| Title | Experimental Assessment of Cement Integrity Under Geopressured Geothermal Reservoir Conditions |
|---|---|
| Authors | Mileva RADONJIC, Kolawole BELLO |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Geothermal Wellbore Cement, strength retrogression, liquid pressure-pulse decay permeameter, zonal isolation |
| Abstract | Geopressured reservoirs in the northern Gulf of Mexico basin along the coast of Louisiana have been identified to be potential source of geothermal energy. These reservoirs are made of unconsolidated sandstone capped by shale layers and possess temperatures as high as 140 ⁰C. Salinity values reaching a high of 100 g/L are associated with these reservoirs due to the dissolution of surrounding salt domes. Currently, novel wellbore system with downhole heat exchanger is being developed for in-situ heat harvesting that would result in zero-mass withdrawal. The techniques for heat extraction from geopressured geothermal reservoirs involve production of hot water and injection of cold water into the wellbore, which expose downhole materials to harsh temperature variations. Heating and cooling make the cement expand and contract as a result of thermal expansion. This volumetric change can initiate cement fractures, leading to failure of annular cement sheath resulting in well integrity issues and subsequently lead to lack of zonal isolation and compromised wellbore mechanical properties. This study measures the effect of cyclic thermal loading on cement slurry designs exposed to brines representative of the local geofluids. Grain volume porosimeter and Liquid Pressure-pulse Decay Permeameter were used to quantify the presence of thermal fractures as they are capable of measuring porosity and brine permeability of cement under reservoir conditions. Scanning Electron Microscopy micrographs with Energy Dispersive Spectroscopy capabilities, and Thermogravimetric analysis were used to study the microstructural and compositional changes in the cement slurry designs. Five cement designs with a range of chemical additives/fibers were subjected to 100 thermal cycles from 90 to 40⁰C at 100% relative humidity in brine. The experimental results indicate leaching of Ca(OH)2 will occur from the cement irrespective of cement composition which causes the porosity and permeability of the cement sheath to increase. Due to the thermal cycling, the strength of the cement sheath decreases as a result of mineralogical alteration that is accompanied with lower compressive strength. The study also shows that glass fiber and steel fiber can be added to the design to reduce the permeability and increase the strength of the cement sheath under thermal cycle loading conditions although they would not prevent it from completely degrading as the well ages, but will increase the life service of the wellbore under given conditions. |