Uniaxial and Triaxial Stress-Strain Behaviour of Ductile Cement Pastes Reinforced With Wollastonite Microfibers

When an oil well is submitted to cyclic steam injection the heating process induces tensile stresses in the cement sheath due to the thermal gradient that take place leading to cement-steel debonding and/or cement cracking. Similar problem can occur if the cement sheath is submitted to high creep deformations coming from the adjacent rock (this is the case for example of oil exploration in salt domes). In both cases sheath cracking can result in loss of hydraulic isolation and consequently in excessive water production with undesired economic and environmental consequences. In order to deal with this challenging scenario oil well cementing systems of special properties (e.g. high tensile strength, low elastic modulus and elevated toughness) should be used as an alternative to conventional high compressive strength systems. In this study cement pastes of high ductility were developed using wollastonite micro-fibers as reinforcement. The mixtures were developed within the framework of the Compressive Packing Model [1] and wollastonite microfibers were added in volume fractions of 2.5, 5.0 and 7.5 %. Uniaxial and triaxial compressive tests were carried out to obtain the unconfined and confined stress-strain behavior of the composites. The crack initiation stress and strain and the fracture process of the pastes under unconfined stress will be reported in this paper. Triaxial tests were performed under confining pressures of 0, 600 and 1200 psi and the Mohr-Coulomb criteria assumed to determine the internal frictional angle and cohesion. The results show that the addition of wollastonite microfibers increased the compressive strength of the pastes keeping the same strain capacity of the matrix. The internal frictional angle was also increased with the increase in the fiber volume fraction. However, the cohesion of the paste was reduced with the fiber addition.