Durability and poro-elastic properties of a leached oil-well cement paste under high temperature and confinement

摘要

Cement-based materials are commonly used by the oil and gas industry for sealing purposes at different stages of the well life. Matured cement grout constitutes a sheath aimed at isolating the fuel well from the surrounding geological formation. In practice, the cement-based grout is mixed at an ambient temperature of ca. 20°C, then injected between the well walls and the casing, and let to mature in situ, under temperature and pressure conditions of up to more than 100°C and 60 MPa. After the fuel production phase, the depleted reservoir needs long-term sealing to avoid residual leakage, either if permanently abandoned, or when considered as an underground CO2 sequestration site. In such context, the original primary cementing sheath should still be able to contribute to isolating fluids within the well from the surrounding geological layers, although it may not have been originally designed to do so. In particular, despite the formation of a protective carbonation layer, on the long term, the noticeable acidification of groundwater fluid by dissolved CO2 is likely to induce cement degradation: this phenomenon is usually named leaching. Currently, extensive leakage of abandoned oil and gas wells is still reported, so that long-term well integrity and durability, and in particular that of its primary cementing, still requests investigation. The practical industrial issue here is to evaluate to what extent the mature cement grout, after extensive leaching, is still able to reasonably isolate gas (i.e. CO2) from the environment. In our laboratory investigations, liquid permeability K is considered a proper durability indicator, while drained compressibility modulus Kb is used as a mechanical performance indicator. Two original experimental set-ups have been used under 90°C temperature, in presence, or not, of a hydrostatic stress Pc representing the in situ stresses. Such loading is applied to a 90°C-cured oil-well cement paste. The effect of extensive leaching upon transport properties (assessed by K) and poro-elastic properties (using ev and drained compressibility modulus Kb) is determined. Porosity is assessed prior to loading by two different methods.