Abstract 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.
Introduction Wellbore phase redistribution (WPR) occurs in wells where more than one phase flows and has an impact on the quality of recorded pressure data. WPR may cause an increase in the wellbore storage coefficient in both drawdowns and build-ups. Phase change, on the other hand, causes the wellbore storage coefficient to decrease during build-ups and to increase during drawdowns. While the impact of phase change on the pressure behaviour is usually limited to early times, WPR may dominate a well test for several hours. When WPR occurs, derivative shapes can be easily misinterpreted as being due to double porosity, partial penetration or composite behaviour. Typical derivative shapes (for gas condensate fields) due to WPR are reported in Fig.1, where curve 5 is typical of situations where the denser phase re-enters into the formation.