Effect of Drainage Height and Grain Size on the Convective Dispersion in the Vapex Process: Experimental Study

Abstract

Abstract Interest in the vapor extraction (Vapex) process for heavy oil and bitumen recovery has considerably grown as a viable and environmentally friendly alternative to the currently used thermal methods. The potential for success of Vapex process is even more attractive in some scenarios that preclude the thermal methods. Presence of overlying gas cap and/or bottom water aquifer, thin pay zones, low thermal conductivity, high water saturation, unacceptable heat losses to overburden and underburden formations etc., are some of the limitations with the thermal techniques, which can be potentially overcome by vapor extraction implementation. However, predicted low production rates by previous researchers for field application of Vapex technique remain a serious barrier to commercial applications of the process. The scale-up methods that have been used by previous workers for translating the laboratory results to field predictions were primarily based on the reservoir transmissibility. An analytical model developed by Butler and Mokrys1 showed that the oil rate should be proportional to the square root of reservoir transmissibility. The effect of convective dispersion between solvent and virgin heavy oil in porous media were ignored in developing this model. The main objective of this work is to develop an improved scale-up method for the Vapex process using physical model experiments carried out in models of different sizes. In this paper we report the results of a new series of experiments that extend the previously reported results of Karmaker and Maini2 to a significantly wider range of model heights. These new experiments employed a new design of slice type physical models that places the sand-pack in the annulus formed by two cylindrical pipes. Combining the new results with the previous data of Karmaker and Maini2, we show that the transmissibility based scaling up method seriously under-predicts the results at larger scales. This observation suggests that much higher rates can be expected in the field implementation of the Vapex process. A new correlation has also been proposed for scaling up the experimental data to the real field cases. It indicates the height dependency of the convective dispersion contribution, which can be the dominant mass-transfer mechanism for the process, to be higher order than previously postulated. Experimental results from this work show that the stabilized rate is a function of drainage height to the power of 1.1 to 1.3, instead of the square root functionality of the Butler and Mokrys2 model. Introduction Cost effective heavy oil and bitumen recovery methods are still challenging issues that have not been fully resolved. The huge volume of almost immobile hydrocarbon resources in the world, especially located in Canada, Venezuela and United States, which are about six times of the total conventional oil reserves, offers unlimited challenges and opportunities to researchers. High viscosity and low mobility of these oils cause the primary recovery to be very low. The adverse mobility ratio problem also limits the application of waterflooding to these reservoirs. The overall recovery that can be achieved prior to the IOR methods does not usually exceed 6–8% of the original oil in place3. The wellknown observation of dramatic decrease in the viscosity of heavy oil with temperature increase makes the thermal recovery methods, such as steamflooding, cyclic steam stimulation (CSS), in-situ combustion and more recently steam assisted gravity drainage (SAGD) process, the obvious choices. However, thermal methods are not universally applicable to highly viscous heavy oil reservoirs. The low recovery factors associated with CSS, inefficient steamflood in highly viscous oils and also relatively high mobility requirement in addition to the process control difficulties for the in-situ combustion technique are some of the obstacles that leave the SAGD process as the only thermal option for heavy oil and bitumen recovery in many reservoirs. In the SAGD process, two horizontal wells located in the same vertical plane are used to inject the steam from the upper well and produce heated oil from the lower well.