| Energy,as an important material foundation and driving force for the progress of human society,has always been closely related to national economy,people’s livelihood and national security.Oil shale has become an important strategic energy source for China because of its huge reserves.Compared with the ground dry distillation technology,the oil shale in-situ conversion technology injects the heat source into the in-situ reservoir through drilling wells,which prompts the kerogen in oil shale to retort to extract oil and gas resources;this technology is more green and environmentally friendly,and is more efficient in the exploitation of deep oil shale resources,and therefore it is the focus of the research on the development technology of oil shale at present.However,the porosity and permeability of oil shale reservoirs are underdeveloped,which hinders the heat exchange in the reservoirs and the transportation of oil and gas resources after pyrolysis,and hydraulic fracturing technology is needed to improve the seepage conditions in the reservoirs.Due to the high clay content and obvious development of laminae in oil shale reservoirs,the fracture length formed by ordinary hydraulic fracturing technology in oil shale reservoirs is short,and the fracture expansion is easily affected by the laminae,which makes it difficult to form a complex fracture network.In view of the characteristics of low-permeability reservoirs,on the basis of hydraulic fracturing,acid fracturing technology is widely used to transform low-permeability reservoirs.The acidic reaction between the acid and the minerals in the rock is utilized to reduce the fracture initiation pressure,change the pore seepage characteristics,and form a complex fracture network in the reservoir.Therefore,this study takes the oil shale in Maquan area as the research object,and uses a combination of theoretical calculations,numerical simulations and physical experiments to investigate the evolution of the physico-mechanical properties of the oil shale under acid damage and its influence on the fracture extension behavior.In this study,first of all,through the oil shale acid damage experiments and inductively coupled plasma emission spectroscopy,nuclear magnetic resonance and other technical means,to explore the microporous structure and seepage evolution characteristics of the oil shale under the acid damage,and clarify the micro reaction mechanism between the acid and the oil shale,so as to provide a basis of reaction mechanism for the change of mechanical properties of the oil shale under the acid damage.The results show that calcite and pyrite are the main minerals that undergo acid erosion reaction in oil shale,and some potassium feldspar and clay minerals also undergo hydrolysis reaction.The dissolution of minerals increased the porosity and pore size of the oil shale,and the single acidification promoted the transformation of pore space from microporous(<10 nm)to mesoporous(10～100 nm),with a maximum increase of 100% in porosity,while the pressure accelerated the penetration and reaction rate of the acid in the oil shale,and the pore space was transformed from microporous(<10 nm)to mesoporous(10～100 nm)and macropore(>100 nm)under the compound effect of the acid and pressure,with a maximum increase in porosity of 346%.Meanwhile,acid damage promoted the conversion of bound fluid pores to free fluid pores in the oil shale,and the free fluid porosity in the oil shale increased significantly.Free fluid porosity is closely related to the fractal dimension of free fluid porosity,and the increase of free fluid porosity reduces the fractal dimension of free fluid porosity,and the distribution of free fluid porosity is more uniform,which is conducive to the transmission of fluids in the pores.The combined effect of the two promoted a significant increase in permeability by 6 and 52.5 times along the vertical and parallel bedding directions,respectively,and the higher permeability of the parallel bedding played a crucial role in the increase of permeability under acid damage.Subsequently,the evolution of key mechanical properties related to fracturing such as compressive strength,tensile strength and elastic modulus of oil shale under acid damage was studied through experiments.Based on the damage theory,the acid damage variables of oil shale were calculated according to the Bellion damage variable form,and the relationship between the acid damage variables and the related physicomechanical properties of oil shale was established,along with the uniaxial compression intrinsic model of oil shale under acid damage,which provides a theoretical basis for the study of hydraulic fracturing behavior of oil shale under acid damage.The relationship between acid damage variables and related physical and mechanical properties of oil shale and the uniaxial compression constitutive model of oil shale under acid damage were established,which provides a theoretical basis for studying the hydraulic fracturing behavior of oil shale under acid damage.The results show that the acid damages the effective load-bearing skeleton of oil shale and reduces the mechanical strength.The highest decreases of compressive strength,modulus of elasticity,and tensile strength of oil shale were 14.39%,22.51%,and 29.55% along the direction of vertical bedding,and 38.13%,25.00%,and 21.79% along the direction of parallel bedding.Meanwhile,the damage ontology model established under uniaxial compression experiments of oil shale based on the Bellion damage variable form can more accurately describe the initial compression-density stage,elastic deformation stage and yielding stage in the process of uniaxial compression deformation of oil shale,and it can be used to analyze the stress-strain relationship as well as the damage evolution law of oil shale specimens after acid damage.Finally,this study combines hydraulic fracturing numerical simulation and true triaxial hydraulic fracturing experiments to investigate the effects of the degree of acid damage,fracturing fluid viscosity,fracturing fluid flow rate,and the orientation of bedding planes on the fracture initiation and extension behaviors of hydraulic fracture in oil shale reservoirs at the mine scale and the indoor experimental scale.The results show that the deterioration of the mechanical properties of the oil shale borehole wall under acid damage reduces the fracture initiation pressure of the oil shale,with the highest fracture initiation pressure decrease of 49.24%,and the cumulative length of hydraulic fractures in the oil shale increases,with the highest increase of 46.17%.Under the effect of acid corrosion,multiple acid damage points appeared at the wall surface of the borehole,and the fracturing fluid showed stress concentration at the acid damage points,and the hydraulic fracture extended along multiple fracture initiation points,which weakened the influence of the bedding planes and contributed to the formation of complex fracture extension patterns.Meanwhile,the presence of bedding planes changes the extension pattern of hydraulic fractures in oil shale formations.When the hydraulic fracture extends to the bedding plane,it will be captured by the bedding plane with lower mechanical strength and turn,which prompts the fracture to extend along the bedding plane,and the increase of fracturing fluid viscosity or displacement will prompt the hydraulic fracture to pass through the bedding plane,forming a complex fracture morphology.High-viscosity fracturing fluid can produce a blocking layer on the surface of the fracture,which makes it difficult for the fracturing fluid to percolate and leach into the specimen,thus prompting the fracture to expand along the direction of the maximum in-situ stress and promoting the longitudinal fracture to pass through the bedding plane;high flow rate fracturing fluid generates a very high pressure inside the specimen within a very short period of time,which results in the instantaneous initiation of the fracture and its extension along the direction of the maximum principal stress,and prompts the fracture to pass through the bedding plane rapidly without turning.The number and complexity of fractures formed increased significantly.At the same time,the direction of the bedding planes also affects the complexity of forming hydraulic fractures in oil shale specimens.When the bedding plane is perpendicular to the direction of maximum in-situ stress,because the mechanical strength of the bedding plane is lower than the strength of the oil shale matrix,bedding fractures may still be generated in the specimen,and the complex fracture patterns of bedding fractures and longitudinal fractures are formed.When the bedding plane is parallel to the direction of maximum in-situ stress,only bedding fractures can be generated in the specimen,and the fracture morphology is simple and it is difficult to form cross-fractures.In this study,a systematic study on the evolution of physico-mechanical properties of oil shale under acid damage and its influence on fracture extension behavior is conducted to provide reference for the selection and optimization of oil shale reservoir reforming technology,and to provide technical support for the realization of the commercial development of oil shale resources in China. |
PDF Full Text Request