Reservoir temperature – a parameter characterizing thermal state of the reservoir; develops under the influence of thermal flux directed to the surface from inner zones of the Earth.

Main mechanisms of heat redistribution in the Earth crust:

• conductive heat transfer defined by the thermal conductivity of rocks;
• convective transfer associated with fluids moving inside rock fissures.

Temperature environment in subsoil is characterized by the following parameters: geothermal gradient (reservoir temperature increment per 1 meter of depth) and geothermal step (inverse geothermal gradient). Along with temperatures normal for the given reservoir areas with abnormal reservoir temperatures may exist. Reservoir temperature in accumulations depends on their occurrence depth and geothermal specifics of the giver Earth crust area. Observed temperatures vary from near 0°С in gas-hydrate accumulations to hundreds °С in deep-lying formations. Reservoir temperature is measured by mercury thermometers, thermistor temperature gauges and other types of thermometers. Drilling and associated operations disturb natural distribution of reservoir temperature. The rate of recovery of natural thermal area in the well depends on the well diameter, on the duration of well flushing-out, temperature differential between the flush fluid and the surrounding rock and on their thermal-physical properties. Normally it takes from 8 to13 days to recover natural reservoir temperature in the well.

Reservoir temperature measurements are especially important in petroleum and geological engineering. Reservoir temperature changes in oil and gas accumulations lead to changes of volumes of gas, fluids and container rock. Temperature increase causes decrease of oil and water viscosity and increase of gas viscosity. Temperature increase in closed reservoir causes formation pressure increase. Reservoir temperature is also associated with changes of phase relation in accumulations, of gas solubility in oil and water and salts solubility in water. Reservoir temperature decrease complicates hydrocarbons production and leads to losses of valuable products (condensate, viscous oil and paraffin), that is why oil fields (especially waxy oil) are developed with increase of reservoir pressure.