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Guar-borate crosslinked fluids have been successfully used in hydraulic fracturing operations for decades. These fluids are often preferred because of cost, ease of use, and robustness. Additionally, borate fluids are renowned for their ability to recover viscosity after exposure to high shear, a process commonly referred to as “rehealing.” With the prevalence and reputation of these fluids, it is easy to become complacent and rely on the assumption of rehealability when pushing borate fluids to their operational limits. This is particularly true in the current climate of cost minimization. However, there are situations in which borate fluids might not reheal, or the rehealing process is significantly retarded. Because common assumptions about borate fluids cannot be assured in all situations, understanding the variables that affect performance is imperative.
This paper explores the properties of instant and delayed borate crosslinked fluids under different shear rates, shear histories, and a range of pH values. Various experiments were conducted to investigate the viscosity and the ability of viscosity to recover under different shear histories, temperatures, and pH values. This paper focuses on optimizing the fluid chemistry to provide the desired viscosities from the surface to deep within the fracture.
The work completed demonstrates how several borate fluid formulations can generate nearly identical viscosity profiles under a single shear rate, but vastly different profiles after exposure to high shear. If shear history is not considered in fluid design, some formulations could appear feasible during initial testing but fail to provide the desired viscosity in the near-wellbore (NWB) region during field operations. Assuming borates will simply reheal without consideration of pH and shear history on rehealing time could give rise to premature screenouts.
Borate crosslinked fluids are viewed as simple and forgiving. This is true to a point, but there are limitations that can be overlooked if the appropriate testing is not performed and formulations are stretched to their limits. The information presented in this paper demonstrates where assumptions about borate fluids' ability to reheal fail, while providing recommendations that can help ensure the desired viscosity is maintained throughout the treatment.
Polymer-based (gelled or in-situ gelled) and emulsified acids have been used for matrix acidization of carbonate reservoirs for several years. Gelled and emulsified acids are typically used for acidization of high-temperature carbonate reservoirs because of their lower reaction rate as compared with nongelled/emulsified acids, resulting in deeper penetration of acid, whereas in-situ gelled acid is used for acid diversion. Literature review indicates that several laboratory-scale experimental studies have been performed to analyze the effect of acid gelation and emulsion on carbonate acidization as compared with nongelled/emulsified acids. However, there are very few modeling or quantitative theoretical studies regarding carbonate acidization with gelled and emulsified acids that can be tested at laboratory or field scale. More specifically, a theoretical analysis of the effect of transport and rheological properties (i.e., shear-thinning behavior) of gelled and emulsified acids on the acidization process is not available in the literature. Therefore, the primary objective of this study is to analyze the effect of transport and rheological properties of gelled and emulsified acids on carbonate acidization in three dimensions, which can help in terms of design of gelled- and emulsified-acid properties to achieve lower leakoff rate and deeper penetration of wormholes.
The authors present 3D numerical simulations of carbonate acidization with hydrochloric acid (HCl), gelled acid, and emulsified acid by use of a two-scale-continuum model. By use of this model, the effect of transport and rheological properties of these non-Newtonian acids on the acidization curve and dissolution pattern is analyzed and compared with the available laboratory-scale experimental data. It has been observed from the numerical simulations that a lower amount of acid is necessary to breakthrough, and thinner wormholes are formed for both gelled and emulsified acids compared with neat HCl. Additionally, acidization remains in the optimum dissolution regime for a large variation in terms of acid-injection rate for both gelled and emulsified acids compared with neat HCl. Finally, the authors develop a wormholing criterion for acids, the rheological behavior of which can be described by the power law. This criterion can be used to estimate the optimum injection rate for vuggy and nonvuggy carbonates.
Dynamic proppant suspension behavior of carboxymethyl hydroxypropyl guar (CMHPG) crosslinked with labile (reversible) and inert (irreversible) crosslinkers are quantified and compared in this work. In reversibly crosslinked CMHPG by borate, particle settling slows with increased imposed orthogonal shear rate and proppants remain suspended for long periods above a critical shear rate. On the contrary, in irreversible zirconium crosslinked CMHPG, particle settling is negligible at low shear rates, dramatically accelerating above a critical shear rate. Local flow field data was obtained to provide insight into the dramatically different particle settling behavior. It was observed that the slowing of particle settling with increased shear rate in borate crosslinked CMHPG originates from shear-induced gelation, which leads to significant increases to both viscosity and elastic stresses. For zirconium crosslinked CMHPG sheared at low shear rates, the local velocity profile is flat, indicating that the network remains intact with negligible flow in the bulk. Above the critical shear rate, the velocity profile becomes sloped, indicating breakup of the network. Therefore, reversibly crosslinked polymers suspend proppant better at high shear rates with an upper critical shear rate for proppant suspension, whereas irreversibly crosslinked polymers suspend proppant better at low shear rates with a lower critical shear rate for proppant suspension. These findings provide valuable guidance in terms of optimizing fracture fluid design to help maximize proppant transport into fractures.