Chemical reactions in geothermal cements

Abstract

The Portland cements used for completion of geothermal wells undergo a series of chemical reactions as they cure in the increased temperatures of the wells. In hydrothermal conditions, the calcium silicate cements undergo transformations changing from the initial amorphous binder formed at room temperature into a crystalline one. On its own, Portland cement will slowly crystallise into a series of high Ca/Si ratio phases which have high porosity and low strength. The crystallisation of these phases causes a reaction known as ‘strength retrogression’, to occur with the actual phases formed depending on temperature. To avoid this phenomenon it is common practice to add ~40% silica flour (quartz) which allows the formation of low Ca/Si ratio phases such as tobermorite or xonotlite which have low porosity and high strength. These cements are placed in a formation that contains large amounts of reactive silica. Migration of silica and calcium during the curing reaction changes the phases that form and alters the durability. In the New Zealand fields, the level of dissolved CO2 in the geothermal fluids creates a further chemical reaction in the hardened cement, that of carbonation. This reaction occurs throughout the depth of the well with the rate dependent on CO2 concentration and inversely proportional to the Ca/Si ratio of the binder, the stronger ones carbonating fastest. In many fields, an impermeable layer traps CO2 so an acidic fluid forms which readily dissolves or corrodes the carbonated cement leaving the casing unprotected so the wells may corrode within a few years. This rapid corrosion of calcium based cements means that new cementing systems are needed to fully exploit our geothermal resources, work now being conducted at IRL.