The mechanism of retention of scale inhibitors (SI) within the reservoir formation is central to a squeeze treatment having a long lifetime. Scale inhibitors are retained within porous media by the two main mechanisms of adsorption (Γ) and precipitation (∏). There is not complete agreement in the literature about when we should use one mechanistic description or another, and indeed both can occur together as coupled adsorption/precipitation (Γ/∏). Previously a general model of coupled (equilibrium) adsorption/precipitation has been derived and the agreement between the model and experiment was very good (Kahrwad et al., 2008). This model was subsequently extended to derive a consistent dynamic coupled Γ/∏ flow model for simulating non-equilibrium (kinetic) coupled processes of any type (Sorbie, 2010). This latter model has not yet been fully validated, but the work in this paper and its companion paper (Paper 2: Ibrahim et al, 2010) provides the type of data required in order to do this. In this paper (Paper 1), new static experimental adsorption/precipitation measurements are presented for two phosphonate inhibitors, DETPMP (a penta-phosphonate) and OMTHP (a hexa-phosphonate) using sand, kaolinite and siderite as the mineral phases. These experiments were carried out at a range of adsorbent mass (m)/ fluid volume (V)/ ratios and it is the "apparent adsorption", Γapp vs. the final scale inhibitor concentration, cf, which is measured and plotted. By observing how the Γapp vs. Cf, curves vary for different values of the (m/V) ratio, this indicates whether we are in the purely adsorbing (Γ) or in the coupled adsorption/precipitation (Γ/∏) regime (Kahrwad et al., 2008). For these static apparent adsorption tests, m = 10g, 20g and 30g samples of each mineral were used (with a fixed volume of SI solution, V = 80ml) to analyse the apparent adsorption behaviour. In addition, related pure precipitation/compatibility tests were carried out in the absence of any minerals in the bulk solutions. The experimental results for both phosphonate scale inhibitors show good agreement with the theory in different regions of pure adsorption and coupled adsorption/precipitation. These results show clearly how such laboratory measurements should be carried out to determine both the levels of SI retention and the precise retention mechanism. This paper characterizes the systems used in subsequent dynamic adsorption/ precipitation sand pack floods which are reported in a related paper (Ibrahim et al., 2012) and which will be used in future to validate fully dynamic coupled Γ/∏ flow models (Sorbie, 2010).
The term mixed scale pertains to the scales found in oil and gas production system containing both organic and inorganic constituents in such a way that either aqueous-based inorganic dissolver or solvent-based organic dissolver fails to act on it. These scales are also known as wetted scales. This research discovers formulations which can effectively dissolve and disperse mixed scales dominated by inorganic content. Micro-emulsion-based solutions are identified as the best in tackling such mixed scales. A few inorganic and organic dissolving chemicals along with surfactants and co-surfactants are considered in this research to develop environment friendly solutions. The stable micro-emulsions are subjected to detailed dissolution study to establish their efficacy. The synthesized chemical solutions are shown to dissolve mixed scales of different composition. A chelant-based micro-emulsion formulation is also found to be effective in dissolving difficult to treat metal naphthenate scales co-precipitated with organic content, which is a novel application.
Scale inhibitors (SI) are retained in the reservoir formation by both adsorption () and precipitation () mechanisms, and these are responsible for the duration of the squeeze lifetime. However, processes such as the adsorption of SI and dissolution of precipitated SI complexes are not instantaneous. During a scale inhibitor treatment in a rock core, the fluid/mineral contact time may be quite short compared to the equilibrium time for the SI/mineral system for many practical experimental flow rates. This is also the case in the near-wellbore region where flow rates are very high and hence dynamic (non-equilibrium) effects may be significant. The SI retention process occurring in the sand packs, and indeed within the reservoir formation, are therefore likely to be non-equilibrium processes for at least some period of the squeeze process. If this hypothesis is correct, non-equilibrium behavior can be observed from the change in observed SI effluent concentrations as the flow rate changes in the sand pack flood. Therefore, non-equilibrium effects must be taken into account in the derivation of the parameters governing the squeeze such as the adsorption isotherms and/or the precipitation/dissolution model.In this paper, we present a range of novel experimental results from a series of variable rate adsorption and precipitation sand pack floods for OMTHP ( hexaphosphonate) scale inhibitor. The sand pack experiments were conducted using silica sand as the adsorbing mineral and all the floods were conducted using identical procedures. The unique feature of this series of floods is that the bulk coupled adsorption/precipitation behaviour of this system (OMTHP/sand) has been fully characterized in previous work (Paper 1, Ibrahim et al., 2012). Therefore, we know precisely when the system is in the "adsorption only ()" or in the "coupled adsorption/precipitation ()" regime. The sand pack effluent results show non-equilibrium behavior as the flow rate changes. These results indicate the importance of flow rate on the derivation of dynamic isotherms () and precipitation models () prior to field application modeling. We believe that the results presented in this work probably yield the most complete dataset of well characterised adsorption/precipitation SI pack floods assembled to date. SPE 155109process has been developed recently and is presented at this conference (Sorbie, 2012).Non-equilibrium adsorption/desorption (Bourne et al., 1992) and non-equilibrium inhibitor return behaviour during precipitation/dissolution (phase separation) core flooding experiments of a phosphonate based scale inhibitor on limestone material (Lawless et al., 1994) have been published previously. The reported results demonstrate that the actual inhibitor desorption process occurring under the conditions tested is a non-equilibrium process, whereby the concentration of scale inhibitor in the post flush effluent is affected by the post flush flow rate. They observed that the steady state SI effluent concentration increased if the flow rat...
Paraffin fouling of pipelines and wells is a dynamic process involving deposition and removal. Deposited wax growth and hardening have been observed in the field. They must be properly evaluated at the design stage in order to develop a suitable and economic flow assurance strategy. Laboratory studies and direct observations on the nature and composition of the deposited wax have been made possible with the aid of a novel, axial wax deposition apparatus. This study resulted in an improved understanding and calculation of wax removal and aging rates. A turbulent burst mechanism was used to explain experimental observations made during this study as well as results recently published by other laboratories and field knowledge of wax removal and the aging/hardening process. Direct observations of the wax deposit nature and composition which were performed on samples removed from the deposited wax, provided essential information on the nature and composition of the deposit resulting from different flow regimes. These observations suggested that two distinct flowing zones (layers) and a wall-attached, thin, solid deposit are part of the flow-related deposition-removal process. A turbulent burst action, which is promoted in the near-wall area by the turbulent flow core zone, was used to explain experimental and field observations suggesting a selective removal of n-paraffin fractions. While the proposed removal mechanism acts selectively depending on the size of nonattached, crystallized wax, the transport of liquid n-alkanes components from the bulk, warmer zone to the near-wall colder region is considered as an overall “n-alkane” transport as accepted and used in calculations so far. The proposed two fluid layer model offers a framework for improving the design and operation (pigging and additive) strategies as well as for revisiting the accepted (safe) ranges of design velocities for waxy crude production and transport limes. This work aims to offer a better tool for estimating the pigging frequency, the optimal flow regime and a better additive testing and application strategy with considerable costs-saving to industry.
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