Research Article Effect of Bentonite Prehydration Time on the Stability of Lightweight Oil-Well Cement System Stephen Adjei, Salaheldin Elkatatny , and Abdulaziz Al-Majed Department of Petroleum Engineering, College of Petroleum & Geosciences, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Correspondence should be addressed to Salaheldin Elkatatny; elkatatny@kfupm.edu.sa Received 14 March 2021; Revised 12 May 2021; Accepted 31 May 2021; Published 12 June 2021 Academic Editor: Qingquan Liu Copyright © 2021 Stephen Adjei et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lightweight cement systems are used in the weak intervals of petroleum wells. Sodium bentonite is used as an extender in lightweight oil-well cement systems as it prevents excess water and sedimentation of particles, thereby ensuring the formation of homogenous and stable cement sheaths. The extending ability of sodium bentonite is enhanced when prehydrated. However, the optimum bentonite prehydration time and its effect on the stability of lightweight cement systems have not been well established. The objective of this study is to investigate the optimum sodium bentonite prehydration time and correlate it to the stability of lightweight oil-well cement systems. Bentonite suspensions were prepared by vigorous preshearing at 12000 rpm for 5 minutes, followed by aging times of 0, 30, 60, and 120 minutes. The swelling behavior of bentonite was investigated using a laser particle size analyzer. The Herschel-Bulkley model was used to determine the rheological parameters of the experimentally measured shear stress vs. shear rate data of the aged suspensions. The effect of calcium chloride salt on aged bentonite suspensions was investigated. Density measurements and pore space analysis with the nuclear magnetic resonance (NMR) technique were used to investigate the homogeneity of cement-based cores. It was observed that bentonite swells with time and, after 30 minutes, the swelling is insignificant; however, the swelling property did not have any observed impact on the properties of cement systems designed with the bentonite aged at different times. In general, all the lightweight cement slurries exhibited similar properties, in terms of rheology, stability, and homogeneity, regardless of the bentonite prehydration time. These findings indicate that aging bentonite suspension after vigorous preshearing in lightweight cement design is unnecessary and would only contribute to nonproductive time. 1. Introduction Bentonite is a clay obtained from volcanic rock [1]. It is a type of smectite clay containing the montmorillonite min- eral and certain amounts of quartz, feldspar, cristobalite, pyrite, illite, and mica [2]. The montmorillonite is classi- fied as a 2 : 1 clay mineral because it is composed of two silicon tetrahedral sheets that share oxygen atoms with an octahedral aluminum sheet placed between them [3–5]. The three-layered montmorillonite clay mineral with ini- tially neutral layers becomes negatively charged through isomorphic substitution which occurs, for instance when aluminum substitutes silicon in a tetrahedron sheet or magnesium substitutes aluminum in an octahedron sheet [1, 6, 7]. Isomorphism is an important phenomenon in bentonite as the mineral gets its unique characteristics when the negatively charged surface is balanced by exchangeable cations (Ca 2+ , Na + , and Mg 2+ ). If the dominant exchangeable cation in the bentonite is Na + , the resultant clay is known as sodium bentonite and it is calcium bentonite if Ca 2+ is exchanged [7–9]. The sodium bentonite is very useful in cement compos- ites because of its ability to swell when dispersed in water and also to form stable dispersions [7, 10]. As an extender in lightweight cement systems, bentonite acts as a free water controller, fluid loss additive, or antisettling agent [11–13]. Hindawi Geofluids Volume 2021, Article ID 9957159, 8 pages https://doi.org/10.1155/2021/9957159