SPE 154455

A Successful Remoaval Inorganic Hard Scale Deposits in an Offshore

Pipeline in Gemsa Oil field, Egypt: Field Study
M.Bakr, A. Abdel Hay, SAPESCO; E.Hamdy, GEMPETCO

Copyright 2012, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE International Conference and Exhibition on Oilfield Scale held in Aberdeen, UK, 30–31 May 2012.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been
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Abstract
Water flooding is the hinge pin for Gemsa Oil Field. Water injection is supplied from shallow water supply wells.
Compatibility tests had indicated probable deposition of calcium sulphate scale on surface and subsurface production
equipment. Calcium sulphate scale has been recognized to be a major operational problem. The bad consequences of scale
formation comprised the contribution to flow restriction thus resulting in oil and gas production decrease. The nature of
calcium sulphate scale is very hard and can’t be dissolved with known dissolver. Sister companies that has similar problem
were always going to the mechanical remover options.
Extensive lab and field work was conducted to determine the suitable chemicals to dissolve calcium sulphate scale.
This paper describes the development and field application of chemical treatment to remove scale in an offshore 8” production
line in Gemsa oil field. Continuous precipitation of calcium sulphate scale caused partial plugging of the pipeline. This partial
plugging created a back pressure on production wells which decreased the productivity. The field production has been
decreased to almost one tenth of the normal field production level(1).
A thorough investigation was conducted to identify the composition and location of the scale, in order to recommend a suitable
chemical to remove the scale, and to assess the effectiveness of the treatment method in the field.
Based on extensive lab studies, SAG-01 was tested and applied in production line to remove the scale efficiently, the program
was designed taking into consideration the nature of the scale.
The treatment program included three - staged process:
1. The use of an organic solvent
2. The main treatment (SAG-01).
3. The post flush stage (injection water),
The job results were outstanding, where as
1- Production increased by about 2,000 BOPD
2- Launcher pressure dropped by about 350 psi.
3- Decrease the back pressure on our producing wells.
4- Additional producing wells into production.
5- Efficient cost /Bbl.
2 SPE 154455

Introduction
Some of the Gemsa Oil Field has been water-flooded with source wells. Compatibility tests have indicated probable deposition
of scale on surface and subsurface production equipment. This case history outlines the physical and theoretical prediction for
down-hole scale deposition in Gemsa oil wells. It also describes the control of scale inhibition programs which carried out to
control down-hole scale deposition by using squeeze technique. Scale deposition on surface and subsurface oil and gas
production equipment has been recognized to be a major operational problem. Scale contributes to equipment wear and
corrosion and flow restriction, thus resulting in a decrease in oil and gas production. Experience in the oil industry has
indicated that many oil wells have suffered flow restriction because of scale deposition within the oil/gas-producing formation
matrix and the down-hole equipment, as well as scale deposits in the surface production equipment and, generally in primary,
secondary, and tertiary oil recovery phase. Oilfield scale costs are high because of drastic oil and gas production decline,
frequent pulling of down-hole equipment for replacement, re-perforation of the scaling producing intervals, reaming and re-
drilling of the plugged oil wells, stimulation of the plugged oil-bearing formation and other remedial work-over. As scale
deposits around the well-bore, the porous formation becomes plugged and may be rendered impermeable to any fluids. Many
case histories of oil well scaling by calcium carbonate, calcium sulfate, and barium sulfate have been reported(1). Problems
pertaining to oil well scaling in North sea fields have been recorded and are similar to case in the USSR where scale has
severity plugged wells, oil field scale problems have occurred as a results of water flowing in Algeria (1), South Sumatra oil
(2)
field and Egypt, where calcium and strontium sulfate scales have been found in surface and subsurface production
equipment. Water is the main substance, which is responsible for scale build up problem. In order to study the circumstances
behind the scale build up problem, this study summarizes a new methodology that can be applied to predict the scale build up
before start precipitation(1).

Methodology and Design

The prerequisite condition for the formation of scale is supersaturation of the scaling minerals in the produced or injected
water. Supersaturation of a mineral occurs when the mineral concentration in brine exceeds the equilibrium concentration.
When brine is supersaturated, salts can precipitate out of solution forming scale. Figure 1 shows a cross section of production
tubing with scale build up. The degree of supersaturation affects precipitation rates and contributes to the scale severity.
Factors leading to supersaturation are:

1. Increased mineral concentration

2. Changes in temperature, pressure or pH
3. Mixing of incompatible waters.
4. Thermodynamic (Press. &Temp.
5. Kinetics (time).
6. Hydrodynamics (agitation & velocity).
7. pH
8. Particle Size
9. Other environmental
Precipitation begins with small particles (nuclei). A certain number of molecules must agglomerate and orient them in a fixed
crystal lattice. The nucleation preferentially occurs on pre-existing mineral or metal surfaces, such as foreign crystalline
matter, corroded surfaces, welds, scratches or nicks on pipe, and sand and clay particles. Once nucleation has occurred, there
SPE 154455 3

must be adequate contact time between the supersaturated solution and the nucleating sites to form scale.
The nuclei constitute the sites for further mineral deposition to form crystals. The continual crystal growth and adherence form
scale (3).
The most common scales are carbonate and sulfate scales. Some of these scales can be removed by treatments using acid and
other solvents, or by mechanical means. The effectiveness of chemical treatments depends on the type of scale formed and
where it is deposited. It is difficult to dissolve sulfate scales and scales deposited deep in the formation far from wellbore.
Mechanical methods are effective against thick scales formed in tubulars, but cannot access scales formed in formation or
proppant pack. Mechanical methods are often more expensive since they require workover rigs or coiled tubing units.
Allowing scale to build up also means the well production will suffer until the scales are removed. Therefore, prevention of
scale deposition is the best strategy in dealing with scale problems (4).
Scale History
Gemsa field has suffered from the scale deposition problems for over four years. The problem started when the production
wells & pipelines were plugged with scale. This scale formed from the incompatibility of injection water with Nukhul
formation water, the analysis of which is shown in table 1. This problem was encountered in production line and production
wells. The scale deposition caused partial plugging in all field facilities. The launcher pressure increased above normal records
and caused back pressure on production wells.
Studies were implemented to solve this problem to predict the type of scale deposition in the system.
And many samples were collected from surface and subsurface to identify the problems and the results shown in table (2).
And after investigation, the problem was calcium sulfate scale precipitations. Therefore, the samples indicated that we have a
problem mainly with calcium sulfate scale in surface and subsurface.
Experimental
Due to the large number of commercially available scale dissolvers in the market place, a series of screening tests were
performed in order to rank the scale dissolvers that had been selected for the study. A total of three scale dissolvers (EDTA,
soda Ash and SAG-01) were provided by different companies.
1- Preparation of sample
A bulk deposit sample was soaked in hot water (90 °C) for one hour and then filtered through Whitman filter paper size 1, then
the precipitate soaked for one hour in hot (70 °C) xylene/diesel mixture and filtered then the precipitate was dried.
2- Method of treatment
The method runs according to NACE TM 0397 Screening Tests for Evaluating the Effectiveness of Gypsum Scale Removers.
A- Gypsum Scale Dissolver Screening Test
a- A number of 12 scale samples each 10.0 ±0.05 g was soaked in Na4EDTA scale dissolver for 24 hours at two
temperatures, 25°C and 50°C.
b- The samples were filtered through 0.45-μm membrane filter and dried in an oven set at 40°C.
c- The residual solids weighed along with the pre-weighed membrane filter.
d- The weight of scale lost determined to the nearest 0.01g and the efficiency of treatment calculated
B- Gypsum Scale Converter Screening Test
a- A number of 12 test bottles each of 6.0 ±0.05 g were treated by soaking in soda ash (Sodium carbonate) solution for 24
hours at two temperatures, 25°C and 50°C.
b- Using a flat-tip needle attached to a 50-mL syringe, suction off most of the clear supernatant fluid from each test bottle
without removing any of the remaining solids.
4 SPE 154455

c- The remaining solids soaked for one hour in 15 wt% HCl solution.
d- The samples were filtered through 0.45-μm membrane filter and dried in an oven set at 40°C.
e- The residual solids weighed along with the pre-weighed membrane filter.
f- The weight of scale lost determined to the nearest 0.01g and the efficiency of treatment calculated. The results showed
in table (3).
The new chemical SAG-01 get the highest efficiency (87%) after flushed with water compared to the others.
Field Application
To remove the scale efficiently, the program was designed taking the nature of the scale type in consideration.
The treatment program included three stages process:
Organic Solvent Stage:
An organic solvent (diesel & xylene) was used to remove most of the hydrocarbon material, impregnated in the scale and that
was followed by a thorough flush with injection water.
N.B:
The recommended mixing ratio is xylene: diesel (1:1).
Main Treatment (SAG-01):
Using line volume capacity (68,502 Gal) of solution of SAG-01 (24.6%) to convert the insoluble Calcium Sulphate to the
soluble form which pumped easily by water. The pipe was soaked in this solution for 24 hours after which the line was
displaced to water injection water.
The Post Flush Stage (water injection):
The post flush water injection volume was taken to remove the reaction product outside the pipeline.
Results
1. Clean the production line completely and back to original diameter.
2. 500 drum of deposits were received from the production line with safe operations.
3. Production increased by about 2,000 BOPD
4. Launcher pressure dropped by about 350 psi.
5. Decrease the back pressure on our producing wells.
6. Additional producing wells into production.
7. Efficient cost /Bbl.
Conclusions
 SAG-01 is a powerful gypsum scale dissolver than other dissolvers.
 SAG-01 can penetrate & dissolve the calcium sulphate scale which in contact with.

Recommendations

 The chemical job by SAG-01 should implemented into steps (related to the condition of the pipeline).
 Pumping diesel & xylene with sufficient amounts before and after each step to avoid the plugging.
 Implement the pigging program periodically to avoid any additional deposits.
SPE 154455 5

Nomenclature
ppm = parts per million
BWPD = barrels of water per day
ppt = pounds per thousand
Mbbl = Thousands of barrels
Bbl = barrels
bpm = barrels per minute
BOPD =barrels oil per day

References
1. Cowan, J.C., and Weintritt, DJ. : Water Formed Scale Deposits, Gulf Publishing Co., Huoston Tx., 1976 .
2. Mitchel, R.W., Grist, D.M., and Boyle, M.T.,: "Chemical Treatments Associated With North Sea Projects", J.P.T.,
(May, 1980), 904 - 912.
3. El- Hattab, M.l. : "Scale Deposition in Surface and Subsurface Production Equipment in The Gulf of Suez" SPE -
11449 Paper Presented in The Middle East Technical Conference & Exhibition Held in Manama, Bahrain, March, 14
- 17, 1983. , lP.T, (Sept., 85), 1640 - 1652.
4. Harms, S.D, Smith, J.M., and King, G.E., : "Novel Workover Treatment Proves Effective in Permian Basin:
Laboratory and Field Results", Paper SPE 17296 presented at SPE Permian Basin Oil and Gas Recovery Conference,
Held in Midland, TX., (March, 10-11,1988).
6 SPE 154455

Fig 1 - Tubular cross section showing scale build up.

Fig 2 - Relation Between Pumping Rate Vs. Pressure

Date Sample Point % Calcium Sulphate

Well SE-5A 75
6/11/2006
Pig Receiver 81.8
(pipeline)

Pig Receiver 79.3

17/11/2006
(pipeline)

8/2/2007 Well SE-5A 58.8

Table 1 The complete water analysis of the formation
Well SE-10B (L/S) 77.3 and injection water used in Gemsa Oil field.
10/3/2007
Pig Receiver(pipeline) 66

11/03/2007 Well SE-5A 78.5

Pig Receiver 71
27/7/2007
(pipeline)

Pig Receiver 89.9

30/11/2007
(pipeline)

30/1/2008 Well SE-5A(L/S) 80.2

SPE 154455 7

Mg/L
Radical
Nukhul Injection
Formation Water
+
Sodium , Na Water
60146 26040
++
Calcium , Ca 6625 1502
++
Magnesium , Mg 1145 2539
+
Potassium , K 12709 1037
++
Iron, Fe 5.47 0.30
++
Strontium , Sr 393 25
++
Barium , Ba 1.296 0.10
-
Chlorides , Cl 120104.6 50309
-
Fluorides , F 47.50 -----
-
Bromides , Br 519 -----
-
Bicarbonates , HCO3 199.8 135
--
Sulphates , SO4 685.25 7800
Total Dissolved Solids. 198208 90391
TDS
Salinity as sodium 198172.59 -----
chlorides
0 0
pH 6.6@ 29.3 C 6.86@ 20 C

Table 2 Percentage of Calcium Sulphate Scale in Surface and Downhole

Fig 3 a - Deposits From Pipeline

Fig 3b - Deposits From Pipeline