Best Practices for Pressure Maintenance and Recovery in Reservoirs with Gas Caps

Abstract

Summary Gas cap reservoirs are a special class of hydrocarbon reservoirs that have segregated gas caps and are examples of reservoirs that are at their saturation pressures. The gas and the oil are in equilibrium at reservoir pressure and temperature. Producing a gas-cap reservoir requires special engineering skills. Waterflooding is generally not an option because the gas-cap acts as a pressure buffer (due to gas compressibility) and increasing the pressure by injecting water is difficult. Other production schemes are generally deployed to extract the oil and gas efficiently. One such method is to produce the oil and reinject the gas back into the gas cap. As reservoir pressure depletes, the gas cap expands, pushing the underlying oil toward the producing oil zone, and wells will begin to produce increasing amounts of gas and ultimately only gas. To maintain the reservoir pressure, gas has to be injected into the gas-cap. The amount of injected gas depends on the size of the gas cap and other reservoir properties. Some options of injected gases includes the produced gas, lean gas, inert gas or any other gas that may be locally available. Produced or lean gas have been the primary gases for maintaining reservoir pressure in gas cap fields. These gases have no adverse effect on facilities and recovery. Another advantage of this practice is that it relies entirely on gravity drainage, especially in the case of negligible water encroachment. Other options to consider include CO2 and acid gases. This paper summarizes several methods for pressure maintenance in fields with gas cap. The injectants that were considered include CO2, flue gas, acid gas (H2S and CO2), sour gas (CH4 and H2S). The main motivation is to inject the produced CO2 as a mechanism of mitigating Co2 emissions. This is part of the oil and gas industry’s sustainability options and part of the framework of the circular carbon economy. The advantages and disadvantages of each method on some typical gas-cap reservoirs are investigated. This includes the interplay of recovery, impact on facility and metallurgy, sustainability and economic impact for these options. For CO2 that may be miscible with the oil the option of injecting it into the oil rim is also considered. The long-term contamination of the gas cap is also evaluated.