Establishing scale inhibitor retention mechanisms in pure adsorption and coupled adsorption/precipitation treatments
Ibrahim, Jamal Mohamad Bin Mohamad
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One of the most common and efficient ways for preventing formation of inorganic solids deposition such as carbonate and sulphate scales in reservoir and near wellbore formation is by applying scale inhibitor (SI) squeeze treatments. The two main mechanisms that govern the scale inhibitor retention and release process in the formation are by adsorption/desorption and precipitation/dissolution. They are described by different but related modelling approaches, and there is not complete agreement in the literature about when to use one mechanistic description or another. The equilibrium adsorption isotherm determines the general nature and extent of the scale inhibitor return process in the low concentration flow regime. However, the additional SI “loading” within the near wellbore formation may be greatly enhanced by precipitation. The dynamic effects of adsorption and precipitation, also have a strong bearing on a field squeeze treatment and may significantly affect the profile of the inhibitor return curve. Field observations are not accurate enough to distinguish between different mechanisms and a detailed analysis of a given retention mechanism (e.g. pure adsorption or coupled adsorption/ precipitation) requires carefully designed laboratory experiments at the appropriate “field relevant” conditions. In this study, we present novel experimental techniques systematically from static to dynamic tests, as follows; 1. Static Adsorption/Compatibility Experiments – these experiments were conducted on two phosphonate scale inhibitors; namely DETPMP (a penta-phosphonate) and OMTHP (an hexa-phosphonate) using sand, kaolinite and siderite as the mineral phase. Adsorption experiments were carried out at a range of adsorbent mass/ fluid volume ratios (m/V), since this indicates whether we are in the purely adsorbing or in the coupled adsorption/precipitation regime. 2. Dynamic Sand Pack Experiments – based on the static tests, OMTHP scale inhibitor and sand mineral were selected for dynamic tests as it has the most clearly interpretable results. The experiments were conducted using a sand pack flow apparatus at different flow rates using identical procedures, which demonstrates the non-equilibrium effects which occur in both adsorption and precipitation treatments. iii The experimental results from static tests show excellent agreement with the theory in different regions of pure adsorption and coupled adsorption/precipitation. Whereas for dynamic sand pack experiments, the effect on post flush effluent inhibitor concentration is in the same direction for each system under test, i.e. reduced flow rate leads to higher effluent concentrations and vice-versa. These results also show clearly how such laboratory measurements should be carried out to determine both the levels of SI retention and the precise retention mechanism. The generated data from this work will be used as a basis to further develop existing coupled adsorption-precipitation () models within the Flow Assurance Scale Team (FAST) in Institute of Petroleum Engineering, Heriot-Watt University to improve the future prediction of scale inhibitor squeeze treatments.