Thermal stress field of high-temperature spray wellbore for underground coal gasification

As a supplement to traditional physical coal mining technology, Underground coal gasification(UCG) is a new generation of chemical coal mining technology. During the gasification process, the wellbore is subjected to a combination of high temperature and internal pressure loads. Aiming at the structural characteristics of the wellbore of UCG production wells, based on the heat transfer theory, the calculation model of the transient temperature of the wellbore under the cooling condition of annular spray water injection is established. Based on the wellbore pressure model, the calculation model of the wellbore is established according to the theory of elasticity and the wall cylinder. The calculation model of the temperature stress field of the casing-cement sheath-formation rock combination is established, and the engineering calculation software of the thermal stress field is formed, and the influence law of different factors on the stress of the casing-cement sheath is analyzed. At high temperature, the stress of each part of the wellbore increases due to the limitation of thermal expansion or thermal shrinkage. Under the condition of natural cooling, the theoretically calculated maximum stresses of the casing and cement sheath are 2 640.6 MPa and 151.3 MPa respectively, which exceed the allowable compressive stress of the material itself. The axial stress of casing and cement sheath is only 28.4 MPa and 15 MPa respectively, which is far less than the result when the temperature is considered. Under the cooling mode of annular water injection, the wellbore stress can be effectively reduced by controlling the temperature of the spray chamber. With the increase of casing internal pressure, the wellbore stress also increases, and the change of casing internal pressure will cause the stress direction of casing and cement sheath to change, and it needs to be discussed on a case-by-case basis when checking its strength. The stress drop at the intersection of both sides of the cement sheath is generally larger, and the performance parameters of the ratio increases, the stress of the casing-cement sheath decreases, that is, the cement sheath material with good bonding performance, high toughness, and high Poisson's ratio can reduce the casing-cement sheath stress. The above results can provide a reference for the structural design and production process of UCG production wells.