In this paper, results are presented from an extensive series of phosphonate inhibitor adsorption experiments using both consolidated and crushed clean (clay free) sandstone core material. A near complete account of phosphonate adsorption in highly quartzitic systems is developed from the results presented. Although some details are known in the literature, we believe that this is the most complete analysis of these effects on phosphonate adsorption that has been assembled to date. The effects on inhibitor adsorption of pH, calcium ion concentration, temperature and inhibitor concentration are investigated in some detail. The appropriate adsorption mechanisms which operate under the various conditions are elucidated from the inhibitor adsorption experiments and from the additional work on the-potential measurement for quartz. In particular, the respective roles of hydrogen bonding at low pH (~2) and calcium binding at higher pH (~6) are clearly demonstrated and explained. At intermediate pH values (~4) at room temperature, the adsorption of inhibitor is found to be lower than at both pH 2 and pH 6 due to a relative weakening of both the hydrogen bonding and calcium binding mechanisms. Mechanistic results from the crushed rock and core adsorption experiments are consistent although the level of adsorption is rather different for each medium. At elevated temperatures, inhibitor adsorption is higher under all conditions and, when Ca2+ is present, the effect of calcium enhanced surface condensation (or surface precipitation) is clearly shown. The importance of this enhanced adsorption mechanism is discussed in the context of field scale inhibitor adsorption/desorption and "precipitation" type squeeze treatments.
This work should be viewed in the context of:
previous work on the effect of pH on phosphonate adsorption and;
forthcoming work on adsorption onto pure clay mineral substrates and in clay-containing reservoir cores.
Scale inhibitor "squeeze" treatments provide one of the most common and efficient methods for preventing the formation of sulphate and carbonate scales in producer wells. Although squeeze treatments have been used widely in the field, the full range of mechanisms controlling inhibitor retention and returns from oil producing formations following a squeeze is not yet fully understood. Two mechanisms due to adsorption and precipitation are generally believed to be involved in the retention and release of inhibitor in the reservoir.
Adsorption is thought to occur through an electrostatic attraction or physical adsorption between the inhibitor and formation minerals. The precise form of the isotherm describing this adsorption process determines the squeeze lifetime as is described in detail elsewhere. "Precipitation" squeezes have been used in field practice in order to attempt to increase the lifetime beyond that attainable by adsorption. Such squeeze processes are usually based upon the precipitation of the calcium salt of a scale inhibitor within the formation. The slow release of inhibitor into the production stream is thought to be a function of the relatively low solubility of the inhibitor/calcium complex. Although this technique has been successfully applied in many reservoir situations, it does, under some circumstances, carry with it the risk of formation damage. As a result, most precipitation squeeze techniques have been specifically applied to high volume, thick zone wells.
It is also noted in the literature that there is no clear-cut line between the two basic types of squeeze method, adsorption and precipitation. Both mechanisms can occur concurrently depending upon the chemical nature of the inhibitor and on formation parameters such as divalent-cation concentration, pH and temperature. There are opposite views with respect to the mechanism controlling a precipitation type of squeeze treatments. It has been reported recently that the conditional solubility of the calcium-phosphonate (DETPMP) complex is of the oder of ~ 10(-4).