Abstract
An accurate evaluation of coal rock fracture conductivity is an important prerequisite for predicting the productivity of CBM wells. Coal rock is soft and fragile, with low elastic modulus and high Poisson ratio. In the process of fracturing flowback, the contact deformation between proppant and fracture wall will affect the fracture conductivity when the proppant is embedded in the coal rock; thus the calculation method of plate fracture conductivity is no longer suitable for the evaluation of coal rock. Based on the contact deformation theory of elastic mechanics, a method for calculating contact deformation of proppant in fracture is proposed. Considering the effect of the deformation and embedded depth of proppant and the tortuosity of pore flow channel between proppant particles on fracture conductivity, a model for calculating fracture conductivity of coal rock fractures under three kinds of proppant arrangement (Model 4-1, Model 3-1, and Model 2-1) is established. Comparison of calculation results of theoretical model and experiments confirmed that the arrangement of proppant in coal rock fracture is closest to Model 3-1, and the influence of mechanical parameters of coal rock and proppant on fracture conductivity is calculated and analyzed by this theoretical model. The study shows that the coal rock fracture conductivity is affected little by Poisson’s ratio of coal rock and proppant, which is greatly influenced by the elastic modulus of them, and the effect of particle size of proppant is especially significant.
Highlights
Hydraulic fracturing is the primary means of stimulating CBM wells and has been widely used in exploration of coalbed methane [1,2,3,4,5,6,7,8,9]
The fractures produced by hydraulic fracturing can communicate with natural fractures in the coal seam [10,11,12], enhance the connectivity between natural fractures, greatly improve the permeability of coal reservoir [9], and have a very significant effect of increasing production [13, 14]
When the elastic modulus of coal rock wall tends to infinity, the proppants will not be embedded, and the value of α will only affected by the deformation of proppants
Summary
Hydraulic fracturing is the primary means of stimulating CBM wells and has been widely used in exploration of coalbed methane [1,2,3,4,5,6,7,8,9]. Researchers have carried out relevant experimental research on fracture conductivity of coal rock, in terms of proppant embedment, crushing and wall compaction, and the damage of fracture fluid residue to fracture conductivity. Gao [29] carried out a long-term diversion experiment of coal rock and tested the conductivity under the condition of closure pressure and proppant embedment. Zhang [34] considered proppant embedment and natural fracture and other factors, tested the coal rock fracture conductivity under the condition of low sand concentration, and found that single layer sanding can support coal rock fractures under lower closed pressure and obtain ideal fracture conductivity. Zheng [35] and Dong [36] considered the influence of natural fractures and the properties of coal rock on fracture conductivity and carried out experiments on fracture conductivity of coalbed methane wells. This paper has important theoretical and practical value to establish the corresponding theoretical model
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