Mechanisms of Cement Degradation in HPHT Carbonic Acid Environment

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Abstract
Cement deteriorates when exposed to acidic environment such as carbonic acid, sulfuric acid and hydrochloric acid. The rate of acid attack is influenced by pressure and temperature among other parameters. Real world scenarios involving practical exposure of well cement to these aggressive fluids include CO2-EOR, carbon capture and storage, CO2/H2S co-sequestration and matrix acidizing. The origin of cement degradation can be due to physical, chemical or mechanical processes or combination of these. Chemical degradation occurs when cement reacts with aggressive fluids to form minerals that are easily leachable or susceptible to further reaction. Cement exposed to brine saturated with Co2 undergoes combination of processes which includes hydration and chemical shrinkage, expansion and thermally induced pressurization, structural transformation of calcium silicates, decalcification, carbonation, bi-carbonation and mineral leaching. The degree of occurrence of these processes are controlled by temperature, CO2 gas composition, pressure, slurry composition and brine concentration. The objective of this study is to better understand the mechanisms governing the degradation process and quantify the rate of deterioration experimentally. This paper describes the mechanisms involved in the degradation of well cement in high pressure and high temperature (HPHT) carbonic acid environment. To understand the cement degradation mechanisms, previously published experimental data (compressive strength, porosity, permeability and FTIR mineralogy) is augmented with aging fluid calcium content measurements and photographic images of bisected specimens to further interpret the results. Aging fluid samples were collected after degradation test and chemical analysis was conducted to quantify calcium content. During the test, brine saturated with a gas mixture of carbon dioxide and methane was used to generate aggressive environment around the cement specimens. Results show improvement in mechanical and transport properties of cement specimens, although the specimens were physically and chemically degraded. Three mechanisms of degradation are identified as the driver of the degradation process. Structural transformation of calcium silicates at elevated temperature leads to slight retrogression in strength. Carbonation reaction improves mechanical strength but reduces porosity and permeability. Bicarbonation and leaching reactions increase porosity and permeability leading to the loss of mechanical strength. These mechanisms occur concurrently; the overriding process governs the rate of degradation. Within the experimental time scale adopted in this study, carbonation is the rate-controlling process because the overall integrity of the cement is improved. These results indicate that cement sheath is chemically degraded when exposed to carbonated brine under HPHT conditions. Very limited studies have been conducted to evaluate cement performance under HPHT acidic environment. The analysis presented in this paper sheds light on how common oil well cements degrade under extreme borehole conditions.

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