A Reservoir/Wellbore Model for Multiphase Injection and Pressure Transient Analysis

Abstract

SPE Members Abstract Phase injectivities for multiphase injection processes are studied using an isothermal black-oil numerical model that properly treats wellbore/reservoir interaction. A fully implicit technique for the coupling of wellbore and reservoir flow equations is described. The effect of gravity segregation in the wellbore is taken into account to simulate cocurrent water and gas injection. It is shown that the current techniques used in reservoir simulation for assigning phase injectivities yield inconsistent results when wellbore phase redistribution takes place. The effect of multiphase flow in the well and in the reservoir during well testing is also investigated. The fully coupled wellbore/reservoir model is employed to study the effect of wellbore phase segregation on buildup pressure response. pressure response. Introduction Multiphase injection of fluids in reservoirs may occur in a variety of oil field operations such as steam and cocurrent water and gas injection processes. In enhanced recovery processes, special techniques are usually required to control and modify injection profiles. Laboratory experiments reported by Elson have showed the effect of phase separation in the wellbore. Uneven injection profiles in the perforated intervals have been clearly observed. Field observations have also shown this behavior with the lighter and less viscous phase being preferentially injected into the top of the formation while the heavier phase goes into the lower part. phase goes into the lower part. In reservoir simulation, phase injectivity is commonly handled by means of simplified approaches which usually ignores the gravity segregation in the wellbore. Multiphase flow in the wellbore and in the surrounding formation may also occur during well tests. Anomalous pressure buildup behavior, as a result of wellbore phase pressure buildup behavior, as a result of wellbore phase segregation, has been observed in high GOR wells. A general review of well test analysis with multiphase flow was presented by Raghavan. However, the effect of wellbore phase segregation was not taken into account. An attempt to account for this effect in pressure buildup analysis was made by Fair. In his work, an empirical relationship for pressure change was introduced in the wellbore storage equation in order to describe phase redistribution in the well. Recently, Winterfeld presented a model that treats rigorously multiphase flow in the wellbore during pressure buildup. The model solves simultaneously for transient multiphase flow in the wellbore and in the reservoir. Counter-current two-phase flow in the wellbore during shut-in is allowed by the use of semi-empirical expressions to account for phase to phase viscous forces. To allow for phase segregation in the simulation of multiphase injection processes and multiphase well tests, a wellbore flow model must be coupled with the reservoir model. Several papers dealing with the simulation of transient multiphase flow in pipes have been published in the technical literature. Liles and Reed developed a semi-implicit drift-flux model for simulating unsteady state two-phase flow in pipes. Later, Millers presented a similar model to study wellbore storage effects during geothermal well testing. Sharma et al. simulated transient gas-oil flow in pipelines using a black-oil type model. A thermal pipelines using a black-oil type model. A thermal compositional model for simulating transient gas-liquid flow in natural gas pipelines was reported by Kohda et al.. P. 123