Gas Permeability and Pore Structure Analysis of an Ultrahigh-Durability Oil Well Cement Mortar Plug Using Micro-CT Scans

Abstract

With the increasing focus on global warming, greenhouse gases emitted from abandoned and/or orphaned oil and gas wells (AOOGWs) has been a topic of increasing interest. The requirements around the world to plug AOOGWs vary, yet the use of cement-based materials remains the common approach. However, traditional approaches provide insufficient performance in preventing the leakage of greenhouse gases. As such, improving the performance of cement-based plugs in terms of gas impermeability and mechanical strength can cut the potential greenhouse leakage pathways. The gas diffusion through an ultrahigh-durability (UHD) cement mortar plug (CMP) developed in this paper was studied in comparison to ordinary cementitious mortars. Experiments were conducted to measure the porosity and gas permeability under steady-state conditions. Micro-computed tomography (micro-CT) scans were utilized to extract the geometric information of the pore structure in the traditional mortar and UHD-CMP samples. Characteristic parameters were estimated for the extracted pore structure, such as porosity, connectivity, tortuosity, fractal dimensions, and pore throat ratio. The gas flow in the extracted pore structure was simulated using the lattice Boltzmann method (LBM). The LBM simulations showed that the UHD-CMP containing distributed fibers and a low water-to-binder (w/b) ratio has a five order of magnitude lower gas permeability than the traditional alternative. The decrease in the permeability is attributed to less porosity and smaller pores. A strong correlation (R2=0.86) was found between gas permeability and the measured porosity. The relationships between other pore structure parameters and the gas permeability were also investigated. In addition, the compressive strength of UHD-CMP was more than three times that of the traditional mortar, indicating a higher resistance to mechanical loads within the well conditions.