Use of liquid pressure-pulse decay permeameter in experimental evaluation of permeability in wellbore cement under geopressured geothermal conditions

Geopressured reservoirs in the northern Gulf of Mexico basin along the coast of Louisiana have been determined to be viable source of geothermal energy and potential sites for carbon sequestration, where CO2 can be utilized to induce convective flow of geofluids and enhance heat harvesting. These reservoirs are made of unconsolidated sandstone capped by shale layers and possess temperatures as high as 140⁰C. At high temperatures, cement strength retrogression occur when calcium silicate hydrate phase in hydrated cement converts to alpha dicalcium silicate hydrate phase. The higher the temperature, the quicker the rate of transformation of calcium silicate hydrate. The conversion changes the structure of the hydrated cement leading to increased porosity, permeability and lowered compressive strength. The real problem lies in the great increase of permeability which makes the cement susceptible to chemical attack by low pH formation fluids which lead to loss of hydraulic barrier capability of cement, the most important function of cement in well bore system. The consequence of the loss of zonal isolation is the environmental release of previously contained geofluids, This study uses liquid pressure-pulse decay permeameter (PDPL) to measure the effect of increased temperature on cement permeability. PDPL is computer operated device capable of measuring permeability of cement to liquid (water) under reservoir conditions. Compared to conventional (steady state) methods, the liquid pressure-pulse decay permeameter cuts down the long time required to stabilize water fluxes from days or weeks to hours. This is very critical as cement permeability could change due to leaching or hydration during the time required in steady state methods. Permeability is calculated using pressure decay across a cement core sample over time. For the experiment, a range of chemical additive were added to portland cement slurry to counteract and curb strength retrogression, changing cement hydration products into chemically more stable phases, with favorable Ca to Si ratio. Four 13.1 pounds per gallon (ppg) (with water to solid ratio of 0.87) cement slurry designs with silica flour, calcined clay, silica sand, steel fiber and glass fiber and 13.1 ppg neat cement slurry were subjected to cycle thermal loading in salt brine. The results indicates that glass fiber and steel fiber cement can be added to the design to improve the permeability and increase the strength of the cement sheath for geopressured geothermal reservoirs in the Gulf of Mexico