New Oxidizing Breaker for Filtercake Removal at Low Temperature.

Abstract

Abstract 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