Abstract:

Drilling of high pressure/high temperature (HPHT) wells and injection of liquefied (cold) CO2 into underground formations produce thermal stresses in the surrounding rocks. The induced tensile stresses may exceed the tensile strength of the formation or other materials (casing, cement). This may create tensile fractures affecting the well integrity. Finite-element modeling of cooling-induced thermal stresses around a cased injection well was carried out. The formation was a sandstone-shale-sandstone or shale-sandstone-shale sequence. The results demonstrated that tensile hoop stresses developed in the rock with relatively high thermal expansion coefficient (shale) upon cooling by 40-50°C. A contrast in the coefficient of thermal expansion between different strata played a key role in the development of tensile stresses. The contrast in the Young's modulus, on the other hand, had little effect. Upon cooling by 80°C, the tensile hoop stress zone extended about 10 cm from the cement into the surrounding shale, and the stresses were sufficiently high to fracture the rock.

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