Modified Cam-clay (MCC) model parameters of new cement formulations for low-enthalpy geothermal wells

The Modified Cam-Clay (MCC) model, primarily used to capture the plastic behavior of cement, can be adapted to assess the impact of cyclical thermal loads on cement sheaths in geothermal wells. We measured the MCC parameters for three cement formulations relevant for low-enthalpy geothermal wells: Class-G (G-cement), Class-G with 15% rubber particles (R-cement), and Class-G with 1.5% polypropylene fibers (PP-cement). Experiments included unconfined compressive strength and hydrostatic compression tests to determine initial yield surface size and slopes of the swelling and compression lines. Cyclic loading was tested under conditions representative of deep (3000m) and shallow (1200m) wells by measuring inelastic deformation and elastic moduli after each cycle. Results show that R-cement is the weakest as rubber particles act as points of stress concentration and crack initiation, while PP-cement is the strongest as PP fibers prevent cracking. PP-cement has the largest yield surface, R-cement the smallest, and G-cement the highest tensile strength. Under cyclic thermal loading, plastic damage accumulation is higher in deep wells and negligible in shallow wells. Both G- and R-cements show small stiffening after each cycle, more significant stiffening occurred in PP-cement. Overall, PP-cement has the largest yield envelope and is less sensitive to thermal cyclic loading. The G-cement exhibit a similar behavior but retains the highest tensile strength.