Abstract
During a scale inhibitor squeeze treatment, the dynamics of the inhibitor return is governed by the inhibitor/rock interaction. In the case of adsorption/desorption inhibitor treatments, this interaction is described through the inhibitor/rock adsorption isotherm. The rock mineralogy plays an important role in determining the nature of the adsorption isotherm and clays in particular are known to strongly affect the adsorption process. However, no previous work has (a) presented a systematic study of the effects of various clays on inhibitor adsorption and (b) clearly demonstrated the significance of this behaviour for the squeeze process. It is the central objective of the work in this paper to carry out these tasks.
In this paper, an extensive series of results from static bulk adsorption tests using pure mineral separates (quartz, kaolinite, chlorite and muscovite) as adsorbent and using phosphonate (DETPMP) as model inhibitor is presented. These allow us to make a systematic study of the factors (e.g. pH, [Ca2+], temperature, etc) affecting inhibitor adsorption onto these substrates. The inhibitor adsorption (measured in mg/g) is found to be much higher on the clays than on quartz, consistent with their higher surface areas. Phosphonate adsorption shows a declining trend as solution pH increases from acidic to near neutral values, which is significant at ambient temperature. At weak acidic conditions (~ pH 4), the inhibitor adsorption is strongly affected(enhanced) by temperature rise from 25°C to 95°C. Calcium has been shown to be involved in the adsorption (or surface condensation) of phosphonate scale inhibitor onto quartz (at pH ~ 6 at 25°C and at pH ~4 at 95°C) and, indeed, it also plays a role in the binding of inhibitor onto the clay surface. However, for clays, no minimum in bulk adsorption level is seen at pH ~ 4 as is observed for quartz. From our results, the order of importance of factors governing the phosphonate inhibitor level onto clays appears to be Temperature > pH > [Ca2+]. Some supporting electrokinetic (electrophoretic) measurements on the quartz and clay mineral samples are reported from which the ζ-potential at the mineral surface has been determined as a function of pH under various solution conditions. It is found that the phosphonate adsorption behaviour onto both quartz and clays correlates very well with the surface charge measurements.
To support the findings from the bulk adsorption tests, results from reservoir condition core floods are presented for reservoir rock from a North Sea field which contains significant quantities of clay (principally kaolinite). By comparing the inhibitor return curves with those from a relatively clean (low clay) outcrop sandstone, we demonstrate that this clay material strongly affects inhibitor adsorption during injection (adsorption) stage, which consequently produces considerably higher inhibitor return concentrations during early stage of seawater postflush (< 400 pore volumes). However, in the long tail (> ~ 800pv), low concentration (< 6 ppm) region, the return curves are quite similar. Calcium was involved in the phosphonate adsorption mechanism in these high clay cores as well as in the outcrop sandstone cores. The dynamic adsorption isotherms for these inhibitor/core systems are derived and compared with similar isotherms from the low clay material. The field significance of these results for the inhibitor squeeze performance is demonstrated by using the field and outcrop isotherms in some model squeeze return calculations.
