For readers who want a good introduction to gas injection processes for extracting more oil from reservoirs, two videotapes might fill the bill.
BP Amoco PLC distributed these tapes at an improved oil recovery conference in Tulsa last April organized by the US Department of Energy and the Society of Petroleum Engineers. One video, Gas Injection: Many Ways to Recover More Oil, explained some of BP Amoco's gas injection applications in oil reservoirs.
The other tape, Miscible Gas Injection, described the basic theory of miscible gas floods and how reservoir engineers can use this understanding for determining when to apply miscible gas displacement to a reservoir.
Gas injection applications
Some fundamental points covered in these tapes are:
- Gas injection into reservoirs can contain light hydrocarbons such as methane, ethane, butane, propane, as well as other gases such as carbon dioxide, nitrogen, and air.
- In miscible hydrocarbon injection, the butane, propane, and ethane-although very effective for extracting oil-should be minimized because of their cost.
- Gas can be injected alone or with water.
- The process can be immiscible or miscible.
- Miscible injectants can lower residual oil saturation to 3%
- Gas is more efficient in sweeping low-permeability reservoirs than oil.
- Gas tends to move up while oil moves down.
Because of this last property, BP Amoco, in its Harding field development project off the UK, is injecting gas with water. It expects the gas to move upward into trapped attic oil, which water will not sweep.
In Magnus field, also off the UK, BP Amoco estimates that gas injection into the upper sands that contain oil trapped below a shale lens could increase oil recovery factors to 70% from the present 50-55% for the current production scheme, which involves injecting water at the field's periphery with oil produced from the crest.
Prudhoe Bay field, on Alaska's North Slope, is BP Amoco's largest gas injection project. It estimates that the gas injection process will recover about 5 billion bbl of oil that otherwise would have remained in the ground, if gas were not available to inject. The main gas injection process in Prudhoe is a pressure maintenance scheme that pushes oil downward. But BP Amoco also is conducting miscible gas injection in Prudhoe's watered-out sands. It expects to gain about 500 million bbl of incremental oil from this process.
In BP Amoco's Andrew field off the UK, gas injection through a single well in the gas cap prevents the oil-gas contact from rising. If oil entered the gas cap, it would leave a residual oil saturation that could not be recovered.
Pressure maintenance with gas injection can also prevent retrograde condensate forming a residual oil saturation such as in BP Amoco's operated Cupiagua field in Colombia. BP estimates its gas injection program there will recover an additional 450 million bbl of liquid hydrocarbons.
Technical challenges
BP Amoco emphasized on its videotapes that a successful miscible flood needs computer simulation combined with extensive laboratory analysis that has characterized the fluid and rock properties.
The right grid size in the computer model is important. Too coarse a grid may not reflect the processes that take place in the reservoir.
As described in these tapes, optimization of the process is essential for a successful project.
