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Solvent vapour extraction (VAPEX) has shown great potential to recover heavy oil reserves. However, the application of the VAPEX technique is limited by its low oil production rate, which is caused by the slow mass transfer and inefficient gravity drainage. Hence, increasing the heavy oil–solvent mixing process and introducing more production mechanisms are imperative to enhance the performance of VAPEX. This paper proposes a new process, namely enhanced VAPEX, in which the solvent-chamber pressure is dynamically operated rather than constantly maintained throughout the production process. During the enhanced VAPEX process, pressure in the solvent chamber is cyclically depleted to induced foamy oil flow, which not only adds oil production but also increases contact area. In addition, a high-pressure non-condensable gas is applied to induce fingering phenomenon, which enlarges the heavy oil–solvent contact area. The enhanced VAPEX process is experimentally evaluated in this study. It is found that gas fingers can penetrate deep into the oil zone. In addition, foamy oil flow not only increases oil production but also improves mass transfer. Experimental results show that the oil production rate of the conventional VAPEX process was increased by 182% by the enhanced VAPEX process.

Diffusion-convection mass-transfer process in porous medium is one of the major enhanced oil recovery (EOR) mechanisms of Vapor extraction (VAPEX). Previous work tried Fick’s Law to model the diffusion process, and mostly assumes the diffusion coefficient as a constant in the equation. However, the diffusion coefficient factually is a function of concentration, and thus the effect of its gradient should be included in the governing equation. In addition, bulk flow exist during the VAPEX especially during the solvent chamber rising phase, hence the process may be a diffusion-convection process rather than a pure diffusion process as presumed in previous works. This paper proposes a new one-dimensional (1D) VAPEX mathematical model on the basis of diffusion–convection equation, gravity-based fluid flow equation (Darcy’s law), and mass-conservation equation. The model is directed at a vertical thin cylindrical VAPEX process with injector and producer both setted at the bottom. The solvent chamber is considered as a two-phase area and the oil in it is completely saturated, while the matiral beyond the chamber is thought as in liquid phase. First, the diffusion-convection model is developed to obtain the concentration distribution in the transition zone; the boundary between solvent chamber and transiton zone is seen as moving with time. Second, the drainage velocity of the saturated oil in solvent chamber is calculated and combined with the mass conservation equation, to model the saturation of the oil phase changing with time. On the basis of the saturation model the oil production rate can be obtained. The recovery factor profile is divided into two distinct stages: solvent diluting-dominated stage and saturated oil flowing-dominated stage. During the first stage, it is found the solvent chamber growing rate tends to be constant; the oil production rate is proportional to the square root of permeability, which constists with the existing VAPEX theoretical model. During the second stage, the production rate is linearly related to the 1.1–1.3 power of the model length rather than the square root of it. This agrees perfectly with the lab observations in previous work.

Solvent-based EOR techniques, such as vapour extraction (VAPEX) and miscible/near-miscible flooding, have been studied and applied in petroleum industry. Diffusion–convection mass-transfer process is one of the most important EOR mechanisms in these techniques. This paper develops an accurate semi-analytical solution to a 1-D diffusion-convection mass-transfer model. The velocity term in the diffusion–convection equation has been assumed as a constant in previous works. This assumption is actually not valid in the solvent-based miscible flooding, since the velocity is a function of local viscosity and density, both of which depend on the local concentration. In this study, the spatially and temporally varying convection velocity is rigorously simulated through an accurate semi-analytical approach. First, we consider a sequence of time steps. In each time step, the convection velocity varies with space. Then the spatially varying velocity profile is divided into multiple sections. The velocity profile in each section is approximated with a linear function, so that the analytical solution for the diffusion-convection equation in each section can be obtained in Laplace domain. Then, the solutions in all sections are couped together and solved to obtain the mass-transfer rate through which the concentration distribution can be computed. Finally, a new convection velocity profile can be acquired by using Darcy's law for the next time step. The semianalytical solution is validated by an analytical solution for a special hyperbolic velocity case. In comparison with the numerical solution, the semi-analytical result is free from truncation error and numerical dispersion, thus it is more accurate and more reliable in computing the concentration distributions. The proposed method can be used in streamline simulation, or coupled with other functions to improve the simulation of solvent-based EOR techniques.

TARIM is the largest basin abundant in carbonate reservoirs in China. Acid fracturing is the main technology for exploration and development in TARIM carbonate reservoirs, which featured with great depth (4500–7200m), high temperature (250–320F Degrees) and extreme heterogeneity due to fracture formation. If the azimuths of natural fractures and in-situ stress match the azimuth of fracture-and-cavern area from the well, the acid fracturing treatment would generate good effect in general; otherwise, the effect will be non-ideal. A novel diversion acid fracturing (acidizing) technology, which combines fracture reorientation technology and fluid diversion technology, has been developed to solve the problem. When hydraulic fractures are initiated and extended in unfavorable direction, the fracture reorientation technology utilizing DCF (Degradable Cleaning Fluid, with great block efficiency and high compressive strength) can be adopted to temporarily block the fracture and ensure the hydraulic fracture would reorientate in advantageous direction. DCF is a mixture of TDF (temperature-degradable fibers), which would remain stable at 260 Degrees Fahrenheit while degrade thoroughly at 280 Degrees F, and soluble particles. By using visco-elastic surfactant based self-diversion acid, the fluid diversion technology could achieve deep penetration and efficient acid placement. Well YM204 has no production before stimulation. The azimuth of maximum principal stress is about NE45 Degrees. FMI data shows that natural fractures in Ordovician carbonate formation are oriented at NW315-SE135 Degrees while there is a fracture-and-cavern area about 150m to the northeast of the wellbore according to the seismic lateral prediction. Thus, acid fracturing treatment combining fracture reorientation technology and fluid diversion technology was conducted in this well. Post-treatment evaluation indicates that the fracture-and-cavern area has been connected to the wellbore and the output remained over 100t/d of oil and 7000m3/d of gas.