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The new generation of Drill-In Fluids have been developed to mitigate the potential of damage the producing formation and eliminate the need for post-completion cleanup. However depending on the reservoir features and the architecture selected for completing the well a significant reduction of the well productivity or injectivity can be caused by filtercake formation. The common approach is the application of breakers such as acids, strong oxidizers or enzymes. When an oxidizer treatment is the preferred option, high concentration solutions are usually applied at temperatures higher than 50–60 °C. Recently, the requirement of an effective oxidant breaker, as an alternative to acid, in low temperatures acid-sensitive reservoirs promoted an extensive research activity, which lead to a new cleanup treatment capable of effectively reducing the damage induced by the polymers which form the filtercake. In this paper the performance at laboratory-scale of a new oxidizing agent is reported and some guidelines for its application including the possibility to delay its action are provided. The new breaker system consists of two components: a solution of hydrogen peroxide at a very low concentration (1–2% v/v) and an iron complex activator. By coreflooding tests the effectiveness of this breaker to remove the external filtercake and the polymers, which entered the formation, was identified. Experimental results showed that this type of oxidants can degrade the polymers contained in the filtercake also at temperatures as low as 35°C, generating non-damaging by-products characterized by very low molecular weight. A delayed action of the breaker can be obtained by modifying the oxidizer concentration and the type of iron complex. Finally some results on the behaviour of the developed system in heavy brines including CaCl2 and CBr2 are presented. Introduction Oxidizing breakers are widely employed in the oil industry for removing the filter-cake after drilling operations or breaking the viscosity of the fracturing fluid when pumping is over. They are usually extremely reactive at high temperatures and promoting the degradation of the polymers allow to minimize formation damage and improve well productivity. The most common oxidizing breaker used is the potassium persulfate. It is very effective in the temperature range from 50 to 80°C.1–2 At higher temperatures it reacts too quickly and does not allow to obtain a gradual degradation of the polymers contained in the treatment fluids. In this case the preferred option is the use of the breaker encapsulation with polymers in order to promote a slow release of the oxidant and a delay in the breaking process.3–7 At lower temperatures it is inactive 8 and therefore an activator like the triethanolamine (TEA) has to be employed to promote the oxidizing action of the persulfate.9–11 The best concentration at which the persulfate is effective is low in the range between 0,1 and 0,3% w/w. At these concentrations it is capable of decreasing the fluid viscosity but does not provide an efficient degradation of the biopolymers to generate polymer fragments with low molecular weight.12–14 In fact, the radicalic degradation mechanism typical of the persulfate promotes the formation of high molecular weight insoluble compounds, which results from a coupling process between two polymer free radicals. These molecules can be strongly damaging for producing formation. Another common oxidizing breaker is the sodium hypochlorite, which is effective at higher concentrations (10–12% w) than the persulfate. Its degradation mechanism is not completely clear 15–17 and at present its use is not the favourite because of environmental matters. On the contrary, the hydrogen peroxide is an oxidizing agent, which is more environmentally friendly than the sodium persulfate and hypochlorite. However because of its esothermal decomposition reaction, it is not commonly applied for biopolymer degradation but preferentially for other well treatments such as steam injection.18–19 A longer delay in the break time can be obtained using the inorganic peroxides like Na2O2 and ZnO2, which produce hydrogen peroxide in-situ by action of acid.20–22
The new generation of Drill-In Fluids have been developed to mitigate the potential of damage the producing formation and eliminate the need for post-completion cleanup. However depending on the reservoir features and the architecture selected for completing the well a significant reduction of the well productivity or injectivity can be caused by filtercake formation. The common approach is the application of breakers such as acids, strong oxidizers or enzymes. When an oxidizer treatment is the preferred option, high concentration solutions are usually applied at temperatures higher than 50–60 °C. Recently, the requirement of an effective oxidant breaker, as an alternative to acid, in low temperatures acid-sensitive reservoirs promoted an extensive research activity, which lead to a new cleanup treatment capable of effectively reducing the damage induced by the polymers which form the filtercake. In this paper the performance at laboratory-scale of a new oxidizing agent is reported and some guidelines for its application including the possibility to delay its action are provided. The new breaker system consists of two components: a solution of hydrogen peroxide at a very low concentration (1–2% v/v) and an iron complex activator. By coreflooding tests the effectiveness of this breaker to remove the external filtercake and the polymers, which entered the formation, was identified. Experimental results showed that this type of oxidants can degrade the polymers contained in the filtercake also at temperatures as low as 35°C, generating non-damaging by-products characterized by very low molecular weight. A delayed action of the breaker can be obtained by modifying the oxidizer concentration and the type of iron complex. Finally some results on the behaviour of the developed system in heavy brines including CaCl2 and CBr2 are presented. Introduction Oxidizing breakers are widely employed in the oil industry for removing the filter-cake after drilling operations or breaking the viscosity of the fracturing fluid when pumping is over. They are usually extremely reactive at high temperatures and promoting the degradation of the polymers allow to minimize formation damage and improve well productivity. The most common oxidizing breaker used is the potassium persulfate. It is very effective in the temperature range from 50 to 80°C.1–2 At higher temperatures it reacts too quickly and does not allow to obtain a gradual degradation of the polymers contained in the treatment fluids. In this case the preferred option is the use of the breaker encapsulation with polymers in order to promote a slow release of the oxidant and a delay in the breaking process.3–7 At lower temperatures it is inactive 8 and therefore an activator like the triethanolamine (TEA) has to be employed to promote the oxidizing action of the persulfate.9–11 The best concentration at which the persulfate is effective is low in the range between 0,1 and 0,3% w/w. At these concentrations it is capable of decreasing the fluid viscosity but does not provide an efficient degradation of the biopolymers to generate polymer fragments with low molecular weight.12–14 In fact, the radicalic degradation mechanism typical of the persulfate promotes the formation of high molecular weight insoluble compounds, which results from a coupling process between two polymer free radicals. These molecules can be strongly damaging for producing formation. Another common oxidizing breaker is the sodium hypochlorite, which is effective at higher concentrations (10–12% w) than the persulfate. Its degradation mechanism is not completely clear 15–17 and at present its use is not the favourite because of environmental matters. On the contrary, the hydrogen peroxide is an oxidizing agent, which is more environmentally friendly than the sodium persulfate and hypochlorite. However because of its esothermal decomposition reaction, it is not commonly applied for biopolymer degradation but preferentially for other well treatments such as steam injection.18–19 A longer delay in the break time can be obtained using the inorganic peroxides like Na2O2 and ZnO2, which produce hydrogen peroxide in-situ by action of acid.20–22
A new generation of guar gum is produced in a manner that provides high crosslinked fluid viscosity. The ultra high efficiency of these polymers enables polymer concentration (loading) to be reduced without sacrificing essential fracturing fluid properties such as rheological performance, proppant transport and fluid loss control. Reduced polymer loading also provides enhanced clean-up allowing for better utilization of fracture length, as measured by lower flow initiation stresses, and higher proppant pack conductivity. These factors positively influence well productivity. These polymers are also more tolerant to both KCl and a wider range of mix waters. Field loadings of the new polymer typically range from 15 to 30 lb/1000 gal of water (i.e., 15 to 30 pptg), with most treatments ranging from 15 to 20 pptg. This paper will describe the results of rheological measurements made on low polymer crosslinked gels using steady shear. Proppant pack conductivity and regained permeability testing will also be presented. In particular, numerous treatments have been pumped into wells completed in the tight sands of the Unita, Piceance, Greater Green River and Wind River Basins of the Rocky Mountain province. Comparative post treatment analysis, utilizing basic indication techniques such as normalized production (e.g., gas production/ net pay) at fixed reference times (e.g., 30-day cumulative gas) will be presented. In addition, a more comprehensive analysis using well testing (drawdown) methods will also be included as a means to evaluate the benefits of using borate crosslinked fluids based on lower polymer loadings. Introduction Hydraulic fracturing is a means of improving oil and gas production. Its success depends, in part, on implementing the best design with the right fluid in the right formation. Historically, water-based fluids were prepared, for the most part, with viscosifiers such as guar gum or its derivatives. Recently, in a spirit of continuous improvement, new water-based viscosifiers have been developed in an effort to overcome deficiencies in the guar-based fluids. These new alternatives include low proppant-concentration slick water fracs1–3, viscoelastic surfactant fluids4–6, micro-polymer fluids7,8 and others. Although there are certainly niche applications that are ideally suited for these new fluids, the vast bulk of candidate wells requiring fracturing are still best treated with guar-based fluids. The reason for this is the guar polymer offers so many favorable properties as a fracturing-fluid viscosifier. These properties include availability, cost efficiency, mixing simplicity, viscosity control, fluid loss control, proppant suspension, controlled degradation and clean-up. However, studies over the last decade have identified fluid degradation and clean-up as the most serious weakness of the guar-based fracturing fluids. The inclusion of specialized tests that measure regained conductivity of proppant packs has identified clean-up as the most problematic factor of these treatments9,10. As a result, the industry has focused on improving the controlled degradation of the fluids with new, improved polymer (gel) breakers11–14. Significant improvement in laboratory test results and actual well performance can be attributed to inclusion of these new gel breaker refinements15. In addition to improvement in gel breakers, regained conductivity tests in proppant packs also suggest borate cross-linked guar fluids can, under certain circumstances, provide high degrees of clean-up. This effect is due to the reversible nature of the borate-guar crosslink junctions, with the reversibility being driven by increasing temperature and declining alkalinity. As the pH declines, the degree of cross-linking is reduced, diminishing gel quality and the viscosity of the fluid. The declining viscosity, due both to reduced cross-linking and gel breakers reducing the molecular weight of the polymer, contributes to improved regained fracture conductivity.
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