Reservoir Simulation of Gas Injection Processes

Abstract

In reservoirs with favorable rock and fluid properties, application of gas injection processes can enhance the recovery efficiency and value of a field development. ExxonMobil employs a systematic approach to evaluating and selecting recovery processes during the full life-cycle of each reservoir. Once a gas injection project has been implemented, ongoing surveillance programs and optimization studies are performed to ensure that maximum value is obtained from the resource. One important aspect of managing these resources has been the use of reservoir flow modeling to understand performance expectations and to develop reservoir management strategies to optimize recovery. Because ExxonMobil's experience with reservoir simulation of gas injection processes has covered many types of injection processes over many years, a full suite of simulation approaches has been used for reservoir flow modeling. The gas injection processes studied and implemented have included miscible floods, immiscible floods, Water-Alternating-Gas (WAG) floods, as well as gas disposal. A full range of injection gases such as CO2, H2S, N2 and hydrocarbon injectants has been used. The fit-for-purpose simulation approach is chosen based on specific needs and objectives for each reservoir. The approaches have ranged from coarsely-gridded black oil simulation to finely-gridded compositional simulation depending upon the specific requirements for a given field. In recent years, the emphasis has been on state-of-the-art compositional simulation. ExxonMobil has applied compositional simulation to design and manage more than 25 gas injection projects covering the full range of injectants and lithologies. Because each gas injection project is unique, the evaluation of benefits must be comprehensive and adaptable. Case studies are presented illustrating a range of gas injection processes and reservoir types. The case studies highlight important aspects of data collection, issue identification, simulation technique evaluation, process evaluation, and long-term project stewardship. Depletion Strategies Utilizing Gas Injection Injection of either hydrocarbon or inert gases is employed in reservoirs where a favorable combination of pressure, fluid properties, and reservoir characteristics result in gas being preferred versus water injection. ExxonMobil participates in over one-third of worldwide gas injection enhanced oil recovery projects and therefore has established a large knowledge base of gas injection project experience at the laboratory, pilot test, and field implementation stages. Projects in which ExxonMobil participates include gas cycling for condensate recovery, immiscible and miscible gas injection for secondary oil recovery, tertiary WAG and foam-assisted WAG projects, and re-injection of sour gas for both oil recovery and sulfur/CO2 disposal. The key factors that impact performance of gas injection projects are reservoir pressure, fluid composition, reservoir characteristics, and relative permeability. Reservoir Pressure Pressure is a key parameter in determining whether or not the injected gas will be miscible with the in-situ oil that will be contacted in the reservoir. Oil recoveries for gas injection processes are usually greatest when the process is operated under conditions where the gas can become miscible with the in-place oil. The gas and oil can be first contact miscible or develop multi-contact miscibility by extracting light components from the oil (the "vaporizing" gas drive process), and/or by losing components to the oil (the "condensing" gas drive process). Miscibility can be achieved either by managing the reservoir pressure and/or by changing the composition of the injected gas by addition of either heavier hydrocarbons or acid gas components. Gas injection can also be used to immiscibly displace oil and for reservoir pressure maintenance. Also, because gas injection will require compression, the pressures of both the source gas and the receiving reservoir are important for facilities cost and design reasons.

Advantages of Up-Dip Water-Miscible Gas Injection

“…Hence, even though stable gas-oil displacement has higher microscopic sweep efficiency than water-oil displacement, considering the conditions in many oil reservoirs, the significant difference in viscosity and density can decrease the sweep efficiency (Lawrence, Teletzke, Hutfilz and Wilkinson, 2003). Waterflooding, compared to gas flooding gives a better volumetric sweep, due to less viscosity and density differences of oil and water.…”

Experimental Study of Tertiary Immiscible WAG Injection

“…As a consequence, immiscible gas flooding without a mobility control strategy is seldom applied as an improveded oil recovery (IOR) process, except in favorable gravity-stable configurations [1]. However, gas as a displacing agent suffers the obvious drawback of its very high mobility.…”