4
SPE-192002-MS
observe the change in the inflow performance. Although diffusion is the mechanism through which gas is
desorbed from the coal matrix, its direct application is not achievable without the use complex methods such as numerical simulation. A simple inflow relationship (Equation 4
) is used that is derived from pseudo pressure and Darcy's law.
(4)
Though applying the fitted Jensen and Smith tank model to the daily gas rate it is possible to estimate the daily reservoir pressure. Applying the daily reservoir pressure and the daily bottom hole pressure into equation allows the inflow performance to be estimated.
There are two major effects that control the fracture permeability of coal formations. The first being compaction as a result of dewatering and lack of support within the coal formation, this causes a reduction in fracture permeability. The second being matrix shrinkage where the coal shrinks as gas
is removed through desorption, this causes an increase in fracture permeability.
The Shi-Durucan model is an accepted geomechanical permeability model for coal formations, it accounts for both major effects of compaction and matrix shrinkage.
(5)
Equation 5
describes how the initial permeability is affected by changes ineffective stress. It is this change ineffective stress that is described by the Shi-Durucan model in equation The Shi-Durucan model is semi empirical as it requires the use of two fitting constants sand b. The model also employs physical material characteristics such as the Young's Modulus and Poisson's ratio. It relates the change in pressure to the change ineffective stress.
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