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

Summary

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

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

1. Introduction

Today, dang nhap 188bet 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 dang nhap 188bet hearts of engineers dang nhap 188bet way scale can. Scale is an assemblage of deposits that cake perforations, casing, production tubing, valves, pumps and downhole completion equipment, thereby clogging dang nhap 188bet wellbore and preventing fluid flow. Scale, just like dang nhap 188bet scale found in home plumbing or tea kettles, can be deposited all along water paths from injectors through dang nhap 188bet reservoir to surface equipment. Most scales found in oil fields in Vietnam form either by direct precipitation from dang nhap 188bet 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 dang nhap 188bet possibility that scale will form.

dang nhap 188bet 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 dang nhap 188bet formation pores near dang nhap 188bet wellbore and reduce formation porosity and permeability. It can block dang nhap 188bet 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. dang nhap 188bet productivity index can fail half in just several weeks and therefore that requires serious study of scale problem to find out dang nhap 188bet suitable cures [1].

In this article, we present dang nhap 188bet mechanism and source of scale buildup during oil production. Knowing dang nhap 188bet 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 dang nhap 188bet 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

dang nhap 188bet production of water is a natural consequence of dang nhap 188bet 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. dang nhap 188bet 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 dang nhap 188bet water surface. Scale begins to form when dang nhap 188bet state of any natural fluid is perturbed such that dang nhap 188bet 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 dang nhap 188bet petroleum reservoir [2]:

- Autoscaling

A reservoir fluid experiences changes in temperature and pressure as it is produced. If such changes take dang nhap 188bet fluid composition beyond dang nhap 188bet 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 dang nhap 188bet 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 dang nhap 188bet fluid, which raises pH and causes scale deposition. dang nhap 188bet deposition of carbonate can extend from dang nhap 188bet near-wellbore matrix, along tubing and into surface equipment as dang nhap 188bet produced water continuously changes in pressure and temperature. For carbonate scales, temperature effects often work against pressure effects. For example, dang nhap 188bet pressure drop at dang nhap 188bet point of entry into dang nhap 188bet wellbore can lead to matrix scale. As dang nhap 188bet fluid progresses up dang nhap 188bet tubing to surface temperatures and wellhead pressure, dang nhap 188bet resulting temperature drop may override dang nhap 188bet pressure effect, reducing scale formation in dang nhap 188bet tubing. On dang nhap 188bet other hand, subsequent release of pressure from dang nhap 188bet wellhead to dang nhap 188bet 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 dang nhap 188bet near-wellbore matrix generally produces new fluids with combined ion concentrations that are above dang nhap 188bet 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 dang nhap 188bet 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 dang nhap 188bet 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. dang nhap 188bet first development within a saturated fluid is a formation of unstable clusters of atoms, a process called homogeneous nucleation. dang nhap 188bet atom clusters form small seed crystals triggered by local fluctuations in dang nhap 188bet equilibrium ion concentration in supersaturated solutions. dang nhap 188bet seed crystals subsequently grow by ions adsorbing onto imperfections on dang nhap 188bet crystal surfaces extending dang nhap 188bet crystal size. dang nhap 188bet energy for seed crystal growth is driven by a reduction in dang nhap 188bet surface free energy of dang nhap 188bet 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, dang nhap 188bet formation of any seed crystal will encourage an increase in dang nhap 188bet growth of scale deposits. dang nhap 188bet 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, dang nhap 188bet accumulation of scale can occur at dang nhap 188bet position of dang nhap 188bet bubble point pressure in dang nhap 188bet flowing system. This explains why scale deposits rapidly build on downhole completion equipment.

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

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

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

- Identifying scale

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

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

dang nhap 188bet direct cost of removing scale from one well can be as high as USD 2.5 milion, and dang nhap 188bet cost of deferred production even higher [3]. Just as prevention is better than cure in medical practice, keeping producing wells healthy is ultimately dang nhap 188bet 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 dang nhap 188bet 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 dang nhap 188bet preferred method of maintaining well productivity. Inhibition technique can range from basic dilution method, to dang nhap 188bet 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 dang nhap 188bet wellbore, tubing or formation environment, and effective at preventing reprecipitation. dang nhap 188bet best scale-removal technique depends on knowing dang nhap 188bet type and quantity of scale, and its physical composition or texture. A poor choice of removal method can actually promote dang nhap 188bet rapid recurrence of scale.

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

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

dang nhap 188bet A oil field has produced oil from fractured basement reservoir with 4 produced wells, in which 1P-SCL has dang nhap 188bet highest production rate. However, dang nhap 188bet 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).dang nhap 188bet pressure survey is carried out in an yearly basis to determine reservoir pressure and well PI. dang nhap 188bet skin factor could not be estimated in dang nhap 188bet 1P-SCL well due to wellbore storage effects. It is essential to quantify and characterise dang nhap 188bet scale formation downhole in 1P-SCL, focusing on near-wellbore area where pressure drop takes place in flowing condition. Therefore, dang nhap 188bet 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 dang nhap 188bet suitable method with dang nhap 188bet scale problem of 1P-SCL in dang nhap 188bet A oil field.

All data relating to drilling, completion and operation phases of dang nhap 188bet 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 dang nhap 188bet 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 dang nhap 188bet support from dang nhap 188bet latest version of ScaleChem software to evaluate dang nhap 188bet scale tendency and scale amount.

4.1. Production water composition

Initially, dang nhap 188bet reservoir pressure decreased continuously with very low water cut < 3%. dang nhap 188bet 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 dang nhap 188bet reservoir pressure. Besides, dang nhap 188bet composition of 1P-SCL water at dang nhap 188bet beginning was very rich in Cl-, then dang nhap 188bet Cl- content decreased right after dang nhap 188bet water breakthrough. dang nhap 188bet 1P-SCl has its production target at basement reservoir (more than 4,000mD), therefore, during dang nhap 188bet 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 dang nhap 188bet basement of dang nhap 188bet referent oil field in dang nhap 188bet South East offshore Vietnam, dang nhap 188bet connate formation water (if available) may be immigrated from above reservoir (Oligocene) with low total dissolved solid (TDS) value (< 40,000mg/L). However, dang nhap 188bet TDS value of 1P-SCL production water is quite high (> 90,000mg/L). Hence, dang nhap 188bet production water of 1P-SCL can be considered as drilling mud losses only (at dang nhap 188bet initial time of production) and dang nhap 188bet 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 dang nhap 188bet 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 dang nhap 188bet start of water production in this well suggesting that breakthrough occurred within dang nhap 188bet oil-leg (Cl- is used as a conservative natural tracer [4]). Before water breakthrough, around 105,000 barrels of drilling losses (brine) were recovered. dang nhap 188bet remaining approximate 25,000 barrels of brine could be mixed with injection water during several months leading to dang nhap 188bet decrease of Cl- content in production water. dang nhap 188bet Cl- content sharply decreased and then kept stably at around 30,000mg/L during 15 months. By calculation, dang nhap 188bet mixing ratio at this period is about 95% of injection water. This ratio is acceptable if compared to dang nhap 188bet remaining drilling mud loss out of total production water.

4.2. Scale scenarios at 1P-SCL well

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

- dang nhap 188bet incompatible mixing between injection water and drilling mud loss at dang nhap 188bet time of water breakthrough. dang nhap 188bet 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 ↓

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

Figure 5. Scale tendency at dang nhap 188bet 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 dang nhap 188bet test

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

Desktop scale study is simulated and predicted dang nhap 188bet 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. dang nhap 188bet results of dang nhap 188bet scaling prediction are frequently used to determine whether scaling is likely to occur and dang nhap 188bet scale species likely to be formed at subsurface/surface condition.

Table 4. Dynamic data of 1P-SCL for dang nhap 188bet 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.

dang nhap 188bet 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. dang nhap 188bet programme has advanced dang nhap 188bet simulation and prediction accuracy for sulphate, carbonate and iron scales [7]. dang nhap 188bet 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, dang nhap 188bet influence of oil and gas compositions, GWR and OWR for accuracy predicting of sulphate and carbonate scale deposition.

• Desktop results:

dang nhap 188bet composition of injection water, drilling mud loss and dang nhap 188bet dynamic data at downhole of 1P-SCL were used as input data of ScaleChem software. dang nhap 188bet 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 dang nhap 188bet fluid comes out to downhole of 1P-SCL, dang nhap 188bet pressure drop leads to dang nhap 188bet formation of CaCO3 scale. dang nhap 188bet detail calculation of scale amount was summarised in Table 2.

4.4. Laboratoryteststosimulatetheformationofscaleatdownhole

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

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

• Laboratory test results:

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

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

dang nhap 188bet 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

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

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

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

 CaSO + Na EDTA → CaNa EDTA + Na SO

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

5. Conclusions

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

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

• dang nhap 188bet selection of scale prevention and remedial method must be based on dang nhap 188bet mechanism and source of scale formation.
 • High density calcium-based brine during drilling and completion phase should be used with special caution to minimise dang nhap 188bet 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. dang nhap 188bet BP Technology Magazine. 1998.

2. Guihong Pei, Chunyang Wang, Lili Liu. Experimental study on dang nhap 188bet cause of inorganic scale formation in dang nhap 188bet 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 dang nhap 188bet Cl/Br ratio as a tracer to identify dang nhap 188bet 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 dang nhap 188bet amount of barium sulfate scale formation in Siri oilfield using OLI ScaleChem software. Asian Journal of Scientific Research. 2010; 3(4): p. 230 - 239.