Multi-rate tests generally provide accurate values for gas reservoir formation parameters and deliverability equation coefficients. This type of results enables the engineer to make proper decisions on whether remedial work is required and the potential improvement that can be achieved. The multi-rate tests, such as isochronal or modified isochronal tests, are however cumbersome to conduct due to need for achieving stabilization or lengthy shut-in periods.
The goal of this study has been to develop a simple and reliable method for gas well deliverability determination based on a single rate build-up or fall-off test. Traditionally, single rate tests have been analyzed using empirical backpressure equation using an average value for the exponent (n). Such approximations often provide inaccurate results and do not reveal any information regarding the conditions of the well. Alternatively, single rate tests can be analyzed using theoretical transient gas flow solutions. However, to accurately estimate mechanical skin factor and the deliverability equation coefficients from a single rate test, turbulence factor, β, must be available. There are numerous correlations in the literature for estimating β. These correlations do not provide unique results and calculated skin factor can range widely as a result.
This study introduces the concept of the reservoir-specific β-factor. The methodology for developing a correlation for β factor using multi-rate tests has been developed. Once the β-factor correlation for a reservoir is established, there would be no need for additional multi-rate tests and single rate can be analyzed to provide accurate estimates of mechanical skin factor and deliverability equation coefficients. The accuracy of the proposed methodology has been verified through application in two separate reservoirs in the Appalachian Basin.