Dispersing Settled Barite: An Approach to Improve Performance and Safety of Cut and Pull in Plug and Abandonment Operations

During P&A of hydrocarbon wells, access to annular space behind the casing is necessary to establish a qualified annular barrier. Three main methods to access the annular space are cut and pull, section milling, and perforate-wash-cement. The cut and pull method is most utilized in the industry. This operation, which is both time-consuming and costly, provides comprehensive access to the pristine geological formation, consequently allowing for the establishment of a barrier between rock formations. However, the operation is associated with health, environment, and safety risks (Liversidge et al. 2006). During the commencement of a cut and pull, the casing may be stuck due to the settled barite in the annulus behind the pipe (Joppe et al. 2017). This issue arises when drilling mud, which often contains barite particles as a weighting material, fills the uncemented sections of a wellbore during the P&A phase (Melder et al. 2017). Thus, the oil and gas sector would benefit from using a method to disperse barite simply and cost-effectively.

Barite particles mixed with drilling and completion fluids tend to settle down behind casings over time due to gravity. Even though settled barite is not generally recognized as a well barrier element on the Norwegian Continental Shelf, it has been used in other regions globally as a substitute in wells with low pressure (Kljucanin 2019). The settled barite can cause problems during intervention and abandonment operations (Kleppan et al. 2016). For instance, in a North Sea well, an operator once attempted a cut and pull operation nearly 40 times and used more than 70 days to remove production casing (Desai et al. 2013). Another wellbore challenge is barium sulfate (BaSO₄) scale formation. This issue arises from the reaction between barium ions from the formation water and sulfate from seawater. BaSO₄ scales are a major concern in the oil industry due to their low solubility in water (Murtaza et al. 2023; Kamal et al. 2018).

A collection of chemical compounds called chelating agents is used to alleviate the issues associated with scaling. These chemicals form stable, soluble complexes with metal ions, including those found in barite, potentially allowing for the dispersion of settled barite and improving fluid circulation (Almubarak et al. 2022). Chelating agents, such as DTPA and EDTA, have emerged as effective solvents for dissolving BaSO₄ (Luo et al. 2020). The solubility of barite in chelating agents has been the subject of numerous studies by researchers. Lakatos et al. (2002) conducted a comparative analysis of seven different barite dissolvers, determining the dissolution capacity of each solute. The chelating agents used in their study included DTPA, EDTA, hydroxyethylethylenediaminetriacetic acid, dioxaoctamethylenedinitrilotetraacetic acid, nitriletriacetic acid, 1,2-diaminocyclohexanetetraacetic acid, and triethylenetetraaminehexaacetic acid. However, their experiments were conducted at 25°C, which does not accurately simulate downhole conditions in oil and gas wells. Putnis et al. (1995) investigated the dissolution rate of barite in DTPA at varying concentrations and temperatures. Their study utilized barite with a particle size range from 104 μm to 150 μm and a DTPA solution with a concentration range from 0.001 M to 0.5 M. However, the barite particle size used in their study differs significantly from typical drilling grade barite (75µ to 6µ size).

The later research by Putnis et al. (2008) demonstrated that a satisfactory dissolution rate could be achieved at lower temperatures with a reduced concentration of DTPA. For instance, they discovered that when barite was soaked in a 0.05 M DTPA solution for 19 hours at 22°C, its solubility reached 650 ppm of Ba. However, when the temperature was increased to 80°C, a higher DTPA concentration (0.5 M) was more effective. In this scenario, 5,280 ppm of barite was dissolved in the 0.5 M DTPA solution, compared to 4,936 ppm in the 0.05 M solution. Nasr-El-Din et al. (2004) studied two primary solutes (DTPA and EDTA) for their effectiveness in dissolving barite scale. The solubility test indicated that using both EDTA and DTPA resulted in a barite solubility of 23,700 ppm. While the outcomes of the two formulations were satisfactory, the exact composition was not revealed. Bageri et al. (2017a, 2017b) used a potassium-based DTPA chelating agent with potassium formate, chloride, and carbonate, increasing barite solubility from 67% to 95%, with potassium carbonate being the most effective. Barite was converted into barium carbonate using a high pH solution of potassium carbonate and potassium hydroxide, then dissolved using hydrochloric acid, which produced toxic barium chloride. Therefore, the combined use of EDTA and a converting agent was proposed for a more economical and environmentally friendly approach. Ivanishin et al. (2021) compared different types of EDTA, finding K5-DTPA and K4-EDTA similarly effective in dissolving BaSO₄, with K4-EDTA being the preferred stimulation fluid in terms of cost. Luo et al. (2020) evaluated the efficacy of DTPA and low molecular weight sodium polyacrylate in removing the barite scale, concluding that dissolving ability improved with increased time, temperature, pH value, and concentration. Sazali et al. (2020) explored the removal of BaSO₄ using EDTA and KCl in synergy at elevated temperatures, suggesting that agitation, high temperatures, and certain pH values were required for maximum solubility. These studies provide valuable insights into the dissolution of barite under various conditions.

Solid barite samples were obtained from a North Sea water injection well drilled in 1998. The well’s history included multiple scab liner installations necessitated by a prior leak (Yousuf et al. 2024). During a 2022 slot recovery operation, solidified barite deposits within the annulus between the later-installed 5-in. and 7-in. liners significantly hindered liner removal. To facilitate detailed sample collection, a section of the liner (1839–1968 m measured depth) was hydraulically cut and retrieved. Due to the isolating nature of the scab liner, the 7×5 in. annulus exclusively contained the completion fluid (KCl polymer mud) and lacked any formation cuttings. Furthermore, the consistent properties of the KCl polymer mud throughout the retrieved section suggest a uniform fluid composition within the settled barite column. However, potential heterogeneity of element distribution or particle compositions can be anticipated. This heterogeneity may arise from variations in iron content at different depths and across the radial profile of the wellbore (Yousuf et al. 2024). This study leverages insights from barite-scale dissolution experiments to address the challenge of debonding/dispersing settled barite created as a result of sagging. Unlike previous research focused on the complete removal of BaSO₄ through dissolution, this study proposes a novel approach—utilizing chelating agents to disperse settled barite. Dispersion aims to break down large barite clumps into smaller, more manageable particles. This could be achieved by partial dissolution around grain boundaries or by removing elements that promote agglomeration. If chelating agents only partially dissolve barium from the barite, no dispersion or particle breakdown might be observed. However, this study hypothesizes that chelating agent solutions can penetrate into settled barite particles and chelate iron, which (as shown by Yousuf et al. 2024) may be acting as binding agents between barite particles. In this scenario, dispersion and particle breakdown are expected. The primary objective is to test this hypothesis and evaluate the efficacy of a cost-effective, single-step dispersion process for barite-utilizing chelating agents under optimized conditions. A comprehensive investigation of key parameters influencing the process was conducted, including EDTA concentration, solution volume, pH, and interaction duration. This study aims to improve our understanding of how these parameters individually and collectively impact barite dispersion.

The aim of this investigation was to assess the efficacy of a chelating agent in dispersing settled barite and examine the impact of various parameters on this dispersion. We designed the experiment to determine whether EDTA could disaggregate settled barite particles. The dissolution of barite or other metallic elements was of secondary interest. Existing experimental methodologies for dispersion or deflocculation were deemed unsuitable for this study (Gregory 2013), as they may confirm dispersion but do not facilitate comparative analysis. Consequently, another method involving manual sieves was explicitly used for this purpose. This approach selected particles of specific sizes for the dispersion experiment. After the investigation, the entire solution was filtered and gently rinsed with deionized water to eliminate residual chelating agents. The particles were then dried in an oven, and their sizes were measured using the same sieves. This procedure enabled the determination of the extent to which the particles were broken down because of the experiment. Fig. 1  illustrates the experimental methodology used in this study.

The materials used in this study were settled barite samples. These samples were extracted from the already mentioned North Sea well that experienced issues with settled barite, making it particularly relevant to this study; see Fig. 2 . Through laboratory experiments, it was found that the samples are mainly composed of BaSO₄. Interestingly, iron acting as a bridging component played an essential role in forming these samples. Previous studies utilizing X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry, and QEMSCAN techniques indicated a dominant barite composition with no other detectable barium phases. Iron oxide/hydroxide acts as a binding agent between the barite particles. Furthermore, these iron-rich particles seem evenly distributed throughout the settled barite material. This characteristic was consistently observed in samples from two additional North Sea wells (Yousuf et al. 2024). However, the mechanism resulting in this behavior remains in question.

Initially, particle analysis was conducted using a Metasizer particle size analyzer. However, for this specific experiment, additional measurements were obtained by manually sieving, using five different sieves with sizes of 2 mm, 1 mm, 500 µm, 250 µm, and 125 µm. This analysis allowed for a more detailed categorization of samples into five groups based on their respective sizes. Particles of varying lengths were combined in the experimental design to ensure constant concentration throughout the experiments. This was done to ensure the integrity and consistency of the investigation. Chelating agents have been used as the dispersing agent in this study. EDTA and DTPA were used as they effectively dissolved barite scales in oilwell reservoirs and production tubing. Sodium hydroxide was used to adjust the pH to optimize experimental conditions. In addition, KCl was tried as an activator in the experiment based on barite-scale dissolution experiments.

The preparation phase was thorough and carefully planned. Each experiment involved drying and segregation of the settled barite samples into specific containers (Fig. 3)  according to the size proportions listed in Table 1 .

These specific proportions were not randomly selected but based on the PSD of settled barites. These proportions ensured that the samples used in each experiment accurately represented the material. However, the whole particle size change results were only used to determine the PSD change experiment. For the rest of the investigation, we only look at the change in particle size for 4 mm to 2 mm particles. In this phase of the experiments, we used EDTA disodium salt as the chelating agent. This choice was made because previous studies have shown its effectiveness. Subsequently, DTPA, another proven chelating agent, was incorporated.

During the process, the following steps were taken:

1. Measure the settled barite (Fig. 4a) .

2. Carefully fill a plastic container with the settled barite materials, taking precautions to avoid any breakage during preparation.

3. Gently pour a solution of the chelating agent into the container, being cautious not to disrupt or damage the settled barite particles.

4. Execute all movements with extreme slowness to ensure minimal particle disturbance and avoid agitation during the experiment. This will minimize the potential for disrupting the particle arrangement or inducing unintended interactions. Allow an hour for interaction between materials (except for time effect experiments), leaving the mixture undisturbed.

5. After this period, separate dispersed particle sizes using the five sieves used during preparations.

6. Dry the samples at 70°C.

7. Measure the weight of the dried materials (see Fig. 4b ).

All experimental tests were conducted under static conditions. The primary objective of these tests was to investigate the effect of varying concentrations of chelating agents, specifically EDTA and DTPA. The philosophy of using these two chelating agents is to identify if the dispersion of settled barite could be done through the family of aminopolycarboxylic chelating agents. The concentrations of EDTA solution examined for dispersion experiments were 0.05 mol/L, 0.1 mol/L, 0.2 mol/L, and 0.4 mol/L. The dispersion tests were usually conducted for an hour. However, the effect of the extended period on the dispersion profile was examined at five different time intervals: 30 minutes, 1 hour, 2 hours, 4 hours, and 24 hours. Furthermore, the dispersion tests were conducted at various pH conditions to evaluate the impact of pH on the profile. The pH levels tested were 6, 8, 10, and 12.

Inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma-optical emission spectrometry (ICP-OES) techniques were utilized to examine filtrates. For ICP-MS, these filtrates were collected, subjected to an additional round of filtration, and subsequently diluted in a 5% HNO₃ solution in preparation for the analysis. The analysis was conducted using an Agilent 7850 ICP-mass spectrometer with external calibration. For ICP-OES, the samples were filtered through 30 mm, 0.45µ polyethersulfone filters. Portions of the filtered samples were diluted with 2% nitric acid to the proper concentrations of targeted elements.

The magnitude of electrostatic forces acting upon barite particles settled in EDTA at a molecular level was examined using a Zetasizer Nano instrument. This instrument operates on the principle of dynamic light scattering and electrophoretic light scattering, which are used to ascertain the size, zeta potential, and molecular weight of particles and molecules suspended in a liquid medium, as described by Clogston and Patri 2011. To maintain uniformity, all samples were subjected to identical conditions. A sample cell was prepared by adding 1.5 g of settled barite to 100.0 mL of 0.1 EDTA solution. Each sample underwent three tests within three pH values of 5, 10, and 13 to elucidate the influence of pH on the dispersion behavior of the samples based on their stability coefficient.

The primary stage of this experiment was designed to assess the ability of chelating agents to disperse barite that had settled. This was done by concentrating on the visual observation of any discernible effects. This inquiry was addressed by a simple mixing experiment in which 20 mL of 0.1 M EDTA disodium salt solution was carefully poured into 2 g of settled barite. The mixing process was performed carefully to ensure a smooth and gradual combination, reducing the risk of particle breakage.

The resulting samples were observed in the laboratory for 4 hours without any disturbances. Remarkably, faint signs of small cracks on the particles were visible within the first minute of mixing. These cracks progressed over the next hour, becoming more evident. After 4 hours, an apparent fracturing of the particles was observed. The impact of EDTA on settled barite was illustrated by accompanying images, providing a concrete depiction of the observed phenomena (Fig. 5) . This experiment was a preliminary investigation into the visual dynamics of chelating agents, especially EDTA, in settled barite dispersion.

The initial stage of the analytical study was designed to assess the dispersion effectiveness of three different EDTA variants: EDTA disodium salt, EDTA tetrasodium salt, and EDTA. The inaugural experiment sought to ascertain which EDTA variant demonstrated superior proficiency in dispersing sedimented barite. To this end, 4 g of each sedimented barite specimen was vigorously amalgamated with six disparate volumes of each solution, followed by a sieve analysis. This analysis quantified the particle count traversing sieves with varying mesh dimensions. Before the study, all barite specimens comprised particles exceeding 500 μm, categorized as fine or substantial particles. After the initial test, the specimens were reevaluated to determine the quantity of particles capable of passing through identical sieves. The pH level of the solutions was maintained at 10 via potassium hydroxide adjustment. The outcomes of this experimental procedure are depicted in Fig. 6 .

The sieve analysis results elucidated the dissolution of barite particles with diverse EDTA salts. It was observed that all three formulations diminished the particle size below 500 μm across all samples, though specific samples manifested either minor or more pronounced size reductions. The principal conclusion drawn from the experiment is that all three solutions exhibited comparable dispersion performance across all concentrations and samples. At reduced volumes, the EDTA disodium salt demonstrated marginally superior performance, whereas at increased volumes, the EDTA tetrasodium salt exhibited slight predominance. Nonetheless, the negligible variances could be ascribed to experimental- or particle-size uncertainties. For all variants, dispersion efficiency escalated with concentration augmentation, indicating that ample solution volume promoted dispersion by curtailing contact time, saturation, and particle dimensions. The experiment’s objective extended beyond comparing the three EDTA solutions and aimed to select the optimal variant for subsequent experiments. Based on the findings, any of the solutions could be utilized. However, due to favorable solubility, pH stability, and availability, EDTA disodium salt was used in ensuing experiments. Notably, the experimental containers underwent a 5-minute agitation for each trial, enhancing the dispersion efficiency significantly. Contrastingly, later tests were executed with minimal or no agitation, resulting in a decline in dispersion efficiency.

The aim of this experiment was to investigate the impact of the amount of dispersant solution on the thickness of the settled barite ratio on the dispersion of barite particles. The dispersant was a 0.1 M EDTA disodium salt solution, and the barite particles used were 2–4 mm in diameter. The empirical data presented in Fig. 7  unequivocally indicate a direct correlation between the concentration of the dispersing agent, specifically EDTA, and the dispersion rate of barite particles.

As the ratio of EDTA volume and the corresponding weight of the settled barite escalates, there is an associated increase in the diffusion percentage. For instance, at a ratio of 5, a mere 50% of the particles were dispersed. However, substantial enhancement was observed when the ratio was augmented to 50, with the dispersion rate soaring to 70%. This trend suggests that a higher ratio results in a more efficacious dispersion of particles. The augmentation in dispersion can be attributed to the chelating properties of EDTA, which facilitate the breakdown of particle agglomerations by capturing the metal ions within the settled barite.

Consequently, as the EDTA concentration increases, the number of available chelating sites also rises, leading to a more pronounced dispersion effect. Moreover, the data indicate that a 10-fold increment in the dispersant volume-to-weight ratio (from 5 to 50) yields an approximate 30% increase in diffusion. This significant elevation in dispersion upon increasing the dispersant weight ratio substantiates the potency of the EDTA solution in the dispersion of barite particles. The enhanced opportunity for interaction between the EDTA molecules and barite particles at higher volumes can explain the justification for such an increase. The increased volume provides a more significant medium for the particles to be separated and dispersed, reducing the likelihood of reagglomeration and promoting a more thorough dispersion process.

This experiment used four distinct concentrations of EDTA disodium salt solution. The different volume of solutions was poured into a sample of settled barite, with particle sizes ranging from 2 mm to 4 mm. Each sample weighed 4 g. The mixtures were then subjected to dispersion for 1 hour.

Fig. 8  offers comprehensive insight that the dispersion process is influenced by both the concentration of the EDTA solution and the volume-to-weight ratio of the dispersant to the settled barite.

For a fixed volume-to-weight ratio, the percentage of particles smaller than 2 mm increases with the concentration of the EDTA solution. This trend suggests that a higher concentration of EDTA is more effective at dispersing the barite particles. Similarly, for a given concentration of EDTA, the percentage of particles smaller than 2 mm also generally increases with the volume-to-weight ratio. This indicates that using more dispersants relative to the amount of settled barite can lead to more effective dispersion. The most effective dispersion was achieved with a volume-to-weight ratio of 50 cm³/g and an EDTA concentration of 0.4 M, resulting in 64% of the particles being smaller than 2 mm. Conversely, the least effective dispersion was observed with the lowest volume-to-weight ratio of 5 cm³/g and lowest EDTA concentration of 0.05, resulting in only 37% of the particles being smaller than 2 mm. However, the limiting factor here is the solubility of EDTA in water, which depends on various factors, including pH, temperature, and the presence of other solutions (Sunda and Huntsman 2003).

In this study, we examined barite particle dispersion over varying intervals to assess its effectiveness when subjected to EDTA disodium salt (Fig. 9) . Initially, a dispersion of 42% was recorded after a duration of half an hour. This percentage reflects the proportion of particles that had been reduced to a size capable of passing through a sieve measuring 2 mm. Over time, an increase in the dispersion of particles was observed. The dispersion percentage rose to 53% after 2 hours and escalated to 63% after 4 hours. The peak dispersion was achieved after a full day (24 hours) at 75%. These findings indicate that the effectiveness of EDTA disodium salt in dispersing barite particles depends on the duration of exposure, with more extended periods resulting in more excellent dispersion. However, it should be noted that the rate of increase in distribution appeared to diminish over time, suggesting the presence of a saturation point in the dispersion process, beyond which the effectiveness of EDTA disodium salt may not be significantly enhanced. The slowing down of the dispersion rate increase might imply that the dissolution of the most dispersible particles had nearly completed, leaving aggregates of more resistant or larger particles less susceptible to dispersion. This aspect merits further exploration to determine the optimal exposure time for achieving maximum dispersion efficiency without unnecessary resource usage.

EDTA can exist in seven different forms depending on the pH of the solution. These forms are H6Y2+, H5Y+, H4Y, H3Y−, H2Y2−, HY3−, and Y4−, where Y represents the EDTA residue. The proportion of each form of EDTA changes with the pH, as shown in Fig. 10 . The anion Y4− is dominant when the pH value exceeds 10. This form of EDTA has the highest complexing capacity, meaning it can form stable complexes with metal ions. This enhances the stability of the complex (Fredd and Fogler 1998).

When EDTA chelates Fe³⁺ or Ba³⁺ ions, it forms a complex. This complex is formed by combining metal ions and aqueous solutions of EDTA (Xue et al. 1995) . The chelation forms a highly stable complex, even at neutral pH. This formation of the complex helps solubilize Fe³⁺ or Ba³⁺ ions in water, making them bioavailable (Heimbach et al. 2000). The pH of the solution significantly influences the stability of the Fe³⁺-EDTA complex or Ba³⁺-EDTA complex based on the pH. The pH-dependent stability of the Fe³⁺-EDTA complex was explained by Schwarzenbach et al. (1957), whose concept of conditional stability constants accounted for concurrent acid-base and complexation phenomena (Burgot 2012). According to Fig. 11 , which represents the data from Zou et al. (2009), Fe³⁺-EDTA complex is most stable in a pH range of 2 to 10, while the Ba³⁺-EDTA complex is most stable in the range of 6 to 14.

The study presented in Fig. 12  shows that the dispersion of barite particles is maintained at a relatively uniform level across the examined pH spectrum.

This outcome may be unexpected, considering the known pH-dependent ion-binding properties of EDTA, which binds iron at lower pH values and barium at higher ones. Nonetheless, the empirical data demonstrate that despite EDTA binding to iron and barite in distinct ways, neither the quantity of chelated material nor the chelation mode significantly affects the dispersion of barite particles. The dispersion of particles within a solution is determined by a multitude of factors, not solely the ion-binding capacity of EDTA. Factors such as particle size and shape, the ionic strength of the solution, and the presence of other interacting entities must also be considered.

In addition, the efficacy of EDTA as a chelating agent is reliant on the deprotonation of its four carboxyl groups. Given that the pKa of the fourth carboxyl group is 10.34, it is inferred that at pH levels below this value, complete deprotonation may not occur, potentially affecting the chelating capabilities of EDTA. Although the findings do not quantify the extent of particle chelation by EDTA salts, they indicate the degree of dispersion or breakdown due to chelation. Considering the higher concentration of barite compared to iron content, the chelation of a small amount of iron could be as effective in dispersing settled barite as the chelation of a larger quantity of BaSO₄.

This part of the experiment was conducted based on the knowledge that KCl can convert BaSO₄ to barium chloride in EDTA disodium salt (Bageri et al. 2017a, 2017b). The chemical equation below shows the reaction of BaSO₄ with KCl (Sazali et al. 2020).

Fig. 13  presents an analysis of the dispersion percentage of barite when different concentrations of KCl are used as an activator for the EDTA chelating reaction. The study reveals that a 40% dispersion of barite was observed at a 0% KCl concentration. However, the dispersion percentage remained at 40% even when the KCl concentration increased to 1%.

A slight increase in dispersion to 49% was noted at a 0.2% KCl concentration. However, the dispersion was slightly reduced to 48% at a 0.4% KCl concentration. In addition, a further increase in the KCl concentration to 1% led to a decrease in dispersion, reverting to 40%. The data suggest that adding KCl does not significantly enhance the dispersion of settled barite using EDTA disodium salt. The maximum dispersion (49%) was achieved at a KCl concentration of 0.2%, but a further increase in concentration did not result in improved dispersion. This could be attributed to the fact that the drilling fluid used to create the settled barite already contained KCl. Therefore, the addition of KCl cannot significantly influence the dispersion process.

We performed experiments by mixing different particle sizes to examine how dispersion affects all the particle sizes. The objective was to observe the PSD before and after the experiment. In both cases, the particles were dried before the measurement to remove moisture or EDTA. The PSD observed by using different concentrations of EDTA is given in Fig. 14 .

Fig. 14  provides a comprehensive representation of the PSD in a dispersion experiment conducted at varying volume-to-weight ratios and concentrations. Each series on the charts corresponds to a distinct volume-to-weight ratio, as indicated in the shared legend. The graphs show how many particles (wt%) are finer than a specific size. A clear trend is observable across all charts: An increase in the volume-to-weight ratio results in a higher percentage of particles breaking up for each particle size. This pattern is consistent across all particle sizes and concentrations, suggesting that a higher volume-to-weight ratio and concentration enhance dispersion effectiveness.

In all instances, the charts indicate that particles of all sizes are undergoing fragmentation. This observation is intriguing, given that intermediate sieves receive some fragmented particles from top sieves. However, the quantity of particles fragmenting from these sieves surpasses those obtained from full sieves. While all charts underscore the efficacy of the EDTA solution in particle dispersion, they also present some noteworthy observations. The volume-to-weight ratio of the dispersant to the settled barite plays a significant role for 0.05 M EDTA, more so than for the other solutions, and has the most negligible effect on 0.4 M EDTA. This means that for a 0.05-M EDTA solution, an increase in the volume of the dispersant enhances the dispersion process. However, for a 0.4-M EDTA solution, the chelation capacity reaches its maximum at the lowest volume of dispersant used, indicating that further increases in volume do not enhance the process much. Interestingly, for a 0.1-M EDTA solution, increasing the volume of the dispersant from 10 times to 25 times the weight of the settled barite decreases the dispersion performance. For 0.1 M EDTA, increasing the volume of dispersant from 10 times to 25 times the weight of settled barite decreases the dispersion performance. A similar trend is observed for a 0.2-M EDTA solution when the volume of solution is increased from 5 to 10 times.

Conversely, for 0.4 M EDTA, all volume-to-weight ratios exhibit similar performance. The best dispersion performance comes from 0.4 M EDTA with 74.4% for a volume-to-weight ratio of 50, but the second-best performance is from 0.1 M EDTA. These data indicate that factors besides concentration and volume-to-weight ratio influence the settled barite dispersion. It also provides some critical observations on how EDTA affects different particle sizes—increasing the volume-to-weight ratio for 0.1 M EDTA involving big particles more than smaller particles. However, 0.4 M EDTA influences all particle sizes at all proportions, indicating the efficiency of increasing the dispersant concentration.

The ICP-MS and ICP-OES tests were conducted to elucidate the mechanism by which EDTA disodium salt disperses settled barite. DTPA is also used for this experiment to investigate if the dispersion process can work with the aminopolycarboxylic acid family. In this experiment, the effect of three samples of 20 mL 0.1 M EDTA, 0.1 M DTPA, and deionized water is observed on 2 mg of settled barite samples (Fig. 15) .

From Fig. 15 , the deionized water, serving as the control sample in this experiment, exhibited no dissolution of either iron or barium from the settled barite, as evidenced by the recorded values of 0.0 for both elements. In contrast, the 0.1 M solution of EDTA demonstrated moderate efficacy in dissolving both elements, with 182 ppm of iron and 96.53 ppm of barium recorded. However, the 0.1-M DTPA solution showed a differential effect on the dissolution of the two elements. It was the most potent solution for iron dissolution, with 236 ppm dissolved, but was less effective than 0.1 M EDTA for barium dissolution, with only 73 ppm dissolved. It is noteworthy that EDTA is chelating only iron, not barium, which is attributable to the low pH of the solution. As depicted in Fig. 11 , iron exhibits a high stability constant at lower pH, while barium exhibits almost zero stability constant, prompting EDTA to chelate iron only. However, the behavior of chelating agents exhibits a marked shift at high pH. The constant stability chart illustrates that EDTA and DTPA form complexes primarily with barium at high pH, with trace amounts of iron also being chelated. Notably, DTPA demonstrates a superior ability to chelate iron and barium compared to EDTA, suggesting its greater efficacy at these elevated pH levels.

Fig. 16  presents the ICP-MS and ICP-OES results, illustrating the impact of the volume of EDTA solution on the weight of the settled barite ratio. The data reveal a significant decrease in the concentration of Fe as the volume of EDTA to settled barite ratio increases. At a volume-to-weight ratio of 5, the Fe concentration peaks at 1,900 ppm, declining to 1,300 and 590 ppm at ratios of 10 and 25, respectively. This trend suggests that an elevated volume-to-weight ratio results in a diminished concentration of Fe. It is noteworthy that EDTA is chelating iron, not barium, attributable to the pH of the solution. As depicted in Fig. 11 , Fe exhibits a high stability constant at lower pH, prompting EDTA to chelate it.

Interestingly, the quantity of Fe decreases with an increasing volume-to-weight ratio despite the sieve experiment indicating that a higher volume-to-weight ratio enhances dispersion performance. This is attributed to the concentration of iron that EDTA can chelate, which is reduced across the larger volume of EDTA solution. The quantification of the absolute volume of chelated materials suggests a positive correlation between the dispersion quantity and the volume of the solution, a phenomenon corroborated by sieve analysis experiments (Fig. 16b) . This observation implies that the dispersion quantity concurrently exhibits an upward trend as the solution volume escalates. Alternatively, the reduced interaction between the high volume-to-weight ratio of EDTA and the settled barite material could be a contributing factor, as agitation enhances barite dissolution. Conversely, the ICP results suggest that the iron chelation by EDTA increases with the concentration of EDTA (Fig. 17) .

As illustrated in Fig. 17 , a fourfold increase in the concentration of EDTA results in a 150% increase in iron chelation. The chelation of barium by EDTA is less pronounced due to pH constraints. These findings corroborate the results of the sieve experiment, which indicated a 127% increase in dispersion when the concentration of EDTA was increased from 0.1 to 0.2. This underscores the significant role of EDTA concentration in the chelation and dispersion processes. Further research could provide more insights into the optimal conditions for maximizing dispersion efficiency. Fig. 18  analyzes the dissolution of iron (Fe) and barium (Ba) from settled barite when exposed to an EDTA disodium salt solution at varying pH levels. It is crucial to consider that EDTA preferentially chelates iron at lower pH levels and barium at higher pH levels. Comparison of iron content across Figs. 16a and 17  reveals a significant discrepancy. The possibility exists that the samples originated from separate experiments, potentially involving different wellbore depths. Such heterogeneity is not unexpected, as the amount of iron leaching from the casing wall and mixing with the drilling mud can vary considerably depending on depth.

An initial look at the ICP-MS data may indicate a pH-dependent chelation behavior of EDTA. At pH 4, significant iron dissolution (520 units) was observed, confirming EDTA’s preferential chelation of iron at lower pH levels. Barium dissolution remained minimal (4.1 ppm) at this acidic condition. As the pH increased to 8.7, the ICP-MS data showed an iron dissolution decrease (130 ppm) while barium dissolution showed a substantial increase (410 ppm). On one hand, this initial rise suggests a shift in EDTA’s chelation preference toward barium at higher pH, potentially aligning with our hypothesis. On the other hand, Fig. 12  indicates that even under conditions unfavorable for iron dissolution (observed at pH 13 with 5.1 ppm iron), some degree of particle breakdown might still occur. One possible explanation for this observation lies in the potential precipitation of iron hydroxide at high pH. In other words, while the ICP-MS analysis may have not detected iron due to its low solubility in water as iron hydroxide, the initial dissolution or detachment of iron from the barite structure at high pH itself could facilitate dispersion. Further investigation is necessary to confirm this new hypothesis and elucidate the exact mechanism(s) governing dispersion at different pH levels.

The empirical evidence substantiates the proposition that the solution of EDTA disodium salt exhibits a preference for chelating iron at diminished pH levels while favoring barium at elevated pH levels. Consequently, the ideal pH level for the dissolution process is contingent upon the target element, iron, or barium. The perfect pH, in turn, hinges on the specific particles required for chelation to achieve efficacious dispersion in our scenario. Furthermore, it is essential to acknowledge that when the pH exceeds 10, the Y4− species, which exhibits the most robust complexation capability, becomes predominant, reflected in the results. However, it is imperative to note that these observations are intrinsically tied to the specific conditions under which the experiments were conducted and may exhibit variability when subjected to different experimental parameters. This study underscores the importance of contextual factors in interpreting and applying these findings. Fig. 19  elucidates the interplay between five distinct pH levels of DTPA and settled barite. It is observed that the chelation preference of DTPA undergoes a transition from iron (Fe) to barium (Ba) as the pH level escalates.

At an acidic pH of 2, DTPA exhibits a high propensity for iron chelation, demonstrated by a value of 94. In contrast, the chelation of barium is negligible, indicated by a value of 1.3. Upon an increase in pH to 5, the chelation of iron remains relatively constant (97), and the chelation of barium continues to be minimal (1.1). A notable shift is discerned at a pH of 8.7, where the chelation of iron plummets dramatically to 44 and the chelation of barium surges to 140. This trend persists with the further increase in pH. At a pH of 10, the chelation of iron dwindles further to 18, while the chelation of barium leaps to 570. At the most alkaline pH of 13, the chelation of iron reaches its nadir (5.7 ppm), while the chelation of barium attains its zenith (1,000 ppm). Like EDTA, DTPA has its most robust capability.

These findings lend credence to the stability constant chart that DTPA exhibits preferential chelation toward iron at lower pH levels and barium at higher pH levels. It is crucial to underscore that these results are contingent upon the specific conditions under which the experiments were executed and may exhibit variations under disparate experimental parameters.

In this investigation, we measured the zeta potential of settled barites in two distinct solutions—water and 0.1 M EDTA—across a pH range of 5 to 12. This range was selected due to the observed variation in EDTA activity at different pH levels. Fig. 20  presents the zeta potential for each solution at different pH values.

The zeta potential, which typically signifies the stability of a solution and thus its propensity to settle, decreased from zero with increasing pH in the EDTA solution. Specifically, it decreased from a maximum of −19 mV at pH 5 to a minimum of −27 mV at pH 12. This trend aligns with the findings of García-Fuentes et al. (2003), who posited that a shift in zeta potential toward zero enhances the repulsive electrostatic barrier, thereby reducing the attractive forces among particles. This suggests that the surface of the settled barite particles becomes increasingly negatively charged as the pH of the EDTA solution rises. However, the zeta potential values of both solutions are close. In other words, changing the solution from water to 0.1 M EDTA did not significantly change the electrical stability of the solution. This indicates that the dispersion behavior of settled barite observed might not be caused by a change in the electrical stability of the particles in the solution.

Initial visual test results showed the significant impact of EDTA on barite particles, causing them to disintegrate and disperse. Subsequent analytical experiments tested various EDTA salts, proving similar effectiveness in dispersing barite. It was observed that an increase in the amount of EDTA leads to better dispersion. However, the volume of the solution also plays a crucial role, with a larger volume resulting in less contact time between the EDTA solutions and the barite, less saturation, and smaller particle sizes.

• The effect of the dispersant solution ratio on settled barite dispersion was also examined. The higher concentrations of EDTA disodium salt and an increased volume-to-weight ratio significantly improve the dispersion mechanism. Although longer exposure times improve dispersion, the rate of increase appears to slow down over time, indicating a potential saturation point in the dispersion process. Despite the chelating properties of EDTA, changes in pH have minimal impact on barite dispersion. This pH behavior suggests that other factors, such as particle size, shape, ionic strength, and potential interactions with other substances in the solution, may also be influential. The addition of KCl does not improve barite dispersion, suggesting that the existing KCl in the drilling fluid may diminish the effect of additional KCl.

• Observing the distribution of particle sizes gained further insights into the effects of EDTA concentration and volume-to-weight ratio on dispersion effectiveness. It was found that the results varied depending on the size of the particles. ICP-MS and ICP-OES analyses provided a quantitative perspective on the dispersion and dissolution processes. EDTA chelates iron and barium, with changes in concentration linked to the volume-to-weight ratio. An increase in EDTA concentration can lead to more iron chelation, while the chelation of barium can exhibit more complex responses, likely due to pH constraints. The EDTA solution does not significantly alter the electrical stability of the solution; the dispersion behavior of settled barite may not be primarily influenced by changes in the electrical stability of the particles in the solution.

• When DTPA was introduced as an alternative chelating agent, it revealed its preference for chelation depending on the pH level, which aligns with stability constant trends. As the pH level increased, the observed shift from iron to barium chelation highlighted the complexities of choosing chelating agents in dispersed systems.

The study was designed based on the hypothesis that dissolution of barium alone might be insufficient to disrupt the settled barite particle network. Iron dissolution was therefore considered as a potential mechanism for dispersion. Interestingly, dispersion was observed across the entire pH range investigated (pH 4–12).

This thorough study confirms the potential usefulness of chelating agents, especially EDTA, in dispersing settled barite and emphasizes the complex nature of the factors that control this process. The findings from this research provide a solid basis for future studies aimed at optimizing dispersion methods in industrial applications using other aminocarboxylic acids, which will be particularly relevant for further mitigation of cut and pull problems and similar systems.

The authors acknowledge the Research Council of Norway (RCN) for financing the Centre for Research-based Innovation "SWIPA - Centre for Subsurface Well Integrity, Plugging and Abandonment," RCN Proj. No. 309646, for which the work has been carried out. The project is also financed by the operating companies AkerBP, Equinor ASA, and Wintershall Dea, Norway, and includes in addition more than 20 in-kind contributing industry partners. The research and development partners in SWIPA are SINTEF, NORCE, IFE, NTNU, and UiS. We would also acknowledge the support of Equinor in supplying the settled barite samples from their wells to perform the experiments.

Original SPE manuscript received for review 23 April 2024. Revised manuscript received for review 10 July 2024. Paper (SPE 223118) peer approved 20 August 2024.