Coal-series Strata Fracture Behavior And Mechanism Of Small Layer Perforated Coal-bearing Rock Section Fracturing Based On The CBM Demonstration Project Of Songhe

Influenced by coal measures reservoir with the low porosity and low permeability characteristics,the gas-bearing reservoir need to be perforated during the development process of coal measures gas.Therefore,exploring the rock mechanics property of gas-bearing coal measures reservoir and the expansion behavior of fracturing fracture are the theoretical basis for enhancing the fracturing reformation effect.Based on the CBM demonstration project of Songhe in Western Guizhou and coal-bearing member characteristic analysis of study area,this paper studied the crack initiction and expansion mechanism of fracturing fracture and the characteristic of fracture shape in the area,constructed fracture model for size calculation.By using the Fracpro PT software to simulate the field hydraulic fracturing process of the third fracturing section of GP-1 well,this paper analyzed the main factors which controlling the fracturing fracture expansion,discussed the favorable conditions of fracturing fracture breakthrough the fracturing section,carried out the optimization of adaptive fracturing technique,contrasted the effect of reservoir reformation before and after fracturing.The research shows that the reservoir reformation pattern of small perforation and staged fracturing technique is suitable for the gas-bearing coal measures reservoir in Songhe Minefield with the characteristics of overpressure,low porosity and low permeability.The rock mechanical properties of coal seam of coal bearing strata is mainly controlling by its petrographic composition,macroscopic coal type and burial depth.The rock mechanical properties of roof and floor is mainly controlling by its lithology,aquosity and burial depth.There are obvious mechanical properties differences between the coal seam and roof and floor rock.In the same burial depth,the elastic modulus value of roof and floor rock is 5 to 9 times its coal seam,the compressive strength value of roof and floor rock 3 to 7 times its coal seam,while the Poisson ratio value of roof and floor rock is obvious lower than its coal seam.The mechanical properties differences between coal seam and roof and floor rock make developing the interior of fracturing section the fracture overall.Along with the burial depth of fracturing section increasing,the fracture formation and expansion become more difficulty and the effect of fracturing is decreasing.The fracturing fracture of study area coal bearing strata is meanly growth vertically split which have plenty of complicated crack near the wellhole and have simple single crack far away wellhole.The KGD model is the adaptive fracture model for size calculation.The expansion of fracturing fracture is mainly controlling by the factors of stress difference,mechanical properties and construction condition.Under the different factor controlling,the fracture length,height and width have present certain law.Based on studying the hydraulic fracturing numerical simulation of third fracturing section of GP-1 well,it is found that the fracturing fracture breakthrough the fracturing section more easily when the thickness of perforation layer adding to 10.8 meter from 9.0 meter,perforating the gas bearing sand stratum by the 32 hole per meter,adjusting the pad fluid volume ratio to 30%,and adopting small displacement on the earlier stage while maintaining 8m3/min displacement on the middle and later stage.Accordingly to the above condition to optimize construction parameters.In addition,the non-dimensional flow conductivity of fracture before and after optimization are 7.23 and 10.90 respectively.The effect of fracturing promote 50.76% as a whole.