Evaluation of Skin Factor from Single-Rate Gas-Well Test

Abstract

Abstract Skin factor is commonly used as well flow efficiency indicator and the criterion for performing stimulation treatment to enhance well productivity. Skin factor, which is generally determined from interpretation of pressure transient well tests, is a composite factor and must be broken down into its various components in order to estimate near-wellbore damage. This is particularly important in the gas wells because they experience additional pressure drop near the wellbore due to high gas velocity. This additional pressure drop near the wellbore is generally referred to as non-Darcy effect and can be approximated by a rate-dependent skin factor. Therefore, the total skin factor obtained from a gas well pressure transient test has two primary components, rate-independent and rate-dependent skins. The rate-independent skin is primarily a function of formation damage, well deviation, completion, and perforation. Rate-dependent skin accounts for the non-Darcy effects. Both of these skins can be determined directly from the interpretation of pressure transient well tests if several transient tests at different rates were conducted. The multi-rate tests are however cumbersome to conduct and it is advantageous to estimate the rate-independent skin factor from a single rate test. In order to obtain a reliable value for the rate-independent skin from a single-rate test, the rate-dependent skin must be evaluated independently. The rate-dependent skin depends, among other parameters, on the coefficient of inertial resistance, β. A number of correlations relating β to permeability are available in the literature. These published correlations are derived from limited set of laboratory measurements on various porous media. Our previous investigations have revealed that these correlations do not provide consistent results. Alternatively, β can be determined from the results of the multi-rate well tests. The calculated β values from such tests reflect the actual field conditions that cannot be replicated in the laboratory. However, the complex dependency of β on the properties of the porous media precludes development of a single β -k correlation that can provide accurate and consistent results in all reservoirs. A more realistic approach would be to develop a relation between permeability and β for a specific porous media. To develop a correlation for β, a database of multi-rate gas well tests was established which contained data from over 100 well tests obtained from a number of operators in the Appalacian Basin. The well tests from several wells in the same reservoir were available and several field-specific correlations for β were developed. The comparison of skin factor determined from these correlations against the skin factors determined from the well test data indicated that reservoir-specific correlations for β provide accurate and consistent results.