A Material Balance Equation for Stress-Sensitive Shale Gas Reservoirs Considering the Contribution of Free, Adsorbed and Dissolved Gas

Abstract

Unconventional shale gas reservoirs around the world have been proven to store gigantic volumes of natural gas. It has been demonstrated with both laboratory and mathematical work that these reservoirs can be represented by a quintuple porosity formulation plus an additional storage mechanism provided by dissolved gas in kerogen. All these storage mechanisms must be considered for estimation of original gas in place (OGIP). Otherwise pessimistic values of OGIP and recoveries will be obtained by ignoring any of these mechanisms. This paper presents a new easy-to-use Material Balance Equation (MBE) for shale gas reservoirs that considers the contribution of free, adsorbed and dissolved gas and their effects on cumulative gas production. Furthermore the proposed MBE takes into account the stress-dependency of permeability and porosity as the reservoir is depleted. Aguilera (2008) formulated a MBE to account for the effect of fracture compressibility on OGIP determination in stress-sensitive naturally fractured reservoirs. Cabrapan et al. (2014) extended the method by incorporating adsorption in shale gas reservoirs. The authors used the Langmuir Adsorption theory for quantifying the adsorbed gas volume as a function of average reservoir pressure. In this paper, the method is further extended to include the effect of production by diffusion of dissolved gas from kerogen. The volume of dissolved gas depends on the total fractional volume of kerogen in shale and the methane concentration in the kerogen body, which is in turn a function of pressure and temperature, as proposed by Swami et al. (2013).Results are presented as crossplots of P/Z (pressure/gas deviation factor) vs. G p (cumulative gas production), G p vs. time and gas rate vs. time. The plots allow detecting four stages of production in a shale gas reservoir: 1) production of free gas from fractures and organic porosity, 2) production of free gas from the inorganic matrix when fractures start closing, 3) production by desorption from the organic material and 4) production by diffusion of dissolved gas. The same trends have been observed in Devonian Shales of the Appalachian Basin where long production histories are available. It is concluded that dissolved gas is not only an additional storage mechanism but it also provides an important pressure and production contribution in shale reservoirs.The new consideration introduced in the MBE proposed in this paper is of importance because although diffusion from kerogen in shale gas reservoirs is by nature a slow process, it provides long term production rates with relatively small declines. To the best of our knowledge, an analytical MBE that includes simultaneously stress-dependent porosity and permeability, free gas, adsorbed gas and dissolved gas has not been published previously in the literature.

Study on the Influence of Shale Storage Space Types on Shale Gas Transport

“…Due to the weak van der Waals force (intermolecular force) between clay mineral molecules and shale gas molecules, it is difficult to adsorb shale gas molecules in this part . The micro–nano storage space model can simulate the transport process of free gas compressed in nanopores, and the adsorbed gas on organic matter pore walls . Under specific temperature and pore pressure conditions, the desorption process and adsorption process of shale gas are in a state of dynamic equilibrium.…”

Critical Review of Fluid Flow Physics at Micro‐ to Nano‐scale Porous Media Applications in the Energy Sector

“…e typical characteristics of a shale reservoir include low porosity, low permeability [185,186], complex network of matrix-fracture systems [187][188][189][190][191], and heterogeneous mineralogy [181,182,[192][193][194][195], and the combination of these characteristics presents a unique challenge in understanding the fluid flow and fluid-rock and fluid-fluid interactions in these reservoirs. Following theoretical considerations, some studies explored the fluid flow in these reservoirs as a function of convection, slip flow, Knudsen diffusion [196][197][198][199][200][201], surface diffusion/sorption [202][203][204][205][206][207][208][209][210], rock-mechanics [189,[211][212][213][214][215], and osmosis [216][217][218]. Unlike conventional reservoir rocks, shales are typically oil-wet or mixed-wet as discussed in a review by Singh [218] for various shale rocks across the world [219][220][221][222].…”

A Novel Analytical Method to Calculate the Amounts of Free and Adsorbed Gas in Shale Gas Production

“…The subscript sc represents the standard condition, p sc is 101325Pa, and T sc is 298.15K. It is noted that (p) can be obtained by (13).…”

“…For shale gas reservoirs, Ambrose et al formulated a new gas-in-place equation accounting for the organic pore space taken up by the adsorbed phase [12]. Orozco and Aguilera introduced a new 2 Mathematical Problems in Engineering material balance equation considering the contributions of free, adsorbed, and dissolved gas [13]. Meanwhile, the free-toadsorbed gas ratio in shale can be obtained by experiments.…”