CO2 and H2S corrosion resistant cementing systems are widely studied, however, most of them based on oil cement could only reduce corrosion rate rather than eliminate it. A novel CO2 and H2S corrosion resistant calcium phosphate cement was developed to eliminate corrosion. After cured at 100 °C with 5 MPa CO2 for 28 days, compressive strength of calcium phosphate cement set increased to 33.9 MPa from 21.8 MPa while permeability decreased to 0.02 mD from 0.03 mD. Thermal analysis shows that less than 1% weight loss at 600–770 °C which is the decomposition temperature of calcium carbonate indicates that calcium phosphate cement performs good CO2 corrosion resistance. Meanwhile, cured at 80 °C with 2 MPa H2S for 28 days, compressive strength increased to 19.8 MPa from 14.2 MPa while permeability remained unchanged. Calcium phosphate slurry performs good rheology behavior, little liquid loss, rapid ultrasound and static gel strength. The comprehensive performance of calcium phosphate cement could meet the requirement of long-term sealing of cement sheath in CO2 and H2S rich wells.
CO2 and H2S are common gases in oil and gas development (Li, 2016; Moroni, 2008; Omosebi, 2017; Qiu, 2012; Zhang, 2011), and destroy the seal integrity of the cement sheath leading to annulus pressure (Garnier, 2012). After the structure of the cement sheath is damaged, CO2 and H2S will also directly corrode the pipe string (Kiran, 2017). Moreover, due to the sulfide stress corrosion cracking, the downhole pipe string will suddenly and brittlely break even when the stress is still much lower than its yield strength. If the pipe string or whole well is discarded, H2S will cause huge damage to people, equipment and environment around the well. Due to unqualified cementing quality, severely corroded casing and failed interval plug lead to oil revesed out from the casing and sulfur water coming out of the ground of well R22 in Rumaila Oilfield of Iraq illicit great environmental, safety and security risks to its nearby area. Thus, abandonment operation was carried out (Zhang, 2015). Therefore, in order to prevent formation fluids containing CO2 and H2S from corroding casings and accessories, the first problem to be solved is the corrosion protection of cement sheath.