Experimental Investigation of Wellbore Phase Redistribution Effects on Pressure-transient Data

Pressure transient analysis is a well established reservoir evaluation method. By analysing pressure and pressure derivative curves from build-up and drawdown tests, it is possible to identify reservoir characteristic parameters and heterogeneities. However, much of the pressure data recorded during a well test may be dominated by wellbore effects that can mask reservoir characteristics and lead to erroneous well test interpretations. This is particularly true when the well production rate is controlled at surface and more than one phase is flowing. These effects, which are transient in nature, include phase change, flow reversal, and re-entry of the denser phase into the producing zone. This paper presents the results of experiments carried out at Imperial College to investigate the effects of phase redistribution and phase re-injection on pressure build-up data. Single-phase and two-phase flow tests were conducted with air and water. An experimental rig was designed to emulate a reservoir connected, via a resistance, to the base of a flowing well. The "reservoir" is recreated by a pressurised vessel, while the "well" is simulated by a vertical pipe. The "well" was flowed at controlled rates to mimic those encountered in gas condensate reservoirs. After steady-state conditions had been attained, the "well" was shut-in at the top of the rig (i.e. at surface) and the associated transient phenomena monitored via distributed measurements of pressure, temperature, liquid hold-up and wall shear stress. Pressure build-up data were interpreted using established well test analysis techniques. The experiments provide a qualitative and quantitative understanding of the effects of gas rates, liquid rates and rising gas bubbles on wellbore phase redistribution and re-injection. The results yield an insight into the corresponding impact on well test transient pressure behaviour