Thoughts on Simulating the Vapex Process

Abstract

The Vapor Extraction (Vapex) process and its many hybrid variants have attracted a great deal of attention as potentially less energy intensive alternatives for exploiting heavy oil and bitumen resources. However, despite much work over the past two decades, uncertainty remains about the basic mechanisms, the scaling aspects and the most appropriate methods of numerically simulating these processes. This paper offers some insights into several of these outstanding questions. The questions are examined in the context of an extensive and well-documented set of Vapex experiments carried out by Maini and his colleagues over the past 10 years.We have experimented with different methods of simulating these experiments using a physics-based reservoir simulator. Despite the high permeability (greater than 200 Darcys in all of the experiments), we find that capillary pressure plays a significant role in the drainage. The simulations suggest that most of the drainage takes place in the capillary transition zone along the edge of the vapor chamber, rather than in the single-phase zone ahead of it which has not yet been contacted by vapor.It has been emphasized in the literature that the near-linear scaling of oil rate with height observed in the experiments is dramatically different from the square root of height dependence predicted by the original analytic model of Vapex. However, the experiments also show an increasing solvent fraction in the produced oil phase as height increases. When this "solvent mixing" effect is separated out of the rates, the remaining height dependence is less than linear, though still greater than square root of height.The relative roles of molecular diffusion and mechanical dispersion in Vapex have been widely discussed in the literature. Generally, mechanical dispersion is expected to play a larger role in these high permeability experiments (vis-à-vis the field), due to larger fluid velocities. We present a method of inferring the diffusion/dispersion present in the simulations, despite a hidden component of numerical dispersion caused by the numerical method itself. We find that the experiments are well matched with values of diffusion and dispersion in line with literature correlations, and that the contribution of mechanical dispersion is perhaps not as large relative to that of molecular diffusion as might be expected.The paper also provides some thoughts on questions we believe are still unanswered, including mechanisms behind the height dependent mixing phenomenon and the scaling of the experimental results to the much greater heights and lower permeabilities characteristic of the field.

Experimental and numerical investigations into the effect of heterogeneities on the recovery of heavy oil by VAPour EXtraction (VAPEX)

“…This difference is similar to that seen by Das and Butler as can be seen from fig:6 where we have plotted the observed and predicted oil drainage rates as a function of the square root of permeability [16]. This discrepancy has also been observed by other workers [1][2][3][4][5][6][7][8]12,13,[15][16][17][20][21][22][23][25][26][27][28]50,52] and suggests that there is some problem with the Butler-Mokrys equation.…”

“…This analysis assumes first contact miscibility between the oil and the vapour. This is clearly much simpler than performing a fine grid simulation to predict VAPEX performance but unfortunately seems to systematically underpredict the drainage rate seen in laboratory experiments [1][2][3][4][5][6][7][8]13,14,[16][17][18][20][21][22][23][24][25][26][27][28] In contrast, the only published field pilot (in Soda Lake, Saskatchewan) obtained oil production rates a factor of 10 lower than predicted. A further uncertainty in the field scale application of VAPEX is the impact of geological heterogeneity.…”

Upscaling study of the cyclic solvent injection process for post‐chops reservoirs through numerical simulation

“…The Pedersen correlation is expected to give better liquid viscosity predictions for light and medium gravity oils than the JST correlation. [20] Therefore, in this study, the viscosity model was constructed as a modified Pedersen model. Valuable information was obtained through history-matching the laboratory experiments.…”

“…WinProp now allows use of the Pedersen corresponding states viscosity correlation in addition to the Jossi‐Stiel‐Thodos (JST) correlation. The Pedersen correlation is expected to give better liquid viscosity predictions for light and medium gravity oils than the JST correlation . Therefore, in this study, the viscosity model was constructed as a modified Pedersen model.…”

“…Capillary pressure has been shown to play a significant role in VAPEX processes . Cuthiell and Edmunds experimented with different methods of simulating VAPEX experiments using a physics‐based reservoir simulator.…”