| According to the requirements of the national energy strategy,unconventional oil and gas reservoirs,especially the imperative for the development of shale gas reservoirs.Shale gas reservoir belongs to the discontinuous gas reservoir,hydraulic fracturing is one of the most important technical for commercial exploitation.According to the USA's successful experience of the unconventional oil and gas development,unconventional oil and gas production will increase with the increasing of reservoir effective transformation volume.Considering the fracturing induced crack,natural fracture and fracture network's communication,the reservoir volume will greatly improved,therefore,in view of the CBM,shale gas and tight gas reservoir's special physical/mechanical properties,analyze the reservoir in-situ stress calculation and cracks network cracks formation mechanism of unconventional oil and gas,put forward the reservoir stimulation measures and improve the storage capacity of unconventional oil and gas has become one of the key problems in domestic oil and gas development.In this paper,the laboratory test,field data analysis,numerical simulation and theoretical analysis methods are adopted,meanwhile,mechanical properties,reservoir rock stress field of unconventional oil and gas,the interference between hydraulic fracture,natural fracture and staged fracturing cracks internal also should be considered.The reservoir deliverability prediction and cracks dynamic propagation of unconventional oil and gas are identified,eventually,puts forward measures to increase production for different lithology reservoir.The specific research work and conclusions are as follows:(1)Considering the influence of water saturation on mechanical properties of shale,sandstone and coal was obvious,in the paper,the laboratory experiment and field data analysis were used to the law of study the relationship between saturation and shale,sandstone and coal's mechanical parameters such as: compressive strength,tensile strength,elastic modulus,Poisson ratio,internal friction angle and cohesion,simultaneously,the relationship between dynamic and static mechanical parameters various kinds of rock were established.Finally,the low permeability rock experimental mechanics parameter evaluation methods were got.This study result shows that with the increase of shale water saturation,shale strength and cohesion are reduced,and the decreases speed of cohesion changed obviously.when the saturation is increased to 0.85,the influence of saturation to the shale strength and cohesion can be neglected;coal hydration expansion is not that strong,With the increase of water saturation of rock,the crack's friction coefficient decreases,coal strength decreases with time,the coal brittle characteristics become weak and the ductility feature enhancement,coal deformation characteristics transfer frangible to elastic-plastic deformation,elastic brittle stress-strain relation is no longer suitable for coal,when the moisture content of coal rise to saturation,the compressive strength in logarithm relation with water content of coal.(2)Compared the induced stress field and displacement field of distribution of crack type I,crack type II and arbitrary oblique shape,the crack tip induced stress influence range was determined,simultaneously,the staged fracturing stress interference between adjacent isometric and different length cracks was also discussed.This study result shows that with the increase of number fracturing clusters the maximum principal stress and minimum principal influence range will be increased.when the staged fracturing segment number less than 4,the influence of the number of fractures to the maximum/minimum principal stress interference critical distance significantly.When the cracks increased to 5,the growth of stress interference distance with the cracks increase,however,the sphere of influence to the maximum principal stress is larger than that of the minimum principal stress.(3)Considering structure and rock mass stress nonuniformity of unconventional oil and gas reservoir,and the crack's boundary has a self similar characteristic,therefore,the fractal theory combined with the discrete element method were used to study the process of fracture dynamic propagation.This study result shows that when distance from the wall less than 0.5m,the circumferential stress and shear stress around the wellbore reservoir decreases with exponential form,when distance from the wall exceeds 0.5m,the influence of the borehole to the reservoir stress could be neglected.The relationship between shale,sandstone,coal and fractal dimension has established based on mercury method,and with the help of PFC discrete element software the relationship between the fractal dimension and crack propagation model was built,and then the laws for the fracturing fluid displacement,fractal dimension,stress non-uniform coefficient and rock lithology development has been studied.To ensure the cracks opening surface area is the largest,fracturing flow must be increased,at the same time,try to adjust the stress nonuniformity coefficient.(4)Through theoretical derivation,the dynamic fracture prediction software was finished based on all three dimensional crack model.By comparing the results from microseismic monitoring,Fracpro-PT software calculation and the self programmed software,the self programmed software's calculation accuracy was validated,the influence law of construction parameters,rock mechanics parameters and reservoir rock mechanics parameters to the crack geometry was studied with the help of the self programmed software,meanwhile,in view of the reservoir temporary plugging and the interference of natural fracture to explore the crack extension process.This study result shows that when the maximum horizontal principal stress is greater than or equal to the minimum horizontal principal stress,and approximation angle less than 25°,the natural fracture can not be penetrated;with the approaching angle increases,when the angle add up to 50°,the crack extends along the natural direction;when the angle reach 60°,the natural fracture can be penetrated easily.Simultaneously,when the principal stress difference is 2.5MPa,the Stress direction change critical distance is 49 m,hence,suggesting staged fracturing spacing is less than 49 m.(5)According to the transport characteristics of coalbed gas,shale gas and tight gas within the reservoir,the gas water two-phase productivity prediction model was built,for this model,the influence law of shale,sandstone and coal hydraulic fracturing cracks effective length,crack number to the gas production capacity was analyzed.Combined with fracture properties,the author presents measures to advance the unconventional oil and gas production,the variable density proppant could be used to increase the crack active length for CBM,and for shale and sandstone,the staged fracturing technology should be applied to hydraulic fracturing,at the same time ensure that interference between cracks. |
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