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

摘要

Assessing reservoir connectivity during the earlieststages of reservoir evaluation is highly desirable forsuccessful field development. Pressure measurementswith wireline formation testers have long been used toassess compartmentalization; if two permeable zonesare not in pressure communication, they are not in flowcommunication. However, the presumption thatpressure communication implies flow communicationhas repeatedly been proven to be incorrect (Elshahawi,et al. 2006; Mullins 2008); better methods to assessconnectivity are needed.Recently, downhole fluid analysis (DFA) has enabledfacile assessment of fluid composition equilibration.Pressure equilibration requires relatively low mass flowcompared to fluid composition equilibration. Thuspressure communication does not impose a stringentcondition on connectivity. In contrast, fluidcomposition equilibration requires mixing of the entirecontent of the reservoir. The mass flow compared topressure communication is significantly higher; thusfluid composition equilibration provides acorrespondingly much more rigorous set of conditionsto determine connectivity.The time constants for pressure versus fluidcomposition equilibration are compared for identicalreservoir parameters. Fluid composition equilibrium isa stationary state in which all components have reachedzero mass flux. A reservoir model is designed tosimulate numerically equilibration processes overgeologic timescales at isothermal conditions in whichdiffusion and gravity are the active mechanisms. Avariety of initial conditions and reservoir fluid types isconsidered. The results are compared with analyticalcalculations. Longer equilibration times correspond totighter constraints on connectivity.Fluid composition equilibration is seen to constrainconnectivity by many (seven or more) orders ofmagnitude beyond constraints imposed by pressureequilibration in reasonable geophysical scenarios. Onlya process that stretches across the entire age of thereservoir is likely to capture geologic events that causecompartmentalization. Consequently, the evaluation ofthe distribution of fluid compositions is shown to be afar better method to test for connectivity than pressurecommunication. Determination of fluid equilibriumshould become part of the standard procedure forreservoir connectivity evaluation.