Propagation Mechanisms And Filling Patterns Of Hydraulic Fracturing Cracks Of Coalbed Methane Reservoir In Southern Qinshui Basin

The coalbed methane wells productivity and the selection of plugging-removing technology are restricted greatly by the complexity of propagation mechanisms and filling patterns of hydraulic fracturing cracks in coalbed methane reservoir. Hydraulic fracturing cracks in coalbed methane reservoir are characterized by fractures complexity, fillings distribution is unknown in fracture, the high degree of fracturing fluid filtration and damage degree of reservoir is serious, plugging-removing is difficult, etc. Following the theory of conventional oil and gas reservoirs, propagation mechanisms and filling patterns of hydraulic fracturing cracks in coalbed methane reservoir were proposed, but the physical model and propagation mechanisms of hydraulic fracturing cracks are unactual, and the internal distribution and migration characteristics of the fillings in hydraulic fracturing cracks of coalbed methane reservoir was based on experimental simulation and numerical calculation, the mechanisms of fracturing fluid filtration, filtration space, and fracture plugging mechanisms has not been studied systematically, therefore, a case study of no.3 coalbed methane reservoir in the southern Qinshui basin is proposed in this paper, based on the coal mine observation, laboratory test and the theory analysis, the propagation mechanisms and filling patterns of hydraulic fracturing cracks are summarized, fracturing fluid filtration mechanisms of coalbed methane reservoir is revealed, the filling blocking mechanisms of fracturing cracks and its impact on hydraulic fracturing effects and coalbed methane wells productivity are analysed. The mainly results of thesis are as follows:(1) By the coal mine observation, the main propagation patterns of hydraulic fracturing cracks in coalbed methane reservoir were found in southern Qinshui basin. The fracturing fluid distribution laws in coalbed methane reservoir were revealed clearly through coal mine sampling and testing method, the propagation of hydraulic fracturing cracks controlled by the natural fracture was verified, the distribution characteristics of proppant and coal fines was identifed.(2) A set of testing system which can be used to test fracturing fluid filtration way and the amount of filtration was designed, fracturing fluid filtration characteristic parameters (filtration volume, filtration path) which contain primary structure of coal and tectonic coal samples were obtained, combined with the experiment phenomenon, the difference of fracturing fluid filtration characteristic were summarized, which provides a test basis on the study of the filtration dynamic mechanisms of fracturing fluid and the building of filtration model.(3) Combining the theory of fluid mechanics, the filtration dynamic mechanisms of fracturing fluid of the primary structure coal, tectonic coal were studied respectively, the fracturing fluid filtration model of tectonic coal was proposed from damage mechanics, the calculation mathematical model of fracturing fluid filtration volume of the primary structure coal, tectonic coal were built, the filtrate locking mechanisms of fracturing fluid and the demage mechanisms of gas reservoir fluid production were studied, and the controlling proposal of reservoir damage was proposed.(4) Based on the coal mine observation, the development process of the hydraulic fracturing cracks of coalbed methane reservoir was proposed, the importance of filter cake was emphasized, with combinations of the theory of material mechanics, the crack initiation mechanisms was analyzed, the crack initiation model of the primary structure coal, and tectonic coal fracturing cracks were established. Based on the theory of force chains, the expansion mechanisms of hydraulic fracturing cracks was studied.(5) The source, classification of the fillings and the plugging mechanisms in the fracturing cracks were studied, the geological model of hydraulic fracturing effects of coalbed methane wells restricted by fracturing cracks plugging was established, the plugging mechanisms of the fracturing cracks near wellbore area during production was proposed, which can provide the theory basis for the optimization deployment and plugging-removing of coalbed methane wells.