Summary
Gases can migrate into the cemented annulus of a wellbore during early gelation when hydrostatic pressure within the cement slurry drops. Different means to describe hydrostatic-pressure reduction have been proposed and reported in the literature. Among them, static gel strength (SGS) is the most widely accepted concept in describing the strength development of hydrating cement. The classic shear-stress theory uses SGS to quantify the hydrostatic-pressure reduction in the cement column. Approaches derived from the concept of SGS have contributed to understanding mechanisms of gas migration and methods of minimizing it. Unfortunately, these approaches do not accurately predict gas migration. Although SGS was originally adopted to describe the shear stress at interfaces, it has also been used to estimate the shear resistance required to deform slurry during the hydration period. Before early gelation, the hydrostatic pressure will overcome the formation gas pressure and prevent gas migrations. During gelation, the cement develops enough rigidity to withstand the gas invasion. This critical hydration period is defined as the transition time. API STD 65-2 (API 2010a) provides standards for determining the transition time by use of the concept of SGS. Current industry practice is to reduce the transition time, thereby lowering the potential for invading gas introducing migration pathways in the cemented annulus. This approach, although certainly helpful in reducing the risk for gas migration, does not eliminate its occurrence. Experimental results presented in this study demonstrate that the relationship between SGS and hydrostatic-pressure reduction is not linear. Characteristics of the transition-time endpoints depend on slurry properties and downhole conditions. Moreover, SGS is not able to characterize the gas-tight property of a cement slurry. When slurry gels, the mechanical properties are governed by its growing solid fraction. The gel can deform under shear loading, but gases and other fluids will need to break or fracture the bond between solids and push them aside for pathways to form within the cement/matrix domain at this point. To fully understand this process, the bond strength between solid particles and the compressibility of the cement matrix are needed. The bond strength and compressibility are mechanical properties dependent on the changing rigidity of the gelling cement. However, SGS does not address these important properties and, therefore, SGS is limited in its ability to predict gas-migration potential. A better means to characterize the cement/matrix strength by use of fundamental concepts and variables for replacing SGS is desired.
A numerical approach is presented to simulate the non-Newtonian flow of a wellbore cementing process to quantify the potential for poor drilling mud displacement efficiency and bond strength development between the cement annulus and rock formation. The approach consists of using the lattice Boltzmann method with a Bingham plastic constitutive model to represent the cement flow behavior. The lattice Boltzmann method is a pseudo-particle, mesoscale approach that naturally models complex flows in a computationally efficient manner, but has thus far seen limited use for capturing such slurry (or similar) flows. Results from the model are presented for a wellbore cementing process with various annular configurations and cement slurry properties. In particular, the results consider irregularities and imperfections in the shape of the rock formation surface, as well as changes in the cement flow properties (e.g., viscosity), as could be affected by variations in mix and/or the pumping process. These results show an array of circumstances in which poor drilling mud displacement efficiency and bond strength development between the cement column and rock formation does or does not occur as the cement is pumped into the wellbore annulus. Lastly, potential future work and developments are discussed for the numerical approach to address other failure mechanisms of zonal isolation, which are still poorly understood mechanistically.
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