Study and Verification of a Physical Simulation System for Formation Pressure Testing while Drilling

Based on a brief overview on the determination methods of formation pressure and their features, the major principle of formation pressure testing while drilling (FPTWD) and existing physical simulation systems was introduced, and the deficiency of the existing physical simulation systems was also discussed. A laboratory high-precision physical simulation system was therefore developed to simulate the downhole testing environment and testing process of FPTWD. The present experimental system was designed to endure pressures up to 20,000 psi, and the relative control accuracy of pressure is approximately 0.02% FS. Two kinds of man-made specimens with the permeability of 10–110 mD were used to test the pressure response and to verify the present physical simulation system. The debugging results indicated that the variation amplitude under the stable condition is approximately 0.07 psi, 0.08 psi, 0.11 psi, and 0.11 kN for the annular pressure, pore pressure, confining pressure, and thrust force, respectively. Thus, the high control accuracy is approximately ±1.0 psi under the stable conditions. The experimental results indicated that the pressure drawdown declines rapidly in the stage of withdrawing formation fluids and then recovers slowly. The pressure drop amplitude also decreases with permeability, while the pressure buildup amplitude increases with permeability. The tendency of pressure change is nearly the same for both the present and the previous systems, and the pressure curve of the present system is much smoother and better than that of the previous system. The relative error of explaining formation pressure is less than 1% and 4% for the present and the previous systems, respectively. In addition, this physical simulation system has important applications to verify the interpretation model, to analyze pressure response recorded by FPTWD tools, to test the capability and design of FPTWD tools, and to calibrate the formation pressure, formation parameters, and instrument factors