Institute of Petroleum Engineering

Project: Heavy Oil (HO)

Cold production of heavy oil in comparison with thermal methods, can offer advantages on capital cost, energy consumption, environmental pollution, safety and in-situ upgrading. Pressure depletion, water injection, gas injection, water alternating gas (WAG) and simultaneous water and gas (SWAG) injections are amongst cold production methods that have been applied in field development. Factors affecting recovery in the above processes are not fully understood. Available laboratory data provide conflicting information.

The drive energy for oil production during pressure depletion is supplied initially by oil expansion but then by the release of gas from solution and the subsequent expansion of reservoir fluids. The growth rate of an individual gas bubble depends on the depletion rate as well as the diffusion properties of the fluids. Formation of gas bubbles in a heavy oil sample with a viscosity of 2500 cp at reservoir conditions is shown in Figure 1. Note a large number of bubbles in (a), where the pressure is depleted at a relatively fast rate, typical of laboratory tests. The formation and growth of gas at a rate close to that in the field are shown in (b) and (c), respectively. Only a single bubble is formed and its growth results in a continuous gas path leading to early gas production hike. The oil recovery is expected to diminish rapidly with a sharp drop in pressure and an increase in gas production rate due to highly unfavourable mobility ratio.

Water flood of heavy oil is considered to be generally an inefficient displacement process due to unfavourable mobility ratio and severe viscous fingering. During the water flooding in a water-wet model, despite of the high viscosity difference between heavy oil and water, viscous fingering was not observed if water is injected at very low rate. Water could move through the existing connate water film and extracting some material from heavy oil. The dissolution of surface active material can result in lowering of interfacial tension between water and hydrocarbon phases, improving water-oil displacement. Figure 2, shows such a process, where the water phase is indicated by the blue colour.

Figure 1: bubble nucleation

Figure 1. Bubble Nucleation and Growth During Depressurisation.

Figure 2: micromodels a-f, 98K

Figure 2. Sequence of Water Flow, Dissolution of Surface Active Material and Displacement of Oil by Water.

Both hydrocarbon and non-hydrocarbon (CO2, Flue gas, CO2 enriched flue gas) gas floods have been used, especially for thin heavy oil reservoirs. Depending on the reservoir conditions, gas injection could be either immiscible or miscible. The main advantage of CO2 is that at most reservoir conditions it is a supercritical fluid with high solvency power to extract hydrocarbon components and displace oil miscibly. Furthermore its high density makes it quite compatible with oil alleviating gravitational segregation. However, in heavy oil reservoirs it lacks acceptable sweeping efficiency due to large viscosity contrast.

Water alternating gas (WAG) and simultaneous water and gas (SWAG) drive mechanisms have been used to improve sweep efficiency of gas injections. Our comprehensive research on both WAG and SWAG injection schemes has shown that apart from large-scale mechanisms of oil recovery by WAG and SWAG, there are also important favourable pore-scale mechanisms operational in these processes.

In HW a novel method of combined water-CO2 flood, which could remove most limitations and shortcomings of the water or gas flood, has been proposed. The method could also be applied in combination with thermal methods, where a large volume of CO2 would be produced and need to be stored for environmental reasons.