Physical simulation of the nonlinear transient flow behavior in closed high-pressure gas reservoirs. Part I: Pressure-depleted flow experiments on matrix cores

Abstract In this work, in order to physically simulate the transient flow process of gas in a closed high-pressure reservoir, a pressure-depleted flow experiment was newly designed. Then five short matrix cores were put into a long core holder with a closed end-face to conduct the experiment. Six pressure gauges were uniformly distributed at six different positions on the holder to monitor the flow pressures at the endpoints of each core, and a flow meter was located at the outlet of the holder to test the flow rate. Before the experiment, an initial pressure distribution was built along the holder. By using the experimental pressure data, a series of curves of pressure dynamics and pressure distribution were plotted and then matched by nonlinear regressions. It was found that the pressure dynamical curves and the pressure distribution curves could be described by a piecewise equation and a quadrinomial equation, respectively. By using the experimental flow rate data, the dynamical curves of flow rate were plotted and matched by a quadrinomial function of the semi-logarithm of time. In addition, it was also found that the relationship of pressure with flow rate could only be described by using a polynomial function, instead of the classic binomial and exponential functions. All the curves related to pressure and flow rate were of strong nonlinearity. Furthermore, an infinitesimal method was presented to deduce the calculation formula of gas recovery factor whose value at any experimental time was calculated. At the end, the nonlinear transient flow characteristics were thoroughly analyzed and summarized. This study can be a good reference for researchers in the related field to study the nonlinear transient flow behavior of geo-fluids.