Research On Well Productivity And Economic Feasibility Of Commingled Coalbed Methane Production In Multi-seam Zone

The southwest region of China has rich coalbed methane(CBM)resources.Coal seam thickness is thin while the gas content is high.It is a key favorable area for me-dium and long term CBM development in China.If multi-seam mining can be carried out effectively,it will provide precious experience and guidance for the multi-seam zone development.In this paper,the multi-seam characteristics and development potential of resource of Songzao mining area is studied.Combined with laboratory experiments,the mechanism of adsorption and seepage characteristics under the conditions of original and proppant surported coal fracture are studied respectively.The inter-layer interfer-ence mechanism caused by the difference of reservoir energy and conductivity was an-alyzed,and the feasibility of commingled production in the study area was analyzed.Based on the history matching,CBM production capacity was predicted and the param-eter sensitivity of co-production capacity was analyzed.The economic benefits of mul-ti-coal seam development were studied based on the results of production prediction and economic evaluation results.From the perspective of economic benefit,the necessity of expanding non-primary coal seam for multi-layer coal seam was demonstrated through the sensitivity analysis of reservoir parameters.The research shows that the tectonic conditions of the Permian Longtan Formation coal-bearing rocks in the study area are relatively simple,the dip of strata is generally small,and the water-richness is weak.Seam roof and floor sealing conditions are good,with the favorable geological conditions of coalbed methane.Coal seams of Longtan Formation has small spacing and concentrated distribution with thin seam thickness.However,the gas content of the Longtan Formation is good and the high gas content compensates for the small thickness of the coal seam,which suggests a good resource condition.A series of measurements were performed with and without proppant for an artifi-cial fracture from a coal sample under hydrostatic condition.The experiments show that proppant supported fracture has little effect on adsorption ability,however swelling be-haviour may be impacted.Proppant supported fracture can significantly enhance per-meability about 2-3 orders of magnitude higher than original fracture depending on proppant type and it leads to cleat compressibility decreases at the same time.Multi-layer proppant filling do not necessarily result in higher permeability than monolayer proppant filling since proppant particle distribution have an effect.Sparsely placed monolayer proppant can have comparable permeability with densely packed multilayer proppant.Due to the differences in reservoir properties,inter-layer interference is inevitable in multi-coal seam production.The greater the difference between the CBM reservoir properties,the more serious the inter-layer interference phenomenon.Differences in reservoir energy and conductivity are the most important factors that cause inter-layer interference in multi-coal seam drainage,which mainly includes the influence of reser-voir pressure gradient and difference of desorption pressure-reservoir pressure ratio.Differences in permeability can mitigate or aggravate inter-layer interference phenom-enon.When carrying out the combination and optimization of the commingled produc-tion layer,reservoir energy and conductivity should be integrated under the premise of meeting certain resource abundance.At the same time,it is necessary to consider the ease of CBM reservoir stimulations.Structurally fractured coal and the coal layers closely related to the strong water-bearing surrounding rock should be avioded,so as to avoid their adverse effects on the post-fracturing engineering and drainage measures.According to the drainage situation of CBM well in the study area,numerical sim-ulation software SIMED II was used to perform the numerical simulation work.History matching was performed to allow more accurate production prediction of multiple su-per-imposed CBM system.The history matching results show uniform pressure of each coal seam.The production prediction results of QC-X well in the study area show that mul-ti-layer production can greatly increase the production.When the single layer M8 is targeted,the peak gas production is only 1460m~3/d,while that of comminggled produc-tion can reach 2220~2370 m~3/d.Sensitivity analysis was performed to semi-quantitatively study the effect of various reservoir parameters on the productivity of multi-layer coalbed methane wells.Different parameters affect different production stage.Desorption pressure,cleat porosity,coal seam permeability and coal seam thick-ness affect the entire stage,which is,drainage stage,stable production stage and pro-duction decline stage.Discounted cash flow method was used to perform the economic evaluation of commingled CBM production.The economic results of well QC-X in the study area show that single layer M8 development plan and the 5-layer M6-1M6-2M7M8M12 de-velopment plan are not economically viable.The comminggled CBM production gener-ally improve the economic benefits,however,it is not that the more the layers targeted,the better the economic results.For well QC-X,the second coal seam section-M7 and M8 commingled production has the best economic benefits,and it can obtain the max-imum net present value,the highest internal rate of return and the shortest payback pe-riod.Therefore,for multiple CBM production,the production and the economic viabil-ity should both be evaluated to maximize the development efficiency of CBM wells.In addition,the economic results of multi-layer and single-layer development under gen-eral conditions with different reservoir parameters are analyzed.Research shows that in most cases,multi-layer development is more profitable than single-layer development.However,when the desorption pressure or the permeability of the extended CBM res-ervoir is too low,the economic attractiveness of multi-layer drainage is less than that of single-layer development even if the gas production increases.Therefore,the mul-ti-layer CBM mining can not be carried out blindly,and it is necessary to calculate the economic benefits by production prediction to assist in the decision-making of the de-velopment scheme in regard to rational combination and optimization of production layers.