Investigation And Application Of Rock Mechanical Characteristics Of Karamay Ultra-heavy Oil Reservoirs In The SAGD Process

At present,the development of the Karamay ultra-heavy oils faces these difficulties such as long circulation period,poor/slow steam chamber propagation,uncertain time for switching to production,and ignorance about geomechanics in the SAGD process.This paper tries to solve these problems above from the aspect of geomechanics.Previous studies can't interpret these critical issues clearly for Karamay oil sands,like the proper geomechanical/thermal conditions for shear dilatancy and tensile parting expansion,the deformation mechanisms at the micro-scale,and coupled relation between the stress/deformation and flow properties.These experiments in this paper,including triaxial drained compression experiments,isotropic drained compression-expansion experiments,and permeability experiments,are employed to investigate the thermoelastoplastic deformation behaviors,microstructures,and flow behaviors of Karamay oil sands in the processes of reservoir reform and SAGD production.The triaxial experiments reveal that the strain-softening and shear dilatancy occur under the effective confining stress of 0.5?2 MPa at room temperature,and the dilatancy volume increases with the decrease of the effective confining stress;the strain-softening and dilatancy are considerable under the effective confining stress of 0.5 MPa at the temperature of 45?70 ~oC;the strain-softening and dilatancy take place under a wider effective confining stress range of 0.5?5 MPa at 100 ~oC.The isotropic loading tests uncover that the volumetric expansion increases with the increase of pore pressure and with the drop of temperature.The SEM images show that the contact points/faces between the undisturbed oil sand grains are few,and the rich-content bitumen/clay mixtures filled in the interparticle regions make Karamay oil sands a unique bitumen-cemented structure;the shear fractures can be induced under room temperature and the effective confining stress of 0.5 MPa,characterized by the grain rolling and big interparticle pores;and the grains with sharp edges and corners,having sufficient contact after bitumen is squeezed out,form the interlocked structures which contain adequate shear dilatancy potentials.The permeability tests show that a lower effective confining stress is beneficial to the shear-induced permeability improvement;the permeability induced by tensile parting expansion rises with the decrease of the effective confining stress in the semi-log coordinates.Conventional models can't incorporate the effects of temperature,phase transition of bitumen,and pore collapse.This paper proposes a generalized modified elastoplastic model suitable for the geomaterials with bitumen-cemented structures,and an improved Drucker-Prager model with cap plasticity considering temperature and the effective oil saturation.Based on these models,it is found that the elastic modulus decreases,and the bulk modulus/Poisson's ratio increase when temperature increases from 20 ~oC to 70 ~oC;while these change tendencies are opposite when temperature increases from 70 ~oC to 100~oC.The D-P internal friction angle and cohesion drop,and the shear failure surface and the cap surface shrink when temperature increases.The shear dilatancy-induced permeability has an approximately linear relation with the shear volumetric strain.The tensile parting-induced permeability increases with the tensile parting volumetric strain,especially at a higher temperature,and this relation can be fitted well using the Touhidi-Baghini equation.The porosity as well as water saturation increases after dilations.Present models can't distinguish the mechanism differences during the different stages of SAGD,and can't incorporate the phase transition of bitumen in the coupled analysis.This paper establishes the comprehensive thermo-hydro-mechanical coupled models in the SAGD life cycle,specifically including the reservoir reform stage,preheating stage,and production stage.These deformation equations,flow equations,heat transfer equations,and phase transition equations in each stage are given.The coupled FEM equations in each stage are derived theoretically,and the numerical solutions are followed.Some typical case studies based on these models display that the predicted BHPs agree well with the actual monitored data;the thermal transfer is only obvious in a narrow near-well region;the stress path for the wellbore rock moves to the left in the p-q coordinates,which means closer to the shear yield surface;there is only porothermoelastic deformation within the reservoir;the porosity increase in the inter-well region is lager.If the heat transfer around the wellbore is neglected,the stress path all moves to the upper left,which means much closer to the shear yield surface compared with the case study considering heat transfer;the reservoir also possesses porothermoelastic deformation;the maximum change in the porosity takes place on the wellbore.For a deeper pay zone,the water injection can make the stress path move to the upper left.During the preheating stage,the thermal communication can be established effectively;the bitumen transition zone shows a shape of an ellipse;the maximum Mises stress occurs on the bottom of the wellbore;the plastic zone occupies a circular region with a radius of about 0.5 m.If the phase transition of bitumen is neglected,the temperature increase in the inter-well region is faster.During the steam breakthrough stage and the steam chamber rise-up stage,the pressure transfer outside the chamber is faster than the temperature transfer;the maximum porosity increase takes place in the regions right above the chamber;plastic deformation happens in the regions within the chamber and around the chamber.During the steam chamber lateral-expansion stage and the steam chamber decline stage,the drained zone increases,and the highest porosity increase occurs on both sides of the chamber.This contribution proposes some evaluation suggestions on the operation effects in the SAGD life cycle.An engineering idea is proposed that uses an assisted vertical well to reform the reservoir embedded with mudstone stringers in SAGD wells,and theoretical evidence is provided.This study shows that the increases in the injection pressure and the volumetric expansion can improve the hydraulic dilation area(HDA);the increases in the distance from the production well to bedrock,the distance between SAGD wells,and the liquid viscosity can decrease the HDA under water injection.During the preheating stage,the moving speed of the bitumen phase change interface and the heat flux through the wellbore decrease with time,and adequate operations can be taken to switch to production as long as the inter-well midpoint temperature reaches 80 ~oC.During the production stage,the predicted output considering geomechanics is higher than traditional models.After water injection for the reservoir embedded with mudstone stringers,the pore pressure in the regions right above the upper stringer changes little;the stress path moves to the left in the p-q coordinates;the reservoir only owns porothermoelastic deformation;the highest porosity increase occurs in the region between two stringers.The porosities along the wellbore extension show a significant difference,which can lead to the non-uniform development of the steam chamber in different well sections in the subsequent preheating stage.If the assisted vertical well is used,pore pressures in the region above the upper stringer can be significantly increased;the stress path of the wellbore rock moves to the left.Within the reservoir embedded with stringers,the porosity can be increased in the vertical direction,and the porosity in the region above the upper stringer is significantly improved.For these reservoirs that can't be effectively developed after water injection using the assisted vertical well,the production strategies should be adjusted.For instance,the SAGD wells can be used to inject steam,and the vertical well can be used to produce oils.