For a recent project in Australia, Portland cement was modified to withstand contact with temperatures above 800 °C. This High Temperature (HT) cement system is applied to cement casing strings used as injection and production wells in a process known as Underground Coal Gasification (UCG). In its simplest form, this process involves drilling a production well and an injector well into an existing coal seam. Once drilling and cementing of the wells are complete the coal is ignited underground, and air and water are then introduced through an injector well. The air reforms with the combustion materials and forms a synthesis gas (syngas) containing carbon monoxide, hydrogen and methane (as a minor component), that moves under pressure through the coal seam to the production well, where it travels uphole to the downstream facility. Since coal typically resides at shallow depths, it was necessary to have cement that would set at relatively low bottomhole static temperatures of less than 39 °C and have the ability to withstand the extreme temperatures of the advancing combustion front, which can exceed 800 °C.
Portland cement with a silica source has been used in geothermal wells in which production temperatures can reach 380 °C or more, but recent testing showed that it can completely disintegrate around 450 °C. The final HT cement system design easily tolerates the elevated dry temperatures at the combustion front and maintains compressive strength in normal API compressive strength (CS) tests at 350 – 450 °C. The slurry mixes and pumps in the field as one would expect any slurry properly prepared to do while following API methods. Fluid loss control was adjusted with common liquid additives. Since the welfare of our planet is dependent on our ability to develop more environmentally friendly alternatives, the UCG process may become the preferred method to extract the energy contained within coal. The new HT cement system design enables a cost- efficient, logistically simple cementing method/material for this application where none existed previously.