Complex Unconventional Wells Addressing Difficult Reservoir Development : A Case Study of West Kuwait Jurassic Reservoirs

Abstract

Abstract Najmah-Sargelu (NJ-SR) unconventional fractured carbonate and Middle Marrat (MMR) tight carbonate Jurassic reservoirs are spread across many fields in West Kuwait where new Jurassic oilfields are still being discovered. Oil is being produced since late eighties mainly from MN, UG, AB and DF structures. Drilling through over pressured and fractured NJ-SR reservoir, lying below thick high pressure Gotnia Evaporites is challenging due to well control issues such as total mud losses and kicks compounded with high H2S- high CO2 corrosive environment. Moreover, pressure reversal in MMR and depleted pressures pose further complications resulting in costly wells. A total of 53 wells drilled in these fields until 2002, were all vertical (Fig.1) except one NJ-SR high angle well drilled in year 2000 which could not be tested due to complications during completion. Although many wells produced at high rates of 5000–10000 bopd, some failed to produce for their inability to intersect productive fractures in NJ-SR or due to low PI in tight MMR carbonates. This paper presents an overview of recent efforts in planning and drilling unconventional wells addressing cost-effective development of multiple Jurassic reservoirs. First such well in 2003 was a long reach horizontal well that was successfully tested and produced from NJ-SR. This was followed by Kuwait's first long reach medium radius re-entry horizontal well in 2004 that was tested and completed open-hole in MMR successfully. A 1100ft lateral drain-hole for productivity enhancement in MMR was geosteered successfully on planned trajectory in 2006 through re-entry. The well showed oil indication during initial testing; however, it was suspended due to post-stimulation complications during testing. Recently, the first deviated development well was drilled and completed where the trajectory was optimized for both NJ-SR and MMR reservoirs. The well successfully intersected planned fracture-swarm in NJ-SR where core was acquired and oriented to characterize the fracture zone. Other complex wells include vertical deepening for transferring water producing NJ-SR well to MMR extending productive life of non-usable well. Well planning for a complex NJ-SR deviated - MMR horizontal well was completed early this year and drilling is expected to be finished in 2007 followed by a long reach NJ-SR re-entry horizontal sidetrack besides the first MMR water injector in 2008. These unconventional complex wells have successfully met technical challenges in planning & drilling while addressing productivity enhancement and cost-effective development of multiple Jurassic reservoirs. Learning gained in these wells will go a long way in finalizing cost-effective field development plans incorporating optimal well trajectory, completion designs and off take strategies for maximizing recoveries from these difficult and challenging reservoirs. Introduction Unconventional fractured Najmah-Sargelu and tight Middle Marrat carbonates constitute Jurassic reservoirs in West Kuwait. Appraisal and development of these deep reservoirs spread over several oil-fields containing light but sour crude, is a challenging task that often requires unconventional 'out-of-box' thinking, and unique well designs with improvised work-flows & methodologies. Several first time applications of new industry trends and cutting-edge oilfield tools and techniques have been employed to address these challenges. This paper describes as to how some of these challenges have been met through integrated multidisciplinary teamwork leveraging latest industry trends, tools and techniques. The oil accumulations in NJ/SR reservoirs are formed as a result of unique geology characterized by organic rich mature source rock interlayered with tight fractured carbonate rocks. These are deep, over pressured reservoirs are sealed at the top by a thick Gotnia Salt-Anhydrite cap-rock, and is underlained by Dharuma shale below. The productivity of this low matrix porosity reservoir (<5%) is enhanced by the occurrence of natural fractures created as a consequence of folding and faulting, some of which remained open through a combination oil recharge, and high reservoir pressures coupled with horizontal stress anisotropy.