Results of Laboratory Experiments to Simulate the Downhole Environment of Formation Testing While Drilling

A laboratory-based experimental study was conducted to more completely understand the fundamentals of pressure transients produced by Formation Testing While Drilling (FTWD) equipment. These tools can be run in open hole, positioned at a desired level, and actuated to measure reservoir pressure and flow capacity while in a dynamic drilling environment. Any number of pressure measurements can be made at various levels. From these measurements, reservoir pressures can be accurately determined and formation mobility estimated. Sperry-sun*’s GeoTAP™ tool uses a testing technique in which a probe connected to a small sample chamber taps into a formation. Following connection, the small sample chamber is used for drawdown and build up tests. In order to validate how accurately FTWD formation tools can determine characteristics of given hydrocarbon reservoirs, a comprehensive lab fixture was built to duplicate the downhole environment. This fixture includes the capability to circulate mud and form a mud cake on a core, to control hydrostatic mud and formation pressures, and to perform a pressure test. The pressure test produces a transient using exactly the same pad, transducers, electronic control board and software used in the current GeoTAP tool. The fixture was run repeatedly for a wide range of simulated downhole conditions by varying formation pressures, hydrostatic overbalance, flow rate, flow line volume, and pretest chamber volume; and using different cores and reservoir fluids. The permeability of the different cores was analyzed according to the experimental results from the fixture, using an analytical spherical-flow equation model. The validity of experiments was initially assessed using a single-phase spherical flow analytical model. To study multiphase invasion effects, the UTCHEM** mud infiltrate simulator was used to estimate the extent of the invasion. Results from UTCHEM were then used as initial conditions for a multiphase transient analysis. In this step a commercial reservoir simulator, Landmark*’s VIP, was used to determine the transient response considering the invaded region in the vicinity of the near wellbore. When these more powerful numerical simulators were used, the match between experimental data and theoretically expected results was better than 90%.