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The comparison of acidizing and ultrasonic waves, and their synergetic effect for the mitigation of inorganic plugs Nasir Khan, jingyang pu, Chunsheng Pu, Xu Li, Lei Zhang, Gu Xiaoyu, and Heng Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00622 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017
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Original Core 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Core
Ultrasonic Treatment
Acidizing Treated Core
Damaged Core
Treated Core
A A Core
Core
A - Particle B-Inorganic plugs
B
Synergetic Effect of Ultrasonic & Acidizing
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Treated Core
Core
Core
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The comparison of acidizing and ultrasonic waves, and their synergetic effect for the mitigation of inorganic plugs
1 2 3 4 5 6 7 8 9
Nasir Khan1, Jingyang Pu2, Chunsheng Pu1,*, Li Xu1, Zhang Lei1, Gu Xiaoyu1, Heng Zheng1 1
School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, 266555, China
2
Dept. of Petroleum Engineering, Missouri University of Science and Technology, Rolla, MO, 65401
ABSTRACT
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Oil and gas industry is plagued by inevitable formation damaged in the wellbore
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proximity during entire life of the well. Therefore, it is indispensable to ameliorate the
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damaged permeability by either using conventional applied techniques or ultrasonic-assisted
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stimulation method. The latter is characterized by efficient, simple and convenient, and
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environmentally secure method. Due to these distinguished characteristics, the demand of this
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physical Enhanced Oil Recovery (EOR) technique increased in petroleum industry. In this
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study, ultrasonic waves and hydrochloric acid (HCl) were used separately as well as in
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combination to recover the lost productivity caused by calcium carbonate (CaCO3) inorganic
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plugs in low permeability sandstone core samples. Results showed that permeability recovery
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increased with irradiation time upto 100 mins; however, it decreased with further irradiation.
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This deviation could be due to particles bridge formation at later stage. In addition, optimum
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frequency and power of ultrasonic waves (20 KHz and 1000 W) significantly recovered the
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damaged permeability. Although maximum frequency (25 KHz) could not achieve maximum
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permeability, but higher power was quite effective. The damaged permeability recoveries of
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HCl and ultrasonic waves were 44.5 % and 37.6 % respectively, but the permeability
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recovery was escalated to 61.5 % when HCl and ultrasonic techniques were applied together.
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Inorganic plugs using ultrasonic waves could chiefly be caused by cavitation, acoustic
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streaming, and heat generation in three different ways, such as cavitation, boundary friction Page 1 of 39
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and transformation upon hitting the medium.
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* Corresponding Author: Email:
[email protected], Tel# 00 86 132 1084 6217
3 4 5
KEYWORDS Inorganic plugs; Formation damage; Permeability recovery; Ultrasonic waves; Core
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flooding
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1. INTRODUCTION
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Formation damage plagues the oil industries in term of the natural permeability descent
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and loss in the total revenue during the entire life of the well. Formation damage can be
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well-defined as the original permeability loss of the sub-surface reservoir rock nearby
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wellbore which does occur during variant well operations at varying degrees1,2 as depicted in
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Figure 1. Long time ago, oil price was the main concern of the oil companies, but recent
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energy crises divert companies’ attention towards reduction and recovery of the productivity
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loss caused by inevitable formation damage.3 This unwanted occurrence can predominantly
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be caused by pore obstruction, perforation, inorganic scale deposition and mechanical
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deformation of formation under the stress, and fluid shear along with fluid/fluid or fluid/rock
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incompatibility1,2 as shown in Figure 2. However, inorganics plugs were considered as the
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principal cause of formation damage.
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Figure 1. Formation damage during various well operations at varying degree.
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Figure 2. Permeability decline due to formation damage.
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Inorganic scaling may occur anywhere from the subsurface reservoir to the production
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facilities at the surface where established equilibrium is disturbed upon exploitation of oil and
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gas.4 These inorganic scaling is mainly caused by the injection of incompatible water which
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leads to severe well productivity loss nearby wellbore due to plugging of the pore throat.4,5 Page 3 of 39
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However, scaling deposition is not restricted to the wellbore and also plague the industries
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due to their formation in the well tubing. As it is beyond the scope of this study, so this
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phenomenon will not be covered in current work. The well-known inorganic scaling at the
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oilfields are calcium carbonate (CaCO3), gypsum (CaSO4), barium sulfate (BaSO4), and
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strontium sulfate (SrSO4). The chief culprit of inorganic scaling is calcium carbonate (CaCO3)
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which can be formed by mixing water from two different sources. Specifically, one water
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source has high calcium ions concentration. Whereas, other water source is abundant in
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carbonates at basic medium. Furthermore, it can alternatively be produced by pressure change
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or alteration in fluid production pH5,6 as illustrated via reaction (1). As far as CaCO3 kinetics 7,8
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is concern, it was presented by J. Moghadasi et al.
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showed that increase in temperature triggers the rate of CaCO3 precipitation. Moreover, they
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revealed that rise in temperature led to increase in supersaturation due to declining of calcium
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carbonate solubility with rise in temperature. They also examined the effect of flow rate on the
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concentration of CaCO3. They showed that increase in flow rate caused rapid supersaturation
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of ions which resulted into increasing rate of CaCO3 precipitation. J. Moghadasi et al.
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demonstrated that pressure drop during production phase of the well triggers CaCO3 scaling
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deposition. Moreover, CO2 gas partial pressure increased pressure drop that resulted into
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increase in scale deposition of calcium carbonate. In addition, higher pH also favored calcium
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carbonate deposition.
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via simulation model. Where they
Ca aq + 2HCO aq ↔ CaCO s + H O + CO
The following reactions took place as shown below;
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7,8
(1)
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Carbonic species and water equilibria:
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CO + H O ↔ H CO
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(2)
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H CO ↔ H + HCO
(3)
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HCO ↔ H + CO
(4)
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H + OH ↔ H O
(5)
Ca + HCO ↔ CaHCO
(6)
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Ion-pair equilibria:
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Ca + CO ↔ CaCO
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Ca + OH ↔ CaOH
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(7)
(8)
Solid- Liquid- Phase equilibrium
Ca + CO ↔ CaCO s
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(9)
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Ionic equilibrium reaction is presumed to be occur spontaneously (within millisecond)
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upon mixing of the solution in shown in reaction (2) to reaction (8). However, reaction (9) is
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comparatively slow.
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depend on the flow rate, pH of the system and temperature of environment.
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Subsequently, reaction (9) decide the precipitation of CaCO3 that is
Similarly, sulfate scaling also results from blending of two water sources. Where one rich in sulfate ions and other has the abundance of divalent ions as illustrated by the reaction (10). Page 5 of 39
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M aq + SO aq ↔ MSO s
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(10)
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Whereas, M2+ can be Ca2+, Sr2+ or Ba2+
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This scale deposition, rate of formation and formation’s place are decided by different
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factors, such as brines in supersaturating condition, temperature and pressure profiles, pH
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alteration of the saline solution due to dissolution or liberation of CO2 or H2S , inappropriate
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brine
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thermal-kinetics.10
solutions mixing,
presence of other inorganic acids, ions strength, and
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The recovery of the lost well productivity needs frequent shut-in of the well for the
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remedial treatment techniques in order to acquire the original permeability of the subsurface
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formation but it directly influences the overall profitability of the company. Moreover,
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restrictive effect of the acid on the insoluble calcium or barium sulfate makes chemical
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removal technique impractical. Therefore, it is first line of defense to inhibit inorganic
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deposition instead of costly and challenging treatments required at the later stages. For this
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purpose, appropriate and threshold concentration of the complex phosphates along with sand
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would be pumped into the formation during fracturing treatment which generates filter bed in
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the subsurface formation. Subsequently, saline water becomes conditioned while passing
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through the filter bed and insoluble carbonate and sulfate deposition would be prevented to
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deposit. Complex phosphate ions act as polyphosphate ions which inhibit the crystal growth
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of given nuclei by coating the available surface of the crystal. Consequently, new crystal
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starts to form somewhere else. This process continues till equilibrium attainment among the
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present ions in the solution where no further crystallization happens. As a result, various
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micro-crystals appear as a colloidal instead of forming large crystal which tend to hamper the Page 6 of 39
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flow channel in the subsurface formation. However, this stable equilibrium exists for certain
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ion concentration which may again form crystallization and precipitation. Therefore,
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sequestering agent is applied for the purpose of delaying insoluble minerals’ precipitation
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which is characterized by low concentration of polyphosphate ions in the produced brine.
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Otherwise, insoluble precipitate of calcium phosphate will be formed that will detrimentally
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plug the formation.11 Hence, readily soluble phosphate such as calgon or trisodiumphosphate
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is inappropriate to be applied for this aim.12 Despite protective measures, if inorganic scale
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begins to form and descent the original permeability of the formation. Thereafter, various
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treatment techniques, such as chemical treatment, thermal treatment and hydraulic fracturing
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would mostly be employed to recover the lost productivity. However, environmental
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implication and labor-intensive nature make the researcher to think about other potential
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alternatives.3,13 In this connection, chemical treatment that involves hectic laboratory tasks
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for better chemical selection according to the particular subsurface formation chemistry.
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Besides, heat sensitive downhole assemblies would not openly allow thermal based recovery
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and deleterious hydraulic fracturing technology. The aforementioned demerits of applied
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techniques highlight the importance of ultrasonic-assisted techniques.
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Ultrasonic waves have promising results in the removal of inorganic plugs in comparison
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to traditional chemical method. In addition, ultrasonic waves have been the active and
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non-trivial part of seismic and logging services since the beginning of the oil and gas
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exploration.14 Conventional well stimulation techniques, particularly acidizing, require the
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preparation of design fluid or acid for specific well job which make this technique
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unattractive in comparison with acoustic technique.15 The acoustic approach is efficient, Page 7 of 39
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simple and convenient, environment friendly, reliable, and low cost-effective. The
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attractiveness of the ultrasonic waves becomes more charming due to the zonal control which
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can be acquired via conveying of ultrasonic waves generation tool down the hole either by
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using coil tubing or wireline in order to avoid water and gas prone region.16 Subsequently,
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wellbore can be cleaned out due to falling of loosely suspended particles caused by the
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vibration of high frequency ultrasonic waves and this phenomenon is more effective in
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underbalance condition. Recent developments in the production equipments and the inception
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of complex well trajectories such as horizontal well make wellbore cleaning a challenging
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task by using conventional stimulation technology.17 Furthermore, wellbore is usually shut-in
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in conventionally stimulation methods. In contrast, acoustic technique cannot only be used
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during production phase, but it presents promising results in underbalance condition, too.18 In
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this way, one can investigate the best exposure time for improved oil recovery by analyzing
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real time effect.15 Moreover, simplicity in assembling the pertaining accessories makes this
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technology more reliable and favorable.
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The available literature in the application of ultrasonic waves for the formation damage
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removal is limited. Some researchers have studied ultrasonic application to mitigate
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formation damage in the proximity of the wellbore. Adinathan Venkitaraman et al.19
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conducted successful experiment to remove fines and mud solids. Where they revealed that
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approximately 20 – 80 KHz frequency and 20 – 250 W/m2 acoustic intensity were
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investigated to successfully mitigate fines and mud solids by improving permeability by
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factor of 3 to 7 of the damaged permeability core samples. Furthermore, they showed that the
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effective depth of removal these kinds of plugs were found to be 2.5 inch. Beyond this limit, Page 8 of 39
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ultrasonic waves technique was found to be ineffective in term of cleaning of fines and mud
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solids. Meanwhile, Peter M. Roberts et al.20 applied ultrasonic waves technique to mitigate
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formation damage caused by polymer and organic deposits. They illustrated that acoustic
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wave worked well to recover the damaged permeability by resuspension of paraffin deposits
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in comparatively short time span. Moreover, they showed that the treatment depth was found
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to be 12-15 cm. According to their research, ultrasonic waves were found to be relatively less
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effective in removing polymer plugs. Pu Chunsheng et al.21 conducted an experiment at
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laboratory scale to remove formation damage and also confirmed the ultrasonic effect in low
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permeability reservoir in northern Shaanxi Oilfield, China and Daqing Oilfield, China. They
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observed that daily oil production was doubled after ultrasonic waves stimulation. Besides,
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significant decrease in pressure and better performance in water-injection was also observed.
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Xu Hongxing and Pu Chunsheng further investigated the influence of ultrasonic waves on the
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removal of five different kinds of plugs.22 According to their results, permeability recovery
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would increase with initial permeability of the core samples in the case of inorganic plugs
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and fines removal. While inverse trend was observed in other kinds of plugs. However,
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despite enormous research in this field, to best of our knowledge, no such in-depth studied is
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reported so far in this research area for the removal of inorganic scaling by using ultrasonic
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waves.
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This paper compares the results acquired by the application of conventional acidizing
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technique with ultrasonic-assisted inorganic plugs mitigation. Where, the latter technology is
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characterized by efficient, simple and convenient, and environmentally secure. Moreover,
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best-performing transducer with particular frequency and power besides optimum exposure Page 9 of 39
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time were inspected for efficient and distinguished outcomes. The rational basis behind
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optimized parameters was also clearly encompassed in current study. Experimental results of
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the acidizing and ultrasonic techniques were compared with each other and also related with
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synergetic effect of HCl and ultrasonic wave.
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2.
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The following section represents the employed materials as well as methods to obtain the
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required research goal.
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2.1. Materials and Equipments
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2.1.1. Chemicals & Preparations
MATERIALS AND METHODS
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The analytical grade sodium chloride (NaCl), anhydrous calcium chloride (CaCl2),
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magnesium chloride hexahydrate (MgCl2.6H2O) and anhydrous sodium carbonate (Na2CO3)
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with the corresponding purity of 99.5 %, 96 %, 98 % and 99.8 % were provided by
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Sinopharm Chemical Reagent Co., Ltd. Different concentrations of brine solution with NaCl,
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CaCl2 and MgCl2.6H2O salts in respective 7, 0.6 and 0.4 proportions. Whereas, for the
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purpose of damaging the core samples, inorganic solution of Na2CO3 and CaCl2 were
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prepared in separate containers with 200 g/L and 210 g/L concentrations respectively in order
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to get exact stoichiometric ratio of 1:1. HCl (36.0 %) was procured from Kang De Chemical
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Company Ltd.
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2.2.2. Core Samples
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Artificial core samples were acquired from Haian Petroleum Research Instrument Co.,
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Ltd. The acquired core samples were categorized into three groups on the basis of their initial
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gas permeability (30×10-3 µm2, 80×10-3 µm2 and 150×10-3 µm2) as given in the Table 1. Core
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sample were numbered in such a way that the first digit indicates the permeability, for Page 10 of 39
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example, core no. 3-35, core no. 8-46, and core no. 15-86 while digits after hyphen shows
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particular core samples in that range.
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Table 1. Core samples properties Permeability Length Constitutes (µm2) (cm) -3 30×10 6.0 Quartz & -3 80×10 6.0 Epoxy Resin 150×10-3 6.0
Diameter (cm) 2.5 2.5 2.5
Porosity (%) 20 22 24
Pore Volume (cm3) 5.88 6.47 7.06
ρ (g/cm3) 1.67
2.2.3. Ultrasonic irradiation system and core holder
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Ultrasonic generator as an indispensable component of the entire system produces high
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frequency ultrasonic waves. These ultrasonic generator were manufactured by Hangzhou
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Success Ultrasonic Equipment Co. Ltd with the Brand Name: FYCG and Model# ZJS-2000.
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The ultrasound transducer were also manufactured by Hangzhou Success Ultrasonic
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Equipment Co. Ltd as depicted in Figure 3. All transducers’ probe are made of titanium alloy
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and each having 1.912 cm diameter. As this experiment involves brine solution, therefore,
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actual output power was found be same as manufacturer mentioned power. The gap between
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core sample and transducers were kept 2-3 cm. Whereas, Haian Petroleum Research
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Instrument Co., Ltd assembled the core holder. The type and dimension of core holder were
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TY-5 and 2.5cm*2.5 cm~8 cm respectively. Where, core samples with the diameter 2.5 cm
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and length ranging from 2.5 cm to 8 cm can be adjusted. The maximum 32 MPa pressure can
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be sustained by the core holder.
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Figure 3. (a) Ultrasonic generator, (b) Ultrasonic transducers number 1 to 6 from left to right successively.
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2.2. Essential prerequisite to the dynamic experiment
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2.2.1. Critical velocity determination
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The maximum fluid velocity which can possibly cause particle-bridge formation at the
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pore throat is termed as critical velocity. In the case of fluid flow below critical velocity,
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repulsive colloidal force among the particles at the pore throat dominates hydrodynamic
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forces. When hydrodynamic forces are supposed to be stronger than repulsive force, the flow
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velocity would be greater than critical velocity. Consequently, it results into particle-bridge
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formation.23
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Critical velocity of the respective sets of core samples were measured in this way. Air was
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completely evacuated for the purpose of complete saturation before charging the high
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pressure vessel with 80 g/L brine solution (NaCl: CaCl2: MgCl2·6H2O=7:0.6:0.4). In next
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step, annular pressure was managed to be 3 MPa greater than the inlet pressure of the core in
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the core holder. Thereafter, priming was carried out to remove all the air from the line
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through the vent valve by adjusting the flow velocity (
