Determination of Fluid Composition Equilibrium—a Substantially Superior Way to Assess Reservoir Connectivity than Formation Pressure Surveys

摘要

Assessing reservoir connectivity during the earliest stages of reservoir evaluation is highly desirable for successful field development. Pressure measurements with wireline formation testers have long been used to assess compartmentalization; if two permeable zones are not in pressure communication, they are not in flow communication. However, the presumption that pressure communication implies flow communication has repeatedly been proven to be incorrect (Elshahawi, et al. 2006; Mullins 2008); better methods to assess connectivity are needed. Recently, downhole fluid analysis (DFA) has enabled facile assessment of fluid composition equilibration. Pressure equilibration requires relatively low mass flow compared to fluid composition equilibration. Thus pressure communication does not impose a stringent condition on connectivity. In contrast, fluid composition equilibration requires mixing of the entire content of the reservoir. The mass flow compared to pressure communication is significantly higher; thus fluid composition equilibration provides a correspondingly much more rigorous set of conditions to determine connectivity. The time constants for pressure versus fluid composition equilibration are compared for identical reservoir parameters. Fluid composition equilibrium is a stationary state in which all components have reached zero mass flux. A reservoir model is designed to simulate numerically equilibration processes over geologic timescales at isothermal conditions in which diffusion and gravity are the active mechanisms. A variety of initial conditions and reservoir fluid types is considered. The results are compared with analytical calculations. Longer equilibration times correspond to tighter constraints on connectivity. Fluid composition equilibration is seen to constrain connectivity by many (seven or more) orders of magnitude beyond constraints imposed by pressure equilibration in reasonable geophysical scenarios. Only a process that stretches across the entire age of the reservoir is likely to capture geologic events that cause compartmentalization. Consequently, the evaluation of the distribution of fluid compositions is shown to be a far better method to test for connectivity than pressure communication. Determination of fluid equilibrium should become part of the standard procedure for reservoir connectivity evaluation.