The Marrat reservoir in Dharif field is a deep, sour, high pressure oil accumulation of Jurassic age containing light under-saturated oil of 36-380 API. The carbonate reservoir has a porosity range of 10-20% with permeability of 1-10 md. The field was put on production in 1989 through one well. Subsequently, 10 wells were added gradually developing the field. As of date, the field has produced about 12.5% of oil in place, lowering the reservoir pressure from 10,525 to 7,000 psi.
At present, oil production from the field is about 13,500 bbls/day. Due to low permeability, some wells produce with high drawdown approaching asphaltene onset pressure (AOP), estimated at 3,400 psi. This causes Asphaltene deposition in the tubing that requires cleaning to maintain the production level. The major challenges now are to produce the wells above AOP to avoid asphaltene precipitation in the wells or reservoir while sustaining the production level and maximizing recovery.
Hence, Full Field Model (FFM) for simulation studies was constructed and history-matched. Under depletion case, where the wells produce above AOP, field produced about 24% STOIIP. The water injection case shows significant increase in recovery to 40% STOIIP. Since no prior experience of water injection is available for such tight deep carbonate reservoirs in West Kuwait Fields, several key studies such as a) RCAL & SCAL b) Core flood Study c) Water Compatibility & Scale Prediction modeling d) Injectivity test, were carried out to address water injection feasibility.
The present paper shares the results of above studies which indicate that water injection is a viable option to maintain the reservoir pressure to produce the wells above AOP as well as to maximize recovery. Pilot water injection is planned through one well for which the area has been optimized using FFM. At present Pilot Water injector and source wells have been drilled and injection will be initiated with commissioning of surface facilities
Dharif field is NNE trending elongated anticlinal structure with faulted western limb. The Marrat reservoir in this field has developed in carbonate aggradational and progradational depositional setting. The field was discovered in 1988, put on production in 1989 and gradually developed with additional producers until 2004 (Fig-1). As of today, total 13 deep wells have been drilled in this field of which eleven are completed in the Marrat reservoir, while two are completed in a shallower Jurassic reservoir. The reservoir porosity ranges between 10-20 % while the average permeability is low, ranging between of 1-10 md with locally higher permeability of about 20 - 30 md in some layers. The average net reservoir thickness is about 200 ft and water saturation is less than 15 %. Initial oil water contact (OWC) was estimated to be 13,360 ft Subsea. The initial reservoir pressure was 10,525 psi at 13,200 ft SS (datum). The oil is under saturated with saturation pressure as 1,959 psi. Oil is light and the density is 36-380 API. The asphaltene onset pressure (AOP) is nearer to 3,400 psi, at a temperature of 2350 F.
Verma, Naveen Kumar (Kuwait Oil Company) | Al Shamali, Adnan Aiesh (Kuwait Oil Company) | AL-Mayyas, Eman Abdullah (Kuwait Oil Company) | Dasma, Mohammed (Kuwait Oil Company) | Al-Khamis, Waleed Khamis (Kuwait Oil Company)
Unconventional fractured Najmah-Sargelu (NJ-SR) and tight carbonates of Middle Marrat constitute Jurassic reservoirs in Minagish Field. Although NJ-SR oil was discovered in mid-eighties, reservoir development remains a challenge in this field where exploratory delineation wells have been drilled until 2002. The reservoir characterisation difficulties in this unconventional fractured reservoir include large well spacing, field compartmentalization, low resolution seismic and oil based well logs besides drilling problems. Therefore, understanding of the productivity controls is primarily based on the image logs and core data collected in vertical exploratory or Marrat development wells. Based on these data in Minagish and adjoining fields, supplemented with seismic facies analysis for identifying sub-seismic 'fracture corridors', a regional 3D Discrete Fracture Network model (DFN) was built for West Kuwait in 2004. It thus became feasible to place a Marrat well through NJ-SR fractured area or even to intercept a fracture corridor using a deviated well profile. Yet, since the model was largely conceptual, a quantitative validation was planned through acquisition of NJ-SR core in such wells, followed by core orientation using Goniometry technique and detailed core to image log fracture calibration to understand relationship between large and small scale features and their role in productivity.
First such deviated well was successfully drilled near a fracture corridor and cored using 'half-moon' core barrel technology despite repeated core jamming. Core was oriented using goniometry technique and striking parallelism was observed between the image log based and core based fractures. However, the well could not be tested in NJ-SR and was completed in Marrat based on reservoir considerations. Similar core-based fracture characterization was successfully done in second such deviated well that traversed through a deformation zone further away from a fracture corridor. The well has recently been tested and produced oil from NJ-SR reservoir fulfilling combined objectives of appraisal and development of multiple Jurassic reservoirs. Present paper provides a detailed account of these fracture characterization efforts through goniomentry and core to image log calibration for understanding the productivity controls, and discusses further well design optimization possibilities.
The oil accumulations in NJ-SR reservoirs of Minagish Field is a result to unique geology characterized by organic rich mature source rock interlayered with tight fractured carbonate rocks (Goff, 2004). This deep, over pressured reservoir sealed by a thick Gotnia Salt-Anhydrite cap-rock from above and by Dharuma shale from below. The productivity of this very low matrix porosity reservoir is enhanced by the occurrence of natural fractures created as a consequence of folding, faulting, high horizontal stress and over pressured reservoir conditions. In West Kuwait oilfields, the NJ-SR reservoir is underlained by another Jurassic carbonate reservoir - the Middle Marrat reservoir (MMR), where oil has accumulated on structural highs in low to moderate porosity limestone and dolomitic limestone layers. The MMR Reservoir is bounded by tight argillaceous limestone & shale layers of Upper & Lower Marrat from above and below respectively.