Durability of Oilwell Cement Formulations Aged in H2S-Containing Fluids

Abstract

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.

H2S–CO2 reaction with hydrated Class H well cement: Acid-gas injection and CO2 Co-sequestration

“…Thus, as expected, ettringite is not observed in the carbonated zone as it is not stable due to the lower pH (<∼10.5) of the cement porewater in this region (Taylor, 1997). In addition, ettringite is unstable at T > 90-100 • C (Lecolier et al, 2006;Taylor, 1997) and therefore was not observed in previous high-temperature experiments by Jacquemet et al (2008).…”

“…The H 2 S pathway is favored in anoxic (reducing) conditions whereas the polysulfide pathway is favored in oxidized conditions (Rickard and Luther, 1997). The conversion of ferrite to pyrite has been observed in previous H 2 S-CO 2 studies using hydrated cement powders (Jacquemet et al, 2008) and solid cementitious materials (Lecolier et al, 2006); however, no reaction pathways were proposed. Further studies utilizing synchrotron spectroscopy techniques are currently in progress in order to better understand the specific reaction pathway of pyrite formation from ferrite.…”

“…could be especially important in this experimental system, where there is a strong pH gradient from the interior pore space of the cement (pH ∼ 13) to the external solution (pH = 2.9). In addition, pH controls the stability and dissolution of the cement phases; as the pH decreases, calcium hydroxide (pH stability ∼12-13), C-S-H (pH stability ∼10-11), and ferrite (pH ∼ 4-7) can be dissolved (Lecolier et al, 2006). The mineralogical changes observed (i.e.…”

Long-term durability of rock-based geopolymers aged at downhole conditions for oil well cementing operations

“…The measured permeability of Portland cement exposed to H 2 S for 12 months, and aged at 120°C and 0.7 MPa has been reported to be 20 μd. [26] UCS measurements In Table 6, the UCS of the specimens is tabulated. The specimens were exposed to the different chemicals during time intervals previously specified.…”

Long-Term Integrity of Well Cements at Downhole Conditions

“…There have been some long-term integrity studies of cement in other relevant downhole environments as well (Noik and Rivereau, 1999, Lecolier et al, 2006, Lecolier et al, 2007, Garnier et al, 2012, but the findings from these few studies are so far inconclusive.…”