Reducing the Scaling Potential of Oil and Gas Produced Waters with Integrated Accelerated Precipitation Softening and Microfiltration

Introduction The burgeoning development of domestic energy resources has been closely tied with the production of large volumes of saline water, termed produced water. As a matter of perspective, in 2007 roughly 21 billion barrels (1 barrel = 42 U.S. gallons) of produced water were generated in the U.S. from oil and natural gas extraction [1]. High concentrations of total dissolved solids (TDS) and sparingly soluble salts are characteristics that make produced water difficult to treat and reuse for beneficial purposes and has led to many stakeholders using deep well injection as their primary disposal strategy. Although desalination is not always required for produced water reuse, it is a necessity for many reuse options like stream flow augmentation, crop irrigation, livestock watering and cooling processes. While many options exist for water desalination, reverse osmosis (RO) is amongst the most popular and has been used in produced water treatment applications [24]. A common challenge for the application of RO processes for produced water treatment is mineral scaling, which often limits the achievable feed water recovery ration [5-7]. This challenge is exasperated for produced water applications as a result of their composition. Reducing the risks associated with mineral scaling are important to realizing the potential benefits of produced water reuse. Mineral scaling results in the formation of relatively dense deposits on surfaces that ultimately affect process performance. For example, scale formation on heat exchangers reduces heat transfer efficiency, while on desalination membranes it results in reduced hydraulic permeability. Common mineral contaminants include calcium, barium, magnesium, iron and strontium that precipitate onto the RO membrane with sulfate and carbonate also present in the produced water [6]. A variety of factors determine the onset of scale formation, such as polyvalent cation concentration, solution pH and temperature. Commonly encountered types of mineral scales in RO systems are calcium carbonate, calcium sulfate and barium sulfate [6]. Development of new, or the refinement of existing, softening processes is further motivated by the emergence of wastewaters, like oil and gas produced waters, that can be highly saline and laden with sparingly soluble salts and minerals [2]. This presents challenges for treating produced waters or reinjecting produced waters as a result if scale formation on pipes or in the formation [8,9]. For membrane processes mineral scaling is a limiting factor that in many cases dictates the recovery ration for a given system [3]. Strategies like acidification and /or anti-scalant (dispersant) addition are widespread and relatively successful scaling mitigation strategies; however, reducing the concentrations of scale forming elements may extend the achievable recovery ration even further. This is particularly relevant for treating produced waters whose complex *Corresponding author: