at the 2005 IADC/SPE Drilling Conference held in Amsterdam, Netherlands, February 2005 7 Holden, Thorsen, Gravem, Busengdal “Application and Interpretation of Multiple Advanced LWD Measurements in Horizontal Wells,” paper presented at the SPWLA 47th Annual Logging Symposium held in Veracruz, Mexico, June 4-7, 2006. 8 Bøen, Eiane, Thorsen, Kiattisak: “New Real-time Applications of Acoustic LWD Measurements in Horizontal Wells to Optimize Well Placement,” prepared for presentation at the 1st SPWLA India Regional Conference held in Mumbai, India, March 19-20, 2007. 9 Meister, M. et al.: "Formation Pressure Testing During Drilling: Challenges and Benefits", SPE 84088, paper presented at the SPE Annual Technical Conference and Exhibition held in Denver, Colorado, U.S.A., 5 – 8 October 2003. 10 Gravem, Kroken, Holden, Norman, Pragt: “Second Generation of LWD Formation Pressure Testing Improves Data Quality, Increases Sealing Efficiency and shortens Test Time,” paper presented at the SPWLA 47th Annual Logging Symposium held in Veracruz, Mexico, June 4-7, 2006. 11 Bell, Hampson, Eadsforth, Chemali, Wang, Helgesen, Meyer, Peveto, Poppitt, Randall, Signorelli, 2006. “Navigating and imaging in complex geology with azimuthal propagation resistivity while drilling” presented at 2006 SPE Annual Technical Conference and Exhibition 12 Wang, Meyer, Yu, 2006. “Dipping bed response and inversion for distance to bed for a new while-drilling resistivity measurement”, Expanded Abstracts, 76th Ann. Mtg., Soc. Explorat. Geophys. 13 Dagestad 2006 IO Dagestad, Nathan, Lilleng, Pedersen, ”Proven Commercial Implementation Of Second Generation Integrated – Remote Drilling Operations Center, IADC/SPE 99066, prepared for presentation at the IADC/SPE Drilling Conference held in Miami, Florida, U.S.A., 21–23 February 2006. 14 Dagestad, Saeverhagen Nathan, Knutsen, ”Multi-Skilling as a Key Factor for Economically Viable Operations in a Mature Oil Province: Oseberg East as a Case Example” SPE/IADC 105065, prepared for presentation at the 2007 SPE/IADC Drilling Conference 15 held in Amsterdam, The Netherlands, 20–22 February 2007. Stavland, Wolter, Evans “Mitigating Application-Specific Challenges Through a Total System Approach” SPE/IADC 99122, IADC/SPE Drilling Conference, Miami, Florida, USA, 21-23 February, 2006 16 Jaggi et al: “Successful PDC/RSS Vibration Management Using Innovative Depth-Of-Cut- Control Technology: Panna Field, Offshore India” – SPE/IADC Drilling Conference 2007, Amsterdam, Netherlands
