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. Field evidence and laboratory experiments indicate that wormholes tend to persist as stable high-permeability flow channels throughout the duration of the CHOPS process.
The existence of wormholes in the reservoir needs to be taken into consideration in the design of post CHOPS recovery processes. One of the key issues is the stability of wormholes during the re-pressurization phase of any follow up process to CHOPS. For example, wormholes could remain as stable high permeability flow channels to deliver injected fluids into a reservoir or they could collapse to improve conformance for fluid injection.
This paper presents the results of experiments testing the stability of wormholes under conditions that could be destabilizing such as cold and hot water circulation, heating, and pressurization with hydrocarbon gases (e.g., methane, ethane). The experiments were performed in a triaxial cell, approximately 19 cm in diameter and 18 cm long, packed with water-wet sand and saturated with heavy oil. Wormholes were generated in the pack by injecting heavy (dead) oil into the cell at a sufficiently high rate to produce sand from the pack. Then, conditions that could be destabilizing to wormholes were applied to test their outcome.
Cold water flowing through the wormhole eroded its walls, eventually causing the pack to collapse after more than a week. The injection of hot water accelerated the process of erosion considerably. Heating the pack did not cause it to collapse, but the wall of the wormhole was destabilized to a limited extent in that the diameter of the wormhole increased by more than a factor of two. The heating reduced the viscosity of the heavy oil in the pack, causing the apparent strength of the sand to decrease. Pressurizing the sand pack with methane and then slowly depressurizing it did not destabilize the wormhole. However, pressurizing the pack with ethane caused the sand matrix surrounding the wormhole to collapse over a period of two weeks. As in the case of heating, dissolution of ethane into the heavy oil caused the apparent strength of the sand to decrease by reducing the viscosity of the heavy oil in the pack.
Although the confining pressure in these experiments was relatively low, the experimental cell did not allow for stress redistribution due to the deformation of the sand matrix, as would occur in the field. For this reason, the wormholes generated in the experiments were less stable than ones in the field. Consequently, the laboratory conditions that resulted in wormhole collapse should be regarded as a guide to the behaviour that would be expected in the field.