Experimental evaluation of attach-and-release mineral scale control

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Experimental evaluation of attach-and-release mineral scale control strategy for aqueous fluid transporting pipelines Ping Zhang, Yuan Liu, Nan Zhang, Amy T. Kan, and Mason B. Tomson Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02044 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 10, 2019

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Experimental evaluation of attach-and-release mineral scale control strategy for aqueous fluid transporting pipelines Ping Zhang a *, Yuan Liu a, Nan Zhang b †, Amy T. Kan b, c and Mason B. Tomson b, c

a Department

of Civil and Environmental Engineering, Faculty of Science and Technology. University of Macau, Macau, China

b Department

c

of Civil and Environmental Engineering, Rice University, Houston, Texas, USA

Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, Rice University, Houston, Texas, USA

†

Presently with at Statoil Inc., 6300 Bridge Point Pkwy, Austin, TX

Manuscript prepared for Energy & Fuels

* To whom correspondence should be addressed: Ping Zhang: [email protected] Tel: (+853) 8822 4917

Electronic supporting data (ESD) available

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Abstract Deposition of mineral scale is a severe operational threat to various industrial facilities, including pipeline and production tubing which are used for transporting aqueous fluid. Chemical inhibition is the main strategy to control scale deposition threat. However, the conventional inhibitor delivery methods can become unfavorable, due to operational risks and personnel and logistics demand. This study evaluated the feasibility of applying an attach-and-release strategy for aqueous fluid transporting pipeline scale control via a series of laboratory studies. It shows that both brine chemistry and flow rate can considerably impact inhibitor release behavior. Moreover, efforts have been made to calculate the effluent inhibitor concentrations from actual pipe surfaces and to predict corresponding inhibition effectiveness. This study promotes the potential field application of the proposed scale control strategy to realize the benefit of the flexibility in locating inhibitor injection point and alleviated maintenance burden compared with conventional methods. The experimental and modeling investigations elaborated in this study suggest that the proposed attach-and-release strategy has the potential to be adopted in the field mineral scale control.

Key words: mineral scale; scale inhibitor; delivery; precipitation; dissolution

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1. Introduction As key components in an aqueous solution, dissolved mineral species play an important role in determining the physiochemical properties of the aqueous solution.1 Elevation in the aqueous concentrations of minerals, such as sodium chloride, can considerably increase the salinity of the solution, turning the aqueous solution into a brine water (solution containing a high concentration of mineral salts). If the water can no longer hold the dissolved minerals, mineral solid precipitation phenomenon will take place, resulting in deposition of mineral solid onto the surface of surrounding environment.2-4 The precipitated mineral solids are also called mineral scales. Scale precipitation and subsequent deposition is a ubiquitous phenomenon in natural environment, and if there is no water then there is no scale. However, uncontrolled scale deposition can cause serious operational problems for many industrial processes, such as transporting aqueous fluid via pipelines and producing hydrocarbons with produced waters inside the production tubing.3, 4 The precipitated mineral scale solids can attach to the interior surface of the pipe and accumulate over time to become significant enough to reduce pipe flow and sometimes completely block the pipe throughput. Safety of pipeline and production tubing operations can be severely affected by mineral scale deposition. Among various types of scales, carbonate scale, such as calcium carbonate (CaCO3), is perhaps the most commonly observed scale in industry.3 As for pipeline operational system, precipitation of carbonate scale is typically driven by the change in the pipe operational conditions particularly temperature and pressure.5-10 In order to manage the scale threat, chemical inhibition has been adopted as the main control strategy to inhibit scale formation at various industrial processes.11-18 Typically, specialty scale inhibition chemical (scale inhibitor) is delivered into the pipeline or production tubing to be dissolved into the fluid to inhibit scale formation. The function of the scale inhibitor is to inhibit or delay scale formation kinetics. In

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other words, in the presence of scale inhibitor, scale formation will be kinetically hindered and it will take a considerably longer period of time for scale particles to precipitate and deposit.6

The methods to deliver scale inhibitor into the pipe and production tubing mainly reply on continuous injection and batch treatment.3, 4, 6 Continuous injection involves delivering inhibitor into the pipe via an injection mandrel so that scale inhibitor can be continuously injected into the fluid flowing inside the pipe. Compared with batch treatment, the advantages of injecting inhibitor continuously include controlled inhibitor injection rate and lower operational risk. The drawbacks of continuous injection include the routine maintenance requirement with associated personnel and logistics demand and also occasional difficulty in identifying a proper location to install chemical injection mandrel. On the other hand, batch treatment delivers inhibitor in a batch mode with a volume of inhibitor injected into the pipe or production tubing during one treatment. For instance, in oilfield scale treatment, it is sometimes required to inject scale inhibitor into the production well in a batch mode to deliver inhibitor into deep reservoir for scale control.3, 13 The advantage of scale batch treatment is the ability to deliver inhibitor into the pipeline or production system where an injection mandrel for continuous injection is not available or difficult to install. The disadvantages of the batch treatment include the inability to control aqueous inhibitor concentration and elevated operational risk. Considering the pros and cons of these two delivering methods, continuous injection method has been predominately adopted for pipeline scale control. However, as elaborated above, installing an injection mandrel for continuous scale inhibitor injection can be operationally difficult and the logistic and personnel demand can be challenging.

In the previous study, a novel “attach-and-release” scale control strategy for aqueous fluid transporting pipeline has been proposed.19 Similar to the batch treatment discussed above, a 4


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volume of inhibitor is initially delivered into the pipe to attach to the pipe interior surface. However, different from batch treatment, there is no need to shut down the pipe flow or to push the fluid reversely for inhibitor delivery, thus a much lower operational risk. Compared with continuous injection, the advantage of this proposed strategy includes the flexibility in inhibitor delivery location without the requirement to install an injection mandrel. In addition, this proposed strategy eliminates the maintenance requirement on personnel and logistics. In the previous study, the feasibility of this strategy has been examined with respect to the retention and release behavior of scale inhibitor. A plug-flow type tube reactor was adopted to represent pipe configuration in laboratory studies. Diethylenetriamine pentakis (methylenephosphonic acid) (DTPMP), a common industrial scale inhibitor, was employed in the previous study as the tested scale inhibitor. Laboratory study showed that during the first step of inhibitor retention, a fraction of the pumped DTPMP inhibitor can be retained by the previously deposited CaCO3 solid on the interior surface of the tube reactor. Furthermore, the release of the retained DTPMP on tube reactor surface was studied by flowing a DTPMP-free solution through the tube reactor. The release of DTPMP was speculated to be controlled by the dissolution of the formed Ca-DTPMP solid during the inhibitor retention stage.

In this study, the proposed attach-and-release scale control strategy was further evaluated to explore the feasibility of applying this strategy for aqueous fluid transporting pipeline scale control by use of the same plug-flow type tube reactor. As for the study of the retention of DTPMP to tube reactor surface, feed brines with a much lower initial DTPMP concentration were employed to understand the retention behavior of DTPMP at lower initial concentrations. The release of DTPMP from the tube reactor surface was investigated by focusing on the impact of feed brine

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chemistry and also the brine flow rate. Subsequently, a one-dimensional convection-dissolution equation has been proposed to characterize the dissolution of DTPMP from tube reactor surface. The effluent inhibitor concentration can be predicted for pipes with different diameters. The laboratory investigations elaborated in this study expand our understanding of the proposed attachand-release scale control strategy with respect to the processes of retaining and releasing inhibitor from pipe surface for scale control. This study promotes the potential field application of the attach-and-release scale control strategy to realize the benefit of the flexibility in locating inhibitor injection point and the alleviated maintenance burden compared with conventional delivery methods.

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2. Materials and methods 2.1 Experimental rationale This study adopts a plug-flow type tube reactor to experimentally examine the precipitation and dissolution behaviors of DTPMP inhibitors to and from the inner surface of a CaCO3-coated stainless tube. The tube was initially coated with a layer of CaCO3 solids. Subsequently, DTPMP inhibitor was attached to the surface of the coated tube via a precipitation process, which process is followed by releasing the attached DTPMP inhibitor via dissolution. A number of experimental conditions, such as brine chemistry and flow rate were evaluated to understand their roles in impacting the precipitation and dissolution of inhibitor from tube surface.

2.2 Chemicals and scale inhibitors Commercial grade diethylenetriamine pentakis (methylenephosphonic acid) (DTPMP) was used as the scale inhibitor. DTPMP stock solution with 50% activity (product Dequest 2060) was purchased from Italmatch Chemicals (Houston, Texas). Sodium chloride (NaCl) solid, calcium chloride (CaCl2) solid, sodium bicarbonate (NaHCO3) solid, potassium chloride (KCl) solid and nitric acid (HNO3) were reagent grade and purchased from Fisher Scientific. Deionized water (DI water) was prepared by reverse osmosis and ion exchange water purification processes.

2.3 Experimental setup of plug-flow tube reactor The details of the plug-flow tube reactor apparatus were elaborated in the previous scale deposition studies.19, 20 Essentially, the experimental setup includes two HPLC pumps, a stainless-steel tube reactor, an online pH meter, a sample collector and coil tubing. The tube reactor has a dimension of 0.91 cm inner diameter and 12.7 cm length. A water bath was used to control the temperature

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of the fluid flowing through the tube reactor. A schematic representation of the tube reactor apparatus is included in the electronic supporting data (ESD). The void space of the apparatus includes the internal volume of the tube reactor and the tubing volume. The void space was measured to be ca. 9.2 mL via a KCl tracer breakthrough experiment (see ESD for details of tracer experiment). CaCl2 and NaHCO3 feed brine solutions were prepared by dissolving respective solids into NaCl solutions. Both feed brine solutions were saturated with 100% CO2 at 1 atm and 25oC. During the inhibitor retention and release experiments detailed below, the CaCl2 feed brine and the NaHCO3 feed brine were simultaneously pumped by the two HPLC pumps at the same flow rate into the preheated coil tubing and mixed right before flowing into the tube reactor. The effluent solution was collected by the sample collector at regular time intervals. A back-pressure regulator was placed between the pH meter and the sample collector to maintain the tube reactor system pressure at 5 atm to prevent CO2 gas breaking out of the aqueous solution.

2.4 Philosophy of the experiment of retention and release of scale inhibitor 2.4.1 Retention of inhibitor by CaCO3 medium on the surface of tube reactor The attachment of DTPMP inhibitor onto the interior surface of pipe was investigated via the laboratory study of the retention of DTPMP on the inner surface of the tube reactor. A layer of CaCO3 was first deposited on the interior surface of the tube reactor by pumping the CaCl2 and NaHCO3 feed brines into a clean tube reactor with a total flow rate of 250 mL h-1 at 70oC for 12 h. The composition of the mixed brine solution pumped into the tube reactor were shown in Table 1 as Solution #1, which contained 0.1 M NaCl, 478 mg L-1 Ca2+ and 1358 mg L-1 HCO3- with a molar ratio of calcium ion to bicarbonate ion of approximately 0.5. By pumping Solution #1 into tube reactor, a uniform layer of CaCO3 was deposited on the interior surface of tube reactor.

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Subsequently, a DTPMP-containing feed brine solution was pumped into the CaCO3-coated tube reactor at 70oC to study the retention of DTPMP by the tube reactor. The DTPMP-containing feed solution was prepared by simultaneously pumping a CaCl2 feed with another DTPMP-containing NaHCO3 feed brine into the tube reactor. The flow rate of the mixed feed brine was 100 mL h-1. The effluent DTPMP concentration during the test was measured in the collected effluent solutions. Two feed brine solutions (Solution #2 and #3 in Table 1) were adopted to pump into the tube reactor for 55 mL. Deposition of a layer of CaCO3 onto the interior surface of a clean tube reactor was repeated for each of the DTPMP retention experiment.

Table 1 Compositions of the brine solutions employed in this study Solution #

NaCl (M)

Ca2+ (mg L-1)

HCO3(mg L-1)

DTPMP (mg L-1)

pHc

SI(CaCO3)d

1a 2b 3b

0.1 1 1

478 680 680

1358 2102 2102

0

6.0

0.48

2.5 5

6.0 6.0

0.59 0.59

Solution #1 was saturated with 100% CO2 at ambient condition of 1 atm and 25oC. This solution was employed to flow into tube reactor to precipitate a layer of CaCO3 on the interior surface. b Solution #2 and #3 were saturated with 100% CO at ambient condition of 1 atm and 25oC. This solution 2 was used to attach DTPMP to the CaCO3-coated tube reactor interior surface c pH values were reported at ambient condition of 1 atm and 25oC d SI values were calculated at the testing condition of 5 atm and 70oC a

2.4.2 Release of inhibitor from CaCO3 medium on the surface of tube reactor Upon the completion of the DTPMP retention on tube reactor surface, the tube reactor was subsequently pumped with a DTPMP-free feed brine to investigate the release behavior of the retained DTPMP from CaCO3 medium on tube reactor. A DTPMP-free feed brine (Solution #4 in Table 2) was pumped into the tube reactor right after the completion of pumping Solution #2 for 55 mL. In addition, this experiment was repeated by pumping Solution #4 into the tube reactor

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following the completion of pumping Solution #3 for 55 mL. Solution #4 contains 1 M NaCl, 612 mg L-1 Ca2+ and 1643 mg L-1 HCO3- with a pH of 5.9 at ambient condition. Solution #4 was pumped at a flow rate of 50 mL h-1 at 70oC. The effluent DTPMP concentration was measured during feed brine flow. To further understand the release behavior of the retained DTPMP and also the impact of feed solution composition on DTPMP release, four different DTPMP-free feed brine solutions were prepared with a wide range of Ca2+ concentrations, as shown in Table 2. Each of these four solutions was pumped in sequence into the tube reactor for 20 mL at the same flow rate of 100 mL h-1 following the completion of pumping Solution #2. The effluent DTPMP and Ca2+ concentrations were measured during the course of the flow of each feed solution. Another effort was made to evaluate the flow rate impact on the release of DTPMP from the tube reactor surface. In a separate experiment, after the completion of pumping Solution #2 for 55 mL, Solution #4 was employed to flow through the tube reactor at 70oC at different flow rates from 10 mL h-1 to 250 mL h-1. The corresponding linear flow velocities ranged from 0.0043 cm sec-1 to 0.11 cm sec-1. The effluent DTPMP concentrations were measured during the flow of Solution #4 at each flow rate (flow velocity).

Table 2 Compositions of the feed brines employed for the experiments of the release of DTPMP Solution #

NaCl (M)

Ca2+ (mg L-1)

HCO3(mg L-1)

DTPMP (mg L-1)

pHa

SI(CaCO3)b

4 5 6 7

1 1 1 1

612 1704 3970 163

1643 1643 1643 1643

0 0 0 0

5.9 5.9 5.9 5.9

0.32 0.79 1.20 -0.27

a

pH values were reported at ambient condition of 1 atm and 25oC

b

SI values were calculated at the testing condition of 5 atm and 70oC

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2.5 Analytical and characterization methods for aqueous chemical species and the formed scale particles Aqueous calcium and potassium concentrations in the collected effluent samples were analyzed by inductively coupled plasma-optical emission spectrometer (ICP-OES) (Optima 4300 Dv, PerkinElmer). DTPMP concentration was calculated from the phosphorus concentration measured by either ICP-OES or inductively coupled plasma-mass spectrometry (ICP-MS, PerkinElmer) (see ESD for details of DTPMP measurement). The surface topographies of the inner surface of tube reactor materials were analyzed by contour optical microscope (COM) (Bruker Corp., Billerica, MA).

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3. Results and discussion 3.1 Retention and release of DTPMP by CaCO3 medium on tube reactor surface In the previous study, retention of DTPMP on the surface of tube reactor was studied using a plugflow type tube reactor.19 First, a layer of CaCO3 was precipitated on the interior surface of the tube reactor by flowing a solution supersaturated with CaCO3 solid (Solution #1 in Table 1). Subsequently, a DTPMP-containing feed solution was pumped through the tube reactor to allow a fraction of the injected DTPMP to be retained by the deposited CaCO3 layer. In the previous study, the minimum concentration of the DTPMP in the feed solution was set to be 10 mg L-1. In this study, feed solutions of much lower DTPMP concentrations of 2.5 and 5 mg L-1 (Solution #2 and #3 in Table 1) were adopted to evaluate the retention and release behavior of DTPMP inhibitor. In many oilfield and pipeline operations, a lower scale inhibitor dosage is frequently employed due to pumping capability limitations, financial considerations and/or environmental regulations. In addition, scale inhibitor is designed to control scale deposition at a concentration of only a few milligrams per liter or less.6 Testing of feed brines with a much lower DTPMP concentration can expand our knowledge of retention and release of DTPMP inhibitor from the surface of pipeline. Table 1 suggests that Solution #1 to #3 were all supersaturated with CaCO3 at the testing condition of 5 atm and 70oC with the calculated CaCO3 saturation index (hereafter referred to as SI(CaCO3)) of each solution higher than 0.4. Figure 1 plots the effluent DTPMP concentration versus the volume of brine flow by individually pumping Solution #2, #3 and also a KCl tracer through a CaCO3-coated tube reactor at a flow rate of 100 mL h-1 at 70oC and 5 atm. Figure 1 suggests that after ca. 55 mL of mixed feed brine flow, a final breakthrough level of almost 100% can be reached by pumping Solution #3 (5 mg L-1 DTPMP). On the other hand, Solution #2 (2.5 mg L-1 DTPMP) reached ca. 90% breakthrough. The breakthrough curve of flowing Solution #3 generally reached

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a higher breakthrough level than that of Solution #2 during the course of feed brine flow. This observation is in agreement with the previous study results that the feed solutions with a higher DTPMP concentration tend to reach the final breakthrough level faster.19

Figure 1 Breakthrough curves for pumping Solution #2 and #3 in Table 1 through tube reactor. The dashed line represents the breakthrough curve of KCl tracer.

Compared with the breakthrough curve of KCl tracer which is assumed to have no adsorption or attachment to the tube reactor surface, the observed breakthrough curves of Solution #2 and #3 suggest that a fraction of the pumped DTPMP was retained by the reactor surface. It has been calculated that the amounts of DTPMP retained during these two flow-through experiments were 25.7 mg and 35.2 mg, respectively. The retained DTPMP will be subsequently released from tube reactor surface by following a DTPMP-free feed brine for scale control. The next sections will focus on the discussion of the release behavior of the retained DTPMP. In addition, surface topography can provide spatial characteristics of the materials, such as surface roughness and waviness.21 In this study, COM was employed to characterize surface topography to provide information regarding the surface roughness of the tube reactor and coated reactor. This piece of information can reveal the presence of CaCO3 and DTPMP on the inner surface of tube reactor.

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As shown in Figure 2a, a pristine stainless steel surface in the absence of CaCO3 or DTPMP was measured to have a minimum average roughness (Ra) of 1.66 μm. Ra is the arithmetic average of the absolute values of the profile heights over the evaluation length.21 After the completion of flowing Solution #1 in Table 1 to accomplish CaCO3 surface coating, the Ra value was measured to increase to 4.66 μm and more orange-colored zones were presented in Figure 2b, illustrating CaCO3 surface coverage. Figure 2c and d presented surface roughness after the completion of flowing Solution #2 (containing 2.5 mg L-1 DTPMP) and Solution #3 (containing 5 mg L-1 DTPMP), respectively. As discussed above, it can be calculated that 25.7 mg and 35.2 mg of DTPMP inhibitors were retained on the surface of tube reactor. Clearly, the presence of DTPMP inhibitor further increased the surface roughness to 8.96 μm and 11.63 μm, respectively. One can argue that the increase in roughness is due to the formation of Ca-DTPMP solids and a higher DTPMP concentration in the brine solution will lead to more Ca-DTPMP solid formation, revealed by the increase in surface roughness.

(b) Ra = 4.66 μm

(a) Ra = 1.66 μm

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(d) Ra = 11.63 μm

(c) Ra = 8.96 μm

Figure 2 COM images of the surface topography to show surface roughness. (a) pristine stainless steel; (b) CaCO3 surface coverage; (c) after completion of pumping Solution #2 and (d) after completion of pumping Solution #3. After the completion of flowing Solution #2 and #3 with DTPMP retained by the CaCO3 medium on tube reactor surface, the retained DTPMP was released from the surface of the tube reactor by flowing a DTPMP-free feed brine through the tube reactor at a flow rate of 50 mL h-1. Solution #4 in Table 2 was employed to flow into the tube reactor post the attachment of DTPMP onto tube reactor surface. Solution #4 doesn’t contain DTPMP and is also supersaturated with CaCO3 with a calculated SI(CaCO3) of 0.32 at testing condition of 5 atm and 70oC. Figure 3 illustrates the released DTPMP concentrations in the effluent from the tube reactor surface while flowing Solution #4 after the completion of pumping Solution #2 and #3. The effluent DTPMP concentrations following pumping Solution #3 were higher than those following pumping Solution #2. For both DTPMP release experiments, the DTPMP concentration profiles can be generally divided into two periods: from the onset till ca. 30 mL of feed brine flow and the remainder of the fluid flow. The effluent DTPMP concentrations before 30 mL of flow were much higher than those after 30 mL. As discussed in the previous study, the release of DTPMP from tube reactor surface is dictated by the dissolution of the formed Ca-DTPMP solid and two different solid phases exist for Ca-DTPMP solid: amorphous and crystalline.19 Compared with the crystalline solid, the 15


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amorphous solid has a higher solubility leading to a higher effluent DTPMP concentration. Thus, it appears that the dissolution of amorphous Ca-DTPMP solid accounts for the higher DTPMP concentration in the first period (0 to 30 mL) and Ca-DTPMP solid was gradually developed into crystalline phase with a lower solubility and the dissolution of crystalline Ca-DTPMP solid is responsible for the effluent DTPMP concentration in the later period (30 to 60 mL) in Figure 3.

Figure 3 Effluent DTPMP concentrations during the release of DTPMP from tube reactor surface following the completion of pumping Solution #2 and #3 in Table 1. 3.2 Experimental evaluation of brine chemistry impact on DTPMP release behavior The release of phosphonate inhibitor from the inner surface of the tube reactor can be impacted by a number of experimental parameters, such as brine chemistry and flow rate. In this study, the impact of brine chemistry of the feed brine solution was examined experimentally. The focus was given to understand the impact of Ca2+ concentration in the feed brine. Another set of DTPMP release experiments were carried out by employing several different brine solutions with a wide range of Ca2+ concentrations. As shown in Table 2, four different solutions (Solution #4 to #7 in Table 2) with the same solution pH and aqueous bicarbonate concentration were adopted. These four solutions differ in Ca2+ concentration from 163 to 3,970 mg L-1 with the corresponding SI(CaCO3) ranging from -0.27 to 1.20 at the testing condition of 5 atm and 70oC. These four

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solutions were pumped into the tube reactor in sequence at the same flow rate of 100 mL h-1 following the completion of pumping Solution #2 to study the impact of aqueous Ca2+ concentration on DTPMP release. In other words, the first step was to carry out attachment of DTPMP on the surface of tube reactor by flowing Solution #2 through tube reactor. Subsequently, the release of DTPMP was tested by flowing four feed brines with different compositions in sequence to examine the impact of brine chemistry on DTPMP release. Each of these four brines were pumped into the tube reactor for a volume of 20 mL. Figure 4 illustrates the effluent Ca2+ and DTPMP concentration variation during brine flow following the completion of flowing Solution #2. Obviously, both Ca2+ and DTPMP concentrations varied noticeably by switching the feed brines flowing through the tube reactor. Solution #4 to #6 are all supersaturated with CaCO3 with an increasing SI(CaCO3) from 0.32 to 1.20 and Ca2+ concentration from 612 to 3,970 mg L1.

The corresponding effluent DTPMP concentrations from 0 to 60 mL of brine flow continuously

reduced from around 0.25 mg L-1 to below 0.1 mg L-1. Note that during the flow of Solution #6 (40 to 60 mL), the effluent Ca2+ concentration decreased from ca. 4000 to 2600 mg L-1. The reduction in Ca2+ concentration can be elucidated by the deposition of CaCO3 solid as a consequence of a reduction in inhibitor concentration and hence inhibition effectiveness. Different from the period of 0 to 40 mL, during the period of 40 to 60 mL the effluent DTPMP concentration was as low as around 0.1 mg L-1, leading to a reduced inhibition effectiveness against CaCO3 solid deposition. Solution #7 contained a lower Ca2+ concentration and was undersaturated with respect to CaCO3 at the experimental condition. The reduction in aqueous Ca2+ concentration in the brine resulted in a higher effluent DTPMP concentration observed from 60 to 80 mL of flowing Solution #7. As detailed above, the effluent DTPMP concentration is determined by the dissolution of CaDTPMP solid during the flow of brine. Therefore, the solubility product of Ca-DTPMP solid

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should be the determining factor in controlling the dissolution of Ca-DTPMP solid and hence the aqueous phase DTPMP concentration. Based on the previous study on Ca-DTPMP precipitation chemistry,22-24 the precipitated solid formed by mixing Ca2+ and DTPMP is assumed to be in the form of Ca3H4DTPMP, Therefore, the corresponding negative base 10 logarithm of the IAP (pIAP) of Ca-DTPMP solid can be calculated as: 3 4 p(IAP) = - log10 [(Ca2 + ) (H + ) (DTPMP10 - )]

(1) Note that the term (Ca2+) is the activity of free Ca2+ species and (DTPMP10-) is the activity of free DTPMP10- species.

Figure 4 Effluent DTPMP and Ca concentrations during the release of DTPMP with four different feed solutions (Solution #4 to #7) after the completion of pumping Solution #2.

Figure 5 plots the calculated pIAP of Ca3H4DTPMP solid as a function of the volume of brine flow. pIAP was calculated based upon the total aqueous phase Ca2+ and DTPMP concentrations via a speciation model reported previously.22 This speciation model considers the acid/base and complex solution chemistry of DTPMP and also the impact of pH, temperature and ionic strength. Regardless of the difference in the compositions of the brine flowing through the tube reactor, the calculated pIAP varied insignificantly with an average of 53.3. This experimentally obtained pIAP value is comparable to the reported crystalline phase Ca-DTPMP solid solubility product of 53.23 18


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This suggests that the measured DTPMP concentration in this study was more likely to be governed by the dissolution of a crystalline phase Ca-DTPMP solid which formed during the previous step of DTPMP retention experiment. Evidently, the calculated pIAP suggests that majority of the Ca-DTPMP solid formed after the completion of flowing Solution #2 was crystalline in nature with a low solubility. It is possible that the formed Ca-DTPMP precipitate during DTPMP retention process underwent a rapid phase transition phenomenon since the onset of flowing Solution #2, leading to the development of crystalline solid. In addition, the effluent DTPMP concentrations shown in Figure 3 were in a similar range as those shown in Figure 4 and the corresponding pIAP values following the pumping of Solution #2 and #3 were calculated to vary unnoticeably with an average of 53.2. This suggests that Ca-DTPMP solids were likely to transform into a more crystalline-like materials after the completion of pumping Solution #2 and #3. The concentration reduction shown in Figure 3 before and after 30 mL of brine flow might be attributed to the dissolution of a middle phase Ca-DTPMP solid during the first 30 mL of brine flow until Ca-DTPMP solid was transformed into a true crystalline material. It was reported that a middle phase Ca-DTPMP solid exists during the phase transition from amorphous to crystalline and the middle phase solid has a solubility higher than the crystalline materials but much lower than the amorphous solids.25 Thus, the dissolution in the first 30 mL of brine flow shown in Figure 3 might be controlled by the dissolution of a middle phase Ca-DTPMP solid with an intermediate solubility product. Further research work will be conducted to use XRD characterization to examine the morphologies of the Ca-DTPMP solids before and after the dissolution experiment. This will provide the direct evidence of the crystallinity of the obtained Ca-DTPMP solids.

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Figure 5 Effluent Ca concentration and calculated pIAP values with four different feed solutions (Solution #4 to #7) after the completion of pumping Solution #2.

3.3 Experimental evaluation of brine flow rate impact on DTPMP release behavior In many pipeline and oilfield scale operations, hydrodynamic conditions, such as brine flow rate, can impact the release and dissolution of scale inhibitor. In this study, the impact of feed brine flow rate on the release of DTPMP from tube reactor was evaluated by flowing Solution 4 in Table 2 at different flow rates after the completion of pumping Solution #2 through the tube reactor. In other words, similar to the previous experiment, the first step was to attach DTPMP to tube reactor surface by flowing Solution #2 through tube reactor. Next, the release of DTPMP was carried out by flowing Solution #4 at different flow rates to investigate feed brine flow rate impact on DTPMP release. As tabulated in Table 3, four different flow rates (Q, mL h-1) from 10 to 250 mL h-1 were tested in this study. The corresponding linear flow velocities (v, cm sec-1) were from 0.0043 to 0.11 cm sec-1. Figure 6 illustrates the effluent DTPMP concentrations at different flow rates (flow velocities) of flowing Solution #4 through the tube reactor. Table 3 also tabulates the effluent DTPMP concentrations at different flow rates. Obviously, the higher the flow rate of flowing the feed brine, the lower the DTPMP concentration in effluent. As discussed previously, the release of DTPMP from CaCO3-coated tube reactor surface is dictated by the dissolution of Ca-DTPMP 20


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solid into the flowing feed brine.19 From a thermodynamic viewpoint, the effluent DTPMP concentration at an equilibrium state is impacted by the Ca2+ concentration in the feed solution. In this study, the same feed solution (Solution #4) was used in this study, suggesting of the same equilibrium DTPMP concentration. From a kinetics standpoint, the increase in the flow rate will lead to a decrease of the residence time of the feed brine solution inside the tube reactor. The decrease in the residence time will result in a lower DTPMP effluent concentration due to incomplete Ca-DTPMP dissolution. Table 3 Parameters from the experiments of the release of DTPMP at different flow rates

a b

(sec)b

DTPMP concentration (mg L-1)c

Equilibrium concentration, Ceq (mg L-1)d

Dissolution Rate constant, k (sec-1)d

2973 594 297 119

1.48 0.49 0.26 0.11

1.90

0.00051

Flow Rate, Q

Flow Velocity, v

L/v

(mL h-1)

(cm sec-1)a

10 50 100 250

0.0043 0.021 0.043 0.11

Flow velocity is calculated based on the flow rate (Q) and the tube reactor inner diameter of 0.91 cm The length of the tube reactor (L) was 12.7 cm

c

These DTPMP concentrations in this column are the measured DTPMP concentrations at each flow rate

d

Both Ceq and k values were calculated via fitting the experimentally obtained concentration values

Figure 6 Impact of feed solution flow rate on the effluent DTPMP concentration during the release of DTPMP from the surface of tube reactor surface.

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The dissolution of DTPMP from the tube reactor surface can be characterized by considering a one-dimensional (1-D) kinetic model combining convection and dissolution processes,26 as shown below: ∂𝐶 ∂𝑡

∂𝐶

= ― 𝑣∂𝑥 + 𝑘(𝐶𝑒𝑞 ―𝐶)

(2)

where v (cm sec-1) denotes the linear velocity. k (sec-1) represents dissolution rate constant. Ceq (mg L-1) is the effluent DTPMP concentration at equilibrium which is also Ca-DTPMP solubility at testing condition. C (mg L-1) represents the effluent DTPMP concentration. The first term on the right side of Eq. (2) represents the convection flow of DTPMP inside the tube reactor and the second term characterizes the kinetic dissolution of DTPMP from tube reactor surface. Note that Eq. (2) was developed with the assumption that Ca-DTPMP solid always exists on the interior surface of the tube reactor during the feed brine flow. If the deposited Ca-DTPMP solid has been depleted from the tube reactor, Eq. (2) will need to be modified by removing the dissolution term. This study is concerned with the scenario that Ca-DTPMP solid always exists on the interior surface and can gradually release DTPMP into the flowing feed brine. Eq. (2) can be solved by introducing dimensionless terms by letting 𝑇 =

𝑣𝑡 𝐿,

𝑋=

𝑥 𝐿

where L (cm) is the total length of the

reactor (12.7 cm). This can yield Eq. (3): ∂𝐶 ∂𝐶 𝑘𝐿 =― + (𝐶 - 𝐶) ∂𝑇 ∂𝑋 𝑣 𝑒𝑞 (3) The term

𝑘𝐿 𝑣

is also known as the Damkohler number, which is to characterize the ratio of the

residence time (L/v) in the tube reactor to the dissolution time (1/k). It can be argued that at the ∂𝐶

steady state, the effluent DTPMP concentration should not vary over time, thus the term ∂𝑇 should equal to zero. Thus, at the steady state, Eq. (4) can be obtained as:

22


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∂𝐶 𝑘𝐿 = (𝐶 - 𝐶) ∂𝑋 𝑣 𝑒𝑞 (4) The analytical solution of Eq. (4) with respect to C can be easily obtained as 𝑘𝐿

𝐶 = 𝐶𝑒𝑞 × (1 ― 𝑒 𝑣 ) (5) Therefore, Ceq and k values can be calculated by curve fitting the experimentally obtained effluent DTPMP concentrations with the calculated concentrations based on Eq. (5) at different flow rates. The results show that the calculated Ceq and k values via curve fitting were 1.90 mg L-1 and 0.00051 sec-1, respectively (Table 3). Hence, the equilibrium DTPMP concentration at the testing condition can be calculated as 1.90 mg L-1, which is very close to the DTPMP concentration of ca. 1.80 mg L-1 obtained based on the previously reported Ca-DTPMP conditional solubility model. As for a pipe with the interior surface coated with Ca-DTPMP solid, the released DTPMP concentration in the effluent can be calculated from Eq. (5). Based upon the obtained Ceq and k values at the testing condition of 70oC and 5 atm, the effluent DTPMP concentration profiles can be predicted for pipelines with diameters of 1 ft (ca. 30 cm) and 2 ft (ca. 60 cm). Figure 7 illustrates the calculated effluent DTPMP concentration profiles as a function of the length of the pipe by assuming a brine flow rate inside the pipe of 155 m3 day-1 or 1,000 barrel day-1. Figure 7 suggests that the pipe with a smaller diameter shows a lower DTPMP concentration within the first 200 meter of pipe length. The eventual effluent DTPMP concentrations from these two profiles converge at the calculated equilibrium concentration of 1.90 mg L-1 after ca. 250 meter.

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Figure 7 Calculated effluent DTPMP concentration profile from the release of DTPMP from pipes with different diameters of 1 ft (30 cm) and 2 ft (60 cm). 3.4 Calculation of inhibition effectiveness of the release DTPMP inhibitor The released DTPMP inhibitor molecules from the tube reactor surface will be commingled with and dissolved into the flowing feed solution to control scale deposition. Obviously, a higher effluent DTPMP concentration will result in a higher scale control effectiveness. Typically, a higher dissolved DTPMP inhibitor concentration in aqueous solution will lead to a higher SI(CaCO3) level against which inhibitor can protect for a fixed period of time.3 He et al.27 studied the relation between the scale inhibitor concentration and the value of SI which can be managed for a period of 2 h at various experimental conditions. According to He et al.27, the below relations are proposed: t0 = 10^(a1 +

a2 SI

+

binhibitor = b1 + b2 × SI +

a3 T

b3 T

+

a4

) SI × T [C𝑎2 + ]

+ b4 × log10 [C𝑂23 ― ]

tinhibitor = 10^(binhibitor × Cinhibitor) ×

t0 f

(6) where t0 (sec) represents the protection time in the absence of inhibitor. T (K) is temperature. [Ca2+] and [CO32-] (mg L-1) denote the aqueous phase concentrations of Ca2+ and CO32- species,

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Energy & Fuels

respectively. f is the safety factor and a value of 2 is assigned for f in this study. tinhibitor represents the protection time in the presence of inhibitor. Cinhibitor is the aqueous inhibitor concentration. a1 to a4 and b1 to b4 are inhibitor-specific constants (see ESD for details). If one assumes that the time duration for the production fluids to flow through the pipeline is within 3 h, an inhibitor protection time (tinhibitor) can be assumed to be 3 h. For each given DTPMP inhibitor concentration (Cinhibitor), the corresponding SI value can be calculated from Eq. (6) via an iterative approach. Figure 8 presents the calculated corresponding SI values for the effluent DTPMP concentrations released from 1 ft diameter pipe. In the absence of DTPMP at distance of 0 m, the calculated SI value is 0.85, suggesting that the pipe can be operated without CaCO3 damage in the absence of inhibitor for at least 3 h if SI is no higher than 0.85. At distance of 300 m, the effluent DTPMP is 1.90 mg L-1 and the corresponding SI is 1.29, indicating that the pipe can be operated with CaCO3 damage free for at least 3 h if SI is less than 1.29 in the presence of 1.90 mg L-1 DTPMP.

Figure 8 Calculated effluent DTPMP concentration profile and the saturation index (SI) profile from the release of DTPMP from a pipe with a diameter of 1 ft (30 cm).

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4. Conclusions The proposed attach-and-release strategy for aqueous fluid transporting pipeline scale control has been further evaluated in this study to examine the applicability of this strategy for field applications. Laboratory investigations were carried out to study the retention of DTPMP inhibitor to tube reactor surface with low initial DTPMP concentrations in the feed brines. It shows that a fraction of the delivered DTPMP can be retained by the interior surface of the tube reactor and the increase in initial DTPMP concentration can lead to a more rapid inhibitor breakthrough from the tube reactor. The release behavior of the retained DTPMP from tube reactor surface was studied to understand the impact of feed brine chemistry and flow rate on DTPMP release. It was found that the increase in Ca2+ concentration in the feed brine can result in a lower DTPMP effluent concentration and the release of DTPMP was controlled by the dissolution chemistry of CaDTPMP solid. In addition, the increase in flow rate will result in a lower DTPMP effluent concentration. Based upon a one-dimensional convection-dissolution equation, the dissolution of DTPMP can be characterized by the equilibrium concentration and dissolution kinetic constant calculated in an iterative manner. Accordingly, the effluent DTPMP concentration from an actual pipe surface with different pipe diameters can be predicted from the obtained convectiondissolution equation. The corresponding saturation index which can be protected against for a period of 3 h by chemical inhibition can be calculated from the effluent DTPMP concentration. The experimental and modeling investigations elaborated in this study suggest that the proposed attach-and-release strategy has the potential to be adopted in the field mineral scale control.

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5. Supporting Information Calculation of saturation index; Experimental setup; KCl tracer breakthrough experiment; DTPMP measurement; Calculation of surface area of the CaCO3 particles; Calculation of inhibition effectiveness of the DTPMP inhibitor

6. Acknowledgement This study was funded by The Science and Technology Development Fund, Macau SAR (File no. 0063/2018/A2). This work was also financially supported by Brine Chemistry Consortium companies of Rice University, including Aegis, Apache, BHGE, BWA, Chevron, ConocoPhillips, Coastal Chemical, EOG Resources, ExxonMobil, Flotek Industries, Halliburton, Hess, Italmatch, JACAM, Kemira, Kinder Morgan, Nalco, Oasis, Occidental Oil and Gas, Range Resources, RSI, Saudi Aramco, Schlumberger, Shell, SNF, Statoil, Suez, Total and the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500).

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7. References 1. W. Stumm, J. J. Morgan, Aquatic Chemistry, Wiley-Interscience, 3rd edn, 1996. 2. J.B. Becker, Corrosion and scale handbook, PennWell, Oklahoma, 1999. 3. A.T. Kan and M.B. Tomson, Scale prediction for oil and gas production. SPE J., 2012, 17, 362378. 4. J. Fink, Petroleum Engineer's Guide to oil field chemicals and fluids, Waltham, MA, 2012. 5. J. W. Mullin, Crystallization, 4th edn., Woburn, Butterworth-Heinemann, 2001. 6. P. Zhang, A.T. Kan and M.B. Tomson, Oil Field Mineral Scale Control, In Mineral Scales and Deposits: Scientific and Technological Approaches, Z. Amjad, K. Demadis, ed., Elsevier Publishing, 2015. 7. P. Zhang, P, D. Shen, G. Ruan, A.T. Kan, M.B. Tomson. Phosphino-polycarboxylic acid modified scale inhibitor nanomaterial: Synthesis, characterization and migration. Ind. Eng. Chem. Res. 2017, 45, 366–374. 8. K. Touati, E. Alia, H. Zendah, H. Elfil, A. Hannachi. Sand filters scaling by calcium carbonate precipitation during groundwater reverse osmosis. Desalination. 2018, 430, 24-32. 9. Y. Chao, O. Horner, F. Hui, J. Lédion, H. Perrot. Direct detection of calcium carbonate scaling via a pre-calcified sensitive area of a quartz crystal microbalance. Desalination. 2014, 352, 103-108. 10. Z. Li, R.V. Linares, S. Bucs, C. Aubry, N. Ghaffour, J.S. Vrouwenvelder, G. Amy. Calcium carbonate scaling in seawater desalination by ammonia–carbon dioxide forward osmosis: Mechanism and implications. J. Membrane Sci. 2015, 481, 36-43. 11. M. Chaussemier, E. Pourmohtasham, D. Gelus, N. Pécoul, H. Perrot, J. Lédion, H. CheapCharpentier, O. Horner. State of art of natural inhibitors of calcium carbonate scaling. A review article. Desalination, 2015, 356, 47-55. 12. Q. Liu, G-R Xu, R. Das. Inorganic scaling in reverse osmosis (RO) desalination: Mechanisms, monitoring, and inhibition strategies. Desalination, 2019, 468, 114065. 13. M.B. Tomson, A.T. Kan, G. Fu, D. Shen, H.A. Nasr-El-Din, H. Al-Saiari, M.A. Al-Thubaiti, Mechanistic understanding of rock/phosphonate interactions and the effect of metal ions on inhibitor retention. SPE J., 2008, 13, 325-336. 14. S. Carvalho, L. Palermo, L. Boak, K. Sorbie, E. F. Lucas, Influence of terpolymer based on amide, carboxylic, and sulfonic groups on the barium sulfate inhibition. Energy Fuels, 2017, 31, 10648–10654 15. M.F. Mady, A. Bagi, M.A. Kelland, Synthesis and evaluation of new bisphosphonates as inhibitors for oilfield carbonate and sulfate scale control. Energy Fuels, 2016, 30, 9329–9338. 16. M.F. Mady, M.A. Kelland, Study on various readily available proteins as new green scale inhibitors for oilfield scale control. Energy Fuels, 2017, 31, 5940–5947. 17. Z. Zhang, P. Zhang, Z. Li, A.T. Kan, M.B. Tomson, Laboratory evaluation and mechanistic understanding of the impact of ferric species on oilfield scale inhibitor performance. Energy Fuels, 2018, 32, 8348–8357. 18. F-A Setta, A. Neville, Efficiency assessment of inhibitors on CaCO3 precipitation kinetics in the bulk and deposition on a stainless steel surface (316 L). Desalination, 2011, 281, 340347.

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19. P. Zhang, Y. Liu, N. Zhang, A.T. Kan and M.B. Tomson, A novel attach-and-release mineral scale control strategy: Laboratory investigation of retention and release of scale inhibitor on pipe surface. J. Ind. Eng. Chem., 2019, 70, 462-471. 20. P. Zhang, N. Zhang, Y. Liu, A.T. Kan and M.B. Tomson, Application of a novel tube reactor for investigation of calcium carbonate mineral scale deposition kinetics. Chem. Eng. Res. Des., 2018, 137, 113-124. 21. J.T. Black, R.A. Kohser, DeGarmo's materials and processes in manufacturing. Wiley, 11th edn., 2011. 22. M.B. Tomson, A.T. Kan and J.E. Oddo, Acid/Base and metal complex solution chemistry of the polyphosphonate DTPMP versus temperature and ionic strength. Langmuir, 1994, 10, 1442-1449. 23. A.T. Kan, J.E. Oddo and M.B. Tomson, Formation of two calcium diethylenetriaminepentakis(methylene phosphonic acid) precipitates and their physical chemical properties. Langmuir 1994, 10, 1450-1455. 24. A. T. Kan, G. Fu and M. B. Tomson, Adsorption and precipitation of an aminoalkylphosphonate onto calcite. J. Colloid Interface Sci., 2005, 281, 275–284. 25. P. Zhang, D. Shen, A.T. Kan, M.B. Tomson, Mechanistic understanding of calciumphosphonate solid dissolution and scale inhibitor return behavior in oilfield reservoir: formation of middle phase. Phys. Chem. Chem. Phys., 2016, 18, 21458-21468. 26. M.M. Clark, Transport Modeling for Environmental Engineers and Scientists, 2nd edn, John Wiley & Sons, Hoboken, NJ, 2009. 27. S. He, A.T. Kan and M.B. Tomson, Inhibition of calcium carbonate precipitation in NaCl brines from 25 to 90°C. Appl. Geochem., 1999, 14, 17-25.

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Experimental evaluation of attach-and-release mineral scale control

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