Abstract
Two indirect parameters influencing coalbed methane (CBM) drainage performances are proposed in this paper, which are effective desorption radius and difference between reservoir pressure and critical desorption pressure (DRPCDP). Variations of the two parameters during CBM drainage are investigated, which shows that they have a linear relationship. By using formula derivations, a theoretical model for gas production prediction is built. It suggests that the cumulative gas production is a product of square of effective desorption radius with DRPCDP, and there is also a cubic polynomial relationship between cumulative gas production and linear average DRPCDP. Furthermore, well PM01 located at southern Qinshui basin of China is selected as a case, and a commercial software is adopted to predict the gas production. Compared with the simulated and modeled cumulative gas productions, the simulated data match well with the modeled data, which indicates that the model has a good accuracy.
Highlights
As an unconventional gas reservoir, a coalbed methane (CBM) reservoir serves as both reservoir rock and source rock for methane (Bustin and Clarkson, 1998; Crosdale et al, 1998)
The first aspect refers to the declines of reservoir pressure and gas content, and the second is the variations of porosity and permeability (Connell and Detournay, 2009; Wei et al, 2007b)
There are enough methods used to analyze above conventional parameters, of which variations have been investigated clearly during gas drainage processes (Sun et al, 2017a, 2018b; Wu et al, 2015). This paper proposes another two indirect parameters, which are effective desorption radius and difference between reservoir pressure and critical desorption pressure (DRPCDP)
Summary
As an unconventional gas reservoir, a coalbed methane (CBM) reservoir serves as both reservoir rock and source rock for methane (Bustin and Clarkson, 1998; Crosdale et al, 1998). During CBM drainage, reservoir varies as a series of processes including dewatering, pressure decline and gas desorption (Ding et al, 2011; Wei et al, 2007a; Zou et al, 2014). For equation (2), we only discuss the second stage-desorbed gas drainage stage, in which the reservoir pressure decreases down to the critical desorption pressure. DRPCDP is a measure of gas content decline, which will be proved in the later section As this difference varies with both positions and drainage time, the entire provenance is divided into n parts. For any CBM well, if values of bottom hole pressure and cumulative gas production are collected sufficiently, a three regression analysis can be used to match them combined with some basic laboratory and field experiments, and all undetermined coefficients in equation (9) can be thereby obtained. Pore compressiblity (Â10À4 MPaÀ1) Matrix compressiblity (Â10À4 MPaÀ1) Elasticity modulus (GPa) Maximum volume strain Poisson ratio Thickness (m) Buried depth (m) Reservoir pressure (MPa) Reservoir temperature (C) Gas content (m3/t) Sorption time (d) Langmuir volume (m3/t) Langmuir pressure (MPa) Critical desorption pressure (MPa) Porosity (%) Permeability (mD)
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