| With global carbon emission,coalbed methane(CBM),as a green and clean energy,is urgent to be developed successfully and reasonably.However,due to the complicated geological conditions,the CBM reservoir has the features of “low permeability,low reservoir pressure,low gas saturation and high gas content” in China.Meanwhile,the coal is stratified because of sedimentary differences and the geological structure in the later period during the coal-forming process,which leads to forming different coal structure combinations.It brings some troubles to reservoir reconstruction.There are many problems when the fracturing is constructed in the soft coal,such as hole collapse,short fractures,formation of fracture networks,coal powder and so on.The hard coal is conducive to the formation of long fractures and has high fracturing pressure because of its hardness.Hard coal has a good effectiveness when it is reformed.The physical,chemical and mechanical properties of coal are quite different when the coal reservoir is composed of hard coal and soft coal.It leads to the huge differences about the crack initiation and propagation of coal and seriously affects the effect of coal reservoir reconstruction.Therefore,it is urgent to find out the mechanism of fracture initiation and propagation in the fracturing process.In this thesis,the crack initiation and propagation mechanism with different coal structure combinations are studied during the hydraulic fracturing process in coal reservoirs.Based on the coal structure combination and its thickness ratio division,the coal structure combination types are divided.It mainly uses the methods of the theoretical analysis,physical test and numerical stimulation to do the systematic research about the influencing factors of crack initiation and propagation with the undeformed coal and the granulated coal combination,the cataclastic coal and the granulated coal combination,the undeformed coal,the granulated coal and the cataclastic coal combination of coal reservoir.The main results and cognition are as follows:(1)The coal structure combination types are classified according to the thickness ratio and combination features of different coal structures.The geological original cause of coal structure is analyzed from the geological perspective and the coal structure has the characteristics of stratification in the coal.Based on the logging data,the different coal structure has the various logging response characteristics and the geological strength index(GSI)was introduced to realize the quantitative characterization of coal structure.According to the relationships between GSI and the key logging parameters,the logging identification model of coal structure was established.On the basis of the thickness ratio and space combination with different coal structures,coal structure combinations can be divided into two-type,three-type and multi-type 3 types and 11 sub-types,respectively.(2)Hydraulic fracturing experiments were carried out on the different coal structure combinations and the characteristics of fracture initiation and propagation were obtained.The hydraulic fracturing experiments were carried out on the coal samples with different coal structure combinations.Through the research,the hard coal overcomes the tensile strength and minimum principal stress to form the longitudinal fractures with the driving of high hydraulic pressure when the coal reservoir is composed of hard coal and soft coal.The increasing of soft coal can inhibit the crack propagation and can cause the transverse deformation of coal reservoir.Meanwhile,the fractures occur along the cross section at the joint part of hard coal and soft coal.When the coal reservoir is between hard coal + soft coal + hard coal,the crack can cut through the soft coal with the thick hard coal and the thin soft coal during the fracturing hard coal.On the contrary,soft coal is difficult to be cut through.When the coal reservoir is soft coal + hard coal + soft coal,it cracks along the cross section in the hard coal.The soft coal can inhibit the crack expansion,resulting in that the hard coal is destroyed under the shear stress.(3)The mechanical mechanism conditions of crack initiation and propagation of hard coal,soft coal and their combinations have been preliminarily identified,respectively.The mechanical mechanism of fracture is theoretically analyzed in the process of fracturing hard coal,such as radial tension fracture,shear fracture,circumference tension fracture and steering fracture.In addition,based on Penny fracturing model,the fracture initiation mechanism of soft coal is analyzed and its model is modified.According to the complete interface model of interface mechanics,the maximum tangential stress theory is used to study the fracture propagation of the combined interface of hard coal and soft coal.When the crack turns to the side of soft coal,the interface failure criterion is reached first and the fracture propagation along the interface is generated if the turning angle is relatively small.However,the steering failure criterion is reached first and the steering propagation occurs if the turning angle is relatively large.When the fracture has a tendency to turn to the side of hard coal,the fracture will propagate along the interface because the failure criterion of the interface is reached first.(4)Based on the numerical simulation,the fracture morphology and the factors of affecting fracture initiation and propagation are discussed in the hydraulic fracturing with different coal structure combinations.The influencing mechanisms are revealed about the geostress field,the coal mechanical properties and fracturing operation parameters.By the numerical simulation,the fracture morphology of coal reservoirs is studied in three different coal combinations,including undeformed coal and granulated coal combination,cataclastic coal and granulated coal combination,undeformed coal,granulated coal and cataclastic coal combination.The initiation time is longer when the undeformed coal is thicker,and the influence range of fracture is limited under the the same discharge.For coal reservoirs with different coal structure combinations,the fracture propagation is concentrated in the undeformed coal and cataclastic coal.With a certain thickness of granulated coal,the fractures are easy to reorient.At the same time,the fractures mainly extend along the interface in the stratified interface,along with the extension to the granulated coal.When the horizontal stress difference is equal,the maximum crack width of the undeformed coal and granulated coal combination is less than that of the combination of undeformed coal,granulated coal and cataclastic coal under the low-medium stress difference.The latter coal structure combination is conducive to the crack initiation and the interface of coal structure is easy to be connected.The larger the elastic modulus of coal structure combination is,the smaller the cracks open.With the increasing of Poisson’s ratio of coal structure combination,the width of fracture increases and the length of fracture decreases.The greater the viscosity of fracturing fluid is,the easier it is to form wide and short fractures.On the contrary,it is easier to form a long and narrow crack.Under the same conditions,the larger the fracturing fluid discharge is,the larger the fracture volume will be.(5)Taking Shizhuangnan Block as an example,the rationality and accuracy of the experiment and simulation results are verified.Based on the analysis of typical wells in the Shizhuangnan Block,the fracture propagation mechanism is further verified with different coal structure combinations.The fracture propagation morphology is seriously affected by the coal structure.The vertical fractures are mostly formed in coal reservoirs with hard coal(undeformed coal and cataclastic coal).The fractures are mostly horizontal fractures with soft coal(granulated coal and mylonite coal).When the coal structure changes from hard coal to soft coal,the fractures are prone to reorient.When there is some soft coal stratification on the bottom,firstly,the fractures extend horizontally within the stratification,but the extension distance of the fractures is short.Then the fractures are toward to the hard coal and form vertical fractures.It is of an important indicating significance to study the fracturing of the complex coal structure reservoir. |
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