A Method To Simultaneously Describe Gravity and Imbibition in Fractured Reservoir Simulations

Abstract

This reference is for an abstract only. A full paper was not submitted for this conference. Abstract In densely fractured reservoirs one has to rely on natural mechanisms like capillary imbibition or gravity to recover oil from the matrix reservoir rock. Capillary forces are an efficient displacement mechanism for rocks that are waterwet: these rocks prefer to hold water over oil and, once exposed to water via the fracture system, strongly absorb water thereby expelling oil to the fracture system, from where it can be produced. The alternative situation, where the rock prefers to hold on to oil, also exists (oil-wet or mixed wetrock). In such cases, the most effective way to produce oil, is to rely on gravity. By depletion or via gas injection a gas cap can be formed in the fractures system. This gas surrounds the oil in the tight matrix rock and, because gas has a lower density than oil, the oil will start to drain from matrix stacks into the fracture system due to gravity. In actual filed cases, usually both gravity and some degree of imbibition contribute. The standard approach in fractured reservoir simulators is to describe fractures and matrix rock as separate systems that can interact with each other(the dual continuum approach), where interaction between matrix and fracture system is governed by shape factors. A proper description of GOGD processes in reservoir simulators is complicated by the nature of gravity to act only vertically and not in all directions as is the case for imbibition and depletion. This paper describes an extension of the dual continuum methods that allows a simultaneous accurate description of gravity and imbibition processes. A comparison is made with methods proposed in the literature and available in commercial simulators. The method has been successfully applied to study fractured reservoirs where gravity and imbibition effects both contribute.