Direct Monitoring of the Performance of Scale Control Programs Across the Produced. Water Life Cycle Via Suspended Solids Analysis. M.M. Jordan, Nalco; J.
SPE 93892 Direct Monitoring of the Performance of Scale Control Programs Across the Produced Water Life Cycle Via Suspended Solids Analysis M.M. Jordan, Nalco; J. Buckman, Heriot-Watt U.; and C.J. Johnston, Nalco
Copyright 2005, Society of Petroleum Engineers This paper was prepared for presentation at the SPE European Formation Damage Conference held in Scheveningen, The Netherlands, 25-27 May 2005. This paper was selected for presentation by an SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
Abstract Effective scale control in production of hydrocarbon deposits is in many fields essential to the economic and safe production of hydrocarbon and associated fluids. Chemical inhibitors of inorganic scale (carbonate and sulphate) have long been applied to subsurface (continual injection or squeeze) to flow lines/process equipment along with the growing area of produced water reinjection. The following paper will outline a method of improved performance monitoring of such chemical treatments. The measurement of suspended solids is a common practice to determine injection water quality but the measurement of the type, amount, texture and composition of solids within produced fluids via environmental scanning electron microscopy (ESEM) combined with energy dispersive analysis (EDX) has not until now been used as a routine method to monitor the effective scale control programs applied downhole, topside and for produced water reinjection. The collection/filtration of small quantizes of produced water that are subsequently analyzed for the texture and composition of sulphate/carbonate solids has been used in a number of fields within the North Sea and Gulf of Mexico as a direct method of scale inhibitor performance. This novel method of measuring chemical performance that does not rely on chemical or brine analysis and allows drilling related solids to be differentiated from scale formed within produced brine. The paper will present results from 4 fields to illustrate the value this method of monitoring was able to bring the field operators to optimize scale squeeze treatments and topside treatment rates.
Introduction The Water Life Cycle (Figure 1) can present many challenges in terms of controlling scale formation1,2 associated with the changes in physical conditions of produced water (carbonate scale) or incompatibility observed when brines mix with incompatible ions (sulphate scales). In this paper an outline will be given of the value of the evaluation of suspended solids via environmental scanning electron microscopy (ESEM) to assess the degree of scale control across the water life cycle from source water wells, production wells from low to high water cut, across the topside process and for produced water reinjection. Scale inhibitor performance monitoring. Historically the methods of monitoring the performance of a scale inhibitor program have relied on analysis of chemical residual and scaling ion concentration. More recently the development of real time monitoring3,4,5 has taken the management of such programs to a new level. In previous publications the integration of these monitoring methods have been outlined.6,7 Results are presented in the following sections that show that an understanding of the types of scale present, their textures within produced fluid via scanning electron microscopy and compositional analysis of these solids via energy dispersive Xray (EDX) analysis adds an extra dimension to the understanding of scale management for downhole and topside applications. Environmental Scanning Electron Microscopy The field emission environmental scanning electron microscope (ESEM) represents several important advances in the scanning electron microscope technology. Whereas conventional scanning electron microscopy (SEM) requires a relatively high vacuum in the specimen chamber to prevent atmospheric interference with the primary or secondary electrons, an ESEM may operate with a poor vacuum up to 10 torr of vapour pressure (conventional SEM working at 10-5 torr). As a result it is possible to examine wet samples and there is no need to coat the samples with gold such as is used to make nonconductive samples conductive in conventional SEM evaluations. When the electrons strike the sample surface some electrons are reflected as “Backscattered Electron” (BSE) which help to image the samples in terms of density of the solid under examination. Low density solids appear darker
2
grey and high density solids appear lighter grey. The emission of electromagnetic radiation from the specimen occurs at various wavelengths also. Of principle interest is X-ray radiation which allows the measurement of the elemental composition of the sample. The device which allows composition information to be obtained on a specimen is called Energy Dispersive X-Ray analysis system (EDX). The use of conventional SEM for evaluation of geological samples is well published8,9 along with the use of EDX to gain insight as to composition of rock minerals.10 A comparison of the conventional scanning electron microcopy method and the environmental scanning electron microscope has previously been published.11 In summary the value that ESEM brings to sample analysis is the ability to examine textures and composition of organic/inorganic scales and reservoir mineral samples which contain or are coated with oil or brine without the need to dry or solvent clean the samples, Figure 2.
Development of Suspended Solids Risk Matrix The above section has outlined a novel evaluation method that can obtain information in terms of amount, type and texture of suspended solids recovered from produced fluids. This information can be combined with water production rate data to create a scale suspended solids risk matrix for individual wells within a production or injection environment. The initial stage of the assessment involves the determination of the amount of scale present on a filter in terms of surface area coverage. The larger the amount of high density (pale grey to white) scale in the fixed volume (50ml) sample the more of the low density (black) filter paper it will cover, Figure 3. The more scale the higher the risk. The amount and type of scale (put into area coverage categories) is then plotted against the amount of water (produced volume categories) that the sampled well produces or receives. The larger the water volume the more potential there is for scale formation and so the higher the risk. Figure 3 shows the method of categorization of the area of scale coverage and the production/injection volume categories. A typical Boston square type plot is then used to visually display the risk so helping to priorities wells which require some form of inhibition is presented in Figure 4. Figure 5 shows a schematic of the risk assessment process as generated for a North Sea operating company the risk values used to set the requirement to repeat monitoring or implement a treatment program would be field specific.
Results From Application of ESEM Methods to the Field In conventional monitoring of scale squeeze treatments evaluation of the scaling ions data is linked to changing proportions of injection vs. formation water to assess dilution vs. scale formation. The change in scaling ions is also linked
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to changes in the concentration of the scale inhibitor within the produced brine and any resulting physical changes in well production rates (total fluid and water rates). The inclusion of suspended solids analysis can greatly help to assess the effectiveness of the scale squeeze or continual inhibitor application treatments by giving an indication of actual scale formation or control within the brine rather than inferred control from ion concentration or scale inhibitor residuals. The following subsections show the value of this method for assessment including scale type identification, scale squeeze treatment effectiveness, when to start/stop scale squeezing wells, evaluation of topside and produced water reinjection scale control programs Suspended solids evaluation used to assess aquifer well production problems. Injection water sources for pressure support and enhanced sweep efficacy are typically the readily available water source available in the location where the field is discovered this may included seawater, river water or aquifer water. In desert locations aquifer water is the most common water source. Field A within North Africa uses aquifer water (Table 1) to enhance oil recovery. This brine was assessed to have a mild sulphate scale risk based on the typical assessment process of collecting water samples, carry out scale prediction and then assign the scale risk, Table 2. Based on the scale type and amount of scale predicted (following the process outlined in Figure 6) from the aquifer wellheads to the aquifer production manifold to the central processing facility (CPF) as small quantises of scale inhibitor was applied at the wellheads. Production of the aquifer water was monitored for several months over that period the downhole flow meters became fouled with calcium sulphate scale, Figure 7. The central process facility where the aquifer water arrived for filtration and then reinjection experienced problems with scale formation with coarse filters and injection pumps. While the problem of scale within the downhole flow meters could be explained due the this location being upstream of any chemical injection questions were raised as to why with adequate scale inhibitor injection should problems be observed in the central process facility. To fully evaluate the scale problem suspended solids samples were collected and evaluated by environmental scanning electron microscopy (ESEM) to determine the amount and type of scale present with samples being collected across the aquifer source water process. Evaluation of suspended solids within aquifer brine retained on filters while the scale inhibitor was not applied to the system is based on Figures 7 to 10 and is as follows:• Calcium sulphate scale particles were present within the aquifer water prior to it reaching surface. This conclusion is reached due the presence of large calcium sulphate grains within fluids sampled at the wellhead and scale within the downhole flow meter, Figures 7 and 8. •
As the pressure drops within the manifold calcium carbonate scale is forming. Carbonate is present in
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manifold samples and within the sample collected at the CPF, Figure 8 and 9. •
After the aquifer brine leaves the manifold no additional sulphate or carbonate scale grains are nucleating within the brine, the existing scale grains continue to grow in size in the flowing fluid. There are clear growth rings on the collected grains and their size increase from wellhead to CPF, Figure 9 and 10. From this evaluation it was clear that two scale types were present not the one suggested from the scale prediction, Table 2, and that if the scale inhibitor levels were not maintained above that required to control both scale types solids could collect within coarse filtration equipment within the CPF. Control measures recommended based on the suspended solids and ESEM/EDX study where as follows:Application of scale inhibitor was required at each wellhead to prevent the further growth of sulphate and formation of carbonate scale within the aquifer brine as it flows to the CPF. Application of a squeeze treatment to aquifer source wells would eliminate the presence of sulphate scale within the produced fluids and reduce the need for additional scale inhibitor on the topside. The scale inhibitor chemical application rate must be adjusted based on the presence of suspended solids within the aquifer fluids. Suspended solids will adsorb some of the topside scale inhibitor so removing some chemical from solution and reducing the effectiveness of the treatment program. Additional chemical would require to be added to compensate for chemical adsorbed onto scale particles. Produced aquifer brine passing thought the filters on the CPF should receive additional treatment of scale inhibitor due to scale inhibitor adsorption onto the filter media and scale particles. This method of improving scale monitoring within the aquifer source water system would not have been possible based on ion chemistry tracking or scale inhibitor residuals. Suspended solids evaluation used to assess low water cut production and separation challenges While the use of ESEM has great values when being applied to low oil content brines its real values come when evaluating solids retained within the oil phase. Conventional SEM would not be able to image hydrocarbon containing samples. ESEM with EDX has been used to successfully evaluate production problems within low water cut wells and wells which produce emulsions. Low water cut case study. An oil production well (Well 1) within a field in the North Sea experienced a reduction in total fluid rate. The well produced at
