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Analytical formulation of nonisothermal injection in geological reservoirs remains challenging, and researchers have adopted a number of simplifying assumptions to theoretically predict the resulting geomechanical alterations. One common assumption behind current thermo‐poroelastic solutions is a constant temperature and constant pore pressure in the reservoir rock adjacent to the source/sink, which results in an unrealistic discontinuity in the rock temperature profile, ignoring the gradual geomechanical changes at this location. Another common assumption in previous studies is the plane strain approximation, thus ignoring vertical interactions between the target layer and the surrounding rocks. This paper presents new closed‐form thermo‐poroelastic solutions for nonisothermal injection in a low permeable porous layer confined with flexible sealing rocks. The transient nature of temperatures in the rock adjacent to the source/sink is incorporated through implication of a temporal heat and fluid flux boundary condition. The impact of vertical confinement is also integrated using the Winkler model. The proposed solutions are proven to capture generation of short‐term thermal‐induced pore pressures near the wellbore, which cannot be captured using conventional solutions. It is also demonstrated that the conventional solutions obtained using constant pore pressure and temperature boundary are in fact a special case of the proposed solutions, and the latter yield the former where the heat transfer between the source/sink and the rock occurs quite rapidly. The vertical confinement is also shown to have a notable impact on the induced stresses‐strains, and thus should be incorporated when studying wellbore stability or assessing seal rock integrity during nonisothermal injection.

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