A new method for evaluation of production decline analysis of a single well in a tight gas formation is presented. The approach is inspired by practical observations from the Cadomin (Lower Cretaceous) and Nikanassin (Upper Jurassic) formations in the Western Canada Sedimentary Basin (WCSB). The Cadomin is encased sometimes in formations of low or ultra-low permeability, which feed the Cadomin, enlarging significantly the ultimate gas recovery (2 to 3 times) of the well and leveling out the gas production rate. The gas feeding can occur through isolated spots due, for example, to the presence of an unconformity.

The proposed method solves the continuity and flow equations for Cadomin-equivalent gas reservoirs and the encasing low and ultra-low permeability formations, which might correspond, for example, to tight or shale gas reservoirs. The new mathematical solution permits integrating the rates at which the well is producing and the rates at which the low or ultra-low permeability source is feeding the Cadomin-equivalent reservoir. Equilibrium is reached when the contribution from the low or ultra-low permeability reservoir is equal to the rate contributed by the Cadomin-equivalent to the wellbore. The proposed method is flexible enough to allow situations in which a higher permeability source might feed a tighter reservoir connected to a wellbore.

The method has application on different types of production declines, for example Arps exponential, hyperbolic and harmonic declines; and more recently developed techniques such as the power law method. The goal of the model, however, is not to replace any of the conventional approaches, which have their place in decline analysis of specific reservoirs, but rather to supplement them on the basis of practical observations that integrate geology of tight gas formations and gas production rates. In the proposed method we get away from Arps’ empirical exponent, b, which has been used (that is good) and abused (that is not good) in the past.

It is concluded that the method developed in this study has application in most types of production declines, including linear and bilinear flow. The solutions are illustrated with actual production rates from tight gas formations in the Western Canada Sedimentary Basin.

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