Rate-Transient Analysis Based on Power-Law Behavior for Permian Wells

Abstract

Abstract In unconventional reservoirs, the application of many rate-transient analysis (RTA) techniques rely 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 than 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 rarely is the characteristic ½ slope observed over long periods. Rate forecasts would be in error if were to assume that the slope would converge to ½ slope at a later time. In this work, we present examples of Permian Wolfcamp horizontal wells with 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). Based on the concept of the power-law behavior, we have developed a workflow to analyze multiphase rate-transient data with high quality measured BHP. Ultimately, the new workflow for rate-transient analysis uses power-law characteristics to evaluate well performance and is a complementary tool to traditional methods such as Arps decline-curve analysis. This paper outlines a power-law analysis workflow scheme and demonstrates that the Chow group is a convenient means to identify 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 frac hits, and to estimate the hyperbolic decline-curve b-factor.