188bet zokb Sources, mechanism and prediction method of scale formation in oil production in Vietnam

Summary

This paper presents 188bet zokb typical sources and mechanism of scale formation in oil production in Vietnam. A workflow of prediction method to find scale formation during oil production was presented. 188bet zokb main sources of scale problem in oil production are incompatible mixing between different water sources and 188bet zokb change of pressure/temperature along 188bet zokb production system. A case study using 188bet zokb prediction method was also introduced in 188bet zokb article. 188bet zokb decrease of productivity index of 188bet zokb observed well can be caused by 188bet zokb dominant build-up of scale at downhole. Based on 188bet zokb mechanism of scale formation, a suitable scale control strategy was given.

Key words: Scale formation, scale prediction, water production, productivity index. 

1. Introduction

Today, 188bet zokb problem of inorganic scaling becomes more and more serious in many oil fields in Vietnam, such as Bach Ho, Te Giac Trang, Ca Ngu Vang, Rang Dong, and Su Tu Vang. Few production problems strike fear into 188bet zokb hearts of engineers 188bet zokb way scale can. Scale is an assemblage of deposits that cake perforations, casing, production tubing, valves, pumps and downhole completion equipment, thereby clogging 188bet zokb wellbore and preventing fluid flow. Scale, just like 188bet zokb scale found in home plumbing or tea kettles, can be deposited all along water paths from injectors through 188bet zokb reservoir to surface equipment. Most scales found in oil fields in Vietnam form either by direct precipitation from 188bet zokb water that occurs naturally in reservoir rocks, or as a result of produced water becoming oversaturated with scale components when two incompatible waters meet downhole. Whenever an oil or gas well produces water, or water injection is used to enhance recovery, there is 188bet zokb possibility that scale will form.

188bet zokb buildup of scale inside wellbore and production equipment (both subsurface and surface) has badly affected oil production efficiency, causing millions of dollars in damage every year. Scale can develop in 188bet zokb formation pores near 188bet zokb wellbore and reduce formation porosity and permeability. It can block 188bet zokb flow by clogging perforations or forming a thick lining in production tubing. It can also coat and damage downhole completion equipment, such as safety valves and gas- lift mandrels. 188bet zokb productivity index can fail half in just several weeks and therefore that requires serious study of scale problem to find out 188bet zokb suitable cures [1].

In this article, we present 188bet zokb mechanism and source of scale buildup during oil production. Knowing 188bet zokb conditions that lead to scaling and when and where it occurs helps in understanding how to prevent and/or remove scale to restore long-term well productivity. A scaling assessment job that has been done at an offshore oil field by 188bet zokb Vietnam Petroleum Institute (VPI) and Hoan Vu Joint Operating Company (Hoan Vu JOC) will also be presented in this paper as a case study.

2. Main mechanism of scale in oil production

188bet zokb production of water is a natural consequence of 188bet zokb production of oil and gas from subterranean reservoirs. Water that is present in these reservoirs contains dissolved salts and dissolved gases. Under certain conditions these salts precipitate and become scale deposits [1]. It is not possible to produce oil and gas for any significant time before water is also produced. 188bet zokb water/oil boundary will rise over time and channelling from water permeable zones will cause increased water to be produced. Scales are formed due to precipitation and crystal growth at 188bet zokb water surface. Scale begins to form when 188bet zokb state of any natural fluid is perturbed such that 188bet zokb solubility limit for one or more components is exceeded. Mineral solubility itself has a complicated dependence on temperature and pressure.

There are two basic mechanisms by which scales are formed in 188bet zokb petroleum reservoir [2]:

- Autoscaling

A reservoir fluid experiences changes in temperature and pressure as it is produced. If such changes take 188bet zokb fluid composition beyond 188bet zokb solubility limit for a mineral, it will precipitate as scale, this phenomenon is called autoscaling or self-scaling. Sulfate and carbonate scales can precipitate as a result of pressure changes within 188bet zokb wellbore or at any restriction downhole. Sodium chloride scale (halite) forms in a similar way from highly saline brines undergoing large temperature drops.

Another serious problem occurs when carbonate scales precipitate from produced fluids containing acid gases. Reduction in pressure during production outgases 188bet zokb fluid, which raises pH and causes scale deposition. 188bet zokb deposition of carbonate can extend from 188bet zokb near-wellbore matrix, along tubing and into surface equipment as 188bet zokb produced water continuously changes in pressure and temperature. For carbonate scales, temperature effects often work against pressure effects. For example, 188bet zokb pressure drop at 188bet zokb point of entry into 188bet zokb wellbore can lead to matrix scale. As 188bet zokb fluid progresses up 188bet zokb tubing to surface temperatures and wellhead pressure, 188bet zokb resulting temperature drop may override 188bet zokb pressure effect, reducing scale formation in 188bet zokb tubing. On 188bet zokb other hand, subsequent release of pressure from 188bet zokb wellhead to 188bet zokb surface can lead to massive deposits of scale in surface equipment and tubing.

 - Incompatible mixing

Mixing incompatible injection and formation waters can cause scale formation. Seawater is often injected into reservoirs during secondary and enhanced-recovery water flooding operations. Seawater is typically rich in SO 2- anions with concentrations often above 2,000mg/L (0.02ppg), while formation waters contain divalent cations Ca2+ and Ba2+. Fluid mixing in 188bet zokb near-wellbore matrix generally produces new fluids with combined ion concentrations that are above 188bet zokb solubility limits for sulfate minerals. Calcium sulfate (CaSO4) scale forms in limestone formations, and barium sulfate (BaSO ) and strontium sulfate (SrSO ) scales form in sandstone formations. If these scales form in 188bet zokb formation, they are difficult to remove chemically and impossible to remove mechanically. Incompatible water mixing can also occur in tubing, producing scales that are accessible to both chemical and mechanical removal.

- Forming scale

Although 188bet zokb driving force for scale formation may be a temperature or pressure change, out-gassing, a pH shift, or contact with incompatible water, many produced waters that have become oversaturated and scale-prone do not always produce scale. In order for a scale to form it must grow from solution. 188bet zokb first development within a saturated fluid is a formation of unstable clusters of atoms, a process called homogeneous nucleation. 188bet zokb atom clusters form small seed crystals triggered by local fluctuations in 188bet zokb equilibrium ion concentration in supersaturated solutions. 188bet zokb seed crystals subsequently grow by ions adsorbing onto imperfections on 188bet zokb crystal surfaces extending 188bet zokb crystal size. 188bet zokb energy for seed crystal growth is driven by a reduction in 188bet zokb surface free energy of 188bet zokb crystal, which decreases rapidly with increasing radius after a critical radius is exceeded. This implies that large crystals favour continuing crystal growth, and also implies that small seed crystals may rediscover. Thus, given a large enough degree of supersaturation, 188bet zokb formation of any seed crystal will encourage an increase in 188bet zokb growth of scale deposits. 188bet zokb seed crystal, in effect, is a catalyst for scale formation. Crystal growth also tends to initiate on a pre-existing fluid-boundary surface, a process called heterogeneous nucleation. Heterogeneous nucleation sites include surface defects such as pipe surface roughness or perforations in production liners, or even joints and seams in tubing and pipelines. A high degree of turbulence can also catalyse scale deposition. Thus, 188bet zokb accumulation of scale can occur at 188bet zokb position of 188bet zokb bubble point pressure in 188bet zokb flowing system. This explains why scale deposits rapidly build on downhole completion equipment.

Figure 1. Two basic mechanisms of scales formed in 188bet zokb petroleum reservoir [1].
 

Table 1. 188bet zokb location and composition of 188bet zokb scale deposit
(a) (b)

Figure 2. Inorganic scale formed in 188bet zokb production system, Vietnam’s oil field (a: at tubular; b: at choke valve).

- Identifying scale

Identifying 188bet zokb location and composition of 188bet zokb scale deposit is 188bet zokb first step in designing a cost-effective remediation programme (Table 1).

3. Scale control method to reduce problem cost by scale formation

188bet zokb direct cost of removing scale from one well can be as high as USD 2.5 milion, and 188bet zokb cost of deferred production even higher [3]. Just as prevention is better than cure in medical practice, keeping producing wells healthy is ultimately 188bet zokb most efficient way to produce hydrocarbons. In most cases, scale control method starts with scale prediction (forecast) and needs to be updated regularly based upon surveillance. This method will balance preventive measures and remediation (removal method). From 188bet zokb results of prediction of scale, a suitable scale control strategy will be described including prevention and remediation methods.

(a) Prevention

Choosing suitable chemical inhibition - continuous or periodic squeezes;

In most cases, scale prevention through chemical inhibition is 188bet zokb preferred method of maintaining well productivity. Inhibition technique can range from basic dilution method, to 188bet zokb most advanced and cost-effective method of threshold scale inhibitor.

(b) Remediation: Scale removal, either chemical or physical.

Scale removal techniques must be quick, non-damaging to 188bet zokb wellbore, tubing or formation environment, and effective at preventing reprecipitation. 188bet zokb best scale-removal technique depends on knowing 188bet zokb type and quantity of scale, and its physical composition or texture. A poor choice of removal method can actually promote 188bet zokb rapid recurrence of scale.

188bet zokb following workflow is presented to summarise 188bet zokb scale control method:

4. Case study using scale prediction meth- od done by VPI and Hoan Vu JOC

188bet zokb A oil field has produced oil from fractured basement reservoir with 4 produced wells, in which 1P-SCL has 188bet zokb highest production rate. However, 188bet zokb well productivity index (PI) of 1P-SCL significantly dropped from 6.2 to 3.0bpd/psi after just 2 years of first oil during a short period (several weeks).188bet zokb pressure survey is carried out in an yearly basis to determine reservoir pressure and well PI. 188bet zokb skin factor could not be estimated in 188bet zokb 1P-SCL well due to wellbore storage effects. It is essential to quantify and characterise 188bet zokb scale formation downhole in 1P-SCL, focusing on near-wellbore area where pressure drop takes place in flowing condition. Therefore, 188bet zokb best strategy in combating losses experienced due to scaling is periodical prevention of scale formation. VPI has conducted this study for Hoan Vu JOC to propose 188bet zokb suitable method with 188bet zokb scale problem of 1P-SCL in 188bet zokb A oil field.

All data relating to drilling, completion and operation phases of 188bet zokb 1P-SCL well were gathered and reviewed. Current production water of 1P-SCL was also sampled and analysed. To support for this study, many experimental tests and analyses were conducted including 188bet zokb filtration membrane tests, self-scaling tests and incompatible mixing test using HTHP (high temperature, high pressure) laboratory system. Additionally, a computational modelling has also been undertaken with 188bet zokb support from 188bet zokb latest version of ScaleChem software to evaluate 188bet zokb scale tendency and scale amount.

4.1. Production water composition

Initially, 188bet zokb reservoir pressure decreased continuously with very low water cut < 3%. 188bet zokb reservoir pressure decreased from around 7,200psia to 5,600psia after 5 years of operation. It means that there has been no aquifer support for 188bet zokb reservoir pressure. Besides, 188bet zokb composition of 1P-SCL water at 188bet zokb beginning was very rich in Cl-, then 188bet zokb Cl- content decreased right after 188bet zokb water breakthrough. 188bet zokb 1P-SCl has its production target at basement reservoir (more than 4,000mD), therefore, during 188bet zokb drilling phase, a very high density drilling mud was used. This drilling mud was CaCl2 with very high content of Cl- (200,000mg/L). Generally, in 188bet zokb basement of 188bet zokb referent oil field in 188bet zokb South East offshore Vietnam, 188bet zokb connate formation water (if available) may be immigrated from above reservoir (Oligocene) with low total dissolved solid (TDS) value (< 40,000mg/L). However, 188bet zokb TDS value of 1P-SCL production water is quite high (> 90,000mg/L). Hence, 188bet zokb production water of 1P-SCL can be considered as drilling mud losses only (at 188bet zokb initial time of production) and 188bet zokb mixture of drilling mud losses with injection water at a later phase.

Figure 3. Cl- content and water cut in historical production water composition of 1P-SCL well.

Figure 4. Scale scenario at downhole of 1P-SCL [6].

After water breakthrough occurred 188bet zokb water rate and cut increased within a few months to approximately 1,500 barrels/dayand 38% respectively. Seawaterbreakthrough, from injection well to 1P-SCL was marked also by a rapid decline in Cl- in production water (Figure 3) and approximately coincided with 188bet zokb start of water production in this well suggesting that breakthrough occurred within 188bet zokb oil-leg (Cl- is used as a conservative natural tracer [4]). Before water breakthrough, around 105,000 barrels of drilling losses (brine) were recovered. 188bet zokb remaining approximate 25,000 barrels of brine could be mixed with injection water during several months leading to 188bet zokb decrease of Cl- content in production water. 188bet zokb Cl- content sharply decreased and then kept stably at around 30,000mg/L during 15 months. By calculation, 188bet zokb mixing ratio at this period is about 95% of injection water. This ratio is acceptable if compared to 188bet zokb remaining drilling mud loss out of total production water.

4.2. Scale scenarios at 1P-SCL well

By analysis of 188bet zokb water composition of production water and 188bet zokb drilling mud loss, 188bet zokb scale may form at downhole of 1P-SCL according to 2 scenarios:

- 188bet zokb incompatible mixing between injection water and drilling mud loss at 188bet zokb time of water breakthrough. 188bet zokb contact of seawater rich in SO 2- with drilling mud loss (CaCl2) will form CaSO4, an inorganic salt with very low solubility at high pressure and high temperature [5]:

Ca2+ + SO 2- → CaSO ↓

- 188bet zokb change of pressure from reservoir to downhole. As these fluids will experience drop in pressure, CO2 leaving leading 188bet zokb formation of CaCO3:

Figure 5. Scale tendency at 188bet zokb different mixing ratio at downhole of 1P-SCL.

Table 2. Scale amount formed at downhole of 1P-SCL by ScaleChem

Figure 6. High pressure, high temperature laboratory system.

Table 3. Composition of injection water and drilling mud loss for 188bet zokb test

 4.3. Prediction of scale tendency at downhole of 1P-SCL by ScaleChem software

Desktop scale study is simulated and predicted 188bet zokb scaling tendency and severity for oil production system including auto- scaling at subsurface and surface facilities and incompatible mixing between different sources of water by ScaleChem software. 188bet zokb results of 188bet zokb scaling prediction are frequently used to determine whether scaling is likely to occur and 188bet zokb scale species likely to be formed at subsurface/surface condition.

Table 4. Dynamic data of 1P-SCL for 188bet zokb test
 

Figure 7. SEM result of solid scale collected from laboratory test.

Figure 8. XRD results of solid scale collected from laboratory test.

All analytical results obtained from laboratory tests will be used for computation and also used as input data for software.

188bet zokb possibility and severity of scale formation will be calculated and simulated using ScalingChem® software. ScalingChem® software predicts mineral scaling potentials of 80+ solids for virtually any oil and gas well and processing facility as CaSO4, CaCO3, CaSO4, CaSO4.2H2O, BaSO4, BaCO3, Fe(OH)3, Fe2O3, CrSO4, CrCO3,NaCl, plus of scales of F-, OH-, Cl-, Br-, S2-, oxides and acids. 188bet zokb programme has advanced 188bet zokb simulation and prediction accuracy for sulphate, carbonate and iron scales [7]. 188bet zokb programme took into consideration a range of factors governing scale precipitation such as activity coefficients, thermodynamics, common ion and ion pair effects, varying pH, and in some cases, 188bet zokb influence of oil and gas compositions, GWR and OWR for accuracy predicting of sulphate and carbonate scale deposition.

• Desktop results:

188bet zokb composition of injection water, drilling mud loss and 188bet zokb dynamic data at downhole of 1P-SCL were used as input data of ScaleChem software. 188bet zokb results of incompatible mixing are given at Figure 5. Thereby, at mixing ratio of 95% injection water, two inorganic materials were found as CaCO3 (calcite) and CaSO4 (anhydrite). When 188bet zokb fluid comes out to downhole of 1P-SCL, 188bet zokb pressure drop leads to 188bet zokb formation of CaCO3 scale. 188bet zokb detail calculation of scale amount was summarised in Table 2.

4.4. Laboratoryteststosimulatetheformationofscaleatdownhole

Scale prediction tests were conducted to confirm 188bet zokb results of 188bet zokb computer scale predictions and capture 188bet zokb types of scale precipitation at downhole of 1P-SCL due to mixing incompatibility or change of pressure/temperature condition. Using 188bet zokb high pressure, high temperature laboratory system to assess 188bet zokb scaling risk associated with several of pressure at different temperatures of oil process system and interaction between water sources.

188bet zokb mixing ratio between 2 sources is 95-5 as calculation above. 188bet zokb solid scale obtained in 188bet zokb tests on 188bet zokb membrane will be analysed by SEM-EDS and XRD to determine 188bet zokb type of scale. 188bet zokb amount of scale is also determined.

• Laboratory test results:

188bet zokb SEM image shows clearly that there are two types of crystal formed in 188bet zokb test, one of needle-shaped crystal and 188bet zokb other of powder shaped and smaller.

EDS results (element composition analysis) on 188bet zokb needle-shaped crystal show 188bet zokb elements of calcium, sulphur and oxygen account for CaSO4 and on 188bet zokb powder-shaped crystal show 188bet zokb elements of calcium, carbon and oxygen account for CaCO3.

188bet zokb XRD results also illustrate two types of peak of two crystals: anhydrite (CaSO4) and calcite (CaCO3) with weight fraction of 96% and 4% accordingly.

4.5.Evaluation and recommendation

188bet zokb desktop work results show abundance of CaSO4 scale with some CaCO3 that can be formed by 188bet zokb incompatible mixing between injection water and drilling mud brine and by 188bet zokb pressure drop. As per 188bet zokb computational modelling results, 188bet zokb worst case predicts a total amount of scale precipitation of approximately 2,373mg/L in near well-bore/downhole area. Using 188bet zokb laboratory test, a solid amount of around 2,089mg/L was collected in 188bet zokb test conducted under near-wellbore/downhole condition. Hence, both results of laboratory and modelling are in 188bet zokb same range.

Through reviewing 188bet zokb data and evaluating 188bet zokb laboratory test results as well as software modelling results, 188bet zokb major cause of PI drop of 188bet zokb 1P-SCL well is recognised. 188bet zokb results show that 188bet zokb incompatibility between calcium-based brine drilling mud fluid and injection water containing high sulfate causes scale precipitation. However, at 188bet zokb moment of water sampling, 188bet zokb existence of drilling mud in 188bet zokb production fluid is almost negligible, therefore this decrease of PI caused by CaSO4 scale will not continue.

To restore 188bet zokb PI, 188bet zokb chelating-based chemical (such as EDTA) should be applied for 188bet zokb downhole of 1P-SCL well. 188bet zokb following reactions illustrate 188bet zokb mechanism of dissolver performance [3]:

 CaSO + Na EDTA → CaNa EDTA + Na SO

Generally, once 188bet zokb scale is dissolved through chelation, there is no re-precipitation. Stable at temperatures up to 250oC, 188bet zokb chelating agent is a low toxicity scale dissolver that is effectively noncorrosive on most steels- making 188bet zokb treatment extremely safe.

5. Conclusions

• Scale monitoring plan and prediction of 188bet zokb scale formation should be carried out frequently to minimise 188bet zokb bad effect of inorganic scale to oil production.

• 188bet zokb incompatibility between injection water (seawater) and formation water or drilling mud brine needs to be studied prior to water injection or EOR application.

• 188bet zokb selection of scale prevention and remedial method must be based on 188bet zokb mechanism and source of scale formation.
 • High density calcium-based brine during drilling and completion phase should be used with special caution to minimise 188bet zokb incompatibility with injection water containing high sulfate content.

References

1. Mike Crabtree, David Eslinger, Phil Fletcher, Ashley Johnson, George King. Fighting scale: Removal and prevention. 188bet zokb BP Technology Magazine. 1998.

2. Guihong Pei, Chunyang Wang, Lili Liu. Experimental study on 188bet zokb cause of inorganic scale formation in 188bet zokb water injection pipeline of Tarim oilfield. Journal of Chemistry. 2014.

3. J.Moghadasi,H.Müller-Steinhagen,M.Jamialahmadi, A.Sharif. Scale deposition in porous media and their removal by EDTA injection. Engineering Conferences International Symposium Series: Proceedings of 7th International Conference on Heat Exchanger Fouling and Cleaning - Challenges and Opportunities, Tomar, Portugal. 1 - 6 July, 2007.

4. Francisco J.Alcalá, Emilio Custodio. Using 188bet zokb Cl/Br ratio as a tracer to identify 188bet zokb origin of salinity in aquifers in Spain and Portugal. Journal of Hydrology. 2008; 359(1 - 2): p. 189 - 207.

5. Amer Badr Bin Merdhah, Abu Azam Mohd Yassin. Study of scale formation in oil reservoir during water injection - A review. Marine Science & Technology Seminar, Kuala Lumpur, Malaysia. 22 - 23 February, 2007.

6. Amer Badr Bin Merdhah, Abu Azam Mohd Yassin. Scale formation in oil reservoir during water injection at high-salinity formation water. Journal of Applied Sciences. 2007; 7(21): p. 3198 - 3207.

7. M.Amiri, J.Moghadasi. Prediction 188bet zokb amount of barium sulfate scale formation in Siri oilfield using OLI ScaleChem software. Asian Journal of Scientific Research. 2010; 3(4): p. 230 - 239.