Challenges of Optimizing Oil Recovery of "Tight Carbonate" Reservoir, Onshore Abu Dhabi Oil Field, United Arab Emirates

Abstract

Abstract Production rates from tight reservoirs are marginal. However, the tight carbonate reservoir under study accounts for a significant percentage of STOIP, which is the key for drilling and producing these reservoirs. The challenges here are to optimize both the production rates and the total amount of the recovered oil. Good understanding and high quality reservoir characterization lead to efficient reservoir management plans. The studied reservoir is over 50 ft thick and covers an area of 140 km2 with porosity ranges from 3 to 25 pu. This reservoir was deposited during a transgressive cycle on a broad and relatively shallow shelf in an open to slightly restricted subtidal marine environment. The depositional environment plays the major role as well as the diagenitic overprint in term of low reservoir quality. The reservoir is subdivided into four sub zones separated by stylolites. The reservoir can be divided into two main facies fining upward. Although the lower facies composed of Algal Skeletal Floatstone-Boundstone while the upper one is Foraminifera Wackestone, the reservoir quality is relatively low in both (permeability does not exceed 6 mD) due to the severe impact of digenesis. Although a production test conducted over this reservoir reached only 300bopd dry oil, it is still encouraging to implement a sound development plan in order to sustain a reasonable plateau and obtain an optimum recovery. Moreover, the reservoir structure is quite complex in term of isolation with the overburden dense formation and juxtaposition with lower reservoirs. Several options are considered to develop this tight reservoir, but basically they consider the use of long horizontals under WAG and Gas injection processes. Introduction As the industry seeks to increasingly exploit reserves of oil contained in low permeability reservoirs, the reservoir under study present an average permeability of 1 mD. Many questions have been raised about how to optimize production rate, the total amount of the recovered oil and the optimum practice to drill and complete this kind of reservoir. The effective stimulation technique to obtain economic production rate is another issue. The reservoir damages during the drilling operations and workover also represent a concern. Geological background The structure of the reservoir is characterized by a highly faulted elongated NE-SW anticline with a main axis length around 25km and a width of approximately 6km covering an area of 140km2. fig.1 The structure was formed during the Upper Cretaceous in response to the compressional event related to the main ophiolites obduction. Subsequent movement of the structure during the Tertiary has occurred due to the Zagros orogene event which has regionally tilted the area down the NNE.