Decoupling Phosphonate Scale Inhibitors Adsorption and Precipitation Mechanism

Abstract

Abstract Scale inhibitor squeeze treatment is a widely used to prevent inorganic scale deposition in oil and gas production operations. Squeeze treatments consist of three steps: firstly, a bullheading of a scale inhibitor (SI) solution in a producer well. Secondly, an overflush stage pushes the SI deeper into the formation, and the well is subsequently shut-in for some time to allow the inhibitor to further retain into the formation. Third and finally, the well is put back in production, and the SI is slowly released in the production brine. The treatment will protect the well if the concentration in the produced brine is above a certain concentration, known as MIC (Minimum Inhibitor Concentration), normally a few ppm. The retention level is crucial in determining the squeeze treatment lifetime, particularly at low concentrations. It is generally accepted that the retention is governed by adsorption and precipitation mechanisms, which will depend on temperature, pH, and divalent ions concentration such as Ca and Mg. Recent experiments concluded that adsorption is more dominant than precipitation at low concentrations. Traditionally, adsorption/precipitation experiments have been performed at the macroscale using bottle test, analyzing the supernatant after filtering through inductively coupled plasma (ICP) spectroscopy, and applying scanning electron microscopy (SEM) imaging to analyze the SI retention on particulates. Although these techniques provide very valuable information, they are not capable of detecting surface deposits occurring when the SI retention is governed by an adsorption and adsorption /precipitation regime. High-resolution X-ray CT scanning is used in an in-house built mini coreflood system, which consists of a plug flow reactor filled with sandstone (0.25-0.5mm grain size, internal diameter of 4 mm). Coverage and volume deposited were detected through advanced imaging processing. The results show the surface deposit morphology, differentiating between regimes of pure adsorption or adsorption/precipitation. In addition, the retention level and release rate were estimated. This study results will lead to more effective scale inhibitor squeeze treatments.