Study On Influencing Factors Of Fracture Propagation In Hydraulic Fracturing Of Coal-rock Complex

Hydraulic fracturing network reconstruction technology is an important measure for safe and efficient exploitation of coalbed methane reservoir resources and prevention and control of mine gas disasters.However,for the low permeability broken soft coal seam,the effect of direct hydraulic fracturing in the coal seam is not ideal.Based on this,based on the previous research on coal and rock hydraulic fracturing,this paper studies the propagation law of fractures at and near the coal rock interface during the hydraulic fracturing of coal rock complex.In order to study the influencing factors of fracture propagation at and near the coal rock interface in the process of hydraulic fracturing of coal rock complex,through the combination of laboratory test,numerical simulation and theoretical analysis,the influencing and controlling factors of cross-border fracture propagation of hydraulic fracturing of coal rock complex are studied,and the hydraulic fracturing test of large-scale true triaxial coal rock complex is carried out;A method of combining cohesive elements and pore pressure nodes to simulate the hydraulic fracturing fracture network of coal and rock mass is proposed.The effects of different fracturing methods,stress field and coal seam bedding on the expansion of hydraulic fracturing fractures in coal rock complex near the interface are analyzed.The results show that:(1)After the fracture starts in the rock stratum and reaches the coal rock interface,the fracturing fluid will first hold the pressure in the interface and expand along the interface.Under the following circumstances,the fracture will pass through the interface and expand in the coal seam:The fracturing fluid is held in the coal rock interface.When the fracturing pressure reaches the coal fracture criterion,the coal seam will produce cracks,which generally do not deflect:if the coal seam has a natural defect weak surface,when the pressure at the weak surface is enough to open the weak surface during the expansion of the fracturing fluid along the coal rock interface,The crack will pass through the interface at the weak plane and produce cracks in the coal seam along the weak plane:because the coal rock interface is uneven,there are a large number of sudden change points in the interface,which makes the stress concentration easy to occur when the fracturing fluid extends here,so as to reach the coal seam fracture criterion,and finally produce cracks in the coal seam:at the same time,because the shape of the sudden change points of the interface is difficult to predict,When the water pressure crack crosses the coal rock interface here,it is very easy to produce offset.The relationship between the deflection direction and the magnitude of the principal stress is not obvious,which is mainly determined by the shape of the mutation,but it will eventually return to the principal stress direction under the influence of the maximum principal stress.(2)Three fracturing methods are used in the true triaxial simulation test.Under the same principal stress difference,the effect of hydraulic fracturing from drilling in coal seam to rock stratum is not ideal;The way of vertical drilling and fracturing from the coal seam roof is that the hydraulic fracture is easier to pass through the coal rock interface and expand in the coal seam;In the case of drilling parallel holes in the coal seam roof for hydraulic fracturing,it is more difficult for the fracture to pass through the interface than the vertical drilling of the roof,but the influence range of the hydraulic fracture after passing through the interface is large.(3)A cohesive force element method for simulating the fracture network of coal rock complex is proposed,and the numerical calculation model of hydraulic fracturing of coal rock complex is established.The numerical simulation results show that with the increase of the stress field gradient,the pore pressure of the coal rock complex increases,and the fracture opens perpendicular to the coal rock interface.The main reason is that in the process of fracture propagation,the hydraulic fracturing fracture is affected by the stress field,spans from a joint to an adjacent joint,and expands along the stress weak surface of the coal rock complex,resulting in secondary fractures.With the increase of bedding dip angle of coal seam,elliptical compressive stress concentration area or X-shaped compressive stress concentration zone are formed on both sides of the main fracture.Under the condition of coal seam bedding dip angle of 30°,the hydraulic fracture expands at the coal rock interface,the other main fracture passes through the bedding plane and opens the fracture at the bedding,and there is obvious crack opening and sliding on the bedding plane.Under the condition of 60° bedding dip angle of coal seam,the main fracture first follows the direction of maximum principal stress,and then expands along the direction of bedding after encountering bedding,indicating that the control effect on the direction of fracture propagation under this bedding condition is stronger than in-situ stress.(4)There is a threshold for the hydraulic fracturing fracture of coal rock complex to pass through the interface and expand in the coal seam.The difference of principal stress and the strength of interface have an important influence on this threshold.The greater the difference of principal stress and the strength of coal rock interface,the more conducive to the Cross-Interface expansion of hydraulic fracturing fracture.Hydraulic fracturing cracks crack in the overlying strata and expand along the direction of the maximum principal stress.With the gradual increase of the principal stress difference,the control effect of hydraulic fracturing cracks by the stress field is more and more obvious.When the principal stress difference is the same and the strength of the coal rock interface is small,the crack will expand along the upper surface of the coal seam to both sides,which cannot meet the coal seam cracking conditions:when the strength of the coal rock interface is large,the water pressure crack will expand a certain distance in the coal rock interface and complete the pressure holding.When the pressure reaches the coal seam cracking conditions,the water pressure crack will pass through the coal rock interface and induce the coal seam to crack and expand.In particular,when the strength of the interface is much higher than that of the coal seam,because the strength of the coal rock interface is large,and the fracture pressure required for fracture is greater than that of the coal seam,it basically does not expand in the coal rock interface,and the fracture directly passes through the interface and expands in the coal seam.Under the same stress conditions,the greater the strength of the interface,the longer the extension length in the coal seam,and the greater the strength of the interface,the wider the width of the hydraulic fracture in the coal seam.When the principal stress difference is the same,the greater the strength of the coal rock interface,the easier it is to form water pressure accumulation and reach the threshold of coal seam fracture expansion.The greater the interface strength is,the more conducive it is for the hydraulic fracturing fracture to expand in the coal seam after passing through the coal rock interface.At the same time,the interface strength remains unchanged,and the greater the principal stress difference is,the more conducive it is for the hydraulic fracturing fracture to expand across the interface.