Decline curve analysis based on rate-time data is the standard method of evaluating on-shore North American oil and gas reserves. Aside from the classical hyperbolic curve fitting methodology originally proposed by Arps (1945), there have been several new empirical techniques developed for unconventional shale and tight gas reservoirs such as the stretched exponential method (Valko et al., 2010) and Duong method (Duong, 2011). The successful application of methods based only on rate data, however, is usually limited to wells with constant (or close to constant) flowing pressure. High deliverability unconventional plays such as the Haynesville and Eagle Ford are characterized by rate-controlled flow for extended periods of time. Interpretation of these flow periods using rate-time techniques can be misleading, as the bulk of the reservoir response is contained in the flowing pressure data.

In this paper, we propose a straightforward methodology for forecasting the production of high pressure unconventional wells under controlled drawdown. The methodology involves the use of a pressure normalized decline curve, and therefore requires measurement of wellhead flowing pressures. As we will show, the results are consistent with those of more complex analytical and numerical models, but the method does not require any knowledge about, nor does it make any assumptions about the physics of fluid flow in the reservoir. The approach is systematic and repeatable, making it an ideal reserves evaluation tool. The method is validated using synthetic and field data.

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