3. • Contents
• Reservoir temperature
• Main mechanisms of heat redistribution in the earth crust
• Geothermal gradient
• Geothermal step
• Importance of reservoir temperature measurements
in petroleum engineering
4. 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.
Reservoir Temperature is the average temperature maintained inside a
hydrocarbon reservoir.
The hydrocarbons that are stored in the reservoir also have the same temperature.
The temperature can be recorded with the help of a bottom hole temperature
recorder during pressure testing or drill stem testing.
The range of reservoir temperature may vary depending on the depth at which a
reservoir is found.
5. Like reservoir pressure, reservoir temperature is also an important factor
governing the phase behavior and the properties of the reservoir fluids
Reservoir temperature, which depends upon the reservoir depth, can be estimated
by the following equation:
T=Ts+Tgradient×D/100
where T = reservoir temperature, °F;
Tgradient = temperature gradient, °F/100 ft.;
Ts = temperature at surface, °F.
Change of temperature with depth can vary from 0.8°F to 1.6°F per 100 ft.
A value of 1.2–1.4°F per 100 ft. is usually assumed for sedimentary basins.
Temperature anomalies may occur due to geothermal processes.
6. Main mechanisms of heat redistribution in the Earth crust:
conduction by the thermal conductivity of rocks; convective transfer with fluids moving
inside rock.
Temperature environment in subsoil is characterized by the following parameters:
GEOTHERMAL GRADIENT: the rate of increasing temperature with respect to increasing
depth in the Earth's interior. (reservoir temperature increment per 1 meter of depth)
GEOTHERMAL STEP: the increase in depth in the earth's crust (in meters) corresponding
to a rise of 1° C in rock temperature. (inverse geothermal gradient).
Reservoir temperature is primarily governed by various factors, its closeness to the mantle of
the earth, if the depth of the hydrocarbon reservoir from the earth crust is more, the
temperature and pressure is also more.
If the depth is more the relative heat exchange capacity and thermal conductivity is higher.
Based on these factors, one can say that the geothermal gradient of a reservoir varies based on
its proximity to the earth's mantle.
On a general note, the geothermal gradient of any hydrocarbon producing area ranges from
0.6 to 1.6 degree Fahrenheit per 100 feet of increase in depth.
7. 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.
8. Importance of Reservoir Temperature Measurements in Petroleum Engineering.
The rate of recovery of natural well depends on the well diameter, duration of well
production, temperature differential between the fluid and the surrounding rock and
on their thermal-physical properties.
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.
decrease in reservoir temperature complicates hydrocarbons production and leads
to losses of valuable products (condensate, viscous oil and paraffin).
From initial reservoir pressure and temperatures we can determine fluid properties with
correlations, estimate the amount of hydrocarbons in place, and predict an overall
recovery factor from a well.
