Study on Critical Parameters of Nitrogen Injection during In Situ Modification in Oil Shale

Abstract

During in situ modification, inappropriate thermal insulation measures applied in annulus of injection wells will cause injection technical problems such as tremendous heat loss, wellhead lifting, serious casing damages, and corrosion. Continuous nitrogen injection into annulus is an effective measure to realize thermal insulation of injection wells. The critical displacement parameters of nitrogen injection in annulus are a key factor in the process of continuous nitrogen injection. However, the critical nitrogen injection rate under various working conditions, such as different well types, different wellhead steam parameters, and insulation pipe parameters, was not fully considered and studied. In this paper, the annular critical nitrogen injection displacement model was established based on the completion structure of injection well by applying the basic principles of heat transfer and fluid mechanics comprehensively. The influencing factors of critical nitrogen injection displacement were calculated and analyzed. The results show that the annular thermal resistance of horizontal wells is larger than that of directional wells for different well types, and the critical nitrogen injection displacement required is higher. The parameter sensitivity analysis shows that with the increase of pressure, steam displacement, and steam dryness, the required critical nitrogen injection displacement rate increases nearly quadratically. However, with the increase of the length and thermal conductivity of insulation pipe, the critical nitrogen injection displacement decreases cubically. The critical nitrogen injection can be reduced by reducing the annular area of the casing annulus and lowering the string deeply. The research results can provide key basic theoretical guidance for improving the heat utilization efficiency of in situ modified wells and reducing the casing damage rate for hot injection wells.