Surface-Modified Graphite Nanoplatelets To Enhance Cement Sheath Durability

Abstract

We propose a novel cement additive made of graphite nanoplatelets (GNPs) for improved hydraulic isolation and durability of oil and gas wells. The primary role of the cement sheath, which is zonal isolation, can be significantly affected by the permeability of set cement (hardened cement slurry). On one hand, it is the inherent microstructural defects of cement, including pores and microcracks, that results in the intrinsic permeability of cement, and on the other hand, cracking, micro-annuli, or other flow paths developed through the disturbed cement by connecting the pre-existing microstructural defects determine the equivalent permeability of set cement. The purpose of this research is containing or at least minimizing the intrinsic and developed flow paths through the cementitious matrix with the help of surface-modified GNPs. GNPs possess high surface area to volume ratios. In this study, we focus on the effect of surface-modified GNPs on the overall mechanical properties of both cement slurry and hardened cement slurry affecting the permeability of cement. We present two dispersion methods on the basis of physical and chemical treatments of the surface properties of GNPs. The efficiency of proposed methods on the overall properties of the cement is examined before and after its setting. To mimic downhole conditions, cement slurries are cured at 3,000 psi and 190°F for 24 hours. Also, some experiments were repeated under the pressure and temperature conditions up to 5,160 psi and 126°F, respectively, to examine pumpability and behavior of cement slurry at bottomhole conditions. To examine the role of spatial distribution of GNPs on the hardened cement nanocomposite, samples with different concentrations of GNPs were tested. We investigated the effect of modified GNPs on the unconfined compressive strength (UCS), shear bond strength, thickening time, rheological characteristics, and the free fluid content. We measured zero free fluid at room temperature for different concentrations of GNPs, demonstrating uniform dispersion of nanoparticles within the cement matrix. On the other hand, the squeeze of water out of the lower parts of the cement slurry and its upward migration can develop preferential paths for oil and gas migration. Therefore, eliminating the above-mentioned water separation can enhance cement sealing properties. We found that an optimum 0.2 vol% concentration of acid-functionalized GNPs improves the compressive and the shear bond strength of the prepared cement by approximately 42 and 175% as compared to the plain cement, respectively.