•One of the main cost of geothermal energy production is the electrical power consumption of the submersible pumps
•Pumps are forced to overcome losses in a closed environment
•Corrosion and mineral precipitation is often observed in geothermal wells
•Bad cement job can result in wellbore breakdown and power plant shutdown
Iron powder and fiberglass composite pipes (zst.ru)
•Electrically resistive composite casing materials are being introduced to the industry
•Composite tubulars are 4 times lighter than made of steel, withstand same pressure and load, resistant to corrosion and lasts more than twice longer
•The use of EM transparent casing materials requires the development of appropriate geophysical methods
•Presence of conductive particles in the cement allows to increase its displacement and check its quality using EM
•We use magnetic sensors to verify the quality of the cement behind the casing
Schematic representation of an idealized 4 layered geoelectric model
Helmholtz equation in cylindrical coordinates (r, j, z):
w, circular frequency; m= mrm0, magnetic permeability (r - relative, 0 - permeability of vacuum) and s is electrical conductivity
Tangential components of magnetic (H) and electrical (E) fields at the boundaries are continuous:
Vector potential in the wellbore:
M – magnetic moment of the coil, I0 – modified Bessel function, l – separation variable, z – axial distance, r – radial distance, C1 – is a constant obtained from boundary conditions that carry information about the borehole environment. Modeling parameters:
Effect of magnetic permeability of cement on the magnetic field at frequency = 1 kHz. Left to right: real part of secondary field, Hz,s, imaginary part of Hz,s
We assume that the tool is properly calibrated when the primary magnetic field has been excluded from the measurements.
Logging signals at the frequency = 1 kHz cross a cement lift, different tool lengths are considered
Logging signals at the frequency = 1 kHz cross a crack filled with cement, different tool lengths are considered
The wellbore model with different resistivities of the cement and logging signals at the frequency of 200 MHz for real and imaginary part of Hz,s
The eccentricity of cement can be determined using a magnetic sensor tool
Tools with lengths 0.25 – 0.6 m are most sensitive to the magnetic properties of cement
The real part of secondary magnetic field, Hz,s, is more sensitive to magnetic cements than the imaginary part
The most suitable frequencies are in the range 0.1 to 10 kHz, typical of magnetic susceptibility logging, inhomogeneities filled with cement are visible on the logs
200 MHz induction tool can be used for cement solidification detection
The proposed method can determine poor-quality cementation through a non-conductive casing for low enthalpy geothermal applications
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