THEORETICAL-MODEL FOR THE DEPRESSURIZATION OF WATERFLOODED RESERVOIRS

A theoretical model has been developed to explain results from laborat ory experiments in which reservoir cores containing oil at residual oi l conditions were depressurized. In these experiments, high levels of supersaturation were obtained before gas was released from solution, w ith the maximum level of supersaturation increasing as the rate of dep ressurization increased. In addition, when gas was first released from solution it was immobile, and the gas saturation had to build up to a critical value before it could start to move through the rock matrix. This critical gas saturation value was found to increase at higher ra tes of depressurization. The theoretical model has two parts. The firs t part reproduces the experimental observation that the maximum level of supersaturation pressure attained increases as the rate of pressure reduction is increased, and also shows that the number of gas bubble nuclei formed increases as the rate of pressure reduction increases. T his confirms other authors' observations that it is necessary to postu late an increase in bubble nucleation with depressurization rate, in o rder to explain their experimental results. The second part of the mod el represents the development and mobilization of the gas phase by sim ulating the simultaneous growth of several gas bubble nuclei within a porous medium. It is shown that when the gas is first formed, it canno t move until one of two conditions are met; either a large enough bubb le is formed by growth of a single bubble, or by several bubbles joini ng together, to provide sufficient buoyancy to overcome the capillary forces that restrain the bubble; or sufficient bubbles join together t o form continuous channels through the matrix, through which gas can h ow. The model calculates the value of the gas saturation at which one of these conditions is met, and this is defined as the critical gas sa turation. The theoretical model is used to examine the effect of grid size, depressurization rate, residual oil saturation, and interfacial tension on the critical gas saturation. In all cases, the results agre e with the trends observed in high pressure core experiments.