Study On Compressible Evaluation Method Of Shale Oil Reservoir Between Salt Rock

Because of the advantages of wide distribution,good preservation,high oil content and abundant formation energy,the intersalt shale oil reservoir is very likely to be a breakthrough point for the large-scale commercial development of shale oil in China,but the lithologic fluctuation of the salt reservoir and the vertical heterogeneity of the stratum are very strong,which seriously affect the fracturing effect of the reservoir.Taking the Qianjiang formation in Qianjiang sag as an example,the Qianjiang formation has developed 193 grade III salt rhythms with a cumulative thickness of more than 2000 m.The upper and lower salt rocks hold a set of mudstone,shale and glauberite rock,the maximum thickness of 20 m,the general thickness of 5-12 m.This set of strata is both a hydrocarbon source and a reservoir,and the upper and lower salt rocks are plugged to form inter-salt shale oil.Therefore,accurately mastering the petrophysical and mechanical properties of inter-salt shale oil reservoirs and establishing an evaluation model for the compressibility of inter-salt shale oil reservoirs have important guiding value for shale oil hydraulic fracturing design.This study takes the Qianjiang formation in the Qianjiang sag as the research object.Through the test of the rock physical properties of inter-salt shale oil reservoir,the influence of the fluid and mineral composition of inter-salt shale oil reservoir on its mechanical properties is analyzed.In combination with the indoor hydraulic fracturing physics simulation experiment,the hydraulic fracturing crack propagation pattern and key influencing factors of shale oil were studied.The compressibility evaluation index was selected based on this,and an evaluation model of inter-salt shale oil compressibility was established.main conclusion:(1)By analyzing the mineral composition and microscopic pore structure characteristics of inter-salt shale oil reservoirs,the results show that the salt-shale oil reservoir minerals are dominated by argillaceous dolomite,the highest dolomite content is 42.23%-67.64%,and the quartz content is 7.71%-10.14%.The reservoir generally exhibits the characteristics of "high carbonate,low quartz,low-clay minerals";the porosity of the inter-salt reservoirs from 10% to 16%,and mainly intergranular pores;(2)Under the condition of saturated crude oil,the three axis mechanical experiments of rock high temperature show that with the increase of confining pressure,the rocks of inter-salt shale oil reservoir all present a certain degree of brittle-ductile transformation,and the single shear fracture surface is the main surface after the peak.The elastic modulus and peak strength of inter-salt muddy dolomite are higher than those of the shale.In addition,salt rock and salt rock containing impurities show obvious plastic deformation characteristics;through the salt-mud interface direct shear test,the results show that the salt-mud interface can withstand a certain shear strength,combined with interface SEM scanning can be seen.The salt-mud particles intercalate with each other at the position of the interface and cement well;(3)Through laboratory physical fracturing simulation experiments,it is concluded that the strength of rocks similar to the lithology of inter-salt shale oil reservoirs is lower,and the pump pressure curve during the hydraulic fracturing process has no obvious fracture characteristics,and the crack surface is dominated by a single simple crack.(4)Based on the above experimental results,a comprehensive evaluation model for the compressibility of inter-salt shale oil reservoirs was established by comprehensively using the rock brittleness index,fracture toughness,and horizontal stress difference,and the compressibility of a well was analyzed using the model.The results show that the compressibility of inter-salt shale oil reservoirs is between 0.4-0.6,which is moderately compressible,and the cracks are mostly multi-slit,and partially complex seams may appear.The inter-salt shale oil reservoirs have large compressibility longitudinal fluctuations,among which the lower section of the four paragraph of the Qianjiang shows high compressibility.With the increase of well depth,the compressibility decreases gradually.