CT Imaging of Wormhole Growth Under Solution-Gas Drive

Summary The cold production process has increased primary heavy oil production and has been applied with commercial success in the Lloydminster area (Alberta, Canada). In this process, the production of sand is encouraged in order to form high permeability channels (wormholes) within the formation. The process depends on the formation and flow of foamy oil into the wormholes as these grow away from the wellbore and into the reservoir. The formation and growth of a wormhole was visualized using a computed tomography scanner, in an experiment in which oil flowed through a horizontal sandpack and out an orifice. The only drive mechanism was the formation and expansion of methane bubbles within the live oil. The pressure gradient at the tip of the wormhole was approximately 1 MPa/m when it started to develop at the orifice. Two conditions appear necessary for wormholes to keep growing:the pressure gradient at the tip of the wormhole must be sufficiently large to dislodge the sand grains,the pressure gradient along the wormhole must be large enough to transport the sand from the tip to the orifice. The pressure gradient at the tip of the wormhole was 2.9 MPa/m when it reached its maximum length. This suggests that, although the pressure gradient at the tip was sufficient for erosion to occur, the sand could not be carried along the wormhole causing the wormhole to stop growing. The pressure depletion experiment suggests that wormholes can easily develop in uncemented sand in the field since the maximum oil production rate during wormhole growth (18 cm3/day) was significantly lower than in the field. The minimum pressure gradient (11 kPa/m) necessary for sand transport along the wormhole is important in calculating the extent of wormhole growth in the field. Introduction Cold production is a nonthermal recovery process used in uncemented heavy oil reservoirs in which sand and oil are produced together. Production rates from wells on cold production can be up to 30 times larger than the rate predicted by Darcy flow without sand production. In order to better understand the role of sand production in the cold production process, tracer injection tests were performed by well operators.1,2 Tracer dye velocities of 7 m/min were measured between certain wells. The dye showed up 18 h later at 2 km away from the injection well.1,2 The rapid flow of the tracer suggested that it flowed through a small channel excluding the possibility of a fracture or cavity around the well. We confirmed directly the development of high conductivity channels "wormholes" in the laboratory in two previous experiments.3,4 An orifice was located at the end of a sandpack and heavy oil was injected into the sandpack at constant flow rates. The heavy oil did not contain any dissolved gas. A high permeability channel (wormhole) was observed to develop at a critical flow rate. The drive mechanism in these experiments was external since a constant flow rate was maintained using a positive displacement pump. The drive mechanism for the cold production process is solution-gas drive.5 We wanted to determine whether or not a wormhole would develop under solution-gas drive. The pressure vessel used in the two previous external drive experiments was modified to handle the live oil. This required maintaining a back pressure at the orifice end of the sandpack. This back pressure was reduced at a constant rate of 205 kPa/day during the experiment. We observed that a wormhole developed in the sandpack even though the only drive mechanism was the expansion of gas bubbles in the heavy oil. The critical pressure gradient required for the wormhole to start growing (1 MPa/m) was significantly lower than in the two previous dead oil experiments: 800 MPa/m in a first experiment3 and 32 MPa/m in a second experiment.4 This significant difference in the critical pressure gradient is attributed to a destabilization of the sand grains at the wormhole tip due to the growth of the gas bubbles in the pressure depletion experiment. The wormhole stopped growing when the pressure gradient along the wormhole was equal to 11 kPa/m. These measurements are required in order to estimate how far these wormholes can extend in the field. This experiment shows that a wormhole can develop in a sandpack by solution gas drive. Materials The Clearwater sand used in preparing the pack was obtained from collection tanks at Suncor's former cold production pilot field in Burnt Lake, Alberta, Canada. The sand was packed in 2 cm layers with a hydraulic press under 27.6 MPa. The high packing stress was necessary to obtain a porosity of 34% representing field conditions (32-34%) and to give the sand a cohesive strength comparable to field values by creating more interlocking between sand grains. The porosity of naturally deposited sand ranges from 37% for a well-sorted, well-rounded, medium to coarse sand, to more than 50% for poorly sorted, fine-grained sands with irregular shaped grains.6 Either compaction or cementation is required to reduce the porosity of naturally deposited sands to field values. Porosity reduction by compaction of sand sediments can occur by plastic flow, crushing, fracturing, or pressure solution at grain contacts.7 An average particle size distribution of the sand after packing at 27.6 MPa is shown in Fig. 1. The average size of the sand grains was 198 microns. The fines content (less than 37 microns) was 8.4% by weight. The permeability of the sand pack was 1.7 Darcy. The pore volume of the sandpack was 2336 cm3.