Emulsified acid systems for oil well stimulation: A review

Abstract This review discusses the application of emulsified acids for oil well stimulation. Types of oil, aqueous solutions, and emulsifiers are discussed in detail along with the solid agents and inhibitors that are utilized for emulsified acid formation. It has been found that there are at least nineteen (19) proposed ways of combining the chemicals that potentially can be employed operationally. The impact of heat and viscosity on the efficiency of the acid emulsions and the carbonate-decomposition kinetics are also in focus. Pumping emulsified acids through pipelines constitutes enormous flow challenges and this review shows that it is where the main obstacle lies. High-level understanding of emulsification is paramount for smooth pumping and transport of emulsions through pipes and here we show that several mechanisms are in play and that need to be considered for a smooth operation that involves an emulsified acid. Again, pipeline design and specifications must be properly handled, as various emulsions possess unique and distinct flow patterns. Core flooding constitutes a mode of investigation that guides how to achieve the optimal emulsified acid volume during injection and field trials; such studies are highlighted. More importantly, part of the objectives herein is to present some of the rheological approaches that will assist in the assessment of drag of a specific emulsion system which is very valuable when it comes to an assessment of how various additives impacts the drag. Further, this review shows that there is still room for improvement when it comes to the formulation of smart acid emulsions with an emphasis on quick pumping into the oil well without extreme pressure build-up or pressure drop. Functionalized polymers, either organic or inorganic (here primarily organoclays), that are soluble in either water or oil and stable in strong acid can mitigate some of the challenges while pumping acid emulsions. Novel nanomaterials with a high surface area in combination with thermal and acid resilience have also recently gained traction when it comes to the formation of new classes of emulsions that would prove valuable in oilfield stimulation. Overall, the application of emulsified acid systems provides a route to better controlled and more efficient acid stimulation jobs with the drag being the remaining challenge that should be properly managed.