Discussion of the feasibility of spontaneous ignition, oil requirement, and air requirement for air injection in shale and tight reservoirs

Because of the nature of low permeability of shale and tight reservoirs, a gas injection method has the advantage of enhancing oil recovery. Among gases, air has its vast and free resources. And one extra benefit is its thermal effect resulting from combustion. However, issues of feasibility of spontaneous ignition, oil requirement, and air requirement for the continuation of combustion in shale and tight reservoirs need to be addressed. This paper is to address these issues with the aid of numerical simulation. The relevant literature information is reviewed when discussing these issues. It is found that failure of spontaneous ignition may be caused by more factors such as vaporization, fuel displacement by air, etc., in addition to lack of oil exothermicity and heat loss that are commonly believed; the oil saturation difference between the initial oil saturation and the remaining oil saturation after air flooding and low-temperature oxidation is proposed to define the oil content; air requirement may not be satisfied in typical shale or tight reservoirs because of the low injectivity. More factors can complicate the issues of feasibility of spontaneous ignition, oil requirement, and air requirement. Therefore, a simulation approach is more appropriate to address those issues. • Factors affecting spontaneous ignition are discussed. • An oil saturation difference is used to define the oil content. • Air requirement may not be satisfied in shale or tight reservoirs. • Simulation approaches are proposed to study air injection.