Strength retrogression in high temperature well cements: microstructural origins and remediation methods – THWellCement

In the structure of oil and geothermal wells, a cement sheath is placed between the tubular and the formations for stability and sealing purpose. This project is related to the integrity of this cement sheath in high temperature zones (>110°C, below 4km depth). The cement hydration under high temperature results in “strength retrogression” phenomenon, which is a progressive reduction of the mechanical properties and a significant increase of its permeability. The strength retrogression is usually remediated by adding a sufficient quantity of silica, generally around 35% by weight of cement, to cement paste composition. We propose to perform a multi-technique and multi-scale experimental study to explore the microstructural and physic-chemical origins of this phenomenon and its impact on mechanical properties and permeability of the material. An important challenge for such a project is the preparation of a sufficient number of samples under controlled temperature and pressure conditions for various temperature ranges, hydration times and cement system compositions. We propose the design and construction of a multi-cell cement curing system with an ensemble of 6 to 8 unit cells with independent temperature and pressure control in the range of 200°C and 150 MPa. Moreover, these cells will be equipped with P- and S-wave velocity measurement systems permitting to monitor continuously the evolution of cement paste mechanical properties. The prepared samples will be subjected to a detailed microstructure characterization by a multi-technique method, including XRD with Rietveld analysis, TGA, Mercury Intrusion Porosimetry, Nitrogen sorption, Water vapor adsorption, 1H NMR, Si NMR, Al NMR and chemical shrinkage evaluation. The macro-scale properties of the prepared samples, including permeability and mechanical properties, will be explored by performing uniaxial compression tests as well as triaxial compression tests and permeability evaluations in a HP-HT triaxial cell (200°C, 150 MPa confining pressure, 1000 MPa Axial stress). The quantitative characterization of the cement paste microstructure will be used in a multiscale homogenization model, along with the experimentally evaluated macro-scale properties, to construct a predictive micromechanical model for strength retrogression prediction and its remediation by silica addition.