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Abstract This paper presents the case history of efforts made in managing the vertical conformance in heavy oil, multi-layer reservoirs. The onshore field discovered in 1985 in south Sultanate of Oman is heavily faulted, compartmentalized with multiple reservoir layers of varying permeabilities. The oil is relatively heavy with average viscosity of 60–150 cp, density of 22 degree API with low GOR. The waterflood was piloted in 1993 by adopting a 5 spot pattern. Full field waterflood was implemented in 1999. As the waterflood in this field is characterized as one with poor vertical conformance due to varying layer permeabilities, effective conformance control is expected to result in more efficient waterflood by increasing volumetric efficiency and hence potentially increasing the recovery by 5%. It is relevant to highlight that one dominant layer (highest permeability) contributes to most of the current oil production. Hence, profile modification changes need to keep a balance in enhancing production from other layers while not adversely affecting the contribution from this dominant layer. A multi-pronged strategy to tackle the vertical conformance control challenges has been adopted in the form of both mechanical and chemical profile control methods. The mechanical methods are to install (a) selective completion with sliding sleeves in producers, (b) dual injection through tubing and annulus after frac of low permeability layer, (c) selective completion in injectors with regulator valves/ ported sleeves, (d) casing patch, bridge plug, cement squeeze in producers and injectors. The chemical methods are (a) profile control with polymer gel in the injectors, (b) fracture shut-off with polymer-silicate gel in the injector. As can be expected, the critical success factors in the vertical conformance control strategy are good surveillance data, understanding of interwell layer connectivity/ crossflow, selection of candidate well and remedial methods. The paper summarizes the conformance control strategies, implementation, results to date with lessons and recommendations. Introduction PDO operates many heavy-oil fields located in the South of Oman. These fields are clastic reservoirs with multiple layers of varying permeabilities. Water flood is a common practice in the development of these fields due to either lack of aquifer or weak aquifer and low depletion drive energy. However the heavy nature of the oil makes these water flood projects challenging due to very unfavorable mobility ratio between oil and water. Water breakthrough happens very quickly and most of the recovery takes place after water breakthrough. More pore volume injection and ability to produce to high water cut is the key to maximize recovery from these fields. Hence it is essential that the waterflood targets all the hydrocarbon bearing layers between the injector and producer to maximize the recovery of the field. |
