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Identification of Diagenetic Footprint and Associated Development Challenges in the Deep Conventional Naturally Fractured Carbonate Reservoirs of North Kuwait

Abstract

Kuwait Oil Company is currently engaged in an aggressive campaign to delineate and develop the deep conventional naturally fractured carbonate reservoirs of Toarcian age in the northern part of Kuwait encompassing eight oil & gas fields as part of the strategy to meet the accelerated Oil & Gas production short term targets. These reservoirs mostly occur in a depth range of 14500′ to 15000′ with HP/HT and sour operational envelope. Till date about 100 vertical / deviated wells have been drilled in the North Kuwait area penetrating these diagenetically altered dolomitic reservoirs of Toarcian age. A systematic campaign of extensive conventional coring (approximately 16000′ cumulative) and meticulous core description in a consistent manner enabled thorough understanding of the depositional setting and building of a robust sequence stratigraphic model with diagenetic overprint. Some of the development wells, despite very good computed reservoir properties, did not perform on testing as per offset wells. One of the reasons for this unexpected poor well behavior is attributed to pore throat impairment due to presence of solid hydrocarbons (bitumen) in selected reservoir intervals. This phenomenon is further established through an integrated analysis of the Well Test, Core, thin section, Geochemical (XRD, XRF and Pyrolysis) and Petrophysical data. This study has helped in demarcating the areas and limited zones that are affected by bitumen presence which has helped in designing suitable testing and stimulation plans. In one of the recently drilled wells, a proactive measure to understand the affected zone was identified prior to undertaking testing. Based on the updated subsurface model, conventional core was selectively taken in the prognosed reservoir interval with potential bitumen impregnation. A calibrated geochemical model was built integrating cutting sample geochemical description along with the core data. As this well had no ECS log data, geochemical model derived from the study was used for calibration and refinement of the petrophysical model resulting in more reliable formation evaluation. The well testing (perforation strategy) was accordingly optimized, taking into consideration the reservoir characterization based on the integrated studies leading to a successful completion of the well with very good initial oil & gas flow rates without stimulation.