非均质热弹塑性损伤模型及其在油页岩地下开发热破裂分析中的应用

THERMOELASTOPLASTIC DAMAGE MODEL OF HETEROGENEOUS MEDIUM AND ITS APPLICATION TO THERMAL CRACKING ANALYSIS OF OIL SHALE IN UNDERGROUND MINING

地下加热开发油页岩面临着储层渗透性极低或不渗透的难题,在油页岩层各井间形成渗透带关系到油页岩地下开发的成败,油页岩加热过程中的热力和化学作用是形成渗透带的重要条件。针对油页岩地下开发存在的上述问题,首先建立油页岩非均质热弹塑性损伤模型;然后较系统地给出确定损伤变量的方法,并将热膨胀系数、热传导性、比热、弹性模量、抗拉压强度等材料参数作为满足Weibull分布的随机变量,利用Matlab和Monte Carlo方法实现数值计算物性参数的随机赋值,对油页岩地下开发加热过程中热力破坏问题进行数值模拟。研究结果表明:组成油页岩各种矿物颗粒热膨胀和弹塑性性质的不同及其分布的随机性,导致油页岩加热过程中温度场、变形场和应力场的非均匀性,使得油页岩颗粒间变形不协调和油页岩热解产物释放引起油页岩材料与结构性能发生不可逆的劣化损伤,这是油页岩发生热破裂的主要原因;热传导方式加热油页岩,温度的传递比较缓慢,加热井和生产井的间距不宜过大,对油页岩储层性质存在差异时,注热井应选择较高热导率的区段;油页岩的层理对油页岩地下原位开发形成渗透带至关重要,油页岩达到热解温度会产生附加的膨胀力,首先在层理处开裂,应充分利用油页岩层理赋存条件改造其渗透性。

The low-permeability and impermeability of the stratum are hot issues for oil shale in underground mining,so it is of vital significance to form permeable belts among various mining shafts,while the thermo-chemical functions during oil shale heating are prerequisites of forming the permeable belts.Based on the problems mentioned above,the oil shale thermoelastoplastic damage model of heterogeneous medium is established;and then a systematic method of defining damage variables is given.Further,the material parameters of thermal expansion coefficient,thermal conductivity,specific heat,elastic modulus and tensile and compressive strengths are used as the random variables of satisfying Weibull distribution;and the parameters of numerical calculation is given in Matlab with Monte Carlo method.In addition,numerical simulation of thermal cracking during the process of heating in oil shale in underground mining is performed.The results indicate that the different thermal expansions and elastoplastic properties and distributed randomness of various mineral particles composed of oil shale will lead to the nonuniformity of thermal field,deformation field and stress field in heating process,which makes the particles of oil shale deform incompatibly;and the release of product pyrolysis results in irreversible damages of oil shale material and structure.These are the main reasons for the thermal fracture of oil shale.When oil shale is heated through thermal conduction,temperature transfers slowly,so the spacing between heating well and production well should not be kept too long.When difference exists in oil shale reservoir′s properties,heating well should be chosen in the high conductivity region.Oil shale′s stratification is vital for developing permeation zone in in-situ retorting.Oil shale will generate additional expansion force as it reaches its pyrolysis temperature,and stratification will be cracked first,so the oil shale stratification should be utilized to enhance its permeability.