Numerical Simulation of Periodic Wellbore Flow Due to the Inflow of Low-Enthalpy Fluid

Abstract

Cycling production wells exhibit periodic changes in their production rate and wellhead pressure. This periodicity often makes it difficult to connect these wells with power plant facilities. We can expect to improve the power plant output by developing effective mitigation measures based on understanding of the mechanisms generating the periodicity. We have developed a numerical code for coupled transient flow in a wellbore and multiple reservoirs intersected by the well. The code can simulate periodic wellbore flow generated by one of several mechanisms, inflow of low-enthalpy fluid from a shallow reservoir, successfully. Through exhaustive numerical experiments under a number of conditions, we have revealed a simple universal condition for generating the periodic flow. The condition is described using a mean specific enthalpy of the shallow and deep reservoirs, which is weighed by the productivity indices of both reservoirs. Using the code, several scenarios of transition from constant to periodic production rates are simulated by assuming changes in the key conditions such as decrease in the shallow reservoir enthalpy and pressure decline in the deep reservoir. Simulated responses of the periodic wellbore flow to adjusting a flow control valve differ depending on conditions generating the periodic flow. Increase in the flow rate stabilizes the wellbore flow if the deep reservoir has sufficient productivity without pressure decline, while the same operation degrades the stability by promoting pressure decline in the deep reservoir with insufficient productivity. The simple universal condition for generating the periodic flow is also useful for estimating the stability of production wells without simulating transient wellbore flow strictly. This technique is essential when coupled with a practical reservoir model simulating production and reinjection for several months or years because the transient wellbore flow simulation requires huge computational loads.