Improved durability of Saudi Class G oil-well cement sheath in CO 2 rich environments using olive waste Ahmed Abdulhamid Mahmoud a , Salaheldin Elkatatny a,b,⇑ a College of Petroleum Engineering & Geosciences, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Saudi Arabia b Petroleum Department, Cairo University, 12613 Giza, Egypt highlights  Improving the carbonation resistance of Saudi Class G oil well cement using Olive Waste.  Improving the mechanical stability of the oil-well cement in CO 2 rich environment using Olive Waste.  Prevention of CO 2 leakage through cement for CO 2 sequestration wells. article info Article history: Received 21 April 2020 Received in revised form 5 August 2020 Accepted 14 August 2020 Keywords: Cement carbonation Carbon sequestration Olive waste Saudi Class G cement abstract The physical properties of the oil-well cement, especially its strength and permeability, are changing sig- nificantly after reacting with CO 2 -saturated brine which will alter the cement hydration products. In this study, the effect of incorporating the olive waste into Saudi Class G oil-well cement formulation subjected to CO 2 sequestration conditions at 130 °C and 10 MPa on the cement permeability, carbonation depth, compressive and tensile strength, and microstructural changes during 20 days of carbonation was eval- uated. The results obtained showed that the addition of 0.1% by weight of cement (BWOC) of the olive waste to the cement enhanced the cement resistance to the carbonation process and decrease the carbon- ation depth from 1469 lm for the base cement to 1269 lm after 20 days of carbonation. Incorporating 0.1% BWOC of the olive waste into the cement slurry also maintained the original cement permeability of 14.3% less than that for the base cement, while the permeability of the cement samples with 0.1% BWOC olive waste after 20 days of carbonation is 33.9% lower than the base cement permeability. The permeability reduction is the main mechanism responsible for the enhancement of carbonation resis- tance for the olive waste-based cement. The ability of the olive waste to delay calcium leaching is attrib- uted to the decrease in the permeability of the sample with 0.1% BWOC of olive waste. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Geologic carbon capturing and storage (GCS) has been consid- ered as an efficient technique that captures the CO 2 emitted by heavy industries and power plants into the atmosphere [1–3]. According to the department of energy & climate change DECC [4], it is expected that by 2050, the GCS projects only could reduce the CO 2 emissions by about 20%. In GCS technology, the CO 2 cap- tured from the power plants and industrial sites will then be injected into suitable geological deep formations such as the con- ventional reservoirs [5], depleted hydrocarbon reservoirs, saline deep aquifers, and unmineable coal seams [6–9]. The integrity of the reservoir formations and the completion of the injection wells especially the cement sheath are the main fac- tors determining the success of the GCS projects [10–12]. However, the integrity of the cement and its durability are considerably reduced by the cement carbonation-induced degradation [13,14]. The hydrated oil-well cement is composed of around 20% of the portlandite with a considerable concentration of calcium-silica- hydrate (CSH) minerals [15,16], in addition to ettringite [17], and unhydrated tri- and di-calcium silicates [18]. The ettringite and unhydrated tri- and di-calcium silicates usually present in low con- centrations; therefore, they do not affect the cement carbonation process [19,20]. According to Yang et al. [20], at a temperature of 127 °C and higher, portlandite is the least resistive available hydration product to degradation under CO 2 -rich conditions. Therefore, it makes the oil-well cement easily degraded, and hence, it may lead to the https://doi.org/10.1016/j.conbuildmat.2020.120623 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: College of Petroleum Engineering & Geosciences, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Saudi Arabia. E-mail address: elkatatny@kfupm.edu.sa (S. Elkatatny). Construction and Building Materials 262 (2020) 120623 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat