Rock Mechanical Characteristics And Fracability Evaluation Of Shale Oil Reservoir

The special physical properties of mudstone and shale(MS)reservoirs,such as their extremely low porosity and permeability,results in the difficulty of shale flow through tight reservoirs naturally,which affects the shale oil production and mining efficiency.Therefore,effective hydraulic fracturing techniques are needed to improve the reservoir quality in order to promote the flow within MS reservoirs.However,the mechanical properties of rocks play a significant role in hydraulic fracturing,so,it's necessary to evaluate the rock mechanical characteristics and fracability of shale oil reservoirs.In this paper,a total of 18 MS samples collected from a shale oil development zone located in the Dongying Depression were carried out for a series of laboratory experiments,including gas permeability measurement and real-time strain monitoring,X-ray CT analysis,porosity and permeability measurement under overburden pressure,mercury porosimetry,X-ray diffraction and organic geochemical analysis.According to the experiments,deformation characteristics,influence factors of mechanical parameters,failure mode and structure evolution,application of mechanical characteristics in the fracability were investigated and researched.Results are as follows.(1)Through the triaxial compression experiment,the stress and strain characteristics of the MS were revealed.The results show that the initial compressive deformation of the MS is not obvious,the linear(elastic)deformation is short,the nonlinear(plastic)deformation is long,with typical brittle failure characteristics.The Young's modulus of the mechanical parameters is relatively dispersed and the Poisson's ratio is concentrated.(2)The effects of sample size,confining pressure,porosity and material composition(inorganic mineral and organic matter)on mechanical parameters were clarified.Young's modulus and Poisson's ratio increase exponentially with the increase of aspect ratio,the energy absorbed by per unit volume,the releasable strain energy and dissipation energy of the mudstone samples decrease exponentially with the increase of the aspect ratio,the failure energy decreases first with the increase of the aspect ratio,and then decrease.The Young's modulus of the MS increases nonlinearly with the increase of confining pressure,and has a good power function relationship.The Young's modulus decreases with the increase of porosity,and the power function is good.The Young's modulus is positively correlated with siliceous minerals and negatively correlated with calcareous minerals.Organic matter has a negative effect on Young's modulus of the MS.(3)According to the real-time strain data monitored as the gas seepages within MS,the variation characteristics of MS permeability to gas under stress and strain conditions are revealed as “decline type” and “increase type”,the maximum permeability of the later exceeds or lags behind the maximum strength.The failure modes of MS and the influence of the macro / microstructure evolution on the permeability are elucidated,the permeability change during the deformation process is the superimposed result of external conditions and its own characteristics.It is considered that the siliceous mudstone with high siliceous content and low clay content has the best reconstruction effect,and has good fracability.(4)Based on the impact of the material compositions on the Young's modulus,an empirical model was established for evaluating MS mechanical parameters under different confining pressure.The energy absorbed by per unit volume when the rock sample is destroyed,material composition and Young's modulus and the coupling relationship between them were revealed,and determine the favorable fracture types.(5)Using conventional logging data to extract shear wave and calculate the rock mechanics parameters of the MS reservoir.The distribution of mechanical parameters in the target zones of Dongying depression is quantitatively analyzed.Combined with the effect of mechanical on the fracability of shale oil reservoir,predict the relatively favorable area of the target zones and determine the project dessert.