Development of wellbore simulator for better understanding oil well cement behavior and gas migration during early gelation

Gases can migrate into the cemented annulus of a wellbore during early hydration when hydrostatic pressure within the cement slurry declines. 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 proposed by Sabins et al. (1982) employs SGS to quantify the hydrostatic pressure reduction. API Standard 65-2 provides a standard for determining the transition time using 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, while certainly helpful in reducing the risk of gas migration, does not eliminate its occurrence. A better means to characterize cement matrix strength using fundamental concepts for replacing SGS is desired.;The results from the simulations ran using the WSC are quite revealing. The wellbore cross-sections from the WSC simulations show the porous cement that resulted in localized regions from pressurized fluids as well as gas channeling during cement gelation. The effects of different factors on hydrostatic pressure reduction are also investigated, including: formation permeability, initial overburden pressure, wellbore temperature, water-cement ratio, cement composition, and CaCl2-based accelerator. Experimental results verify that shortening transition time cannot change the occurrence of gas migration, as the microstructural development for the same slurry may be identical although the hydration occurs at different rate. The introduction of fundamental concepts in analysis provides the opportunity to parameterize slurry designs and other important factors associated with wellbore conditions.;In this study, an enhanced wellbore simulation chamber (WSC) is developed to simulate hydrostatic pressure reduction in the cemented annulus and possible gas invasion under representative borehole conditions. In addition to the device itself, specific casting and testing protocols have been developed, which detail the procedures required for proper operation of the apparatus. The WSC makes adjustments to the existing cement hydration analyzers by providing representative wellbore conditions, which accounts for rock formation, real-scale wellbore section, and varying overburden pressure.