Mineralogical Controls on Inhibitor Adsorption/Desorption in Brent Group Sandstone and Their Importance in Predicting and Extending Field Squeeze Lifetimes

Abstract

ABSTRACT The problem of downhole mineral scale formation is most commonly remedied by carrying out scale inhibitor "squeeze" treatments. The success of this process depends on there being an appropriate level of interaction between the scale inhibitor species and the rock formation. This interaction is described by an adsorption isotherm, Γ(C), in adsorption/desorption type squeeze treatments and the nature of this isotherm governs the dynamics of the inhibitor return profile. The isotherm depends on the factors relating to (i) the chemical species itself e.g. acid phosphonate, phosphinocarboxylic acid, polymer etc.; (ii) the conditions in the fluid e.g. [Ca2+], pH, iron content, temperature etc.; and (iii) the nature of the adsorbing surface i.e. mineralogy, surface charge, wettability etc. In reservoir formations, all of these factors may be important in determining the inhibitor/rock interaction and, hence, the squeeze lifetime which is defined as the "time" - in days or in terms of produced fluid - that the squeeze lasts before the scale inhibitor concentration falls below the "threshold concentration", Ct, required to inhibit scale in that particular case. In this paper, we focus on the effects of the reservoir mineralogy and surface conditioning of the rock on the squeeze lifetime. Commercial barium sulphate scale inhibitors (acid phosphonate and poly phosphinocarboxylic acid) were tested in core flooding experiments using oil-reconditioned Brent Group Sandstones (North Sea) in order to evaluate their performance prior to field squeeze application. Representative results are presented from an extensive series of phosphonate and poly phosphinocarboxylic acid (PPCA) scale inhibitor core flooding experiments using sandstone cores from different formations within the Brent Group as the adsorbing substrate. In order to isolate the mineralogical effects, results from comparative floods using identically oil-conditioned, clean (highly quartzitic) outcrop sandstone cores are presented. The mineralogy of these cores is described and is found to be significant when inhibitors are being selected for field application. From the experimental work presented, the presence of clay minerals (principally kaolinite) is particularly important because of their influence on the adsorption of inhibitor species. Variation in mineralogy between individual formations within a single reservoir must be addressed prior to inhibitor and core selection so that more appropriate experiments that can support the modelling of field squeeze treatments can be carried out. The results from this experimental work are then used in the computer modelling of the data in order to develop the "Field Squeeze Strategy". An example of this is presented for a field case from the North Sea where the computer predictions were used to design an improved squeeze strategy. These recommendations were implemented by the operator and the subsequent field observations are compared with the design predictions. We find that this experimental/modelling approach can result in extended lifetimes of field squeeze treatments.