Numerical Analysis of Dissolution Process on the Casing Design Integrity Near Salt Caverns

Well design thought salt layer is still a challenge for the industry because of the special features like creep and solubility of the salt rocks (specially water-base fluids). This work focuses on the dissolution in salt rocks that can occur around injection wells, during production phase, when water percolates through permeable rocks reaching evaporite rock layers. In the scenario of a completed cemented salt rock with a dissolution cavity, the cement has no more structural stability to confine the casing and becoming susceptible to failure by laterally bucking or plastication. In this context, the present work focuses on the analysis of the casing integrity using a numerical model, developed in Abaqus finite element software, to account for the influence of the dissolution cavity and creep deformation of the rock salt. The results showed that high horizontal dissolution rates resulted in greater vertical displacements in the cavern salt and it is also observed that greater displacements occurred at the corners of the cavity due to stress concentration in those regions. The casing and the cements experience extreme stress, with casing stress close to the yield stress and the cement, considered elastic in this work, reaching very high stresses, indicating that integrity of the well barriers was not met. 1. INTRODUCTION Many hazards in wells drilled across salt are related to the time-dependent behavior of these rocks. This flow is associated with hardening and softening mechanisms during the rearrangements of its microstructure. The flow rate is influenced by the magnitude of the differential stress, temperature, mineralogy composition and impurities (Maia C. et.al., 2005). For the well design thought salt layers, one of the most challenger is the excessive loads on the casing strings after the deformation of the salt layer touch it. Another important hazards associated with creep deformation can occur during field development, if water percolates upward through the cement sheath or the permeable reservoir, it may reach the bottom of the salt layer. Salt rocks present a meaningful solubility when in contact with non-saturated water-based fluids, and the leaching process can generate salt caverns. Extensive dissolution of salt requires a supply of non-saturated fluid, an outlet where the resultant saturated brine can escape, and a pressure potential capable of causing flow toward the outlet (Johnson, 1981). The dissolution process depends on the flow rate and salinity profile of the fluid injected (Durie and Jessen, 1964). The rapid rate of salt leaching can generate uncontrolled caverns, resulting in successive overlying strata failure and induced subsidence (Johnson 1997, 2005, 2008). The salt creep toward the cavern space near the well will result in a subsidence effect that can significantly stress the cement sheath and the casing strings. Therefore, to guarantee the long-term well integrity and account for safety barrier envelope, the effects of a salt cavern near the well must be addressed.