Scale Inhibitor Core Floods in Carbonate Cores: the Influence of pH on Phosphonate-Carbonate Interactions

Abstract

Abstract Scale inhibitor core floods were performed using outcrop Jurassic Portlandian chalk cores. The effect of pH on the phosphonate-carbonate interaction was studied by performing core floods at various fixed injected pH values (pH ~6, 4, and 2). The scale inhibitor (SI) used in these core floods was 5000 ppm active DETPMP adjusted to the application pH for that core flood. Effluent concentrations of scale inhibitor, lithium tracer, calcium and magnesium are measured, as are the effluent pH values. This thorough set of measurements makes it possible to interpret the inhibitor/carbonate interaction mechanisms quite clearly. Some carbonate dissolution is evident in all core floods which is quantified in our floods and, as expected, the degree of dissolution increases as pH decreases. The pH 2 core flood (C4) showed the highest carbonate dissolution to the extent that the fluid induced the formation of a worm hole through the core. Flood C3 carried out at pH 4 showed more carbonate dissolution than in floods carried out at pH 6, as well as giving high pH (~7) effluents. There is higher interaction between the injected solutions and the rock material in flood C3 (SI retention, G ˜ 4.5 mg/g), and an increase in permeability between the pre-treatment and post treatment stages (~13 %). All floods were modelled using a well established methodology [1, 2]. For the high pH floods (pH 6) the SI return curves were modelled very accurately using an adsorption isotherm which appears to provide an excellent description of the SI/carbonate rock interaction. For the lower pH floods (pH 4 and 2), a reasonably good, but not perfect, match was obtained using a "pseudo-adsorption isotherm" approach. However, a more complete description of the SI /rock interaction involving the role of Ca2+ is required in order to accurately model all of the features of the phosphonate/carbonate interaction. Introduction Huge oil reserves exist in carbonate systems in the Middle East and in other locations around the world. One of the aims of this study is extending our knowledge and understanding of scale inhibitor squeeze treatments from sandstone to carbonate reservoirs. Field studies have appeared in the literature which look at the deployment of scale inhibitors in carbonate reservoirs [e.g. 3, 4, 5]. It has also been noted that the SI/carbonate interaction may be more complex than a simple adsorption/desorption process [4]. In this work, we present the initial findings of a systematic study of these interactions focusing here on the effect of pH on the SI/carbonate interactions for a phosphonate (DETPMP) scale inhibitor. Mineralogy of Carbonate Rock Material In order to carry out a systematic study in well-characterized carbonate cores, a moderately permeable carbonate rock material has been sourced. Sample material was acquired from the Albion Stone Quarries Ltd, Isle of Portland. This rock originates from the Portland Basebed stone and is an open textured oolitic limestone from the Jurassic Portlandian Formation. The <3.5 m thick Basebed is the lower part of the Freestone series, which comprises the Portland Roach, Portland Whitbed and Portland Basebed Formations and is underlain by a cherty series. In this study, four methods were used to characterise the rock material; X-ray diffraction (XRD), petrographic thin section, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). XRD analysis was carried out both on whole rock and on the fine particle fraction. The results of the bulk rock XRD analysis show that the chalk is composed mainly of pure calcite (99.9%). Quartz represents ~0.1% of the rock. Analysis of the fines showed that no clay minerals are present.