Predictive and production analysis models for the unconventional gas reservoirs
Creators: name: Gerami, Shahab
Publisher: name: University of Calgary
Year: 2007

description: Bibliography: p. 221-234
description: With the decline of oil production and increase in demand of energy, worldwide reserves of natural gas play significant role in filling the gap between the demand and supply. This growing demand stimulates the development of the unconventional gas resources which are available in enormous quantities around the world. Two specific kinds of unconventional gas reservoirs are (i) hydrate reservoirs and (ii) coal bed methane (CBM) reservoirs. Whereas gas production from hydrate reservoirs is governed by a combination of heat transfer, fluid flow, and kinetic decomposition of hydrates, gas production from CBM reservoirs is governed by fluid flow, desorption and diffusion mechanisms. Efficient development and operation of any natural gas reservo1r depend on understanding of the reservoir characteristics and well performance. Developing accurate predictive models incorporating appropriate recovery mechanisms is an essential element in the study of any contemplated scheme for gas production from unconventional gas reservoirs. In this work, analytical models are developed to perform production analysis and performance predictions from (i) hydrate reservoirs in contact with an underlying free gas zone and (ii) CBM reservoirs with immobile initial water saturation. In the first part of this work, the predictive and production analysis models for transient and boundary-dominated flow regimes in a hydrate-capped gas reservoir are developed. Using a comprehensive numerical simulator, an extensive simulation study indicates that in some cases some of the non-linear processes involved in gas production from hydrate reservoirs have a negligible effect on the overall physics of the process. This significantly reduces the complexity of the heat and fluid flow equations and allows construction of the analytical transient and boundary-dominated models. The success of these models relies on coupling of the energy and mass balance equations, where the energy equation accounts for the endothermic nature of hydrate decomposition. Verification of the method is obtained by comparing results with those of a hydrate reservoir numerical simulator. In its "forward solution" mode, the models developed here are used as an engineering tool for evaluating the role of hydrates in improving the productivity and extending life of hydrate-capped gas reservoirs. In addition, in its "backward solution" or inverse approach mode, the applications of these new models provide an estimate of initial free gas-in-place and reservoir permeability from production data. In the second part of this work, it is shown that how the traditional type curve analysis models which were originally developed for conventional oil and gas reservoirs can be treated in a manner to analyze dry CBM reservoirs data. The methodology developed here is validated by CMG-GEM (a commercial numerical simulator) for two production scenarios including constant rate and constant pressure production over a wide range of reservoir parameters. The simple model developed here can therefore be used to analyze the dry CBM production data by the methods available for analyzing the production data of conventional gas reservoirs.