Cement Mechanical Property Measurements Under Wellbore Conditions

Abstract

Abstract Cement integrity preservation during completion, stimulation, production, and even, during well abandonment is of critical importance for an operator from long-term economic, productivity, and safety perspectives. Traditionally, compressive strengths have been considered indicators of cement integrity. However, numerous squeeze cementing jobs regularly performed on completed wells are testament to the poor correlation between compressive strengths and cement integrity. Additional mechanical properties such as tensile and flexural strengths, elastic modulus, and Poisson's ratio are being taken into account with increasing frequency for maximizing the cement sheath performance during the life of the well. Unfortunately, all such measurements are performed on samples that have been cured, either under wellbore conditions (for example, pressure and temperature), or laboratory conditions (for example, atmospheric pressure) but tested at atmospheric pressure and temperature. Such properties may at best be useful for comparing different formulations in the selection process but do not provide information about the cement properties under downhole conditions. Using ultrasonic shear wave and compression wave combination measurements, dynamic mechanical properties, such as elastic modulus, bulk modulus, and Poisson's ratio and compressive strength, are measured under pressure and temperature. These measurements are compared with mechanical properties obtained from load vs. displacement under static conditions and acoustic compression and shear wave measurements at atmospheric pressure and temperature. Correlations are made for several slurries. The results are presented. The results also will present cases where the measurements made using this method demonstrated unique advantages over the conventional load vs. displacement techniques. Introduction The stresses exerted on the cement sheath from wellbore operations during construction/production could be severe and could damage the cement sheath. Examples of well operations that could exert stress on the cement sheath are:Cementing at multiple depthsCement hydrationCement shrinkagePressure testingChangeover from displacement fluid to completion fluidHydraulic stimulationHydrocarbon productionFluid injectionGas liftContinued drilling operations after cementingThermal cycling due to production, steam flood, fire flood, water injection, etc. These types of operations could change the pressure and temperature of the cement sheath after the slurry is placed in the annulus. The cement sheath could be damaged if the magnitude of pressure or temperature change is large and the stresses in the cement sheath exceed key values of the cement sheath. The key values are measured values and vary depending on cement slurry formulation. Some major consequences of damage to the cement sheath, such as sustained pressure on the annulus side or damage to the casing, could force well shutdown or result in high remedial costs. Other consequences of damage to the cement sheath, such as loss of hydrocarbon production, production of unwanted fluids (e.g. water), and growth of wellhead, could negatively affect the safety and economics of oil and gas assets because the remedial jobs are expensive to impossible in some cases. Hence, the integrity of the cement sheath should be considered during the early stages of well construction and designed for uninterrupted, safe, and economic production of hydrocarbons. A detailed engineering analysis should be conducted to evaluate how the different well operations affect the integrity of the cement sheath. This has become increasingly important because of a combination of increased risk to cement-sheath integrity in expensive wells operating in extreme operating environments, and increased safety standards.[1–7]