The Influence of Fracture Closure from Petroleum Production from Naturally Fractured Reservoirs: A Simulation Modelling Approach
The term 'naturally fractured reservoirs' generally refers to reservoirs where the fractures have an effect on the fluid flow (Nelson, 1985). Fractured petroleum reservoirs make up more than 20% of the world’s oil and gas reserves (Saidi, 1983) but are considered to be among the most complicated classes of reservoirs. In many cases the fractures are open and contribute to flow, while in other cases fractures are cemented or filled and behave as local barriers to flow. However, fractures not only affect production as static features, but also react to changes in the stress field, both locally and far field so that the aperture and shape of the fracture is altered. This limits/enhances the fracture permeability, which ultimately affects the production of oil/gas. The effect of fractures opening and closing with pressure changes in the reservoir during production has been recorded in naturally fractured reservoirs.
Laboratory-based tests on fracture closure have been conducted in studies described in the literature. Some of these tests have determined differences in responses for weathered versus fresh fracture surfaces, and also with normal versus shear stress application to the samples. With repeated tests it was also noticed that there was a hysteresis effect and that as the fractures underwent more opening and closing cycles, the fractures began to open and close by smaller and smaller amounts, compared to the initial case of opening and closing.
The purpose of this study is to investigate the opening and closure of fractures through pressure changes in a reservoir using a combination of coupled fluid flow - production simulation models and a reservoir simulator without geomechanical coupling. Simplified coupled fluid flow - geomechanical models are first used to check whether phenomena such as stress arching are likely to have a significant impact on reservoir stresses. Empirical relationships between fracture closure and applied stress, based on the literature where testing was completed under laboratory conditions, are then used to form a relationship between pressure and a directional permeability multiplier which is then incorporated into the simulator to model the impact of fracture closure on hydrocarbon production.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009