Going Against the Gradient
When a DTS system is initially run in the hole, geoscientists use its temperature measurements to determine a well’s geothermal gradient, based on changes in temperature that occur naturally with depth. Although temperature gradients can be useful in certain well log corrections, it is not necessarily the gradient that interests most geoscientists. Rather, it is deviations from the gradient that catch their attention. From these deviations, they can infer certain characteristics about the fluids that flow from a reservoir. The temperature profile of a well changes as fluids are withdrawn or injected. The magni tude of this change varies from one formation to another, depending on injection or production time and rate, formation permea bility and thermal properties of the fluid and the rock.3
A
DTS system can monitor disturbances in thermal equilibrium over time to detect such events. Although production or injection may initially introduce fluids of a different temperature into the wellbore, other noticeable thermal changes take place as a result of the fluid flow. These changes are explained by the Joule-Thomson effect, which is directly associated with the pressure drawdown experienced by fluids as they pass from the reservoir into the wellbore.4
This
type of temperature change occurs both when fluids flow into the wellbore, where a large pressure drop often occurs, and when they flow up the wellbore, where a more gradual pressure drop typically takes place (right).
The drop in pressure drives a corresponding change in volume of the liquid or gas, which is accompanied by a change in temperature. Through this phenomenon, it is common to see warming when oil or water enters a wellbore, or cooling when gas enters.5
The geothermal
gradient and the Joule-Thomson effect can be modeled using sophisticated nodal pressure and finite-element thermal-modeling tools, such as THERMA analysis software for wells with distributed temperature sensing.
3. Brown G and Walker I: “Light Fantastic,” Middle East & Asia Reservoir Reviewno. 5 (2004): 32–49. Available online at
http://www.slb.com/media/services/ resources/mewr/num5/light_fantastic.pdf (accessed February 18, 2009).
4. The Joule-Thomson effect accounts for the change in temperature of a fluid upon its expansion in a steady- flow process involving no heat transfer or constant enthalpy. This effect occurs in “throttling-type” processes such as adiabatic flow through a porous plug or through an expansion valve.
5. Al-Asimi et al, reference 2. Temperature
> Deviations from the geothermal gradient. Gas resides in a reservoir at a temperature corresponding to that of the local geothermal gradient (green dashed line). In a typical flowing well, gas will cool as it expands upon entering the wellbore, as dictated by the Joule-Thomson effect. The gas then flows up the well, exchanging heat with its surroundings by conduction through the casing (losing heat if gas temperature is above the geothermal gradient, and taking heat if its temperature is below the gradient). The resulting temperature profile is a function of the flow rate and fluid, borehole and formation thermal properties. This process continues as the gas flows up the well until the temperature curve eventually becomes parallel to the geothermal gradient.
Flowing temperature
Rayleigh band Laser Light
Brillouin lines Brillouin lines
Anti-Stokes Raman band
Wavelength
> Backscatter spectrum. To obtain temperature measurements, the DTS system analyzes the Raman signals. The ratio of Stokes to anti-Stokes signals is proportional to the temperature.
Raman band
Stokes
Reservoir interval
Joule-Thomson cooling on inflow
Winter 2008/2009
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Geothermal gradient
Depth
Number of backscattered photons detected
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