Durability of Oilwell Cement Formulations Aged in H2S-Containing Fluids

In the next few decades, the production of oil fields with high contents of associated sour gases will increase. For instance, it is estimated that 40% of the world's remaining gas reserves contain more than 2% of carbon dioxide (CO2) and/or more than 100 ppm of hydrogen sulphide (H2S). Therefore, investigations on technologies to produce such fields are of utmost importance. Because of the presence of corrosive vas, special attention has to be paid to the design and the selection of materials (steel and cement) used for well construction. Corrosion of steel caused by acid-gas-containing brines is well documented in the literature, and to a lesser extent, data about the degradation by wet CO, or wet H2S can be found. For cement-based materials, one can find abundant literature dealing with deterioration of cement pastes because of the CO, environment. Published data on degradation mechanisms of cement-based materials exposed to H2S environments are more scarce. This paper addresses the problem of durability of oilwell cement in different H2S environments. Different periods of time can be identified in the lifetime of the wells-the production period (typically between 20 and 40 years), the post-abandonment period (some tens of years following the permanent well abandonment), and the abandonment period (several centuries). For each period of time, cementing materials (primary cementing, plugs) are in contact with different types of fluids. Therefore, to correctly assess the behavior of oilwell cement, aging tests have to be carried out in fluids representative of these different periods of time. In this paper, we present both the methodology implemented under high-pressure/high-temperature conditions for testing materials in H2S-containing fluids and the results obtained on cement-based materials. Main physicochemical degradation mechanisms of cement-based materials caused by H2S are identified using various characterization techniques. Depending on the nature of the fluid in contact with cement materials, severe degradation can occur with a strong impairment of macroscopic properties.