This paper presents an integrated approach for evaluating the post-fracture performance of gas wells completed in tight gas sands. Our technique focuses on a methodology for evaluating the stimulation effectiveness of hydraulically fractured gas wells. Rather than relying on a single evaluation technique, we integrate short-term pressure buildup testing with long-term production data analysis using decline type curves. We illustrate the applicability and efficacy of our technique with examples from more than twenty wells completed in tight gas sands. The results of our paper also demonstrate the value and function of short-term pressure buildup tests performed in tight gas sands.
Most wells completed in tight gas sands require stimulation to achieve economic production. Depending on the type and size of stimulation treatment, hydraulic fracturing may be very expensive, often representing a significant portion of the total well completion costs. Since the economic viability of many wells completed in low-permeability reservoirs depends on minimizing costs, then it is imperative to optimize fracture treatments. Fracture optimization is achieved by finding the proper balance between stimulation costs and well productivity. A key component in achieving this balance is a post-fracture evaluation program to determine stimulation effectiveness, principally effective fracture conductivity and propped fracture length.
A number of techniques have been developed by the petroleum industry for evaluating hydraulically-fractured gas well performance. Unfortunately, no single methodology is perfect. Theoretical assumptions, model applicability, and/or data requirements limit each analysis technique. Therefore, we employ an integrated approach in which we capture the benefits and utilize the strengths of several types of fractured well diagnostic techniques. Several previous papers have illustrated the effectiveness of such an integrated approach.1–7
Cipolla and Wright8,9 have identified and grouped fractured well diagnostic techniques into three general categories: direct far-field, direct near-wellbore, and indirect. We focus on the integration of indirect techniques, particularly pressure transient testing and production data analysis.
Pressure buildup testing is the most effective indirect technique for evaluating the stimulation effectiveness of hydraulically fractured gas wells. But, 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 time to reach the pseudoradial flow period, then we can uniquely determine reservoir permeability. Unfortunately, wells completed in tight gas sands usually require very long shut-in times to reach pseudoradial flow. Most operators are reluctant to shut in a well for extended periods, especially under favorable gas product pricing scenarios. If, however, we have an estimate of reservoir permeability from an independent source, then shorter duration pressure buildup tests in tight gas sands become practical.
Conventional decline type curve analysis10–13 of production data is a viable alternative for evaluating well performance without shutting in the well. Unlike pressure transient test analysis, decline type curves do not rely upon identification of characteristic flow regimes for the analysis. As a result, we cannot always obtain unique estimates of fracture half-length, especially when using poor-quality production data. Other production analysis techniques consider the production data to be an extended drawdown test.14,15 Accordingly, these techniques use variable-rate pressure transient testing theory and superposition plotting functions to analyze the production data. Unlike conventional decline type curve analysis techniques, these methods allow us to identify specific flow regimes. However, we still cannot quantify fracture properties unless we have an estimate of reservoir permeability.