Abstract

Abstract The average reservoir pressure is a key parameter in material balance calculations, but its determination is challenging when dealing with shales because of their low and ultra-low permeabilities. This paper presents an easy-to-reproduce methodology for calculating the average reservoir pressure from flowing data, and its use in a new Material Balance Equation (MBE) that considers the simultaneous contribution of free, adsorbed and dissolved gas. The procedure developed in this paper uses a modified gas compressibility factor (Z') introduced in the new MBE. Since Z' accounts for the combined effect of free, adsorbed and dissolved gas, then total original gas-in-place (OGIP) can be determined from extrapolation of the MBE straight line to an average reservoir pressure equal to zero. Drainage area can be estimated on the basis of the calculated OGIP and volumetric equations. As such, the methodology offers the potential to help improve well spacing in shale gas reservoirs in such a way that no stranded gas is left in the reservoir, or that not excess wells are drilled in the field. This can help to improve recoveries from shales by assisting in the determination of the optimum number of wells needed to drain efficiently a given play. In conventional reservoirs, a well is shut-in and the average reservoir pressure is determined from the corresponding pressure build-up test. But, for the case of unconventional shale gas reservoirs, shutting the wells in is unacceptable due to the long time it would require for estimating average reservoir pressure. The methodology developed in this paper for shale gas reservoirs circumvents this problem by using dynamic data. Production data from multi-stage hydraulically fractured horizontal wells completed in a Canadian shale gas reservoir are used for testing the effectiveness of the new methodology. Comparison of typical well spacing values vs. the drainage area calculated with the new methodology leads to the conclusion that, probably, only 40% of the gas is being drained efficiently. The novelty of this work relies on the development of a methodology for calculating average reservoir pressure, OGIP, drainage area, and optimum well spacing in shale reservoirs through the combination of dynamic data and a new MBE that considers simultaneously the effects of free, adsorbed and dissolved gas.

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