Combined Scale Removal and Scale Inhibition Treatments

SPE 60222 Combined Scale Removal and Scale Inhibition Treatments P.S.Smith, C.C.Clement C.C.Clement Jr., BP Amoco; and A.Mendoza Rojas, Ecopetrol - Instituto Colombiano del Petróleo

Copyright 2000, Society of Petroleum Engineers Engineers Inc.

Introduction

This paper was prepared for presentation at the 2000 Second International Symposium on Oilfield Scale held i n Aberdeen, UK, 26–27 January 2000.

Scale precipitation is a common cause of impaired well productivity, with calcium carbonate being the most common scale that is formed. Hydrochloric acid is frequently used for removal of carbonate scale, 1-3  as it generally offers both the best performance and the lowest cost. Such acid treatments can be very effective in providing short term stimulation benefits to such wells, but the treatments are often short lived. 3 For high temperature applications, applications, organic acids have been used in preference to hydrochloric acid, due to corrosion concerns. 4-5 The dissolution of calcium carbonate by chelating agents is also well known 6-10. Treatments with chelating agents to remove calcium carbonate scale have been performed, 6 but dissolution rates are generally lower than with acid and treatment economics economics tend to restrict their use. Calcium carbonate scale deposition can be effectively inhibited in most fields, with scale inhibitor squeeze treatments being widely used to prevent scale build-up 11-13. In these squeeze treatments scale inhibitor is retained in the formation either via adsorption onto the rock surface 14-16, or by precipitation (or phase separation) of the calcium salt of the inhibitor. Precipitation squeezes offer increased squeeze life 17, but may have an associated risk of formation damage during treatment, 18 especially in damage sensitive formations. Although scaling potential can be predicted 19-21 and proactively treated, it is still common for scale to form in some wells, either before an inhibition treatment is pumped or after the end of the treatment life. This can occur if there is insufficient early warning of the onset of scale, or if limitations in well access or equipment availability delay a planned treatment. Consequently, even when inhibition is the scale management strategy, the need still arises for scale removal treatments. Conversely, when acid stimulation is the preferred scale management tool, there would be a benefit in reducing treatment frequency if the job life could be extended by simply adding an effective scale inhibitor to the acid system. system. Although the potential benefits of such combined treatments have been recognised, it has been reported that the post-acid treatment environment prevents scale inhibitors from performing effectively. 22 The current approach to this problem, therefore, is to pump sequential scale removal and scale inhibition treatments.

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Abstract Calcium carbonate scale impacts oil production in a large number of fields worldwide. This scale is generally managed by acid washing to removal the scale and/or by performing scale inhibition treatments. The choice between inhibition and regular stimulation is cost driven, with high cost operations generally selecting inhibition. However, even in wells where inhibition is planned, some scale can be deposited either prior to scale inhibitor deployment, or after the end of the scale inhibition treatment life. Consequently, stimulation treatments are required in many wells, in order to remove calcium carbonate scale. Combining scale inhibition with scale removal treatments offers several advantages. Firstly, in many operating areas, it would reduce the well intervention cost by making the operation a single intervention, offering significant economic benefits and a reduction in well intervention risk. Secondly, pumping a combined treatment not only reduces the risk of  scale re-precipitation during the stimulation treatment, but it ensures that the zones that are stimulated are also inhibited. This directly protects value added by the scale removal treatment. This paper details the development of combined scale removal and inhibition treatments, from project initiation to readiness for field trials. The main challenges that need to be addressed in order to achieve an effective combined treatment are discussed. Data from a laboratory study, investigating the potential for combining scale inhibitors in hydrochloric acid, organic acid and scale dissolver systems are presented and the most effective combined systems are identified.

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P.S.SMITH, C.C.CLEMENT JR. AND A.MENDOZA ROJAS

SPE 60222

Challenges

Scale Dissolver Baseline Performance

Several challenges must be overcome in order to develop an effective combined scale removal and scale inhibition treatment. The most notable are : (a) Cost   - the combined treatment must offer an economic advantage when compared to sequential stimulation and inhibition treatments. (b) Corrosion Control  - the scale inhibitor must not cause a significant change in the corrosivity of the stimulation system. If the stimulation system requires the use of a corrosion inhibitor then the scale inhibitor must not prevent its function. (c) System Compatibility Compatibility  - The scale inhibitor must be totally compatible with the stimulation system, both live and spent. The combined treatment must also be compatible with formation fluids. (d)  Inhibitor Adsorption - The scale inhibitor must effectively adsorb onto the formation, throughout the potential pH range of the stimulation system (live to spent). (e) Process Compatibility   - The flowback after a combined scale removal and inhibition treatment (live or spent) must have no adverse effect on the process system operation.

A series of experiments were performed in order to rank the scale dissolvers and acid systems selected for the study. Corrosivity, scale dissolution capacity and compatibility with the scale inhibitor were assessed. At the end of each test, any dissolver that failed to meet a prescribed minimum standard was eliminated from the study.

Several combined scale removal and inhibition systems could be considered in order to meet these challenges. Hydrochloric acid (HCl) may be the most cost effective treatment to remove calcium carbonate, but corrosion control, system compatibility and inhibitor adsorption may all be difficult in a combined treatment. Conversely, scale dissolvers (based on chelating agents) may offer better corrosion control and scale inhibitor compatibility when spent, but will be higher cost. Organic acids could offer a compromise which allows most of the system requirements to be met. An experimental programme was initiated to identify the best options for combined scale removal and inhibition treatments. This study included inorganic and organic acids, along with chelating scale dissolvers.

Experimental Due to the large number of commercially available scale dissolvers in the market place, a series of screening tests were performed in order to rank the scale dissolvers that had been selected for the study. A total of ten scale dissolvers were provided by seven companies (referenced as D1,D2....D10 in this study). A 15% acetic acid system and a 7.5% HCl acid system were also included in the study (referenced as A1 and A2 in this study, respectively). These acids were used as performance benchmarks and ensured that a wide range of  scale dissolution options were evaluated. A second stage of the study investigated the performance of the best dissolvers and acid systems when combined with scale inhibitor. More extensive tests, and more rigorous test conditions were used for this performance evaluation.

Corrosion Rate : Initial corrosion testing was performed at ambient pressure and elevated temperature (175ºF), using 13Cr coupons prepared from a section of L-80 completion liner. The acceptable corrosion rate was set at 0.02 lbs/sq.ft., based on field requirements, with coupon exposure times of 8, 16 and 24 hours being assessed. All tests were performed in duplicate and the results averaged. Figure 1 shows results after 24 hours of exposure. Five of the scale dissolvers exhibited much lower corrosion rates than the acid systems systems included in the study, but three of the dissolver systems failed to meet the acceptable corrosion rate. The corrosion rate of these dissolvers was an order of magnitude higher than the acceptable standard. Both of the acid systems had acceptable corrosion rates. Dissolution Capacity :  :   Calcium carbonate dissolution was assessed at ambient pressure and elevated temperature (150ºF and 200ºF) using both the neat scale dissolvers (as supplied) and 50% v/v solutions of the dissolvers in a 2% w/v KCl brine. Calcium carbonate chips, graded to 1-2mm, were used for these tests and the reaction was allowed to go to completion (24 hour test time). The minimum performance standard was set as 50% of the dissolution capacity of the reference 15% acetic acid system (A1). Most of the scale dissolvers tested exceeded this standard, but two of the dissolvers showed poor dissolution potential. Results for the neat scale dissolvers at 200ºF are shown in Figure 2. Three of the scale dissolvers gave results better than the acetic acid system but poorer than the 7.5% HCl system and one of the scale dissolvers outperformed outperformed the 7.5% HCl system. system. Scale Inhibitor Compatibility : Basic compatibility tests were performed to ensure that all of the scale dissolver/acid systems were compatible with the selected formation water, crude oil and scale inhibitor. An analysis of the formation water used in this study is shown in Table 1. A top ranked scale inhibitor from a separate study using the same formation water chemistry was used in these scale inhibitor compatibility tests. Some incompatibilities were found when mixing the scale inhibitor with each of the five remaining scale dissolvers, and with both of the acid systems. Generally some precipitation was observed, although for some blends the quantity of  precipitate was small and only developed after aging for 24 hours. A further series of tests was performed using spent scale dissolvers and acid systems, so as to identify any adverse impact of reaction by-products. A major incompatibility was

SPE 60222

COMBINED SCALE REMOVAL AND SCALE INHIBITION TREATMENTS

observed with all of the scale dissolver and acid-inhibitor blends resulting in abundant precipitation. Calcium Tolerant Scale Inhibitors : The calcium tolerance of the selected inhibitor (SI-1) was assessed. Although the inhibitor was stable in the presence of high calcium at low pH, precipitation occurred as the pH increased (Figure 3). It was concluded that the precipitate observed was the calcium salt of  the phosphonate scale inhibitor. This incompatibility incompatibility with SI-1 required an alternative, more calcium tolerant scale inhibitors to be identified. Several inhibitors were tested, and those with the best calcium tolerance were used for further compatibility tests with the spent dissolvers and acid systems. No single inhibitor was found that was compatible with all of acid and dissolver systems. One inhibitor was identified for the scale dissolvers and the acetic acid system (SI-2) and a separate inhibitor for the HCl system (SI-3). Ranking : The screening tests had clearly ranked the scale dissolver systems, with 50% of the dissolvers tested being rejected for not meeting a minimum performance standard. Of  the remaining five dissolvers, two ranked highest in both of the screening tests. The dissolution performance of these two dissolvers were comparable to that of 7.5% HCl, with the added benefit of lower corrosivity - the corrosion rates to 13Cr steel were an order of magnitude lower than the 7.5% HCl.

Combined Scale Removal and Inhibitor Performance A series of more rigorous tests were performed, both in terms of incorporating the scale inhibitor into the stimulation system and in terms of tests conditions. The two highest ranked scale dissolvers (D1 and D2) and the two acid systems (A1 and A2) were used for this stage of testing. Downhole Corrosion : Corrosion rates were measured under high pressure and high temperature conditions (4000 psi and 270ºF) with a 16 hour exposure time to a 13Cr coupon. All of  the systems had corrosion rates less than 0.02 lbs/sq.ft. The corrosion rates of the scale dissolver systems were an order of  magnitude less than the 7.5% HCl acid system. Corrosion Screening with Scale Inhibitor : Tests at ambient pressure and elevated temperature (175ºF) showed no negative impact on the corrosion rate after 8, 16 or 24 hours of  exposure when 15% v/v scale inhibitor (SI-2 or SI-3, based on compatibility) was blended with the scale dissolver or acid systems (Figure 4). The corrosion rate of the acetic acid system actually reduced substantially when the scale inhibitor was included. Impact of Scale Inhibitor on Scale Solubility : The impact on the scale dissolution capacity of incorporating the scale inhibitor (SI-2 and SI-3) in the scale dissolver or acid system was assessed. In the scale dissolver and HCl systems there was no significant impact, but the scale dissolution performance of 

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the acetic acid system was reduced by more than 50% (Figure 5). This reduction in performance coincides with the observed reduction in corrosion rate, and may be due to the buffering of  the acetic system by the scale inhibitor. The combined acetic acid-inhibitor system did not meet the minimum performance standard set and was therefore eliminated from subsequent tests. Downhole Corrosion Rate with Scale Inhibitor : Corrosion tests, at high pressure, and high temperature (4000psi and 270ºF, 16 hour exposure time) were performed with the two scale dissolver-scale inhibitor systems and the HCl-scale inhibitor system. system. Very low corrosion rates were observed with the scale dissolver systems (