Decoupling Phosphonate Scale-Inhibitor Adsorption and Precipitation Mechanisms

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 218743, “Decoupling Phosphonate Scale Inhibitors Adsorption and Precipitation Mechanism,” by Isaac A. Løge, Technical University of Denmark, and Oscar Vazquez, Heriot-Watt University. The paper has not been peer reviewed. _ Scale-inhibitor (SI) squeeze treatments will protect the well if the concentration in the produced brine is above a certain concentration, known as the minimum inhibitor concentration. It is generally accepted that retention level is governed by adsorption and precipitation mechanisms. Recent experiments concluded that adsorption is more dominant than precipitation at low concentrations. To detect surface deposits occurring when the SI retention is governed by an adsorption and adsorption/precipitation regime, high-resolution X-ray computed tomography (CT) scanning was used in an in-house-built minicoreflood system. The results showed the surface deposit morphology, differentiating between regimes of pure adsorption or adsorption and precipitation. Introduction The focus in the complete paper is on squeeze treatments, which consist of the deployment of threshold SIs, such as phosphonates and polymers. The efficacy of this type of treatment, or how long the well will be protected against scale deposition, depends on the level of retention of the SI in the rock formation, which, in turn, depends on the interaction of the SI and the minerals of the rock formation. It is generally accepted that retention is governed by adsorption and precipitation. Sis are essentially weak polyacids that dissociate as a function of pH described by acid-dissociation constants. They generally are quite strong ligands that will bind to metal ions such as calcium (Ca2+) and magnesium (Mg2+). At low concentrations of SI and divalent cations, the SI molecules will be retained on the rock surface through electrostatic interactions governed by an adsorption mechanism. On the other hand, SI-dissociated species can create complexes with divalent cations such as Ca2+ and Mg2+, which usually exist in the formation and injected brines and may be released from the reservoir rock, especially in carbonate formations. Some complexes may precipitate depending on their solubilities. A precipitation mechanism is the dominant mechanism in systems rich in divalent cations such as Ca2+ and Mg2+, for example in carbonate reservoirs. The authors write that they have shown in previous work that, in pipelike flows, the mechanism at the surface plays a major role in how SI interacts with scaling minerals. To assess SI retention, high-resolution X-ray CT scanning is used combined with an in-house-built minicoreflood system consisting of a plug flow reactor of 4-mm internal diameter filled with sandstone grains (between 0.25-mm and 0.5-mm grain size). This technique is capable of detecting the location and magnitude of surface deposits. The aim is to bridge the observations between bottle tests and coreflooding experiments. In a bottle, the detection capabilities are enormous; however, the mass/volume ratio (mass of substrate to volume of solution) is orders of magnitude smaller than in a coreflood. In addition, no real capability exists to visualize the experiments, apart from monitoring the coreflood effluent. Although it is possible to scan a coreflood, it is very difficult to scan the whole core to detect precipitate at sufficiently small scale. Therefore, a minicoreflood seems to be a suitable technique to study SI retention.