Experimental Studies On Effects of Reservoir Heterogeneity On the Vapex Process
Authors: Jiang Q. , Butler R.M. 
Published: March 2010

In thermal recovery processes, heat can penetrate by conduction through solids into low-permeability layers and shales. In the Vapex process, vapour is confined to the pore spaces and its ingress into fine pores can be limited by capillarity. In steam processes, heat can penetrate relatively rapidly beyond the interface and mobilize the oil at depth; in Vapex, the diffusion of vapour is much slower and the thickness of the mobilized oil is much less. Therefore, reservoir heterogeneity, such as that created by low-permeability layers and shales, becomes more important in affecting the performance of the Vapex process than with SAGD because it is more difficult for the gas phase to penetrate the low-permeability regions.

In this study, experiments were carried out using a 2-D packed model to investigate the effects of low-permeability layers and lenses on Vapex. The model was packed in horizontal layers with two different sized sands. Crude from the Tangleflags North field, Lloydminster, was extracted with butane. The effects of both continuous, low-permeability layers and discontinuous, low-permeability lenses were investigated using different well configurations.

The results using these models have been compared with those from uniformly packed models. All experiments involving low-permeability layers gave lower production rates than those involving uniformly high-permeability packing. When the horizontal low permeability layers were continuous, the production rate was even lower than that for a model uniformly packed with the same low-permeability sand. A higher oil production rate was found with discontinuous low-permeability lenses than with continuous layers. This is because the vapour chamber could rise around these lenses to penetrate further into the high-permeability material above; thus the oil could be drained concurrently from different heights.

Capillarity, due to interfacial tension between the oil and solvent vapour phases, has a significant influence on the Vapex process. Its effects are both beneficial, because of the increased contact area between solvent vapour and oil, and detrimental, because of the resistance to the rising of vapour fingers, on a pore scale, from a coarse sand layer into a fine sand layer. This latter effect changes the shape and growth rate of the solvent chamber. A much higher residual oil saturation was found in the fine sands than in the adjacent coarse sands.

Introduction

Thermal methods are considered to be effective for the recovery of heavy oil and bitumen and they have been applied commercially world wide. They contribute 80% of the total production from all types of EOR. However, the large heat requirement makes thermal methods, such as steam injection, inefficient and uneconomic in many reservoirs. Reservoir characteristics that tend to reduce the performance of thermal processes are: thin pay zones, low porosity, high water saturation, low rock thermal conductivity and the presence of aquifers.

A process using hydrocarbon vapours to extract bitumen or heavy oil, known as Vapex, has been described and studied(1,2,3) (Butler and Mokrys, 1989; 1991; 1993). The Vapex process, although an evolution of steam assisted gravity drainage (SAGD), is a non-thermal process.