Scale Inhibitor Core Floods in Carbonate Cores: Chemical Interactions and Modelling

Abstract

Abstract In previous work (SPE 87447), the effect of pH on phosphonate/carbonate interaction was studied by performing variable pH core floods in short outcrop carbonate cores. Effluent concentrations of scale inhibitor (DETPMP), lithium tracer, calcium and magnesium were measured, as were the corresponding pH profiles. Such detailed sets of measurements are required to interpret the inhibitor/carbonate interaction mechanisms. This previous study showed that carbonate dissolution was evident in all core floods and on how this depended on pH. It also indicated where simple adsorption isotherms could describe this accurately and where this was not the case. Recently, we have performed additional core floods using carbonate core from a second block of this outcrop chalk using DETPMP scale inhibitor slugs at various concentrations and pH values. These results significantly extend previous work and are presented under the following two headings: Scale inhibitor/carbonate/pH interactions in short cores + modelling: Simple flooding cycles are described on the propagation of (Li traced) slugs of pH 6, 4, and 2 (no scale inhibitor) travelling through the carbonate core in order to study the behaviour of calcium, magnesium and effluent pH for various injected brine pH conditions. These results have been modelled and explained using the MultiScale software. "Contained" scale inhibitor floods in long (12″) carbonate cores: Contrasting floods using long (12″) carbonate cores have been performed where <1 PV of DETPMP is injected at the inlet and then back produced. In these floods, the core is never fully saturated and these floods are referred to as "contained" core floods [1]. Very extensive data on SI, cation (Ca, Mg and Li) and pH effluents are collected. These observations lead us to a number of conclusions on the factors that must be included in a full carbonate model. In particular, our experimental results - along with some simple modeling - greatly clarify the role of both calcium and magnesium in the mechanism SI retention in carbonate systems. Introduction Mineral scale is a major problem in many oilfields and it most often prevented by carrying out scale inhibitor (SI) squeeze treatments. Adsorption is often the primary mechanism of SI retention during squeeze treatments of sandstone formations. The scale inhibitor retention mechanism can be more complex than simple adsorption within carbonate reservoirs because the carbonate rock is a much more reactive substrate. In carbonate formations, scale inhibitors are deliberately allowed to react with the formation and precipitate as the slightly soluble calcium salt. This can result in longer scale protection times. Even neutralized inhibitors react with carbonates to form precipitates [2]. Reactions that govern the inhibitor squeeze and return are very complicated. Several factors, such as pH, [Ca2+], [Mg2+], temperature, rock mineralogy etc, affect the adsorption level and the shape of the adsorption isotherm [3, 4]. Diethylenetriamine pentamethylenephosphoric acid (DETPMP) is known to be a strongly adsorbing SI onto carbonate.