ANALYSIS OF GEOTHERMAL CASING THINNING RATE OF LEYTE GEOTHERMAL PRODUCTION FIELD
Corrosion is one of the technological problems that geothermal management faces in the operation of geothermal wells. The primary cause of corrosion in geothermal wells is exposure to geothermal brine where conductive and convective heat transfer happens from the reservoir to the surface. The geothermal well supplies the motive two-phase steam fluid from the reservoir to the surface fluid collection facility, and then to the separator where steam is separate from brine. The casing of the well is the first equipment that encounters the field’s geothermal fluid. The flow of geothermal brine from the reservoir causes the casing to deteriorate by way of corrosion. The impact of turbulence, corrosion and erosion significantly reduce the integrity of the casing. Thus, monitoring these facilities is vitally important to minimize the risks to geothermal operations. Caliper measurements are taken to monitor the thickness of the casing for safety and environmental concerns. The effective management of a geothermal well necessitates a very thorough knowledge of the extent to which the aforementioned factors contribute to casing degradation. In this paper, the contribution of factors affecting the thinning rate of a geothermal casing, such as temperature, flow velocity, pH, well geometry and corrosion species, was studied. The effects of several corrosive species on casings were also studied. The corrosive species and the factors affecting the thinning rate of the production wells of the Leyte geothermal production field were analysed with multiple linear regression models in order to determine the dependency of the thinning rate on these parameters. Fluid discharge measurements and sampled chemistry concentrations at the surface were simulated at downhole conditions using software HOLA and WATCH. The results were analysed statistically using ANOVA, a P value test, and multiple regression coefficients. Adjusted R2 of the regression analysis and mean absolute percentage error (MAPE) aided in selecting a good regression model. The study showed that fluid discharge characteristics such as temperature, velocity, pH, steam fraction and geometry of the well contribute about 56.71% to the thinning rate of the casing. The predicted thinning rate model also shows similar trending of the average thinning rate compared to the measured thinning rate for the first and second surveys of repeatedly measured wells.
Corrosion is one of the technological problems that geothermal management faces
in the operation of geothermal wells. The primary cause of corrosion in geothermal
wells is exposure to geothermal brine where conductive and convective heat transfer
happens from the reservoir to the surface. The geothermal well supplies the motive
two-phase steam fluid from the reservoir to the surface fluid collection facility, and
then to the separator where steam is separate from brine. The casing of the well is
the first equipment that encounters the field’s geothermal fluid. The flow of
geothermal brine from the reservoir causes the casing to deteriorate by way of
corrosion. The impact of turbulence, corrosion and erosion significantly reduce the
integrity of the casing. Thus, monitoring these facilities is vitally important to
minimize the risks to geothermal operations. Caliper measurements are taken to
monitor the thickness of the casing for safety and environmental concerns. The
effective management of a geothermal well necessitates a very thorough knowledge
of the extent to which the aforementioned factors contribute to casing degradation.
In this paper, the contribution of factors affecting the thinning rate of a geothermal
casing, such as temperature, flow velocity, pH, well geometry and corrosion species,
was studied. The effects of several corrosive species on casings were also studied.
The corrosive species and the factors affecting the thinning rate of the production
wells of the Leyte geothermal production field were analysed with multiple linear
regression models in order to determine the dependency of the thinning rate on these
parameters. Fluid discharge measurements and sampled chemistry concentrations at
the surface were simulated at downhole conditions using software HOLA and
WATCH. The results were analysed statistically using ANOVA, a P value test, and
multiple regression coefficients. Adjusted R2 of the regression analysis and mean
absolute percentage error (MAPE) aided in selecting a good regression model. The
study showed that fluid discharge characteristics such as temperature, velocity, pH,
steam fraction and geometry of the well contribute about 56.71% to the thinning rate
of the casing. The predicted thinning rate model also shows similar trending of the
average thinning rate compared to the measured thinning rate for the first and second
surveys of repeatedly measured wells.
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The flow within the well was assumed to be in steady state at all times, but the reservoir pressure was
allowed to vary with time. Top-down simulations were conducted where the downhole well conditions
were simulated based on given wellhead conditions, such as mass flowrates and enthalpy values. There
was no comparison made between the simulated temperature and pressure profiles and the measured
flowing temperature and pressure profiles of all the wells, because flowing surveys were not available
for all of them. To simplify the simulation process and since the cased hole section is the main point of
interest, it was assumed that the depth of the feedzone was at the production casing shoe with the
properties of the flow discharge. Table 2 shows the well discharge data used at wellhead conditions,
while Table 3 summarizes the simulated downhole results given by HOLA.
TABLE 2: Fluid discharge data of two-phase and dry wells studied for
predicting the thinning rate of the casing
Well
Well
Date sampled WHP MF H
name
name
Date sampled WHP MF H
Well 1 6-May-12 1.10 48.5 1736 Well 20 30-Oct-11 1.1 18.4 2779
Well 2 11-May-11 1.10 4.7 2435 Well 21 4-May-11 1.0 7.5 2777
Well 3 4-Nov-10 1.10 36.8 1687 Well 22 15-Sep-11 1.0 6.8 2777
Well 4 8-Dec-11 1.10 28.0 1611 Well 23 23-Jul-11 1.0 16.7 2777
Well 5 27-Nov-10 1.10 24.0 1632 Well 24 26-Jan-12 1.0 3.0 2777
Well 6 22-Aug-12 1.1 55.2 1056 Well 25 28-Apr-11 1.5 19.1 2788
Well 7 3-Apr-12 1.1 40.6 1085 Well 26 2-Sep-11 1.8 20.0 2796
Well 8 30-Oct-10 1.1 50.6 1315 Well 27 13-Sep-11 1.23 25.2 2787
Well 9 11-Nov-10 1.1 54.4 1194 Well 28 12-Aug-11 1.02 8.5 2779
Well 10 19-Jun-09 1.1 39.1 1178 Well 29 10-Mar-11 1.1 9.9 2781
Well 11 4-Nov-12 1.3 32.2 1659 Well 30 9-Aug-12 1.4 13.6 2787
Well 12 26-Feb-11 1.22 29.1 1283 Well 31 4-Sep-10 1.4 23.9 2549
Well 13 13-Sep-11 1.25 42.4 2476 Well 32 26-Jan-13 1.5 5.3 2778
Well 14 26-Nov-12 1.11 30.04 1165 Well 33 7-Dec-11 1.5 19.6 2697
Well 15 20-May-11 1.8 20.9 1482 Well 34 13-Oct-10 1.45 14.2 2789
Well 16 22-Dec-12 1.35 18.86 2413 Well 35 11-Jan-12 1.2 6.5 2789
Well 17 11-Nov-12 1.7 26.4 1325
Well 18 11-Aug-12 1.42-1.55 25.1 2536
Well 19 23-Jul-12 1.2 10.6 2312
WHP: Wellhead pressure; MF: Mass Flow; H: Enthalpy
TABLE 3: Summary of simulated downhole data using software HOLA
Type
No of
wells
Range of
temperature
( o C)
Range of fluid
velocity
(m/s)
Two-phase well 19 188 – 288 3.5 - 110
Dry well 16 184 – 236 13 - 98
The software WATCH simulated the fluid chemistry concentrations of pH, Cl, SO 4 , H 2 S, CO 2 and H 2
in the geothermal brine at downhole conditions. The Icelandic Water Chemistry group presented the
software WATCH as a chemical speciation program for UNU fellows in 1993. It interprets the chemical
composition of geothermal fluids. The program reads the chemical analyses of water, gas and steam
condensate samples, collected at the surface, and computes the chemical composition of downhole or
aquifer fluids. This includes the pH, aqueous speciation, partial pressures of gases, redox potentials,
and activity products for mineral dissolution reactions (Arason et al., 2003). Figures 3 and 4 shows the
water and gas chemistry data of each well surveyed, while Table 4 summarizes the simulated downhole
results given by WATCH.
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