The CHOPS process involves the growth of high permeability channels (wormholes) into the reservoir. The wormholes provide improved access to the reservoir, thereby substantially increasing oil production rates. As production from a group of CHOPS wells matures, wormhole networks that have developed from different wells often interconnect. This can have negative consequences. For example, water influx into one well will spread readily to a system of interconnected wells.
A better understanding of preferential trends in the development of wormhole networks could allow more optimal placement of CHOPS wells, in terms of their spatial distribution. There are several factors that could influence the direction of wormhole growth in a heavy oil reservoir. The direction of growth may be affected by heterogeneity in the distribution of petrophysical and fluid properties such as permeability, porosity, water saturation, and oil viscosity. The stress field could be another factor.
This paper presents the results of an experimental investigation on the influence of anisotropic (unequal) horizontal principal stresses on the direction of wormhole growth in a sand pack saturated with viscous oil. The experiments were performed in a large rectangular triaxial cell (box). Each of the two perpendicular horizontal stresses and the vertical stress applied to the sand pack could be controlled independently. Oil was injected into the sand pack through flow distributors located on adjacent vertical walls of the cell. The oil flowed through the sand toward a production well in the cell. When a critical flow rate was reached, the sand failed around one of the openings in the well. The failed sand was transported by the oil into the well, resulting in the growth of a wormhole inside the pack.
The experiments indicated that, under the conditions of stress and flow that were applied, there was a tendency for wormholes to grow in the direction of the lower horizontal stress. However, this tendency was overcome when there were significant directional differences in the oil flux (pressure gradient). Then, the wormholes tended to grow in the direction of the largest oil flux. Both behaviours are consistent with the local effective stress conditions at the tip of a wormhole that lead to continuing sand failure there.