Fracture Propagation Mechanism And Parameters Optimization Study On Multi-Gas Combined Production In Coal Measure Strata

There remains a series of problems on the hydraulic fracture of the coal measure strata in Lin-Xing block of China.For example,the fracture is hard to penetrate adjacent layers to realize the multi-gas production in deep coal.The mechanism of the effect of transition zone and so on.So,it is necessary to find the fracture propagation mechanism on the multi-layered coal strata and guide the construction in the field.In this paper,some numerical simulations and true tri-axial hydraulic fracturing stimulation experiments were conducted.And the multi-scaled fracture propagation mechanism and parameters optimization were studied,where the effect of transition zone was mainly focused.In the mechanism study part,the fracture initiation,penetration and propagation were learned.In the parameter optimization part,the fluid viscosity,fracturing layer selection and the perforation parameters were the main topics.The main conclusions of this paper are as follows:(1)Fracture multi-scaled propagation mechanism on coal measure strata.The near-wellbore fracture initiation process can be divided into three stages.(a)Stage 1: the vertical microfracture bands develop along the perforation hole;(b)stage 2:micro-annulus fracture forms around the wellbore;(c)stage 3: fractures break through along the perforation holes perpendicular to the minimum in-situ stress.When fracture initiating in coal layer,the fracture near the wellbore was complex with many secondary fractures.when initiated in both sandstone and coal layers,the main fracture developed quickly in coal and the smooth fracture surface formed near the wellbore.Four patterns were shown in the coal,that was the T-shaped fracture,H-shaped fracture,step-wise fracture and the complex fracture network.(2)The effect of the lithology transition zone on fracture vertical propagation behavior for coal measure strata.Three functions of the transition zone are shown on fracture propagation in layered coal strata.1)Magnify the fracture diversion through turning inside the transition zone from hard to soft layer.2)Restrict fracture vertical crossing layers from soft to hard layer with a thick transition zone.3)Increase the fracture complexity through fracture branching in the transition zone.The thickness of transition zone has more effect than the strength on the fracture vertical propagation in coal measure strata.(3)Fracturing parameters optimization on the multi-gas production in coal measure strata.The fracturing viscosity showed a stepped manner as function of fracture complexity in hydraulic fracturing.When fracturing in both sandstone and coal layers,high viscosity helped to penetrate more layers and low viscosity formed complex fracture inside the single layer when fracturing in coal.It was suggested that the high viscosity should be used firstly to penetrate more layers and low viscosity then to increase the overall fracture complexity.When the thickness of coal is more than 10 meters and the buried layers are more than 15 meters,single fracturing in coal is better and perforation in middle-upper part of coal is better than that in the middle.When the multi-gas combined,fracturing in the both coal and sandstone is better,and the perforation section should lie most in the sandstone layer and small part in the coal seam to improve the fracture stimulation volume.