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, ß, must be available. There are numerous correlations in the literature for estimating ß. 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 ß-factor. The methodology for developing a correlation for ß factor using multi-rate tests has been developed. Once the ß-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.
Skin factor is the commonly used well flow efficiency indicator. A large positive skin factor is considered to represent near-wellbore formation damage and is frequently used as a criterion to perform stimulation treatment to enhance well productivity. Skin factor, which is generally determined from interpretation of pressure transient well tests, does not only represent near-wellbore damage rather it is a composite factor. Therefore, the skin factor computed from a well test must be broken down into its various components in order to estimate true near-wellbore damage. This is particularly important in gas wells due to high gas velocity especially when gas approaches the wellbore. Darcy's law is not adequate to describe high gas velocity in porous media. To account for additional pressure drop due to high gas velocity near the wellbore, referred to as non-Darcy effect, a rate dependent skin factor is introduced. Therefore, the total skin factor determined from a test on a gas well will have two primary component, rate-dependent skin and rate-independent skin. The rate-independent skin is primarily a function of formation damage, well deviation, completion, and perforation. Rate-dependent skin must be first evaluated in order to obtain a reliable value for the rate-independent skin. The rate-dependent skin depends, among other parameters, on the coefficient of inertial resistance, ß. A number of correlations are available in the literature for predicting this coefficient from rock permeability and porosity. However, the currently available correlations have been derived from limited set of laboratory data that are found to be inaccurate. One of the key reasons for inaccuracy of these correlations is that the dependency of ß on structure of porous media cannot be completely represented by its relation with permeability. Therefore, ß need to be correlated with number of properties such as porosity, permeability, turtuosity, specific surface area, pore size distribution, etc. However, the information on such properties may not be easily available. Alternatively, the rate-dependent skin can be determined directly from the transient well test data. However, to achieve this several transient tests at various rate rates are required.