| Summary: | Cement exposed to brine saturated with CO2 gas undergoes combination of chemical processes leading to mechanical degradation after placement behind the casing. These processes are influenced by downhole conditions such as temperature, pressure, and the composition of CO2 gas. The objectives of this study are to understand the mechanisms governing the degradation of well cement and to quantify the rate of deterioration using experimental and modeling techniques. Experiments were conducted by exposing Classes G and H cement to CO2-saturated brine solution under HPHT conditions. Temperature was varied between 100°F and 430°F while pressure was varied between 3,000 psi and 9,000 psi. To generate aggressive fluid around the cement, the composition of CO2 gas was varied by injecting a mixture of gases, containing varying compositions of carbon dioxide and methane, into the HPHT autoclave. Compressive strength was measured before and after exposure to determine the alteration in mechanical integrity due to chemical attack. In addition, fluid samples were collected after the test and chemical analysis was conducted to quantify pH and calcium content. These are supported with porosity, permeability, FTIR, XRD, EDX, SEM, and visual inspection of the degraded specimens. Besides, a methodology for coupling the governing mechanisms involved in cement degradation is proposed and a fully-coupled model is developed. In both classes of cement, overall mechanical behavior shows improvement, although the specimens were chemically degraded. Three mechanisms of degradation were 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 are interrelated; the overriding process governs the rate of degradation. ...
|
|---|