Prediction Model of Coal Reservoir Pressure and its Implication for the Law of Coal Reservoir Depressurization

Abstract

The main methods of coalbed methane (CBM) development are drainage and depressurization, and a precise prediction of coal reservoir pressure is thus crucial for the evaluation of reservoir potentials and the formulation of reasonable development plans. This work established a new reservoir pressure prediction model based on the material balance equation (MBE) of coal reservoir, which considers the self‐regulating effects of coal reservoirs and the dynamic change of equivalent drainage area (EDA). According to the proposed model, the reservoir pressure can be predicted based on reservoir condition data and the actual production data of a single well. Compared with traditional reservoir pressure prediction models which regard EDA as a fixed value, the proposed model can better predict the average pressure of reservoirs. Moreover, orthogonal experiments were designed to evaluate the sensitivity of reservoir parameters on the reservoir pressure prediction results of this proposed model. The results show that the saturation of irreducible water is the most sensitive parameter, followed by Langmuir volume and reservoir porosity, and Langmuir pressure is the least sensitive parameter. In addition, the pressure drop of reservoirs is negatively correlated with the saturation of irreducible water and the Langmuir volume, while it is positively correlated with porosity. This work analyzed the reservoir pressure drop characteristics of the CBM wells in the Shizhuangnan Block of the Qinshui Basin, and the results show that the CBM reservoir depressurization can be divided into three types, i.e., rapidly drop type, medium‐term stability type, and slowly drop type. The drainage features of wells were reasonably interpreted based on the comprehensive analysis of the reservoir depressurization type; the latter was coupled to the corresponding permeability dynamic change characteristics, eventually proving the applicability of the proposed model.

Analysis of productivity differences in vertical coalbed methane wells in the Shizhuangnan Block, Southern Qinshui Basin, and their influencing factors

“…(a) Comparison of reservoir pressure, (b) comparison of drainage radius, and (c) comparison of gas content. The calculation methods of reservoir average pressure and drainage radius are quoted from Yan et al. (2019).…”

“…Production at the mine-field is currently mainly from coal seam #3, which is dominantly anthracite; its coal vitrinite reflectance (R o max ) ranges from 2.92 to 3.02% (Zhang et al., 2015). The thickness of this coal seam is stable, with an average of 6.35 m. The gas content of the coal is between 7 and 21 m 3 /t (Yan et al., 2019). In addition, there are five aquifers in this block, and P 1 x-P 1 s aquifer and C 3 t aquifer have significant influences on CBM development (Figure 2).…”

Quantitative optimization of drainage strategy of coalbed methane well based on the dynamic behavior of coal reservoir permeability

“…A reservoir pressure prediction model based on the material balance equation was used to study the dynamic characteristics of reservoir pressure of the target wells during the CBM production process 38 . This model makes full use of the actual geological parameters and production data.…”

A Comprehensive Coal Reservoir Classification Method Base on Permeability Dynamic Change and Its Application

“…Based on the above methods, six demarcating pressures for each well were calculated, and the recovery factor of CBM wells (Rc) was obtained by combining the numerical simulation and volumetric method. Additionally, a reservoir pressure prediction model based on the material balance equation (MBE) for coal reservoirs was used, which considers the self-regulating effects of coal reservoirs and the dynamic change of the equivalent drainage area, such that the actual reservoir average pressure and permeability can be calculated based on the reservoir conditions and actual production data [38]. The initial reservoir pressure of well Z26 is greater than the invulnerable pressure, while the rebound and recovery pressure is 2.7 MPa and 2.3 MPa, respectively ( Table 5), indicating that permeability transitions from the alleviative stage and gradually increases in the later period.…”

“…In this contribution, the effects of effective stress and sorption-induced strain, which have been fully considered relative to the permeability of a high-rank coal reservoir in China [12,38], were applied to examine the permeability dynamic characterization in the single-phase water flow stage and gas desorption stage. Some demarcating pressures and corresponding permeabilities were proposed to quantitatively characterize the dynamic permeability in various stages.…”