Post-Fracture Performance Diagnostics for Gas Wells With Finite-Conductivity Vertical Fractures

This paper presents an integrated technique for evaluating the production performance of gas wells with finite-conductivity vertical fractures. Our methodology combines conventional pressure transient test analysis with new material balance decline type curves developed specifically for gas wells with finite-conductivity, vertical fractures. We utilize short-term pressure buildup test analysis to enhance the production data analysis, particularly for interpretation of early-time transient flow behavior. We illustrate—with several field cases—that both techniques can be integrated to provide not only a more consistent and systematic analysis methodology, but also a more accurate assessment of stimulation effectiveness. Introduction Wells producing from tight gas sands require stimulation to achieve economic rates and to maximize ultimate recoveries. The most common stimulation technique is hydraulic fracturing. Depending on the type and size of the treatment, hydraulic fracturing may be expensive—often representing a significant percentage of the total completion costs. Since the economic viability of wells completed in tight gas sands depends on minimizing costs, then it is essential that we optimize fracture treatments, i.e., find the proper balance between stimulation costs and well productivity. A key component in achieving this balance is a post-fracture diagnostics program to determine stimulation effectiveness. Many diagnostic techniques for evaluating hydraulicallyfractured gas well performance have been documented in the petroleum industry, but theoretical model assumptions, model applicability and simplicity, data requirements, and/or data quality and quantity may limit the effectiveness of any single analysis technique. Therefore, we employ an integrated approach in which we capture the benefits and utilize the strengths of several types of hydraulically-fractured well diagnostic techniques. Cipolla and Wright and Barree, et al. have identified and grouped fractured-well diagnostic techniques into three general categories—direct far-field, direct near-wellbore, and indirect. Our methodology focuses on two indirect diagnostic techniques—pressure transient testing and production data analysis. Specifically, we illustrate how short-term pressure buildup testing integrated with long-term production data analysis can be an effective method for evaluating stimulation effectiveness. Indirect Fractured-Well Diagnostic Techniques Although pressure buildup testing is the most effective indirect technique for evaluating the stimulation effectiveness of hydraulically fractured gas wells, knowledge of reservoir permeability—either from the well test or from an independent source—is required to compute fracture properties. If a well is shut in for a sufficient duration to reach the pseudoradial flow period, then we can uniquely determine reservoir permeability from the test data (and very likely also estimate effective fracture half-length and fracture conductivity since these are dependent on the permeability estimate). Wells completed in tight gas sands require very long shut-in times to reach pseudoradial flow, but operators are reluctant to shut in a well for extended periods. If, however, we have an independent estimate of reservoir permeability, then shorter duration pressure buildup tests become practical. Decline type curve analysis of production data has become a common alternative for estimating reservoir permeability without shutting in the well. Fetkovich was the first to incorporate transient flow models with decline curve analysis. He developed the standard “decline type curves” by combining an analytical model for transient, radial flow at constant bottomhole pressure with Arps’ empirical exponential and hyperbolic rate decline models. We note for completeness that the exponential rate decline model is the analytical solution for a well produced at a constant bottomhole flowing pressure and boundary-dominated flow conditions. The original Fetkovich decline type curves are useful for a range of reservoir pressure conditions, but we have observed cases where the boundary-dominated rate-time data changes evaluation curves over time (i.e., changes from one empirical model or stem to another). These changes have been attributed to changes in gas properties as a function of reservoir pressure. To address the impact of pressuredependent gas properties on the evaluation of gas production SPE 97972 Post-Fracture Performance Diagnostics for Gas Wells With Finite-Conductivity Vertical Fractures J.A. Rushing, SPE, and R.B. Sullivan, SPE, Anadarko Petroleum Corp., and T.A. Blasingame, SPE, Texas A&M U.