Visco-plastic Behavior Of Rock In Tight Reservoir And Its Application In Network Fracturing

Tight oil is another new hotspot of the unconventional oil and gas exploration and development in the world after shale gas,but the research on the geomechanical characteristics of tight oil reservoirs is in the ascendant.Previous studies on the geomechanical characteristics of tight reservoirs were mainly focused on the linear elastic model,and their plasticity and viscous properties were less studied.In this thesis,the plastic and viscous characteristics of tight sandstones are studied by means of laboratory tests and theoretical modeling,and then applied them to the hydraulic fracturing of tight sandstone reservoirs.Firstly,the national standard calculation formula of Poisson's ratio under triaxial compression is revised.The triaxial compression tests of tight sandstone samples are conducted and unfortunately the ISRM suggested method and the national standard method are failed to determine the Poisson's ratios of those samples.Thus a method based on the critical point of plastic internal variable is proposed.Secondly,according to the deformation characteristics of porous rocks under confining pressures,the concept of dilatancy partition of tight sandstone is put forward.At the same time,the change of dilatancy angle of tight sandstone during deformation is also studied.Thirdly,based on the compression tests of the tight sandstone samples,the mobilization of strength parameters based on the Drucker-Prager criterion—the coefficient of pressure sensitivity ? and the shearing cohesion k of the tight sandstones are studied during the plastic deformation process.The five pairs of commonly used conversion formulae of strength parameters between the Mohr-Coulomb criterion and the Drucker-Prager criterion are compared with experimentally calibrated results.The coefficient of equivalent plastic strain under the condition of Drucker-Prager yield criterion is deduced,and its mobilization with plastic deformation is also studied.The fractional Maxwell model is used to fit the uniaxial creep test of tight sandstone.Based on the theory of numerical simulation of hydraulic fracturing,a subroutine program based on the fractional Maxwell model is compiled and applied to the numerical simulation of hydraulic fracturing in viscoelastic tight sandstone reservoirs.The influence of viscoelasticity on the geometries of hydraulic fractures is investigated.Finally,the Hertz contact theory is applied to the study of the embedding problem of proppant in the viscoelastic strata,and theoretical solutions of dimensionless embedding depth and dimensionless fracture width based on fractional Maxwell model are deduced.And the sensitivity of seven related parameters is compared.Through the above research,the main conclusions are as follows:1.The method of determining Poisson's ratio based on the critical point of plastic internal variable has clear physical meaning and good operability and practicability.2.The concept of dilatancy partition can be used to study the dilatancy of rocks under different confining pressures and porosities.3.In the process of plastic deformation,the coefficient of pressure sensitivity ? increases with the plastic internal variable ? in a decreasing rate,while the shearing cohesion k increases firstly with the monotonic increasing plastic internal variable ? and then decreases with plastic internal variable ?.4.The coefficient of equivalent plastic strain C is dependent on the yield criterion chosen and C(28)2 3 is a special case when the Mises yield criterion is chosen.After entering the plastic stage,the coefficient of equivalent plastic strain C is not a constant,but varies with the plastic internal variable ?.5.The fractional Maxwell model nicely describes the creep behavior of the tight sandstone.Comparing to conventional integer order models,the fractional Maxwell model shows its superiority in a higher fitting accuracy with less fitting parameters.6.The viscous characteristics of the tight sandstone reservoirs have an important effect on the geometries of hydraulic fractures.Compared with the elastic formations,the hydraulic fractures in the viscoelastic strata are wider,but the crack length are smaller.7.The elastic modulus and Poisson's ratio of the proppant,the elastic modulus and the Poisson's ratio of the formation,the closure stress,the viscosity coefficient and the fractional order have direct effects on the dimensionless embedding depth and the dimensionless fracture width,and the closure stress has the greatest influence.