Gypsum scaling in Solution Mining Wells

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Gypsum Scaling in Solution Mining Wells: Detection,
Geochemical Understanding and Mitigation Options
Arnaud Réveillère, Patrick de Laguérie, Rémi Gruget, Thomas Nancy, Louis Guénel,
Géostock, France.
SMRI Spring 2015 Technical Conference
27 – 28 April 2015
Rochester, New York, USA
Solution Mining Research Institute Spring 2015 Technical Conference
Rochester, New York, USA, 27 – 28 April 2015
GYPSUM SCALING IN SOLUTION MINING WELLS: DETECTION, GEOCHEMICAL
UNDERSTANDING AND MITIGATION OPTIONS
Arnaud Réveillère, Patrick de Laguérie, Rémi Gruget, Thomas Nancy, Louis Guénel.
Géostock, France
Abstract
In January 2013, 1.1 mm thick Gypsum scale deposits on the leaching strings were responsible for a 2
months delay and additional costs in the leaching operations for the creation of a new gas storage cavern
in Manosque, France. This delay was due to reduced flow rate and difficulties for pulling out a leaching
string during a work-over. In May 2013, evidences of thicker scale deposits were identified on the same
cavern, leading to the review of the options available for mitigating the risks of reduced leaching rate and
being stuck during a work-over.
The present article presents methods for the early detection of the deposits, reviews the options for
mitigating existing scale deposits and details the geochemical understanding of the Gypsum scale
formation and ways to prevent it.
Early detection of the gypsum deposits is based on deducing the scale thickness from the continuous
pressure and brine density monitoring and the resulting pressure loss computation. Frequent
geochemical sampling and analysis did not prove useful for detecting the scaling formation.
Mitigating options can be grouped in mechanical action in the well or at the surface and chemical
dissolution. In the well, the increase of pulling strength (possibly using a jack-up rig) and the lubrication of
the deposits have been applied successfully, whereas in-situ removal using a scrapper or a coiled tubing
jet have been considered. At surface, “hammer” cleaning on the outside of the casings and high pressure
jet cleaning for the inside has been implemented. Laboratory analyses of scale dissolution by water,
different brines and 4 chemical products available on the market suggest that the equilibrium
concentration could be reached during in-situ circulation of water, which is more applicable than brine or
chemical product based options.
The presence of Anhydrite and Langbeinite in the leached geological layer and the solubility increase of
Gypsum with pressure and temperature are the main cause for scale deposits. Anhydrite and Langbeinite
are dissolving into the cavern brine. From a Gyspum-saturated brine in cavern conditions, a
supersaturation that deposits gypsum is obtained upon release of pressure when the brine is flowing out.
This analysis is supported by the equilibrium results from the Pitzer model and database implemented in
PHREEQC. In order to avoid this unwelcome Gypsum precipitation, implementing high leaching flow
rates, avoiding stops in operation and minimizing injection in the sump are recommended. If not sufficient,
lowering the concentration in the brine annulus can be achieved through casing perforation.
In June 2013, increasing the pulling strength was the first option implemented option and proved
successful, revealing up to 8 mm thick scale deposits (4.5 mm in average) and 23 tons of Gypsum scale
material on the leaching strings. In parallel, other options were ready to be implemented in order to avoid
major inconvenience for the project.
Key words: Geochemistry, Manosque, Gypsum, scale, PHREEQC, Pitzer, solution mining.
1
1. Introduction
The underground gas storage complex of Geomethane GIE (Southern French Alps) has a capacity of
3
nearly 300 Mnm working gas. It came on stream in the early 1990s and encompasses currently 7 salt
leached caverns, plus 2 caverns under leaching (GA and GB). Each salt-leached cavern, located at a
depth between 900 and 1500 meters, is up to 400 meters height and has an average geometrical volume
3
of 370 000 m . The caverns are located in a saliferous diapiric structure, which is also hosting the nearby
liquid hydrocarbons storage of Geosel-Manosque, which comprises 28 salt leached caverns located
between 300m and 500m deep. This site is further presented in Colin and de Laguérie, 1990; de Laguérie
and Durup, 1994, de Laguérie and Cambon, 2010.
Geomethane GIE is equally owned by Geosud (subsidiary of Total, Geostock and Ineos) and Storengy
(GDF-Suez). Storengy sells storage services to third parties and operates the site. Geostock delivers the
technical (cf. Karimi et al., 2014.) and administrative support services, and is responsible for the leaching
of GA and GB. The location of the caverns is illustrated in Figure 2.
Figure 1. Location of the GA and GB caverns under leaching, and the existing Géomethane gas caverns
that are in gas operation since the early 90s.
Created for the purpose of gas storage, GA and GB are located in the deepest area of the salt structure.
The solution mining of these caverns started in 2012 and is still on-going. Solution mining is performed
using a single deviated well equipped with 2 leaching strings: a 9’’5/8 outer string and a 5”1/2 inner string,
as depicted in Figure 2.
2
Figure 2. Schematics of a solution mining well in direct mode with scale deposits in the brine outlet. Note
that the well may be deviated, in which case the inner string lies on the outer string.
Fresh water is pumped into the formation through the 5’’1/2 inner leaching string. As the salt wall
dissolves, brine forms and a void or cavern develops in the salt deposit. The brine (consisting of water
with dissolved Halite and traces of other evaporitic minerals) flows out of the cavity through the 9’’5/8 x
5’’1/2 annulus. During the first 2 leaching stages on GB, while the cavern was leached in direct mode at a
3
flow rate of 50 to 100 m /h, and had the inner injection string covered by the insoluble minerals deposited
in the sump, evidences of scale deposits in the 9’’5/8 x 5’’1/2 annulus were observed. At a later stage this
could be attributed to Gypsum precipitation.
Such scale deposits raise two industrial risks for the project:
-
being stuck when trying to pull out the inner leaching string during a leaching string move
(latter called “work-over”), due to the increased friction forces between the rough scalecovered walls of the casings in the annulus.
3
-
increase of pressure loss in the brine outlet, entailing reduction of the leaching flow rate and
higher leaching duration and costs.
Problems related to Gypsum scale were documented twice during the leaching of the 35 caverns of
Géosel and Géomethane conducted between 1968 and 1978:
- On well EW (Géosel) in 1975, a 120 tons tension had to be applied for pulling out the 58 tons inner
string, revealing casings covered with 1.5 cm thick Gypsum scale.
- On well EO (Géosel) in 1975, a 100 tons tension had to be applied for pulling out the 60 tons inner
casing string, revealing casings covered with Gypsum scale.
No treatment had to be implemented at that time. Relatively to the above-mentioned two risks, we note
that:
- the risk of being stuck due to increased friction forces are limited by the fact that these 2 wells are
vertical and relatively shallow (less than 800m).
- the consequences of the reduced leaching pace were less stringent due to the low leaching rates that
were implemented at that time.
In 2010-2011, during the leaching of TA, a new relatively shallow liquid storage cavern (last cemented
casing at 650 m bgl), the gypsum scale phenomena has been observed again. The industrial risks were
limited since the cavern was leached using two wells (cf. Gruget,. 2013):
- there is no need for “inner leaching string” pulling out. The only issue related to the gypsum deposit is
the weight increase of the strings, which is manageable,
- the relatively shallow cavern location and the large diameters of the casings left enough margin with
respect to pressure loss increase.
Such phenomenon is a problem also encountered by oil wells producing from reservoirs containing
calcium sulfate (see e.g. Fulford, 1968), and is documented in the FRIMA project in Netherlands, which is
mining salt at depths down to 2600m. As detailed in Hellberg and al., 2005, shortly after starting direct
leaching with seawater, the leaching operation was interrupted due to a blockage created by Gypsum
scale in the production annulus. At that time hot fresh water was pumped down the annulus and “had the
effect of a mechanical and chemical cleaning of the tubing walls”.
GB well is deviated (30° inclination on 1 km length) and reaches a Total Depth of 1830 m MD (1745 m
TVD). There is no centralizer separating the two leaching strings, and the inner casing is therefore lying
on the outer casing in the deviated section. The friction forces are therefore particularly high in this long
deviated well section, and possibly the two casings are bonded by the scale deposits.
Moreover, Manosque salt is composed of 15% to 20% insoluble minerals embedded in the halite. During
the first leaching stage, these insoluble minerals are released from the halite and accumulate in the
sump, where they rapidly cover the inner leaching string. This makes reverse leaching impossible since
the sump would be sucked up into the inner casing which would cause an immediate blockage. A
significant water injection in the annulus is therefore not applicable in GB well in its leaching configuration.
This unwelcome Gypsum precipitation on GB well materialized therefore as a serious and costly problem,
more critical than previously situations experienced since the risk of being stuck during the was
increased, as a result of the deep deviated well, and the possibilities to deal with it easily were reduced as
a result of the impossible reverse leaching.
The evidences of scale deposits in GB well during the first and then the second leaching stages are
presented in section 2. The review of the available options for mitigating the risks of reduced leaching rate
and stuck casing are presented in section 3. The geochemical understanding of the conditions that led to
this situation and the preventive options available are discussed in section 4.
4
2. History and detection of scale deposits in well GB
A continuous monitoring of leaching parameters is performed during GB leaching. Most notably, the well
pressures, flow rates and the brine density are continuously monitored and analyzed daily. This
monitoring aims at the early detection and understanding of potential deviations from the expected
behavior. Water and brine sampling and composition analysis is conducted every 15 days, but this did not
prove useful to detect or to monitor the evolution of the Gypsum scale phenomena.
GB leaching started in October 1, 2012. The first signs of increasing pressure loss in the brine outlet
annulus appeared in late November, 2012, and the phenomena kept increasing until the leaching stop on
December 5.
Several attempts of pulling out the 35 tons inner leaching string proved unsuccessful even when applying
the maximum pulling strength of the work-over rig (i.e. 108 tons). At that point lubricant oil was injected in
the annulus, and a pulling strength of 108 tons lead to successfully pulling out the inner string, revealing a
1.1 mm thick Gypsum scale on both the inside of the outer casing and the outside of the inner string. The
latter was removed at surface, while the former was left in place in the well.
The string positions were then adjusted for the leaching of stage 2, which started on February 12 in direct
mode. The inner casing was covered by insoluble material approximately 1 month after.
On mid-March, 2013, the pressure loss monitoring revealed a progressively increasing scale deposit (Fig.
3) which had a significant impact on the leaching rate. For instance, with the same water injection
3
pressure (44 bar) and brine outlet density (1.182 Sp.Gr.), a 90 m /h flow-rate was achieved on
3
13/04/2013, while only 72 m /h were achieved on 06/06/2013. The only possible reason for such a strong
flow rate decrease is an increased pressure loss related to thick scale deposits. At this stage it was
decided to stop leaching operation and to pull out the leaching string with the work over rig.
Figure 3. Estimation and measurements of the Gypsum scale deposits thickness and mass.
5
Knowing that at the end of stage 1, the rig maximum pulling strength (108 tons) was nearly unable to
POOH the 35 tons inner leaching string with only 1.1 mm scale, the risk of not being able to POOH that
string at the end of stage 2, with much thicker scale, was deemed very high.
The easiest option, i.e. switching from direct to reverse leaching mode, was impossible to achieve since
the injection was taking place in the sump. A set of mitigation options were therefore reviewed, as
presented in the following section 3.
Ultimately, the 35 tons leaching string could be POOH by applying a pulling strength of 105 tons, and
revealed a 4.55 mm thick Gypsum deposit in average, which corresponds to 23 tons of Gypsum scale. In
case of failure of this primary option, back up options were ready to be implemented in order to avoid
major inconvenience for the project.
Such gypsum scale deposits did not happen again so far (in March 2015), even if the cavern leaching has
continued, partly injecting in the sump.
Figure 4. Scale sample taken from the exterior of the inner string in different view angles: face (left), tilted
(center) and profil cut (right). The scale is in cm. The white tipping visible on the left 2 pictures
are crushed crystals, which happened very probably when pulling out the casings and does
not correspond to the in situ conditions. The tilted growth of the gypsum crystal, visible on the
right figure, is related to upwards flowing in the brine stream.
6
Figure 5. Scale thickness measured on the inner 5’’1/2 string and in the outer 9’’5/8 string. The average
scale thickness on the 5’’1/2 is 4.55 mm, and it is assume to be the same in the 9’’5/8.
Thermo analytic investigations on these scale samples were conducted by IBZ-Salzchemie and revealed
with no doubt the signature of Gypsum. Photometric analysis of the dissolved samples showed that iron
made up around 0.05 %wt of the scale, explaining its brown color. Small inclusions of NaCl, CaSO4 or
MgSO4 crystals are also possible.
3-Available risk treatment options
While monitoring the thickness of the scale in the brine annulus (cf. Fig. 3), the treatment options
available to manage the risks were reviewed as presented in Table 1. Some options did not necessarily
cover both risks: for instance, cleaning out the scale at surface helps reducing pressure loss for future
leaching but does not help pulling out leaching strings.
Table 1. Risk management options successfully (green), unsuccessfully applied (red) or
reviewed (all)
Risk
Stuck 5"½ string during a workover
-
Risk
treatment
options
Increase pulling strength: bigger
rig, hydraulic jacks
Lubricant
Flow rate loss
-
- Outer wall “hammer” cleaning
- Inner wall “Kärcher” cleaning
-
- In-situ dissolution with chemicals
- In-situ dissolution with water or brine
These options are detailed in the followings.
7
Surface:
In the well: scrapper, coiled tubing and jet
3.1. Increased traction during work-overs
This is the privileged solution that has proven successful in solving problems related to scale deposits
experienced during the leaching of the Géosel and Géométhane caverns. Increasing the pulling strength
is possible as long as the maximum tension of the casings is not reached, and is easy to implement as
long as the rig maximum hook load is not reached.
In GB case, the maximum tension of the casings is 161 tons. The resident work-over rig has a maximum
pulling strength of 108 tons (90% of maximum rig capacity, 120 tons), which has been successful for
pulling out the leaching strings so far, as detailed in the introduction.
However, these successes were obtained with a very thin margin. On GB well, pulling out the 35 tons
st
inner string required 108 tons traction during the 1 work-over, i.e. maximum rig hookload and 105 tons
nd
during the 2 .
Hydraulic jacks or bringing a bigger rig are then options available for increasing the pulling strength.
3.2. Lubricant injection in the annulus
During the first work-over on GB, several attempts for pulling out the string at 108 tons were
3
unsuccessful. After the injection of 45 m (the annulus volume) of lubricant in the annulus, the string was
finally successfully POOH at 108 tons. This pulling strength then immediately went down to 50 tons, an
acceptable weight for the column. The preceding attempts however may also contributed by breaking part
of the scale bounding the casings, which cuttings then stayed underground, and/or crushing part of the
scale roughness.
The POOH revealed 1.1 mm thick deposits in average, corresponding to 2 tons of Gypsum scale material
on the leaching strings, and an additional weight of 1 “ton”, taking into account the brine buoyancy. The
additional weight of the scale is therefore not significant compared to the required tension increase: 108
tons were necessary to lift the 35 tons string + 1 ton scale material. The main explanation to the stuck
casing therefore is the friction forces increase, which can be partly balanced by the lubricant injection.
3.3. “Hammer” cleaning of the outer casing walls of the 5’’1/2 casing string during POOH
After both work-overs on GB well, removing the scale on the casings was conducted during POOH on the
rig and/or on the tally. Tons of Gypsum were taken out and disposed, as presented on figure 6.
8
Figure 6. 5’’1/2 tally after surface “hammer” cleaning of the outside wall of the inner leaching string. GB
well at the end of the 2nd work-over, Manosque, France, 04/07/2013
3.4. “Kärcher” cleaning of the inner casing walls at surface
High pressure (800 bar) Kärcher enables to take out the scale inside the casings. It was successfully
applied on TA well as presented on figure 7.
Figure 7. Kärcher cleaning on TA well. Manosque, France, 27/10/2010
9
3.5. Scrapper in the well
On TA well in 2010, prior to pulling out the leaching string for Kärcher-cleaning the inner wall (cf. 2.4.), a
scrapper tool was used to remove the deposits. The tool was immediately eroded and that solution did
not prove effective for such abrasive scale.
Figure 8. Eroded scrapper after trying an in situ cleaning on TA. Manosque, France, 2010
3.6. Coiled tubing high pressure jet cleaning in the well
The possibility of calling on a coiled tubing high pressure jet operation was reviewed and its cost
evaluated but was not tested. The mechanical action is similar to that of the Kärcher cleaning presented
in section 3.4, and so would probably be the effectiveness unless the few differences undermine it. Most
notably, the cuttings would be much more difficult to take away with from the jet as compared to the
surface situation, where each 10 m long casing is treated independently in air. An experience of such
1
coiled tubing scrapping of a heavily corroded geothermal well had the issue of producing an upwards
vertical annular speed to lift the cuttings efficiently up to the surface. However in our case the cuttings
may well also fall in the cavern without inconvenience.
3.7. In-situ dissolution using chemical products
As mentioned in the introduction, reverse leaching was not possible in the well configuration, but injection
of several batches in the annulus was possible taking advantage of :
3
3
- the cavern compressibility, which enables to inject 60 m , i.e. more than the 40 m brine annulus, per
cycle of compression and decompression;
3
- the saturation of the non-saturated cavern, which would have enable to inject 590 m , i.e. 15 times the
brine volume in the annulus, once.
1
Personnal communication, Eric Lasnes, CFG-Services
10
Technical advices and products samples were offered by several chemical and oil and gas service
companies. IBZ-Salzchemie was subcontracted for conducting dissolution tests using 4 commercial
products samples offered by these companies. Several products concentrations were tested, 125 g/L was
found as the global optimum and is presented in table 2 below in order to allow for comparison (except for
product C, that has only been tested pure), even if some products may have performed slightly better at
different concentrations.
The experimental protocol was to put scale samples taken at the end of GB first work-over in a solution of
mixed product, water and NaCl, at pH=11, for 24h without stirring. Such batch reaction aimed at
reproducing a possible injection in GB annulus.
Table 2: Dissolved Gypsum after 24h in static conditions.
In water
Solution
Temp.
Dissolved Gypsum
(g/l)
In water with 150 g/L NaCl
Equivalent scale
removal in GB
annulus (mm)
Dissolved Gypsum
(g/l)
Equivalent scale
removal in GB
annulus (mm)
Product A
20 C
22.49
0.21
27.8
0.25
125 g/L
50 C
25.39
0.23
24.3
0.22
Product B
20 C
9.8
0.09
13.6
0.12
125 g/L
50 C
17.2
0.16
18.2
0.17
20 C
18.03
0.16
16.6
0.15
50 C
31.27
0.29
29.22
0.27
Product D
20 C
10.5
0.10
17.3
0.16
125 g/L
50 C
10.4
0.09
17.4
0.16
Water
20°C
0.15
0.00
1.1
0.01
Product
pure
C
Dissolution tests were also conducted over time, as presented in figure 9.
Figure 9. Dissolved Gypsum over time with product A.
11
Products A, B and C were mostly based on chelating agents, such as EDTA, which have the ability to
2+
"sequester" metal ions, and therefore take part of the Ca out of the solution, which in turn tends to
dissolve Gypsum in order to re-establish the equilibrium. Product D is an acid.
These tests suggest the following:
-
24h waiting time was necessary for all chelating agents. The saturation is reached at that
point, waiting longer does not bring major improvement in terms of Gypsum dissolution.
-
Increasing the concentration of the product beyond 125 g/L mostly influences the duration,
and not the final Gypsum dissolution.
-
Besides product D, adding NaCl does not have a significant impact.
Depending on the product, the effect of temperature increase ranges from null to doubling
the efficiency of the Gypsum removal.
The 50°C conditions cannot be maintained during 24h in a static well, and a 20°C temperature condition
is more representative of actual well conditions. In the best case, one batch of the most efficient product
might therefore dissolve some 0.25 mm of Gypsum. Dissolving the expected 4.5 mm with these products
would require approximately 20 successive batches.
The mild effectiveness of such solutions for the extreme scale deposit we expected, the safety aspects of
the operation and the environmental issues raised by the fact that part of the product would be mixed with
brine, which is ultimately disposed of in the sea, contributed to this risk management option being
discarded.
3.8. Dissolution with water or brine
Contrarily to the above-mentioned chemical products, brine or water injections does raise no major
safety, cost or environmental issue, but has a lower effectiveness. It can be implemented by single
batches in the annulus (as presented in section 3.7) or by cutting the 5’’1/2 string above the sump, which
enables to carry out reverse leaching.
In the following, the investigation of the Gypsum dissolution capacity of the solutions is primarily based on
batch reactions simulated using the computer program PHREEQC (Parkhurst and Appelo, 1999, 2013).
The ion-interaction aqueous model used is the Pitzer model (Pitzer, 1973) implemented in PHREEQCv3.
According to Parkhurst and Appelo, 2013, “The Pitzer aqueous model can be used for high-salinity
waters”.
Gypsum is more soluble in presence of NaCl, the maximum being reached with a 150g/l NaCl solution. It
is therefore a priori interesting to use NaCl solutions rather than water, which can be obtained by
dissolving pure salt crystals in the available water, or by mixing this water with brine readily available on
site. Using available brines has an economical and operational advantage, but these solutions contain
Na, Cl and other dissolved elements as well, which can affect their effectiveness for Gypsum removal.
The composition of leaching water and of different brine types available on site (GA and TB caverns were
under leaching operation and produced non-saturated brine, PS3 is an existing cavern with saturated
brine) are detailed below.
12
Table 3: Composition of the brines and water available on site.
TB
Na (g/l)
K (g/l)
Ca (g/l)
Mg (g/l)
Cl (g/l)
SO4 (g/l)
GA
106.45
2.85
0.68
1.56
162.28
11.04
PS3
94.9
2.3
1.0
1.3
143.9
11.1
116.9
3.7
0.6
2.1
181.2
13.0
Leaching
water
0.051
0.001
0.110
0.005
0.320
0.130
GB
95.2
2.1
1.0
1.1
143.7
11.0
The following is obtained. The first interesting result is that all three brines are Gypsum-saturated. GA and
PS3 brines are even found slightly over-saturated regarding to Gypsum.
Figure 10. Gypsum removal capacity when mixing leaching water with NaCl crystals or available brines
(TB, PS3, GA). Batch reactions at 20°C and 1 bar.
These results indicate the equilibrium situation. In order to evaluate the kinetics, and therefore the
chances to reach that equilibrium, IBZ-Salzchemie conducted experiments in static (as described by the
protocol in section 3.7) and dynamic conditions (with stirring, the duration and Reynolds number being
3
adapted to reproduce a 100 m /h flow in GB well). The results presented in figure 11 suggest that the
dynamic experiments enable to reach the equilibrium.
13
Figure 11. Dissolved gypsum obtained experimentally in static and dynamic conditions.
From these results and on the basis of the operational costs of setting up the measures, (relatively high
cost of the chelating agents, operating costs) it was concluded that the most interesting option was to cut
the 5’’1/2 casing and to carry out reverse leaching with water. The expected Gypsum removal was 0.3
3
3
mm/24h at 30m /h, which would enable to remove the scale in 15 days. Using 30 m /h instead of 100
3
m /h is a margin considered for upscaling the laboratory results to the field.
4. Interpretation of the deposits and preventive measures
The precipitation of Gypsum is driven by the over-saturation of the brine relatively to the Gypsum
equilibrium:
2+
Ca
(aq)
2(aq)
+ SO4
+ 2H2O ↔ CaSO4*2H2O(s)
This equilibrium is influenced by several parameters, including:
-
-
-
The temperature. The Gypsum crystal dissolution requires energy, and its solubility increases
with temperature. The cooling of the ascending brine can therefore cause Gypsum scale
deposit. The enthalpy of this reaction is -0.109 kcal/mol in the PHREEQC computations.
The Pressure. Experiments show that there is a net loss of volume when Gypsum is
dissolved in water (Fulford, 1967). Pressure decrease therefore induces a lowering of
Gypsum solubility and Gypsum scale deposit phenomena.
The NaCl concentration and the presence of other components in the solution.
Moreover, several observations can help discriminating which are the dominant factors displacing this
equilibrium:
-
-
-
No scale deposits are noticed to occur in the surface pipelines. If the brine composition was
the only explanation (Gypsum supersaturation due to e.g. Anhydrite dissolution), the scale
deposition that takes place in the well would be unlikely to stop abruptly at the wellhead. The
main condition that is drastically modified downflow after the wellhead is that there is almost
no more pressure drop and temperature variation in the surface piping system.
Scale deposit also happened on the inner string below the outer string shoe. In this region,
the leaching string is hanging in the cavern. Brine is flowing upwards at very low velocities,
the hydrostatic pressure gradient is present, but the temperature can be reasonably assumed
constant.
GA well is leached in parallel with GB, injecting the same fresh water to leach in the same
salt structure at similar depth and according to the same leaching program. The pressure and
14
temperature decreases while flowing up are similar. The composition of the brine might not
be exactly the same due to the diffuse insoluble rocks that are present on site. No sign of
Gypsum was observed on GA.
4.1. Influence of dissolved minerals and brine composition
The geological structure in which GB is being developed is rich in Halite, and also Anhydrite and
Langbeinite, as shown by the dissolution test made on several cores of the TB1 well (from the same salt
formation). This explains the high sulfate content found in the brine composition analyses.
Table 4: Mineralogical interpretation of salt cores dissolution tests on TB1.
Mass fraction (%)
Core #1
Core #2
Core #3
Core #4
1
0.7
3.4
4.5
Langbeinite K2Mg2(SO4)3
2.2
2.6
4.1
2.5
Halite
NaCl
96.8
96.7
92.6
Quartz
SiO4
Anhydrite
Chlorite
Illite
Core #6
Average
0.8
2.4
2.4
3.5
2.9
93
93.7
95.7
94.8
traces
traces
traces
traces
traces
traces
traces
traces
CaSO4
Traces
traces
traces
Core #5
3.9
traces
The presence of Anhydrite is confirmed by the ELAN interpretation of the logs performed at the end of GB
well drilling. In the layer leached in stages 1 and 2, the estimated volumetric fractions are 79% Halite,
16% Anhydrite, 4% Langbeinite and 1% Illite.
In the ternary system NaCl-CaSO4-H2O, at equilibrium conditions, only Gypsum (CaSO4*2H2O) or
Anhydrite (CaSO4) can coexist with the aqueous solution, depending on the temperature, pressure and
NaCl concentration (cf. for instance Blount and Dickson, 1973).
The influence of NaCl is visible in Figure 10 (green curve), showing that gypsum solubility reaches a
maximum around 150 g/L.
The solubility of both minerals also increases with pressure, this effect being enhanced at low
temperature.
The influence of temperature is similar for all pressures and NaCl concentrations: Anhydrite is the stable
solid phase at high temperature; Gypsum is the one at lower temperature, as presented in figure 12 for
NaCl concentrations of 0 and 150 g/L and 188 bar (GB cavern pressure).
15
Figure 12. Gypsum and Anhydrite solubility at 0 and 150 g/l of NaCl, at 188bar. For each of the 2 NaCl
concentrations, the less soluble mineral is the stable one.
Figure 12 also suggests that the transition temperature separating the most stable solid is not the same
for both concentrations. The evolution of this transition temperature against NaCl concentration is
presented in figure 13, together with the GB cavern conditions when known (i.e. during logs).
Figure 13. Most stable solid in parallel with GB cavern conditions during leaching phase 1 (01/10/2012 to
05/12/2012), phase 2 (from 10/02/2013) or work-over 1 (in-between, in which case the brine
concentration is estimated and not measured).
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Therefore, GB cavern has remained in conditions where the stable solid is Gypsum while leaching, which
explains the dissolution of anhydrite (19% of the layer according to the log) that is providing calcium and
sulfate ions to the solution, and possibly Gypsum precipitation in the cavern. The start of the scale deposit
behavior cannot be directly related to the crossing of this transition line, since it did not happen during
leaching, but during stops while brine was getting warmer and more saturated.
Lastly, the brine composition does not only depend on the equilibrium conditions in the cavern. As
summarized in Hellberg et al., 2005, the composition is influenced by the contact time between the
mineral phases and the solution. The ions that originate from fast dissolution minerals are found
preferentially. When brine is saturated for a longer time, the concentration of minerals that dissolve very
slowly increases significantly.
It is also influenced by the contact surface with these minerals. The sump is essentially made of the low
soluble minerals, initially embedded in halite, that have deposited when the halite matrix was dissolved.
Injecting in the sump therefore strongly increases the contact surface with these minerals, and enhances
their dissolution.
4.2. Effects of pressure and temperature on the flowing brine
In GB well conditions, based on the 05/03/2013 temperature and pressure log, the temperature cools
down from 25.7 °C in the cavern to 9.9°C at surface, whereas the pressure drops from 188 to 4 bar. The
evolution of Gypsum solubility is presented in Figure 12, using GB brine composition from Table 3.
Figure 14. Gypsum solubility in GB well pressure and temperature conditions (red). The two other plots
assume unrealistically that the pressure (blue) or temperature (green) do not vary.
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Therefore, from a Gypsum-saturated brine at the entrance of the annulus, a supersaturation would be
obtained and Gypsum would precipitate. The analysis of GB brine (Table 3) shows that it is Gypsumsaturated (at 20°C and 1 bar, the condition in which brine samples have been taken and kept), similarly to
other brines of this site.
These results and observations suggest that the pressure drop and temperature decrease experienced
by the upwards flowing brine are the main factors triggering the scale deposits. It is supported by the fact
that there is no trace of scale in the surface piping, once the temperature and pressure change is
relatively negligible. The temperature drop is particularly true during the first leaching months: in GB case,
the 20°C temperature decrease at the leaching start progressively decreased to approximately 5 to 10°C
one year later, once the salt rock has been progressively cooled down by the cold water injection and the
effect of the endothermal Halite dissolution. The pressure drop has the same order of magnitude during
the whole leaching duration, and slightly increases over time with density and flow rate increase.
4.3. Scale initiation and scale growth
According to Hellberg et al., 2005, the scale initiation may be induced by small amounts of calcium
sulfates that act as seeds. The scale growth is then influenced by the Gypsum crystal growth and capture
of flowing particules by the Gypsum matrix.
Edinger, 1973, indicates that the relative crystal size is a function of the supersaturation, bigger Gypsum
crystals being observed at lower supersaturations. The scale samples (cf. Figure 4) suggests relatively
large crystals (a few millimeters), which therefore tends to indicate that the precipitation rate is high
enough for maintaining a low supersaturation, even while the solubility is decreasing as presented in
Figure 14.
The growing Gypsum matrix can trap particles that are transported in the brine, either originating from the
insoluble minerals (e.g. Anhydrite) or from other precipitated Gypsum crystals that were in suspension
with the brine. In the Gypsum scale deposit issue faced by the FRIMA project, the XRD, IR-spectrometry
and optical microscope analyses that were carried out enabled to identify Anhydrite in the Gypsum matrix,
which was not precipitated but an insoluble transported there by the brine (Hellberg et al., 2005). Higher
flow velocity is deemed likely to decrease the influence this trapping mechanism.
4.4. Preventive measures
To prevent further crystallization on the casing walls, it is necessary to discriminate between unmodifiable
parameters and modifiable ones. Among the previously-exposed conditions necessary for gypsum
precipitation, pressure drop while brine is produced back to the surface is inevitable, unless stopping the
leaching process. Reducing the temperature cooling is hardly feasible at reasonable cost for such high
st
flow rates. Preventive measures can therefore only focus on the 1 condition: the solution composition.
The brine composition is primarily influenced by the minerals present in the leached layers, which is
imposed by the local geological settings. The brine composition, however, also depends on the kinetics of
the various dissolution processes. It is therefore recommended to leach as little as possible in the
insoluble layers, and to leach with high flow rates while avoiding leaching stops. If not sufficient, there is
an additional way of preventing the scale formation: lowering the concentration at the entrance of the
brine annulus. This can be done by perforating the inner leaching string at the entrance of the brine
annulus. In GB well conditions, the Gypsum equilibrium drops with pressure, from 4.4 g/kgw in the cavern
to 3.9 g/kgw at the surface. Diverting 9% of the water flow directly from the inner leaching string to the
annulus would lower the brine concentration below 3.9 g/kgw, and thus make the brine under-saturated
regarding to Gypsum up to the top of the well.
The existing scale also is an excellent nuclei for initiating (or continuing) the crystal growth. Scale should
therefore be removed when possible.
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5. Conclusion and discussion
As part of an expansion project of the Géométhane natural gas storage site, the GB deviated well under
leaching experienced unwelcome Gypsum precipitation, which induced risks of reduced flow-rate and of
nd
being stuck during casing moves (“work-over”). Prior to the 2 work-over, this latter risk was particularly
high, knowing that reverse leaching was impossible and considering the hookload limitations of the
st
resident rig: during the 1 work-over, it had been unable to pull out the 35 tons inner leaching string
several times, before finally succeeding at 108 tons, its maximum pulling strength. This revealed 1.1 mm
thick scale deposits. The expected scale thickness prior to the second work-over was 4.5 mm.
This risk was managed by conducting the following actions:
- the continuous well monitoring, which enabled an accurate estimation of the scale thickness present in
the well annulus, and allowed the assessment and the monitoring of the risk evolution (the scale could
have disappeared)
- the analysis and the evaluation (effectiveness, cost and applicability) of the available options for treating
this risk.
Regarding the operational problem faced on GB, i.e. having an estimated scale thickness of 4.5 mm in
both sides of the annulus prior to the work-over, the following decisions were made:
1. Continue the monitoring of the scale thickness, in order to detect if it starts decreasing. In that
case adapt the leaching program for continuing leaching until the scale disappears, if possible.
Stop if it starts increasing again.
2. If not disappearing, try pulling out the leaching string with the resident work-over rig up to its
maximum pulling strength of 108 tons.
3. If not successful, inject lubricant in the annulus.
4. If not successful, cut the 5’’1/2 inner leaching string in order to implement reverse leaching,
injecting water in the annulus while monitoring the evolution with time of the scale thickness.
The next option in the queue was bringing hydraulic jacks.
A tension of 105 tons proved successful for pulling out the leaching string at the first attempt. Although it
is the “usual” option, this analysis gave the required confidence that the risks could be managed. It led to
continuing leaching and avoided the cost and delay of an anticipated work-over.
rd
In parallel, an analysis of the causes that led to this situation was conducted prior to start the leaching 3
stage, in order to implement preventive measures if possible. The main factors that are necessary for the
scale deposit to happen are:
-
-
The Gypsum-supersaturation in the brine outlet, which is a result of :
o The saturation of the brine relatively to Gypsum at the entrance of the annulus, as a
result of (mostly) Anhydrite dissolution.
o Pressure and temperature drop while flowing up in the well: from a saturated brine in the
cavern, a supersaturation is obtained in the well which tends to precipitate Gypsum
Conditions that favored initiation of the scale deposit on the casing walls, in order to have part
of the precipitating Gypsum stick to the casing walls and to not flow upwards in suspension
with the brine.
A combination of these factors is the likely cause of the scale deposits. Although, the identification of the
causes that initiated the whole deposition process remains unclear since some of the observations
remain unexplained, such as the increase of the deposits thickness with depth, and more importantly the
fact that GA cavern, which was leached in parallel with GB in similar geological settings and using the
same leaching design, did not experience any scale.
Among the necessary factors leading to scale deposits, the distinction should be made between those
that are unchangeable, and the changeable ones. The geological settings and the fact that brine
experiences a strong pressure drop and a temperature decrease while flowing out are a given
(unchangeable). But concentration in the brine of elements coming from minerals that dissolve slowly,
19
such as Anhydrite, can be limited by not having leaching breaks and avoiding injecting at low flow rate or
in the sump. For leaching stage 3 on GB, the leaching injection was therefore positioned in a way that it
3
3
would never be covered by the sump, and the flow rate was increased from 100 m /h to 200 m /h, which
was compatible with the leaching program. The existing deposits were cleared from the outside of the
inner string, and most of those in the outer string were cleared during the POOH and RIH of the inner
string in order to reduce the impact of this scale initiator. No evidence of scale was found again.
Acknowledgement
The authors are very thankful to Tony Appelo for reviewing and enhancing the PHREEQC input file, and
for presenting this case in some of his training courses. They are also thankful to Storengy and
Géométhane for their support on the project, and to P. Cauchois, G. Delahaye, L. Mazan and the other
people in charge of the operations in Manosque.
References
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