Permanent downhole gauges (PDGs) provide vast amounts of pressure-transient and rate data which may be interpreted with improved pressure-transient-analysis (PTA) approaches to gain more knowledge about reservoir dynamics. Permanent pressure and rate measurements allow for analysis of time-lapse pressure transients and comparative interpretation of flowing and shut-in periods. The approaches used provided the basis for an improved methodology of interpreting permanent pressure measurements, where the scope of the standard PTA application may be extended to integrate new data sources.
Practical Remarks on Comparison of Pressure Transients and Choosing a Model. Comparison of different pressure transients is usually carried out on the basis of plotting all the transients and derivatives on the same log-log plot. In practice, a difference between time-lapse shut-in pressure transients does not necessarily indicate change in well reservoir parameters because such a comparison is usually carried out for rate-normalized data with a chosen reference transient. The rate before the shut-in period of interest governs the pressure-transient location on the log-log plot. An approximate value may be attributed to this rate because of the averaging of flow data, while permanent rate measurements may help in reducing this uncertainty. Flowing pressure transients are usually normalized subject to variable rates during the flowing period. This makes comparison of these pressure transients more reliable.
Pressure derivatives are more representative in this sense, because well history before and during a pressure transient is accounted for by use of the superposition principle commonly used for the derivative calculation. At the same time, assuming radial flow as the main flow regime in the superposition calculation—as well as averaging, cutting, or possible errors in rate history before the pressure transient of interest—may have an impact on the derivative trend, especially for late elapsed times and interpreting boundary effects. Simulation of the well history in the linear scale with the analytical model used for the pressure and derivative interpretations in the log-log scale improves reliability of the analysis. The simulated pressure response may help in evaluating the impact of superposition effects and in revealing changes in well/reservoir parameters. Comparison of time-lapse pressure transients and derivatives may be used for diagnostics of changes in well/reservoir parameters, while only simulation of the well history, or at least a part of the history, would provide reliable conclusions on such changes.
Pressure-Transient Analysis as an Element of Permanent Reservoir Monitoring
25 August 2015