One of the key issues in creating a good reservoir model in carbonate reservoirs is identifying the horizontal permeability conduits--"thief zones"--if there are any. In the Sabriyah field in Kuwait, dynamic measurements showed evidence of thief zones in the Lower Cretaceous (Albian) Mauddud formation. Early water breakthrough has occurred in some wells. Previous studies indicated that it was very challenging to detect the thinly layered thief zones using conventional openhole logs. This paper describes a method of recognizing the different types of thief zones in the Mauddud carbonate reservoirs using high-resolution image logs with calibration from core and dynamic measurements and by integrating image logs with nuclear magnetic resonance (NMR) and conventional openhole logs.
The Mauddud carbonates are Early Albian in age and consist of grainstones, wackstones, and mudstones deposited in a ramp setting. Observations from production logging tools (PLTs) and production data indicated that there are a few thief zones in different levels within the vertical Mauddud sequence. A previous core study shows that the fractures in the Mauddud formation are short (<10 cm) and concentrated in diagenetically cemented layers. The fractured thin layers are believed to be the principal type of thief zone. Another type of thief zone is associated with better-developed vuggy porosity. This study shows that both fractured and vuggy porosity-related types of thief zones can potentially be detected through integration of high-resolution image logs with PLT, NMR, and conventional logs. In addition, methods of estimating fracture permeability and porosity-related permeability based on logs are also proposed. The log-estimated permeability determined using this approach fits better with the production profile and can then be used to evaluate the thief zones in a more quantitative manner.
Li, Bingjian (Schlumberger) | Najeh, Hamad (Kuwait Oil Company) | Lantz, Jim (BP) | Rampurawala, Masoor (Schlumberger) | Gok, Ihsan Murat (Schlumberger Data & Consulting Services) | Al-Khabbaz, Mohammed (Schlumberger)
One of the key issues in creating a good reservoir model in carbonate reservoirs is the identification of the horizontal permeability conduits— "thief zones??— if any. In the Sabriyah field in Kuwait, dynamic measurements showed evidence of thief zones in the Mauddud formation. Early water breakthrough has occurred in some wells.
Previous studies indicated that it was very challenging to detect the thinly layered thief zones using conventional openhole logs. This paper describes a methodology of recognizing the different types of thief zones in the Mauddud carbonate reservoirs using high-resolution image logs with calibration from core and dynamic measurements and by integrating image logs with nuclear magnetic resonance (NMR) and conventional openhole logs.
The Mauddud carbonates are Early Aptian in age and consist of grainstones, wackstones, and mudstones deposited in a ramp setting. Observations from production logging tool (PLT) and production data indicated that there are a few thief zones in different levels within the vertical Mauddud sequence. A previous core study shows that the fractures in Mauddud formation are short (<10 cm) and concentrated in diagenetically cemented layers. The fractured thin layers are believed to be the principal type of thief zone. Another type of thief zone is associated with better-developed vuggy porosity. This study shows both fractured and vuggy porosity-related types of thief zones can be potentially detected through integration of high-resolution image logs with PLT, NMR, and conventional logs. In addition, methodologies of estimating fracture permeability and porosity-related permeability based on logs are also proposed. The log-estimated permeability determined using this approach fits better with the production profile and can then be used to evaluate the thief zones in a more quantitative manner.
Chetri, H.B. (Kuwait Oil Co.) | Najeh, Hamad (Kuwait Oil Co.) | Al-Janaie, Dalal Ali (Kuwait Oil Co.) | Al-Mutawa, Majdi (Kuwait Oil Co.) | Chandran, T.P. (Kuwait Oil Co.) | Al-Anzi, Ealian H.D. (Kuwait Oil Co.) | Wells, John (Kuwait Oil Co.) | Cig, Koksal (Schlumberger)
The Sabiriyah Mauddud limestone of North Kuwait is a super giant depletion drive oil reservoir. Massive development efforts are underway, with a planned enhancement in oil production through phased pattern water flood. The Phase1 development addresses the crestal area (50 km2) of the structure, which was the focus for development efforts in the past. The Phase 2 development consists of all remaining peripheral acreage (200 km2), with immense reservoir rock & fluid uncertainties, leading to the identification of reservoir segments for re-appraisal through drilling & testing of new wells.
The fluid description had been a challenge for the reservoir, especially in the mid flank and down flank areas with poor rock / fluid quality. The past data on fluid quality had been the average for the whole of the Mauddud unit perforated together. A serious data gap existed with respect to the vertical variation of the reservoir oil quality. Sampling of formation fluids has been historically not possible in the formation due to low permeability and sharp viscosity variation with depth, to fully develop and discover the potential of the formation. The permeability and viscosity combination of the formation has not allowed fluids to be sampled with a probe formation tester tool. The slick line sampling operation has been also attempted with nitrogen lifting without any result. In early 2002, in order to match the field development plan, formation tester with an inflatable dual packer module was utilized to achieve sampling. The operation was completed successfully in the 9 5/8" cased hole environment. Second half of the year, sampling was also achieved in 8.5" open hole conditions. Late 2002 and onwards, the sampling operation became part of the open hole logging program and the versatility of the wireline tester tool also allowed performing mini-DST and vertical interference applications in required wells. Re-appraisal & testing work, along with enhanced fluid description, led to the conclusion that a large segment within the Phase 2 area, originally considered as a regime of light oil - turned out to be viscous oil.
Prior to progressing on the water flood development in Phase 2 area, significant amount of data was acquired through a rational placement of new wells so as to reduce the uncertainty on rock quality. Numerous injectivity tests / step rate tests were conducted, the results of which were suggestive of the concept of injecting above the formation parting pressure in down flank areas with poorer rock & fluid quality and design the facilities accordingly to meet the subsurface requirements.
The earlier estimates of the water flood recovery were discovered to be quite optimistic as reflected by the latest full field reservoir simulation model runs. The above realizations changed the past outlook of development plan and led to the fine-tuning of the way forward.
The paper proposes to share the unique experience & benefits of progressing through a phase development approach for this giant & challenging carbonate reservoir in North Kuwait.
Mauddud reservoir is a giant oil bearing carbonate reservoir in the Sabriyah field of North Kuwait (location map placed at figure-1). It is overlain by a minor carbonate reservoir viz. Tuba and underlain by Lower/ Upper Burgan reservoirs. The reservoir is comprised predominantly of mid ramp packstones. Presence of higher energy grainstones and floatstones typically result in higher permeability layers within the reservoir.
Although there appeared relatively good lateral correlation between the facies types, the lateral continuity remained a major unknown. Major pressure barriers are stratigraphically confined and result from intense cementation - these barriers are laterally extensive. A number of potential flow baffles are also recognised, their effect on recovery is uncertain at this time. Sabriyah fluid movement appears to be dominated by the grainstone/packstone interval.
Azim, Saikh Abdul (Kuwait Oil Co.) | Abdullah, Shehab A.M. (KOC) | Al Zaabi, Hamad (KOC) | Al-Awadi, Waleed (KOC) | Najeh, Hamad (BP) | Bryant, William (Baker Atlas) | Aldred, Richard D. (Baker Atlas) | Steel, Mark Robert
Laminated shaly sand sequences are often overlooked when determining pay intervals in clastic reservoirs. This is because traditional methods of calculating hydrocarbon saturations in these formations use resistivity measurements and one of the numerous shaly-sand saturation equations. These techniques generally understimate the hydrocarbon content because conventional resistivity measurements are dominated by the conductive component of the formation, in this case shale. Even when the excess conductivity due to the shale is accurately accounted for in the calculation, there is still a high degree of uncertainty in the results.
Laminated sequences such as these are commonly found in the Burgan Formation in the Raudhatain and Sabiriyah Fields in North Kuwait. They have traditionally been considered as non-pay and the hydrocarbon content of marginally shaly formations has generally been underestimated.
Recent work re-evaluating wireline log measurements in these fields, taking the shaly formations into account, has significantly increased the volume of hydrocarbons in place. However, there remained a high degree of uncertainty in the results. These new estimates can now be confirmed, and the uncertainty removed, by the combined use of vertical and horizontal resistivity measurements. The measurements, made using a multi-component induction logging tool, measure the electrical anisotropy of the formation, and as such can give a greatly improved measure of the conductivity of the thin sand layers within the laminated sequences. Additionally, these measurements can be used to determine whether the shaly sand is a laterally extensive laminated formation or a disturbed non-reservoir quality rock.
This paper will review the technology used to acquire the measurements, show examples from the wells logged in North Kuwait, and describe the impact on reserves estimates. The paper also highlights a successful case of active interaction between KOC Field Development and Research & Technology Groups and Baker Atlas as a service company.
Chandran, T.P.R. (Kuwait Oil Company) | Vervest, Edwin (BP) | Brown, Matt (BP) | Al Hajeri, Mubarak (Kuwait Oil Company) | Al-Anzi, Ealian (Kuwait Oil Company) | Najeh, Hamad (Kuwait Oil Company) | Chakravarthi, Ravula (Kuwait Oil Company)
This paper details a program to test saturation logging in a Middle East carbonate reservoir that recently began a secondary recovery project. The ability to measure oil and water saturations in the target flood zones is critical for optimizing recovery and finding unswept zones. Water saturation is typically determined using a pulsed neutron capture logging tool (PNC). The PNC differentiates between the neutron capture cross-section of high salinity water and hydrocarbon. In the case of this waterflood, seawater is injected, with a salinity of 25,000 ppm. This seawater has a similar neutron capture cross-section to the oil in the formation, making PNC tools unsuitable for determining water saturation in break through zones.
A test program was developed to determine the reservoir fluid saturation using carbon-oxygen (C/O) logging techniques. The new generation through- tubing C/O tools with higher measurement resolution enables detection of low salinity seawater. The test program consisted of a log-off between three logging service companies providing C/O tools. The C/O derived saturations were compared to the open hole log saturations and to PNC saturations in two wells in the waterflood pilot area - a comparison that was possible since the injected fluid in the pilot was high salinity formation water.
Most of the wells in the waterflood area are completed with dual tubing strings. The saturation logging target interval is generally in the short string of the dual completion, making wireline surveillance difficult. The test program also evaluated the ability of the contractors to log in a dual environment. This paper details the results of the C/O test logging program and provides recommendations on future saturation logging requirements for this type of reservoir.
The Sabiriyah field was first brought on production in the 1950's and produced intermittently on primary recovery up through the Iraqi invasion of Kuwait. Many of the wells in the field were sabotaged and were redrilled or recompleted. The Mauddud carbonate is the shallowest horizon under development in the field. Beginning in late 2000, a secondary project was initiated in the Mauddud using inverted 9 spot patterns of approximately 1000 acres. Twelve patterns cover the crest of the field for the first phase of secondary development (Figure 1).
The Mauddud is a limestone reservoir with a gross thickness of about 365 feet. The reservoir is divided up into stratigraphic layers 1 through 11, with the main target of the waterflood consisting of layers 2 through 7. Porosities in the pay intervals vary from 8% to 30% with low (<10%) initial water saturation. Matrix permeability ranges from 10 to 80 mD, with high permeability streaks measuring 5 to 10 times matrix. Fractures and vuggy zones are distributed in thin layers in the Mauddud but primarily in the uppermost C zone. Virtually all of the water in the crestal area of the field is immobile. Formation water salinity is approximately 230,000 ppm.