Analyzing Production Data From Unconventional Gas Reservoirs With Linear Flow and Apparent Skin

Many horizontal wells with multiple fractures producing from unconventional gas reservoirs have been observed to exhibit linear flow. Often it is the only flow regime, and it can last several years. Classically, this flow regime is characterized by a half slope on type curves when there is no skin effect. However, most of the time, a skin effect is observed in these wells (caused by flow convergence or finite conductivity in the fractures). The presence of skin changes the shape of data points when plotted on log-log scales, and this can have a huge effect on the interpretation when using type curves. For example, a well with purely linear flow and with skin in a reservoir that is infinite acting may appear like and be interpreted as a finite-acting reservoir simply because of the skin effect. This paper discusses different methods that can be used to eliminate the misinterpretation caused by the presence of skin when analyzing linear flow by use of type curves. First, it is shown that among all the derivative and integral functions that are currently used, only well-test-style semilog derivative (DER) and pressure integral-derivative are not affected by the skin. However, these two functions have other issues that usually make them unfit for use in production-data analysis. DER is noisy most of the time, and the process of integration often introduces errors at early times that can significantly distort the shape of the pressure integral-derivative. Therefore, an easier method is presented to analyze the production data from shale gas reservoirs with extended periods of linear flow and significant skin. This method uses the square-root-of-time plot to remove the apparent skin effect from the data. Then, the data (excluding skin) are used for type-curve analysis. This simple procedure prevents potentially significant misinterpretation.