How do you design a gas injection system for an offshore reservoir?

1 Reservoir evaluation

The first step in designing a gas injection system is to evaluate the reservoir's potential for gas injection. This involves studying the reservoir's geology, petrophysics, fluid properties, pressure, temperature, and production history. You also need to identify the optimal injection locations, rates, and patterns, as well as the expected recovery factor and performance. To do this, you can use various methods, such as material balance, reservoir simulation, decline curve analysis, and reservoir modeling.

2 Gas selection

When selecting the most suitable gas for injection, you need to consider the availability, cost, and compatibility of the gas with the reservoir fluids and rocks. Additionally, you must take into account the impact of the gas on the oil recovery mechanism, injectivity, and environmental regulations. Common types of gas injection include hydrocarbon gas injection, nitrogen injection, and carbon dioxide injection. Hydrocarbon gas injection is often preferred for light oil reservoirs as it can maintain reservoir pressure and increase the oil's solution gas drive. However, it may cause retrograde condensation or gas cycling if pressure is below dew point. Nitrogen injection is usually employed for heavy oil reservoirs as it can reduce viscosity and increase oil's gas drive. But there is a risk of nitrogen breakthrough or low injectivity if the reservoir has high permeability or low pressure. Carbon dioxide injection is often used for medium to heavy oil reservoirs since it can dissolve in the oil and lower its viscosity, density, and interfacial tension. It may also create a miscible or immiscible displacement depending on pressure and temperature. Nevertheless, it could lead to corrosion, scaling, or asphaltene precipitation if the reservoir has high salinity or acidity.

3 Facility design

The third step in gas injection is to design the surface facilities and equipment. This includes selecting the gas source, treatment, compression, transportation, and injection systems. You must also consider safety, reliability, operability, environmental and regulatory requirements. When designing the facilities, you need to assess the availability and quality of the gas source, whether it is from the same field, a nearby field, or an external source. Additionally, you must treat the gas to remove any impurities that may affect injection performance or cause corrosion or scaling. Gas compression is necessary to increase pressure and reduce volume for injection. Additionally, you need to consider power consumption, efficiency, and maintenance of the compression system. For transportation of the gas from source to injection wells, pipelines or vessels can be used. Considerations for this include flow assurance, hydrate prevention, leak detection, pressure losses and surge protection. Lastly, when injecting gas into the reservoir through dedicated injection wells or converted production wells, you must consider well design, completion, monitoring and control of the injection system as well as injectivity, interference and conformance issues.

4 Economic analysis

The final step is to conduct an economic analysis of the gas injection system. This involves estimating the capital and operating costs, the revenues, and the profits of the project. You also need to consider the uncertainties, risks, and sensitivities of the project, as well as the discount rate, tax rate, and oil price scenarios. To do this, you can use various tools, such as cash flow analysis, net present value, internal rate of return, and break-even analysis.

Gas injection is a powerful technique to improve oil recovery from offshore reservoirs, but it requires careful planning and design. By following these steps and factors, you can design a gas injection system that suits your reservoir's characteristics, your gas's properties, your facility's capabilities, and your project's profitability.

5 Here’s what else to consider

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