Coupled Formation, Wellbore, and Flowline Simulation to Explain Formation Tester Dual-Packer Cleanup

Abstract

A computational fluid dynamics (CFD) and hydraulic system cosimulation method was implemented to understand formation tester dual-inlet dual-packer cleanup in oil-based mud. The simulation includes miscible multispecies fluids in the formation, the dual-packer interval (sump), and the formation tester flowlines, with the objective of reconciling a long-observed dual-packer discrepancy between simulated and observed formation fluid cleanup and generating operational best practices. A commercial CFD simulator is used to perform coupled simulations for flow in formation rock and in the dual-packer sump. Unlike reservoir simulators, which apply a simplified sandface condition using a well model, the CFD simulator includes the dynamics of fluid flow in the wellbore. Next, a simplified hydraulic tool flowline model is coded using JAVA, and a JAVA scripting functionality within the CFD simulator connects the CFD solver with the hydraulic model. This method includes mud, mud filtrate (filtrate), and reservoir hydrocarbons and honors the full pressure and flow rate history from the reservoir to the tool. Results show that when fluid sampling cleanup is initiated, flow into the dual-packer sump always starts from the top of the interval. This is caused by the pressure gradient difference between the sump (mud) and formation fluid (filtrate and native hydrocarbon). Simulations with a light hydrocarbon formation fluid result in native fluids fingering through the higher-viscosity filtrate. As the sump pressure continues to decrease, fluids start to enter the interval farther down, creating some density-driven flow circulation in the sump, particularly when the pumps are stopped for an early pressure buildup. The sump region below the lower inlet cleans up much more slowly because the heavier mud is not easily displaced with filtrate or hydrocarbon. Cosimulation that includes the flowline adds significant insights. While sump pressure is driven by flow from formation and by the dual-packer interval fluid densities, measured pressure at the tool pressure gauge also depends on the friction loss between the tool inlet and gauge (which is a function of flow rate, valve positions, and flowing fluid viscosity), flowline fluid head, and potential inlet filter restrictions. The method represents the dynamic responses of the system which is used to explain counterintuitive gauge pressure observations and cleanup behavior. Current commercial reservoir simulators routinely model flow through porous media, but they do not include detailed flow behavior in the wellbore section that is isolated by the dual packer. Similarly, hydraulic flow simulators exist, but are not often coupled to the reservoir. The novelty of this cosimulation is the ability to simulate the interactions between the formation rock, the dual-packer interval, and the flowlines, which are used to explain counterintuitive pressure phenomena and dual-packer cleanup.