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Water injection can effectively improve the reservoir porosity and permeability by shear dilation in the vicinity of wellbores. In this paper, shear dilation and permeability improvement capability/potential are proposed to describe the evolutions of porosity and permeability under water injection-induced shear. The mathematical models based on Kar...
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... The terrestrial Karamay oil sand possesses an 'oil-in-sand' structure, which means the rich solid bitumen makes the oil sand grains almost isolated. In our published paper (Lin et al., 2016;Gao et al., 2019d;Pang et al., 2019), the microstructures of Karamay oil sands were studied by the environmental scanning electron microscope (ESEM), and it was found that there are few contacts between neighboring grains. However, when the bitumen was washed off, these grains with sharp edges and corners can sufficiently contact each other and can form an interlocked structure like the Canadian oil sands. ...
Coupled thermo-hydro-mechanical analyses in heavy oil reservoirs are essential for engineers to evaluate and optimize the field operations in terms of safety, economy, and environment during thermal stimulation. The terrestrial Karamay oil sands were used for a series of high-temperature high-pressure triaxial drained compression tests to understand the deformation and flow behaviors at multiple stress, pore pressure, and temperature conditions. This article was dedicated to the experimental investigations on the elastoplastic deformation and permeability evolution of Karamay oil sands under shear and isotropic compressions. The geomechanical conditions of shear dilatancy and tensile parting behaviors were observed, and the phenomena of shear yield and isotropic compression yield of Karamay oil sands were proven during thermal stimulation. All the elastoplastic and flow parameters at different temperatures were determined. The major conclusions can be drawn that the effective confining stress and temperature both have very significant effects on the elastoplastic deformation and flow behaviors of Karamay oil sands. The shear dilation can be more meaningful with a lower effective confining pressure and a higher temperature. A proper shear dilatancy can occur at 100 °C both at low and high effective confining pressures. A higher temperature is generally not beneficial to the tensile parting induced volumetric expansion. There are both elastic and irrecoverable plastic deformations under isotropic drained compressions. 70 °C is a critical temperature for the elastic property and permeability changes with temperature for Karamay oil sands. The cohesion and friction angle are much lower at a higher temperature, and the cohesion at 100 °C is appropriately 0. The cap yield surfaces at different temperatures show little difference. The relation of shear-induced volumetric strain and permeability can be well fitted using a linear function. The permeability improvement declines exponentially with the effective confining stress increasing. The permeability generally rises with the tensile parting induced volumetric strain at all temperatures, and it can be improved much more at a higher temperature. The permeability declines linearly with the effective stress increase in the semi-logarithmic coordinates.
Mudstone stringers in an oil sand reservoir can significantly affect the propagation of the steam chamber in the process of steam-assisted gravity drainage (SAGD). The micro-fracturing by water injection in strongly heterogeneous oil sands can result in these problems such as hard stringer breakthrough and aggravating heterogeneity. This paper proposed a comprehensive numerical model for conventional SAGD wells and vertical well-assisted SAGD (VWA-SAGD) wells to predict the coupled thermo-hydro-mechanical responses under hot water injection in a typical Karamay oil sand reservoir with two stringers, considering skeleton shear dilation, the ''phase change'' of bitumen, formation heterogeneity, the permeability evolution induced by elastoplastic deformations, and the theory of heat and mass transfer. Major conclusions were drawn that there are three relatively narrow separate transition zones effectively heated around the SAGD wells and vertical well. The vertical well makes about 0.5 MPa increase to the pore pressure of the reservoir embedded with mudstone stringers in vertical directions and rises by about 0.6 MPa for the reservoir pore pressure above the upper stringer to reduce the pore pressure differential along the wellbore. Water injection-induced ultimate stress states can't reach the shear failure line under field operations, so the shear dilation can't be induced. The thermoporoelastic deformation determines whole reservoir deformation. The reservoir between the two stringers contributes to most of the caprock uplift. The VWA-SAGD technique can improve the porosity by about 1% in the whole vertical direction and reduce the anisotropy of porosity along the wellbore, which is beneficial to the uniform and fast propagation of the steam chamber in subsequent preheating and production stages. These findings can be employed to accurately predict the temperature, pore pressure, stress/displacement, and porosity evolutions for the field engineers to properly evaluate the uplift of reservoir and caprock, oil output changes, and heat utilization efficiency.
Viscosity-temperature behavior is important for the thermal recovery of bitumen: firstly, it is a criterion for evaluating the evolution of mobility with temperature, which affects oil production; and secondly, it determines the role of bitumen in the geomechanics in oil sand reservoir. To quantitatively evaluate the effect of temperature on dynamic viscosity, some experimental and modeling investigations were conducted. The viscosity of Karamay oil sand bitumen over a temperature range of 20 ~ 350 °C was measured, and the viscosity-temperature behavior was simulated by a modified three-parameter model using the least square method. It was found that the viscosity of Karamay bitumen drops sharply with the temperature increase. The effect of bitumen viscosity on the geomechanics in the SAGD process was discussed in terms of reservoir deformation, fluid flow, and heat transfer behaviors. The reservoir deformation, fluid flow, and heat transfer behaviors at varying bitumen viscosity show significant differences in the drained, partially drained, and undrained geomechanical zones. The structure’s bulk modulus, effective stress, and volumetric strain in the drained zone are lower than those in the undrained zone; while the oil mobility and Peclet number show the opposite tendency. The changes in the structure’s bulk modulus, effective stress, volumetric strain, oil mobility, and Peclet number due to the phase change of bitumen increase with the increase of oil saturation. This study can provide the field engineers with guidance for the design of a proper oil recovery scheme before its implementation, and with a useful relation for the coupled thermal-flow-structure analyses.
Steam assisted gravity drainage technology has been widely applied to bitumen recovery in Karamay, northwest China. Steam circulation prior to production is designed to establish the thermal inter-well communication, aiming to create a desired initial steady steam chamber. Phenomenologically, the in situ immobile extremely viscous bitumen undergoes the change from solid to fluid state due to rheological fusion, and correspondingly the pay zone displays heated fusion and cold solid regions, within which thermal and mechanical properties differ considerably. To investigate these complex geomechanical responses, the heat transfer with phase change here was treated as a heat conduction with a moving boundary, and the modified Drucker-Prager model with cap plasticity was adopted to depict the mechanical behavior of oil sands. All models here used temperature-dependent constitutive parameters. This contribution coupled heat transfer, phase change and thermoelastoplastic deformation behaviors using a finite element code. A case study on SAGD well pairs using field operation parameters was conducted, and all reservoir information concerning temperature, phase change, deformation, stress and plastic zones were predicted. Temperature, deformation and stress along some typical paths at varying circulation periods were exhibited and discussed. The evolutions of phase change interfaces and plastic zones were provided. The changes of reservoir temperature as well as bitumen fluidity have significant impacts on the geomechanics, characterized by these interesting phenomena such as a maximum reservoir uplift occurring right above injection well, a high stress interference zone with shapes of arch or ellipse, and a plastic zone related to temperature distribution. This paper provides a coupled approach for the evaluation of reservoir thermoelastoplastic responses in steam circulation process, and it can also predict deformation change in permafrost and hydrate formation where transient heat transfer occurs incorporating phase change behavior.
