Structural Integrity of Well Cements

Abstract

This paper investigates the response of geothermal wells to operational loads associated with pressure and thermal conditions. The approach taken combines well data from actual configurations and material properties from laboratory tests with engineering finite element analysis. The analysis takes into account that the response of any geothermal well is the product of a coupled action between three components: (1) casing (2) cement annulus and (3) formation. The interaction of these three components is taken into account by finite element analysis. It is confirmed that the traditional approach, which is based on compressive strength as indicator of cement integrity, is generally inadequate. Other mechanical properties (Young’s modulus, tensile strength) are equally or even more critical to the cement integrity. Work is underway to obtain such properties under wellbore conditions. Until such data become available and more sophisticated calculations be undertaken, this paper will quantify their significance by using an engineering-based approach. The input parameters used in the finite element analysis were gathered from different geothermal sites in California. Development of cements with enhanced tensile strength, continuation of cement testing under downhole performance as well as continuation of refined structural analysis are important tasks needed in future studies.