Abstract
The development of coalbed methane (CBM) is not only affected by geological factors, but also by engineering factors, such as artificial fracturing and drainage strategies. In order to optimize drainage strategies for wells in unique geological conditions, the characteristics of different stages of CBM production are accurately described based on the dynamic behavior of the pressure drop funnel and coal reservoir permeability. Effective depressurization is achieved by extending the pressure propagation radius and gas desorption radius to the well-controlled boundary, in the single-phase water flow stage and the gas–water flow stage, respectively, with inter-well pressure interference accomplished in the single-phase gas flow stage. A mathematic model was developed to quantitatively optimize drainage strategies for each stage, with the maximum bottom hole flow pressure (BHFP) drop rate and the maximum daily gas production calculated to guide the optimization of CBM production. Finally, six wells from the Shizhuangnan Block in the southern Qinshui Basin of China were used as a case study to verify the practical applicability of the model. Calculation results clearly indicate the differences in production characteristics as a result of different drainage strategies. Overall, if the applied drainage strategies do not achieve optimal drainage results, the coal reservoir could be irreversibly damaged, which is not conducive to expansion of the pressure drop funnel. Therefore, this optimization model provides valuable guidance for rational CBM drainage strategy development and efficient CBM production.
Highlights
The development of coalbed methane (CBM) is affected by geological factors, and by engineering factors, such as artificial fracturing and drainage strategies
In the early stage of CBM production, the rapid drop in bottom hole flow pressure (BHFP) causes the reservoir pressure to be lower than the critical desorption
In the numerical simulation, [1] the actual BHFP was used to fit the actual CBM production, with geological parameters corrected by history matching; [2] production strategies were optimized by substituting the corrected parameters into the proposed model; [3] ideal CBM production rates were predicted by combining the optimized production strategies; [4] the practical applicability of the model was verified by comparing the actual and predicted gas production rates
Summary
The development of coalbed methane (CBM) is affected by geological factors, and by engineering factors, such as artificial fracturing and drainage strategies. In order to optimize drainage strategies for wells in unique geological conditions, the characteristics of different stages of CBM production are accurately described based on the dynamic behavior of the pressure drop funnel and coal reservoir permeability. A mathematic model was developed to quantitatively optimize drainage strategies for each stage, with the maximum bottom hole flow pressure (BHFP) drop rate and the maximum daily gas production calculated to guide the optimization of CBM production. If the applied drainage strategies do not achieve optimal drainage results, the coal reservoir could be irreversibly damaged, which is not conducive to expansion of the pressure drop funnel. This optimization model provides valuable guidance for rational CBM drainage strategy development and efficient CBM production. There have been few discussions on the coupling relationship between pressure drop funnel expansion characteristics and the mechanism of permeability dynamic behavior on productivity, despite the importance of these factors for accurately guiding CBM production
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