Research On Fracture And Damage Behavior Of Oil Shale Under Thermal-mechanical Coupling Field

Oil shale, as a kind of unconventional oil and gas resource, is promising toprovide energy and mitigate petroleum pressure in future because of organicmatter called Kerogen in it. As it appears more environment pollution andsource waste for ground utilization of oil shale, the in-situ conversion methodwill be the certain trend for its utilization.This study concerns more about in-situ fracturing of oil shale. Oil shale is akind of sedimentary rock with well-grown beddings, which lead to itsanisotropic mechanical and thermal properties. Profound study of damage andfracture behavior of oil shale can provide vital theories and practical values foroil shale in-situ conversion methods in future.Main contents and results of this study are as follows:(1) Constitutive relation under coupling effects of temperature and stress.The problem of oil shale in-situ fracturing was simplified as plane orthotropic model, results of which were allied with common program solver to solve oilshale fracture mechanics numerical simulations.(2) Oil shale thermo-physical experiments. The change laws of thermalexpansion and thermal conductivity with temperature of direction parallel andvertical to oil shale beddings were tested within25℃~300℃. Change lawswere fitted approximately at300℃~600℃. Function relations of thermalparameters of oil shale were used for fracture simulation.(3) Numerical damage analysis of oil shale under coupling effects of heatand stress. FLAC3Dwas used to establish standard oil shale specimen model. Acommon program aimed at making elements’ material property accord toWeibull random distribution was developed to reflect heterogeneity of rock.Distribution of stress and displacement together with change laws temperatureand heating velocity with time were analyzed. Results show that damage of oilshale appears anisotropic characteristics when it is under thermo-stress effect.Information of damage will be used for fracture numerical simulation.(4) Numerical analysis for single fracture extension. ANSYS andWeibull-distribution-program in (3) were used to establish oil shale model.Rules of cracking and fracture extension of oil shale were investigated. Changelaws of fracture were analyzed using stress intensity factor as representativeparameter. Fracture mechanical properties of oil shale deposit were alsoanalyzed. Distribution properties of stress, strain and displacement coincidewith results in fracture mechanics. (5) Analysis of mutual effect of multi-fracture extension. Concept called“Displacement-Compression Load” was introduced for analysis of mutual effectproperty of multi-fracturing process qualitatively, by the same time, Conceptscalled “Disturbance Factor” and “Mutual Disturbance Factor” were introducedto analyze mutual effects and effect intensity between fractures quantitatively.Results show that mutual effects of multi-fracture appear to restrain fractureextension. The effect has close relation with fracture interval. Mutual effects offractures are not identical; therefore, from the aspect for mitigating restraineffect of fracture extension, optimization for sequence of multi-fracturingprocess can be established.The results obtained in this study have some meaning and preliminarilypractical values for guiding in-situ exploitation oil shale in future.