Environmental Engineering Reference
In-Depth Information
14
Well and Field Monitoring
It is now common to complete wells with permanent gauges that permit continuous or
semi-continuous monitoring of the movement of fluids in the injection or production
string. These gauges can come as wired or via the use of fiber optics, or both. Of these
the fiber optics variety offers superior data gathering potential but at a higher cost.
Fiber Optic Sensing
Fiber optics are employed for two principal measurements:
Distributed Temperature Sensing ( DTS ) which allows for a continuous monitoring of
temperature along the entire length of a fiber optic cable. A pulse of light lasting
5 ns is sent down an optical fiber. This produces a 1 m band of light 1 that moves
along the fiber and relays back temperature related information at each point along
the way. Effectively the system measures the difference in temperature between the
surface where the light pulse initiates and the point from which light is backscat-
tered to the surface from any point downhole. If the surface temperature is known,
then the actual downhole temperatures can be measured. The depth of the backscat-
tering is determined by the time required for the pulse to return to the wellhead.
Distributed Acoustic Sensing ( DAS ) which allows a continuous monitoring of
sound noise, temperature, and strain related phenomenon along the entire length
of a wellbore. DAS may also be referred as DSTS which stands for Distributed
Strain and Temperature Sensing or DTSS short for Distributed Temperature and
Strain Sensing. Only in the case where there is a pressure gauge at the end of the
fiber will the pressures sensed along the well be absolute.
1 Light travels at ≅300,000,000 m/s in vacuum. In a fiber optics cable light travels more slowly
since the refractive index of the optical cable is ≅1.5. Thus in 5 ns a pulse travels 300,000,000 × 5
× 10 −9 /1.5 = 1.0 m.
 
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