The premise of this work is the development and application of a new methodology to forecast production data in unconventional reservoirs where variable rate and pressure drop data are typically observed throughout production. Decline curve analysis techniques for the estimation of ultimate recovery (EUR) require the constant bottomhole pressure condition during the producing life of the well -whereas it is not regular practice to maintain a constant bottomhole pressure profile throughout production in unconventional reservoirs. Therefore, the applicability of the time-rate decline relations is questionable, and methods to remove pressure variations from rate response are needed for generating future production forecasts.From a conceptual view point, we propose the utilization of the convolution/superposition theory along with the recently developed "empirical" time-rate equations, which are normalized by pressure drop data. In order to avoid non-uniqueness, a workflow is used where model parameters for the "normalized" decline curve equations are identified using diagnostic "qDb" plots. Normalized decline curve equations are then convolved with the pressure drop data to achieve a history match and to forecast production.We provide demonstrative application of this technique using an example from an high pressure high temperature shale gas reservoir. For varying bottomhole pressure cases, we show that our proposed techniques effectively remove pressure variations from the rate history. We present the differences in computed EUR values using decline curve analysis with and without corrections for varying pressures. In addition, forecasts are generated using supplementary plots such as pressure drop normalized rate versus cumulative production.

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Material balance analysis is a fundamental technique for estimating gas-in-place. It can be achieved using:Static material balance, using static (shut-in) reservoir pressures, where a plot of static p/z versus cumulative gas production is created to estimate original-gas-in-place (OGIP) orFlowing material balance where gas rates and flowing pressures are used to estimate average reservoir pressure. The flowing material balance concept of Agarwal-Gardner (1999) was extended to dry coalbed methane (CBM) reservoirs by Clarkson et al. (2007a) and Gerami et al. (2007) and to 2-phase (gas and water) CBM wells by Clarkson et al. (2007b). The present study further enhances the flowing material balance for dry CBM reservoirs by presenting a p/z* implementation of the concept. This application, while accounting for the distinguishing characteristics of a CBM reservoir, uses the industry-standard practice of p/z material balance to calculate original-gas-in-place. As with the Agarwal-Gardner approach, the flowing p/z* method can be applied to variable gas rates and/or flowing pressures conditions. In the present work, the derivation and iterative procedure of calculations are explained. Several test cases based on dry/immobile water saturation using real and synthetic data were generated. The resulting estimates of OGIP calculated from implementation of flowing p/z* material balance show excellent agreement and the estimated OGIP's are reliable. Introduction Material balance is the application of mass balance to a producing reservoir. As gas is produced, the reservoir pressure declines. By monitoring the cumulative gas production and the average reservoir pressure, and using the PVT properties of gas, one can determine OGIP and the remaining gas-in-place. Material balance analysis, although simple and more reliable than volumetrics calculation, does suffer from a number of shortcomings and limitations. For example, to measure the average reservoir pressure the well has to be shut-in and that means loss of production. Among other complexities are:low permeabilities lead to poor pressure build-ups (long-buildup times required)pressure build-up can be masked in multiple coal seamsreservoir can be recharged from aquifers CBM reservoirs have additional complexities. The gas storage mechanism as well as the compressibility of CBM reservoirs is dominated by adsorption. These and other CBM-specific characteristics have to be accounted for in any material balance calculations.