Modified Cam-Clay Model Parameters for Well-Cement

Abstract

Abstract Geothermal heat is expected to become an important sustainable energy source for the Netherlands. Geothermal wells are expected to last for decades and provide an acceptable return on investment. The well cement in geothermal wells experiences a unique stress condition due to the injection/production of cold/hot water in a doublet. The impact of these cyclical loads on cement has yet to be discovered. Critical state constitutive models such as the Modified cam-clay (MCC) are most suitable to describe cement's mechanical behavior. In this work, we present a new experimental protocol to measure the MCC model parameters for well-cement and to quantify the plastic damage accumulation under realistic cyclical conditions relevant to the Dutch geothermal wells. The protocol was tested on class G cement. The experiments were carried out in a triaxial apparatus and consisted of (1) unconfined compressive strength (UCS) tests; (2) hydrostatic compression tests to find the initial size of the yield surface (p0) and the slope of the swelling and compression lines (κ and λ); (3) triaxial tests on both the dry and wet side of the yield surface; and finally (4) cyclic loading tests under conditions relevant to a mid-enthalpy doublet. The effect of cyclic loading on cement integrity is quantified by measuring the inelastic deformation and elastic moduli after each cycle, followed by a triaxial test. The UCS tests on class G cement show peak stresses of 34-41 MPa, and critical state stresses (CSS) of approximately 10 MPa. The triaxial tests performed on the dry and wet sides display a strain softening and hardening behavior, respectively, as predicted by the MCC model. CSS values from all the tests align along the newly defined critical state line (CSL) for class-G cement. The CSL has a slope M = 1.5 and an intercept (p) of −3.7 MPa, close to the anticipated tensile strength of class-G cement. Hydrostatic tests give p0 of 23-29 MPa and κ and λ of 0.0046 and 0.02. The cyclic test simulating the producer well shows negligible plastic deformation after 10 cycles. Conversely, the deep injector well shows cement damage accumulation.