Summary

In unconventional reservoirs, the application of many rate–transient–analysis (RTA) techniques relies heavily on the identification and analysis of the linear flow regime, which is characterized by a ½–slope on a log–log plot of Δp vs. t. Through our analysis of more than 400 wells with downhole pressure gauges in the Wolfcamp Shale of the Permian Basin, we observed power–law behavior, but with slopes much different from ½ over long periods of time. In many cases, the duration of the straight line with a slope different from ½ lasts for years, without ever converging to ½. In some cases, the slope changes over time but is rarely the characteristic ½–slope observed over long periods. Rate forecasts would be in error if we were to assume that the slope would converge to a ½–slope at a later time.

In this work, we present examples of Permian Wolfcamp horizontal wells each with a measured bottomhole pressure (BHP) to demonstrate the characteristic power–law behavior with slopes different from ½. Power–law behaviors are typical in heterogeneous systems and are identified using the Chow pressure group (CPG).

On the basis of the concept of power–law behavior, we developed a workflow to analyze single–phase rate–transient data with high–quality measured BHP. Ultimately, the new workflow for RTA uses power–law characteristics to evaluate well performance and is a complementary tool to traditional methods such as the Arps decline–curve analysis (Arps 1945). In this paper, we outline a power–law analysis workflow scheme and demonstrate that the CPG is a convenient means for identifying the exponents of straight lines. In addition, we present case studies to demonstrate the application of this technique to predict the long–term well performance from choked–back wells, to evaluate long–term performance changes associated with offset fracture hits, and to estimate the hyperbolic–decline–curve b–factor.

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