Cementing of geothermal wells – radius of thermal influence

Temperature and pressure are two basic influences on the downhole performance of cement slurries. They affect how long the slurry will pump and how it develops the strength necessary to support the pipe. Temperature has the more pronounced influence. The downhole temperature controls the pace of chemical reactions during cement hydration resulting in cement setting and strength development. The shut-in temperature affects how long the slurry will pump and how well it develops. Assessment of the temperature development during hydration is necessary to determine how fast the cement will reach an acceptable compressive strength before the casing can be released. Cementing of casing of geothermal wells is conducted at high downhole temperatures. For this reason cement hydration retarders are often used. Thus is very important to predict the temperature increase during the cement setting. This will enable to determine the optimal time lapse between cementing and temperature survey. A semi-analytical formula which allow one to estimate the temperature increase versus setting time is used to describe the transient temperature at the cylinder's wall, while at the surface of the cylinder the radial heat flow rate (into formations) is a quadratic function of time. In order to more closely test the mechanical properties of cement under well conditions, the cement must typically be cured or hydrated for the appropriate amount of time under the temperature and pressure conditions as close as possible to downhole conditions in the well. At laboratory tests of cement slurries it is important to specify the radius of thermal influence. This will permit to determine the distance from the axis of the wellbore's model where a constant temperature should be maintained. The abovementioned formula is also used to estimate the radius of thermal influence. An example of calculations is presented.