Design and Analysis of a Well Test for Determining Two-Phase Hydraulic Properties

This paper describes the design and analysis of a well test for determining hydraulic properties of a low permeability, low porosity formation that potentially contains a small amount of free methane. Estimation of gas-related parameters in such formations is difficult using standard pumping tests because (i) pressures and flow rates may fluctuate as a consequence of gas exsolution during the test, (ii) data may not allow discriminating among alternative conceptual models, making the parameter estimates ambiguous, and (iii) the key parameters of interest are highly correlated. In this study we adopt an inverse modeling perspective to examine a test sequence that can be appended to a standard hydraulic testing program. The design calculations show that a series of water and gas injections can significantly reduce parameter correlations, thus decreasing estimation uncertainty. Moreover, the extended test sequence enhances the possibility of identifying the model describing relative permeabilities and capillary pressures. A prerequisite for a successful inversion is that data of high accuracy are collected under controlled test conditions. We discuss the assumptions and limitations of the procedure, and propose some recommendations for testing. Finally, we analyze pressure and gas flow data from a well test performed by the Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) that partially followed the proposed test sequence.