Abstract An oil & gas operator in XiJiang block offshore South China has shifted the development of highly mature oil fields that have been producing for 17 years toward the remaining undeveloped marginal reservoirs using a horizontal well drilling program.

The main targets of the new development campaign are the remaining thin oil column reserve in attic locations of the reservoirs as well as the unproduced thin laminated reservoirs in the area. Upon understanding the uncertainties and challenges associated with drilling horizontal wells in these complex reservoirs, an innovative drilling approach was initiated for accurate horizontal placement in thin sands, channel sands, and thin oil column reservoirs with strong bottom-water. The approach includes the integration of an advance multifunction formation evaluation Logging-While-Drilling (LWD) tool that provides real-time formation evaluation and structural interpretation along with a bed boundary mapper LWD tool with the ability to map multiple key boundaries that are the prerequisite parameters that must be identified during the execution stage and which include the water contact and top and bottom of the reservoir structure simultaneously in distance.

Outstanding outcomes have been observed by implementing this new approach in the complex target reservoirs. The approach was applied to multiple challenging wells, each of which has produced from 2,000 to 6,000 BOPD with very low water cut, far exceeding the set production goals of 1,500 to 2,000 BOPD. These are very promising development economics in the oil fields that have 90% to 95% water cut on average.

The successful implementation of the new development strategy in highly mature oil fields will lead to sustained and extended production, increasing the ultimate recovery and well economics. The approach provides an example of using integrated technology solutions to overcome the challenges in complex target reservoirs.

Introduction The XiJiang oilfields block is located in the Pearl River basin approximately 135 km southeast of Hong Kong, South China Sea (Fig. 1). The field was discovered in 1985 and started to produce oil in 1995. The block is commercially operated by China National Offshore Oil Corporation (CNOOC) Shenzhen branch. There are two platforms (Platform-2 and Platform-3) with a total of 48 slots installed in the vicinity of these two structures.

After many years of production, the reservoirs have entered the maturity phase. By the end of April 2010, XiJiang had produced total 391 MMBO, three times more than the Oilfield Development Plan (ODP) estimate. The field production rapidly declined and water production increased. By this time, the average water cut reached more than 90%.

The field development of these highly mature oil fields then shifted toward using a horizontal well drilling program for the remaining undeveloped marginal reservoirs. The main targets of the new development campaign are the remaining thin oil column reserve in attic locations of the reservoirs as well as at the unproduced thin laminated reservoirs in the area.

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Unlocking the Potential of Complex Marginal Reservoir in Highly Mature Oil Field, XiJiang 24 South China Sea Luo Donghong, Yan Zhenghe, Gao Xiaofei, Rao Zhihua, and Du Ding Yu, CNOOC; Parlindungan Monris Halomoan, Amarjit Singh Bisain, Liu Yang, and Olivia Azwar, Schlumberger

Copyright 2013, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Beijing, China, 2628 March 2013. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference 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 where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435

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Fig. 1XiJiang field location.

Background The Pearl River Mouth basin is a continental margin sedimentary basin formed during the rifting of the South China Sea from the Late Mesozoic to Early Tertiary. Reservoir profile

XiJiang oilfield consists of multiple reservoirs deposited along the Zhu Jiang Formation up to the bottom of Han Jiang Formation during the Cenozoic. The section consists of interbedded sandstone, siltstone, and shale with some presence of limestone, dolomite, and coal. In general, the reservoirs are thick, with a bottom-water and stratified edge-water system (Fig. 2).

Fig. 2Reservoir distribution, XiJiang oilfield.

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XiJiang Structure The structure of XiJiang Platform-3 is branched anticline controlled by a basement fault with clear reverse drag

phenomenon. The structure is relatively flat at top and the sides slope steeply downwards. The XiJiang Platform-2 structure is located 12 km away from XiJiang Platform-3. The structure is a low-amplitude drape anticline reservoir which is flat in general, but steeper toward the south. Production History

The XiJiang oilfield was put into production in 1994. Maximum production of 120,000 BOPD was reached in 1997, and production started to decline with low decline rate of 2.4%. However, after 2007, the decline rate rapidly increased, reaching 28% (Fig. 3). By 2009, the field average water cut reached more than 90%.

Fig. 3XiJiang oilfield production history.

Bypassed Reserve

A new 3D reservoir model (static and dynamic) of XiJiang oilfield was generated based on an integrated reservoir study. The model was verified by the information collected from core, seismic, logging, and reservoir surveillance data. The study revealed that the remaining oil reserves are located in attic locations of the reservoirs and in the unproduced thin laminated reservoirs in the area.

The study further explained that the remaining oil reserves in the field are controlled by following factors: 1. Attic oil controlled by structure

The oil is accumulated in the crest area of structure; additional new and sidetrack wells are required to be drilled in this crest area to recover the remaining oil reserve.

2. Bypassed oil controlled by poor swept area Remaining oil has been left between well patterns because of water coning; additional new/sidetrack wells are required to be drilled between existing wells.

3. Bypassed oil controlled by low permeability reservoir The oil remains unproduced despite several years of commingled production with high-quality reservoirs; additional new and sidetrack long horizontal wells are required to be drilled to improve reserve recovery in the low-permeability reservoirs.

4. Attic oil controlled by interbed roof layers The oil remains unproduced within certain interbed layers; a low production flow rate required to produce bypassed oil within these interbed layers.

The field development of these highly mature oil fields then shifted toward a horizontal well drilling program for the remaining undeveloped marginal reservoir. The main targets of the new development campaign are the remaining thin oil column reserve in attic locations of the reservoirs and the unproduced thin laminated reservoirs in the area. Challenges

During the horizontal well drilling planning stage, the team identified several major subsurface challenges that needed to be overcome during the drilling execution:

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1. The structural and stratigraphic profile of the field with the sudden change in dips, presence of faults, lateral property change, formation thickness change, and the presence of thin diverse carbonate layers (tight streak) could cause exiting from the reservoir and result in low reservoir contact, low drainage area, and low recovery factor.

2. The thin oil column and reservoir has a strong Bottom-water drive system. Therefore, the well laterals must be placed near the top of the target zone. An inability to maintain close distance to reservoir top would result in bypassed oil, early water breakthrough, and low recovery factor.

Solution

The team acknowledged that a fit-for-purpose technology was needed to overcome the challenges and to achieve the objective of drilling horizontal wells in these mature oil fields. Upon the completion of the feasibility study, real-time near-bit density images provided by the multifunctional formation evaluation (FE) LWD suite (Fig. 4) were included in the plan to enable the team to land the well optimally inside the reservoir targets.

Fig. 4Multifunction LWD service integrates a full suite of formation evaluation, well placement, and drilling optimization measurements into a single collar.

The well placement distance-to-boundary technique using an advance deep directional electromagnetic azimuthal LWD (Fig. 5) was confirmed to be applicable to the remaining reservoir targets. It was selected as the fit-for-purpose application along the horizontal section given that it has the ability to map multiple key boundaries in distance, including the fluid contact, as well as the top and bottom of the reservoir structure. In addition, the rotary steerable drilling system was also applied to optimize drilling efficiency and provide full control of the well path within the small drilling or steering window.

Fig. 5Top: Deep directional electromagnetic azimuthal LWD technology has multiple frequencies and transmitter-receiver spacings and integrates two tilted receivers top, R3 and R4) and one transverse transmitter (T6). Bottom: Mapping of multiple key boundaries in distance including the fluid contact, the top and bottom of the reservoir structure, and the resistivity value of the target, upper bed, and lower bed.

The five case studies that follow represent different subsurface challenges and profiles encountered during the execution stage in XiJiang oilfield.

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Execution

Case Study 1 - Fault, fractures and dip uncertainties This particular well was recognized as a very complex reservoir in which the thin target zone is located near the main fault

with high structural uncertainties; minor faults are expected along the lateral, and the existing oil water contact (OWC) is unknown (Fig. 6). Limited data available from the offset wells made this well even more challenging, and staying in the target was extremely important because of main fault near to the toe of the well (Fig. 7).

Fig. 6Proposed lateral trajectory depicted on the structure map of the top reservoir target, with location and distance to offset wells.

Fig. 7Reservoir target profiles and stratigraphy variation based on few offset wells.

Eventually, the well was properly landed utilizing the real-time density image and the triple combo measurements from the

formation evaluation LWD tool (Fig. 8). There was no fault detected during landing, but there were several mud lost circulation events as the well drilled through fractured a carbonate layer.

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Fig. 8Smooth landing section with the aid of real-time close-to-the-bit density image; real-time (RT) trajectory was 5 m deeper than planned trajectory.

During the lateral section drilling, the well placement distance-to-boundary technique using advance deep directional electromagnetic azimuthal LWD was utilized and proved its advance ability by clearly mapping multiple boundaries (Fig. 9) including reservoir top and the boundary between more the conductive zone and more resistive zone in the upper part of the reservoir. As a result, the trajectory could be placed accurately near the reservoir top. Multiple minor faults were also clearly identified, and this information allowed the team to make adjustments to put the trajectory back to the target zone in timely manner.

Fig. 9Mapping multiple boundaries and faults along the horizontal section with the advance deep directional electromagnetic azimuthal LWD tool.

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Case Study 2 - Lateral Facies Change and Bottom-water This well in this case study was also planned to be drilled close and parallel to a known fault (Fig. 10).

Fig. 10The proposed horizontal well planned to be drilled close and parallel to the fault.

The challenges identified included structure dip changes, possible lateral facies change, strong Bottom-water drive, and the

current OWC uncertainty. The lateral section was set to be placed within a 1-m window below the reservoir top (Fig. 11).

Fig. 11The lateral section was set to be placed within 1 m window below the reservoir top target.

The well was successfully landed within a 1-m window below the reservoir top despite the fact that the top was 2.3 m

deeper than predicted (Fig. 12). This was achieved by early well monitoring which allowed the team to made adjustments in time.

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Fig. 12Successful landing within a 1-m window below the reservoir top; the top was 2.3 m deeper than expected.

During the lateral drilling, the deep directional electromagnetic azimuthal LWD tool showed that there was a lateral facies

change in the upper part of the reservoir at the early part of the horizontal section. At the beginning, the upper part of the reservoir was found to be shaly (sandstone with some shale content). Then, after a

30-m interval, the shaly sand zone in the upper part of the reservoir disappeared, and the formation was much cleaner sandstone. The trajectory was then accurately kept between 0.5 m to 0.7 m below the reservoir top throughout the horizontal section. The possible current OWC level was also identified at approximately 3 m below the reservoir top. This valuable information helped the team to optimize the completion design using inflow control device (ICD) and to update the existing reservoir model for further well development planning. The detail of the reservoir profile can be seen in Fig. 13.

Fig. 13Detailed reservoir profile of the target reservoir based on the data from the deep directional electromagnetic azimuthal LWD bed boundary mapper tool.

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Case Study 3 - Oil Column Thickness Change and Bottom-water This well drilled from the Platform-3 targeted one of the upper reservoirs in the block. The plan was to drill 250 m of

lateral section and place the well close to reservoir top (Fig. 14).

Fig. 14The planned well trajectory to drill 250 m of lateral section; several producing offset wells exist near the landing point and toe area of the lateral section, and no wells exist near the middle part of the lateral section.

Most offset wells showed that the reservoir has at least a 6-m oil column with Bottom-water (Fig. 15). However, the team expected that the current OWC has risen as a result of several years of production. The trajectory was set to be placed near the reservoir top, and it was expected that the current OWC would be identified while drilling.

Fig. 15Offset wells showed that the reservoir originally has at least 6 m oil column with Bottom-water.

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The well was landed as per the original plan below the target top as all markers came in at the expected depth (Fig. 16). The rotary steerable system delivered the desired dogleg to get to the landing point, and the near-bit gamma ray and inclination provided early information that allowed the team to call the section total depth (TD) at the desired 89.3 inclination and prevent the well from penetrating deeper into the target.

Fig. 16The well was landed as per original plan below the target top.

The top of the reservoir was clearly identified and mapped along the lateral section, and the trajectory was placed within a

1-m window below the reservoir top. The Bottom-water was also identified throughout the lateral section. Interestingly, the Bottom-water level was not flat and was found to be varying along the trajectory (Fig. 17).

Fig. 17The top of the reservoir was clearly identified and mapped along the lateral section, and the trajectory was placed within a 1-m window below the reservoir top. The Bottom-water was also identified throughout the lateral section. The fluid contact level was not flat and found to be varying along the trajectory.

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The OWC level along the landing point area was found to be closer to the reservoir top. The OWC level along the middle part of the lateral section was farther from the reservoir top and closer again toward the end part of the lateral section (Fig. 17). This phenomenon was caused by the coning effects and can be explained by the fact the several producing offset wells exist near the landing point and toe area of the lateral section and no wells exist near the middle part of the lateral section (Fig. 14). The team used this valuable information to design the completion using the inflow control device (ICD).

Case Study 4 - Thickness variations and detection of resistive carbonate layer below and conductive shale layer above The well was proposed to drill near the crest area of an anticline. It was predicted that during the landing, the formation

would be slightly updip then level off to be flat (Fig. 18).

Fig. 18 The well was proposed to drill near the crest area of an anticline.

The study during planning stage suggested that a lateral property change might occur along the lateral section. There was

also indication of carbonate layer developed in the upper part of the reservoir from the middle to end part of the lateral section. The structural and reservoir depth near the landing point was not a major concern since several offset wells exist very close

to the landing point. However, during drilling the landing section it was found that the reservoir deeper by 6 m compared to the original plan. Despite this large discrepancy, the well was landed smoothly below the reservoir top (Fig. 19).

Fig. 19The reservoir was found deeper by 6 m compare to the original prognoses. Despite of this big discrepancy, the well was landed smoothly below the reservoir top.

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Several surprises were found when drilling the lateral section. The closest offset well at landing point does not indicate the presence of a carbonate layer in the upper part of the reservoir. In actuality, the deep directional electromagnetic azimuthal LWD tool showed that the carbonate layer developed along the landing point interval approximately 1 m below the reservoir top (Fig. 20). The boundary mapping showed the distance from the trajectory to the top carbonate was approximately 0.7 m, and the distance to the reservoir target was 0.3 m. This means not only that the target zone had become thinner, but also that the steering window became smaller.

The top of the reservoir and top of the carbonate layer were clearly delineated and mapped along the lateral section, and the trajectory was placed close to reservoir top. It was found that the target zone was getting thinner toward the middle part of the lateral section because the standoff between the reservoir top and the carbonate layer top decreased. In an interval toward the toe, the standoff increased to 1.5 m, and it then decreased again close to the toe of the well.

Although the target zone thickness varies from 0.4 m to 1.5 m, because of the undulating carbonate layer below the reservoir top, most of the lateral was placed less than 0.5 m from the top along the horizontal section.

Fig. 20Detailed reservoir profile based on the deep directional electromagnetic azimuthal LWD bed boundary mapper information.

Formation dip varied, starting out relatively flat, and then dipping down 1 before it became flat again towards the end. After drilling to the original plan of 200-m horizontal section length, the team decided to extend this section by 50 m to compensate for the reservoir quality, which was slightly less than the expectation. The real-time information from both multifunctional formation evaluation and deep directional electromagnetic azimuthal LWD tools played a vital role in making this decision on the fly and ensuring that the extended plan could be achieved.

Case Study 5 Channel Sand and pinch out reservoir This particular well was set to target thin channel sand. The trajectory was planned to be placed along the middle of the

target since the Bottom-water was not expected to exist. There were only a few offset wells available around the planned horizontal section (Fig. 21). The study from the offset wells showed that the target thickness varied from 1.5 m to 4.4 m (Fig. 22). The team recognized that when dealing with thin target, a slight change in dip or thickness would significantly increase the risk of exiting from the target zone.

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Fig. 21The planned well trajectory on target top structure map.

Fig. 22Well correlation analysis indicated thickness changes and property changes on the target in each offset well.

During the landing drilling operation, the real-time density image from the multifunctional formation evaluation tool was

utilized to determine the structural dip. The well was landed smoothly 0.8 m below the target top with 90.5 inclination, leveling off with the 0.5 structure dipping up, identified from the real-time density image (Fig. 23).

Fig. 23The well was landed smoothly 0.8 m below the target top with 90.5 deg inclination, leveling off with the 0.5 structure dipping up, identified from the real-time density image.

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A reservoir resistivity value of approximately 10 ohm.m was observed when the lateral section was started. After further investigation using the real-time density porosity data, the team determined that the interval had some clay content and so should be defined as shaly sand. At the same time, the deep directional electromagnetic azimuthal LWD tool detected and mapped a higher resistive zone above the trajectory. The decision was then made to chase the higher resistive zone above the shaly sand interval with the help of clear bed boundary mapping provided by the deep directional electromagnetic azimuthal LWD tool (Fig. 24).

Eventually, the trajectory then entered the higher resistive zone after less than a 20-m drilling interval. The real-time data from the multifunctional formation evaluation tool then identified the higher resistive interval as a clean sand zone. The well was then placed optimally within this clean channel sand until the planned TD was reached. During the placement within this lateral section, multiple boundaries, including top and bottom of clean channel sand and top and bottom of the shaly sand, were clearly delineated and mapped. The bed boundary mapping feature clearly showed the geometry profile of the channel sand. In the beginning, the thickness of the channel sand was about 0.8 m; toward the middle of the lateral, the channel sand became thicker, approximately 4.5 m, and then thinning to 2 m toward the toe.

Fig. 24The geometry of the channel sand mapped by deep directional electromagnetic azimuthal LWD tool.

Results The use of multifunctional formation evaluation and azimuthal bed boundary mapping LWD technology has consistently delivered excellent well performance. As of October 2012, five horizontal wells have been drilled using the multifunctional formation evaluation and azimuthal bed boundary mapping LWD tools with results of an average 3,180 BOPD initial production per well and 1% water cut. The results show incredibly higher production of 280% compared to the set targets (Table 1).

Table 1Production performance of re-entry horizontal well in XiJiang oilfield

Well Horizontal Section (m)

KPI Initial Production (BOPD)

Plan Actual Plan Actual Well#1 300 304 100% 1,000 4,060 Well#2 200 200 100% 800 1,320 Well#3 250 268 100% 800 3,516 Well#4 200 295 100% 800 4,000 Well#5 200 250 100% 800 3,000 Total 1,150 1,317 100% 4,200 15,896

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During this new horizontal well campaign in XiJiang oilfield, the production decline rate of the fields was arrested and is currently running at a 0% decline rate (Fig. 25).

Fig. 25The XiJiang fields showed a 0% production decline rate with the sidetrack (re-entry) horizontal well program.

This achievement is very critical since more than 15% of production decline rate would have been occurred without the

horizontal well re-entry (sidetrack) program (Fig. 26).

Fig. 26More than 15% production decline rate of XiJiang fields without the sidetrack (re-entry) horizontal well program.

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Conclusion The ultimate objectives of sustaining and extending the production and increasing the ultimate recovery and well economics of XiJiang mature oil fields has been successfully achieved with the following key factors:

1. Strong team work within multifunctional disciplines of the assets, drilling teams, and service providers. 2. The horizontal well drilling approach used fit-for-purpose technology for optimum well placement drilling. 3. The applications of fit-for-purpose technologies including advance multifunction formation evaluation and

advance deep directional electromagnetic azimuthal LWD with distance-to-boundary technique for well placement in XiJiang horizontal well drilling campaign has improved our understanding of the reservoir characteristics by accurately mapping multiple key boundaries in distance, including the top and bottom of the reservoir structure and OWC level. These advantages have enabled the team to make effective real-time decisions for placing the trajectory in the best place to drain the remaining hydrocarbon and to keep the trajectory away from the OWC.

These key factors can be applied in similar mature fields to optimize base business reserve recovery and extend the economic life of the fields.

Acknowledgments The authors would like to thank CCLS (CNOOC Shenzhen) and Schlumberger for the permission to publish this information.

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