Summary 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 gas, 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 CO2 or wet H2S can be found. For cement-based materials, one can find abundant literature dealing with deterioration of cement pastes because of the CO2 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.

In the next few decades, the production of oilfields with high contents of associated sour gases will increase. For instance, we estimate that 40% of the world remaining gas reserves contain more than 2% of CO2 and/or more than 100 ppm of H2S. Therefore, investigations on technologies to produce such fields are of utmost importance. Due to the presence of corrosive gas, special attention has to be paid to the design of well materials (casing, cementing materials). Corrosion of steel due to acid gas containing brines is well documented in the literature, and to a lesser extent, data about the degradation by wet CO2 or wet H2S can be found. For cementitious materials, one can find abundant literature dealing with deterioration of cement pastes due to CO2 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 hydrogen sulphide environments. Different periods of time can be identified in the lifetime of the wells: the production period (about 20 to 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 behaviour of oilwell cement, ageing tests have to be carried out in fluids representative of these different periods of time. In this paper, we present the methodology implemented under HP/HT conditions for ageing tests of materials in H2S-containing fluids and results obtained on cement-based materials. Main physico-chemical degradation mechanisms of cement-based materials due to H2S are identified using different characterization techniques. Depending of the nature of the fluid in contact with cementitious materials, severe degradation can occur with dramatic impairment of macroscopic properties. Introduction The major goal of the primary cementing is to provide a complete and permanent zonal isolation of the well. This means that the cement sheath must prevent any fluid circulation (gas, oil, brine...) between different rocks layers. To achieve this goal, many events must be successfully addressed from the beginning of the cementing job to the well plugging. Figure 1 exemplifies the different steps that must be successful during the life span of the well. This paper focuses on the durability of cement-based materials used for the cementing of wells that are drilled in fields with high contents of sour gases. For the authors, durability means keeping the initial qualities of the cementitious sheath: mechanical integrity (absence of mechanical failure) and low hydraulic conductivity (no increase of porosity and/or connectivity versus time which would facilitate transport of aggressive species and/or pollutants). Modifications of macroscopic properties of cementitious materials are generally induced by chemical evolutions. Nonetheless, chemical evolutions of the cementing materials are acceptable only if they do not impair mechanical resistance and/or hydraulic conductivity. As one can see on figure 1, durability concerns both production and well abandonment periods. The major differences between these two periods of well life are the followings:Mechanical loading: stress variations within cement sheath occurring during the production period can be induced by thermal or/and pressure changes in the well [d, e]. After the plugging of the well, a mechanical loading can still exist but, contrary to what happens during the production time, the stress changes occur more slowly and last over a very long period of time [f, g].Downhole environment: downhole pressure and temperature, as well as the nature of the fluids in contact with cementitious sheath, are generally varying with time and well locations. For instance, field developments in areas with high contents of sour gases may require acid gas re-injection within the reservoir. In this case, the nature of chemical species varies with time but also with the type of wells (producers, water injectors or gas injectors).

In the near future, the production of oilfields with high contents of associated sour gases will increase. Research on technologies to produce such fields are therefore of utmost importance. Due to the presence of corrosive gas, special attention has to be paid to the design of well materials (casing, cementing materials). However, published data on degradation mechanisms of cement-based materials exposed to H2S environments are very scarce. When placed in contact with acid fluid, portland cement will dissolve and become, at some time, fully degraded. In this case, the zonal isolation will be no more effective. New solutions are required to ensure long-term integrity of cementing materials. One potential solution is to incorporate chemicals in the cementitious matrix. The key idea is to incorporate additives that react preferentially with H2S and not with hydrated phases of the cement. The goal of this work was to assess this solution. The paper is dealing with cement formulations that incorporate mineral admixtures. These mineral admixtures have been chosen because of their high chemical affinity with hydrogen sulphide. We first give properties (density, compressive strength, rheological behaviour) of the novel formulations. Then we describe the static ageing tests performed in H2S-saturated brine at 120°C and 15 MPa. By combining different analytical techniques, the chemical evolution of the new formulations has been investigated. The results are dependent on the nature of the reactive mineral admixture. We show that, in some particular case, the incorporation of specific mineral admixtures leads to a strong damaging effect for the cement matrix. Consequently, for the design of H2S-resistant cement, these specific mineral admixtures should be discarded. The proposed solution (addition of reactive chemicals) is promising but requires more investigation in order to carefully control the reactivity of the different chemical species. Introduction In the next decades, the hydrocarbon production will have to be increased to meet the global energy demand. A solution to increase hydrocarbons production will be to develop new reserves located in extreme environments (ultra-deep offshore, very sour gas, very heavy crude,…) thanks to innovative technologies. For instance, in the near future, production of fields with high contents of associated sour gases will increase. We estimate that 40% of the world remaining gas reserves contain more than 2% mol. CO2 and/or more than 100 ppm H2S. High contents of acid gases make the exploitation more difficult because these gases induce a degradation of wells construction materials. So, due to their presence, special attention has to be paid to the design of well construction and to the selection of materials (casing alloy, cementing material) to ensure long term borehole infrastructures. Therefore, research on technologies, and especially on materials, are of utmost importance to produce such fields. Corrosion of steel due to brines containing acid gas is well documented in the literature, and to a lesser extent, data about the degradation mechanisms by wet CO2 or wet H2S can be found. For cementitious materials, one can find abundant literature dealing with deterioration of cement pastes due to CO2 environments. However, published data on degradation mechanisms of cement-based materials exposed to H2S environments are more scarce. When placed in contact with acid fluids, portland cement will dissolve and will be, at some time, fully degraded. When this happens for oil wells, there is a risk that the zonal isolation becomes ineffective. So, development of new solutions to ensure long-term integrity of cementing materials in such harsh environments is required.