Non-equilibrium adsorption and precipitation of scale inhibitors: corefloods and mathematical modelling

Adsorption/desorption and precipitation/dissolution are thought to be the two major mechanisms operating in the retention and release of scale inhibitor in squeeze treatments in an oil reservoir. The general nature and extent of the scale inhibitor adsorption process is determined principally by the equilibrium isotherm, although the shape of the inhibitor return curve may be significantly modified by kinetic effects for a specific squeeze practice. For a precipitation squeeze process, the solubility of the inhibitor-calcium complex and the rate of dissolution are currently thought to be two main factors that govern the return curves in our computer modelling studies. Experimental coreflood techniques can be used to provide inhibitor breakthrough profiles and these effluent concentration data can then be used to derive isotherms for the adsorption systems or to test the validity of the above precipitation mechanisms. This paper presents results from a series of flow rate varying adsorption and precipitation laboratory corefloods for a penta-phosphonate (DETPMP) and a poly-carboxylate (PPCA). This experimental data is the most complete and accurate ever produced and is of a quality that allows us to test the details of the above mechanisms. A general analysis and discussion of the adsorption isotherm derivation is provided and modelling results for the non-equilibrium adsorption corefloods are reported. The significance of these results for field applications is discussed.