Modeling Of High PH Sandstone Dissolution
Authors: Labrid J. 
Published: March 2010

One of the critical factors to be considered when using alkaline additives in enhanced oil recovery processes is alkali consumption. The mechanisms that control alkalinity decrease are ion exchange, precipitation, reaction with crude oil components, and dissolution of minerals. The aim of this paper is to describe the alkaline dissolution process in sandstone reservoirs.

An experimental approach was followed to identify the key features of the process. Various operating conditions were investigated concerning temperature, nature of the alkaline agent and rock mineralogy. The attack proceeds through the adsorption of silicon and aluminum species on solid surfaces, and the adsorbed species play a more or less important role according to the extent of the reaction.

Precipitation of an aluminosilicate was observed, and its chemical composition was determined from the interpretation of effluent compositions and ESCA analysis.

Kinetic equations were deduced and tested, providing initial reaction rates for the quartz matrix and the associated clay fraction. These equations emphasize the coupling of elementary dissolution processes, the role played by the clay fraction in over-all alkali consumption and the inhibiting effect of the new aluminosilicate formed.

A reservoir simulator was designed, which combines the detailed reaction chemistry and the flow properties. This model gives some new insight into the impact of kinetic phenomena on the propagation of alkaline additives at a reservoir scale.

Introduction

Many alkaline flooding field tests have been undertaken over the last twenty years, which have revealed that alkali injection is not a simplistic method but requires careful project design and monitoring techniques(1). Profitability of these field tests was not clearly established. One reason why conventional alkaline floods have been so discouraging is the lack of mobility control during alkali injection. Another reason is that low caustic soda concentrations required for obtaining low interfacial tensions prevent propagation of alkali because of its consumption by ion exchange and dissolution precipitation processes. It is, therefore, essential to identify the occurring mechanisms and to be able to estimate properly the amount of alkali loss by rock reactions. We can expect from this evaluation a better optimization of me process, i.e. the choice of the most appropriate type and concentration of alkali to obtain a high oil recovery efficiency.

This paper concerns the dissolution process. There is a controversy concerning the way of approaching the problem. Same workers consider that, on account of the high residence times of the alkaline fluids in the reservoir during a field application, the system can be described by assuming that thermodynamic equilibrium is reached in each point of space(2,3). Calculation of pH breakthrough time is then made by using solubility products data for minerals typically encountered in sandstones as well as for new minerals that could appear during the alkaline dissolution(4). Other authors deduce from laboratory experiments that dissolution is kinetically limited(5,6). In that case, this limitation is liable to affect the pH front propagation and consequently reduce the alkali consumption, even at field times scale.