Comparison of Salinity Dependent Mechanical Behaviours of Geopolymer and OPC: An Application to CCS and Oil / Gas Wells

Abstract

Global warming is a severe issue in which the emission of greenhouse gases, especially carbon dioxide (CO2), plays a vital role. Methods that are proposed to mitigate CO2 emissions include improving the energy conversion efficiency of fossil fuels, shifting energy production to low carbon sources, enhancing CO2 uptake by terrestrial and marine biomass and capturing CO2 and storing it deep underground. Of all the methods, carbon capture and storage (CCS) deep underground is the best way to reduce atmospheric greenhouse gases which is often responsible for global warming. One of the major issues faced when storing CO2 in depleted oil or gas reservoirs is its leakage through the injection wells. Ordinary Portland Cement (OPC) based well-cement has been used in these oil and gas wells. OPC, however, has issues such as cement degradation, leakage, strength reduction, and durability reduction when used in down-hole environments. Geopolymer can be a good alternative to OPC because of its excellent acid resistance characteristics, high strength and low shrinkage. In a typical wellbore, wellcement will be exposed to saline water that can have different salinity levels. Hence, the main aim of this study was to compare the mechanical behaviours of OPC and geopolymer cement cured in saline water for 7, 28 and 45 days at different salinity levels (0 %, 10 %, 20 %, 30 % and 40 % of NaCl). A series of experiments were conducted to compare the compressive strength and Young's modulus of OPC and geopolymer. It was observed that the compressive strength of OPC based well-cement increases as the salinity level of the brine water is increased up to 10 % NaCl and that it thereafter decreases as the NaCl percentage is further increased. On the other hand, the compressive strength of geopolymer well-cement generally increases as the brine concentration is increased. Based on the findings of this research, it can be concluded that geopolymer as a well-cement can perform better in saline water than OPC based well-cement, and that therefore geopolymer is suitable as a primary well-cement material.