Geosteering in a Complex Dolomitic Reservoir

Abstract

This paper presents the key of success for the development of a complex prospect, the Sarr dolomitic reservoir in the Kharir field, located onshore Yemen. The drilling of horizontal wells was considered to limit the early water encroachment observed on vertical producers and to improve both the oil production and the final recovery. A careful data acquisition program, a close cooperation between all the parties involved in the drilling process, with the implementation of common decision procedures, allowed geosteering in optimized reservoir targets, while preserving the safety of the horizontal drilling operation. The Kharir Sarr field is a multi-layered oil bearing dolomitic reservoir interbedded with tight limestones. Variable petrophysical properties characterize the dolomite in this field, following the geometry of elementary sedimentary facies, their stacking, their structural position, and finally their diagenetic history. On top of the structure, the reservoir is interpreted as dolomitized patchy biostromes, very difficult to target and to appraise with vertical wells. Monitoring the data acquired in real-time, referenced to pre-job-modeling, provided the key elements for cautious decision-making on trajectory definition. The expertise developed in geological steering has contributed to boost oil production and add significant field reserves. Introduction Field data overview TOTALFINAELF E&P YEMEN (interest 28,57%) operates the Block 10, located in the Hadramaout area, east of the Republic of Yemen, on behalf of the East-Shabwa association, which comprises also COMECO 28.57%, OCCIDENTAL 28,57% & KUFPEC PETROLEUM LTD 14,29%. The Kharir Sarr field has been discovered in April 1993 by well A (Fig. 1). 2D infill seismic was shot in 1994. Two additional delineation wells (Wells B & C), drilled in 1998, have cored the complete Sarr dolomitic section. The Kharir Sarr formation is draping a basement high, created by Jurassic rifting, and reactivated during the Oligocene Aden Gulf rifting. The Sarr formation, dated Valanginian (Cretaceous) is constituted of interbeds of limestone, chalky limestone, dolomite and shale. Reservoir definition The Sarr dolomite reservoir S4 has been subdivided into five layers, namely S4a to S4e (Fig. 2). The Sarr dolomite cycle started with Early Valanginian middle ramp limestones overlain by argillaceous material of the Sarr Shale (S5 reservoir layer), over which the reservoir-forming bivalve bioaccumulations developed. This evolution illustrates a shallowing-up evolution in terms of depositional environment, from the middle ramp limestones at bottom, to the more proximal, inner ramp mollusk facies. Two pulses of slight re-opening and deepening of the depositional environment are noticed at the base of S4b & S4a layers, interbedded with dolomitized algae laminations, i.e. very shallow deposits. At the end of the interval, a marked deepening (S3) is associated with the middle ramp limestones very similar to the base of the cycle (S5). The reservoir facies consists of pervasively dolomitised mollusk bioaccumulations (Fig. 3). Those are considered lens-shaped mounds of 1 to 2 meters in thickness, with lateral extension that may reach a ten's of meters. Their stacking provides interesting continuity of the distribution of vuggy porosity. The embedding mudstone, although devoid of vugs, presents good moldic and intercrystalline, relatively well connected porosity. Field data overview TOTALFINAELF E&P YEMEN (interest 28,57%) operates the Block 10, located in the Hadramaout area, east of the Republic of Yemen, on behalf of the East-Shabwa association, which comprises also COMECO 28.57%, OCCIDENTAL 28,57% & KUFPEC PETROLEUM LTD 14,29%. The Kharir Sarr field has been discovered in April 1993 by well A (Fig. 1). 2D infill seismic was shot in 1994. Two additional delineation wells (Wells B & C), drilled in 1998, have cored the complete Sarr dolomitic section. The Kharir Sarr formation is draping a basement high, created by Jurassic rifting, and reactivated during the Oligocene Aden Gulf rifting. The Sarr formation, dated Valanginian (Cretaceous) is constituted of interbeds of limestone, chalky limestone, dolomite and shale.