Production Analysis and Forecasting of Shale Gas Reservoirs: Case History-Based Approach

In this work we present the current status of production evaluation and prediction for shale gas reservoirs. We focus primarily on the effect of wellbore geometry (vertical vs. horizontal) and fracture stimulation, and consider only relatively simple shale reservoir characteristics. Theoretical production responses from hydraulically-fractured vertical and horizontal wells are first described. Analytical and empirical methods for production data analysis and forecasting of shale reservoirs are then described and demonstrated. The empirical approach used is the recently developed “power-law exponential” rate relation [Ilk, et al (2008b and 2008b)] in addition to the more traditional "hyperbolic" rate relation [Johnson and Bollens (1927)], Arps (1945)]. The analytical methods utilized include production type-curves developed for vertical wells completed in single-porosity, single-phase reservoirs. Finally, numerical approaches for forecasting complex, multifractured horizontal wells are also presented. Two approaches for simulating these complex wells, one simple and the other more rigorous, are provided. In the more rigorous approach, the “compound linear flow regime” concept introduced by van Kruysdijk and Dullaert (1989) is verified. The production analysis methods are applied to field and simulated cases to demonstrate their accuracy and practicality. The empirical power-law exponential method, when applied to both simulated and field cases, is shown to be a reliable method for forecasting shale wells. The analytical (type-curve) methods developed for vertical wells can be useful for analyzing vertical shale wells; when applied to horizontal wells, the resulting estimates of reservoir and fracture properties may be inaccurate. A special case of analysis of a 2-phase shale reservoir is also presented; in this case, the analytical tools developed recently for 2-phase CBM reservoirs [Clarkson, et al (2008)], including 2-phase flowing material balance and type-curves, prove useful. Introduction Shale gas reservoirs are currently being aggressively explored for and developed in North America, with technology, such as multi-fractured horizontal wells, being a key driver for success. The shale reservoirs that have been exploited to date have varied and complex reservoir and production characteristics, making quantitative production analysis and forecasting difficult. The characterization of shale gas reservoirs in terms of production performance has essentially 2 components: 1. Estimation of reservoir properties (permeability, effectiveness of the well completion, and contacted-gas-in-place). 2. Forecasting of the production performance as a means of estimating reserves and to evaluate economic viability. The analysis of production data is often the only practical way of evaluating tight gas and shale gas reservoirs. The modern methods of production data analysis are quite robust and in many respects these methods parallel those of pressure transient analysis. Obviously, the basic theory is the same — but the assumptions are different. Each methodology (pressure transient analysis and production analysis) is appropriate in its own domain. In simple terms, neither the quality nor the accuracy of production data is comparable to that of well test data, but an extensive production rate and pressure history is data that can be utilized to provide a "full life analysis" of the well — whereas, even when available, pressure transient tests are very short, at best on the order of a few days.