Experimental Investigation On The Crack Propagation Of Hydraulic Fracturing In Coal-rock Combination

Coalbed methane?CBM?is a low-carbon,clean and efficient unconventional natural gas resources,which has become an important direction for the development of new energy.China has a wealth of CBM resources,but the geological conditions of the CBM reservoirs are complicated,in particular,CBM in"weaker"coal seams is harder to develop especially.In the process of fracturing in weak coal seam,there are many problems such as the collapse of the fracturing well,the difficulty of maintenance in the later stage of the wellbore,the forming crack pattern single,short and width and so on.The existing hydraylic fracturing technology introduced from abroad can not effectively develop the CBM resources in China.Therefore,there is an urgent mining technology and theory suitable for the efficient and economical exploitation of CBM reservoir resources in China.In this paper,aiming at the different hydraulic fracturing characteristics of"soft coal-sandy mudstone",the fracturing experiments of coal,sandy mudstone and its combination are conducted.Based on the experimental research,theoretical analysis and numerical simulation,the characteristics of coal-rock failure,the hydro-fracture propagation,fractured surface morphology,and the two-dimensional and three-dimensional hydraulic fracturing behaviors of coal-rock and its combination in different bedding directions are systematically studied in this article.The main contents and results are as follows:?1?Investigations on anisotropic fracture mechanical and crack propagation characteristics of coal and sandy mudstone in different bedding plane directions.The experimental results indicate that layered coal and sandy mudstone demonstrate strong anisotropic mechanical properties and crack propagation behaviors in different bedding directions by the experiments of uniaxial compressive test,Brazil disk split test and I fracture toughness test.The higher brittleness(B_RIT-T)in the vertical direction is favorable for the crack propagation.The released strain energy of the fracture tip per unit length in the sandy mudstone in direction of vertical to the bedding plane could make 10.69 to25.53 times area or length of fracture in the coal mass under the ideal conditions.It is convenience for the fracture propagation and the formation of the fracture system.The additional effective length of the crack in the vertical or orthogonal to the bedding direction is 1.36 times higher than that of the parallel stratified structure under the condition of fracturing the same SCB specimen.The maximum deflection angle of the I-type crack in the vertical orthogonal bedding is about 30°under the influence of the cleat,natural fissures,while the maximum deflection angle of the I-type crack along the bedding orientation is usually within the range of 10°under the influence of the bedding.?2?Experimental study on hydraulic fracturing of 2D plate-like coal-rock combination.In order to intuitively observe the crack propagation behaviors at the coal-rock interface,two-dimensional plate coal-rock combination hydraulic fracturing tests were conducted.The study found that taking the technology of fracturing through the hard sandy mudstone to coal seam as the object,which fracturing effect is better than fracturing in the coal seam.The types of"penetrating","offset or diversion"and"bifurcation"cracks are beneficial to the crack propagation to the coal seam,which can be used in the vertical direction of the coal seam to communicate multiple stratifications and form a multi-crack structure.When it comes to the fracturing in the coal itself,the fracturing effect in the direction of vertical to the bedding is superior than that of fracturing along the bedding,because of the crack can expand longer and communicate multiple stratifications in the vertical direction,and main fracture generates secondary cracks in the progress of the fracture propagation.The"bifurcation"type cracks are advantageous to the crack of complicated structure,but the"T"type,"offset or diversion"cracks at the interface leads to the great energy loss,which go against to the propagation of the fracture.?3?Experimental study on hydraulic fracturing of 3D large scale coal-rock combination.The fracturing tests of 3D larges scale coal-sandy mudstone combination form the complementary research with the 2D combination.The experimental results indicate that the 3D fracturing pattern of the crack is more complicated tnan that of the 2D plane combination.The maximum hydro-fracture opening width of the coal in the direction of parallel to the stratification is 1.292mm,and0.952mm in the direction of vertical to the bedding plane,when it comes to the sandy mudstone,the maximum opening width in the direction of vertical to the bedding plane is 0.816mm.?4?Study on the hydro-fracture propagation patterns of coal-sandy mudstone combination by 2D phase field numerical simulation.The numerical simulation results indicate that the fracture pattern is single in the coal itself and it is hard to penetrante through the interface into the sandy mudstone.When it comes to fracturing from the sandy mudstone,the coal-rock interface and the coal begin to damage under the influence of the stress concentration of the crack tip when the fracture extends near the interface.The crack tends to propagate along the damaged area to form a multi-crack structure.In addition,the energy consumed by the crack in the process of offset or diversion is obviously higher,and the damage area is obviously wider.Under the influence of each other,the cracks can appear multi-level bifurcation and fusion,which is beneficial to the formation of multi-crack structure.