Mechanistic study of precipitation squeeze treatments using phosphonate and phosphate ester scale inhibitors
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One of the challenges associated with water injection during oil and gas production is mineral scale formation within the production systems. Scale deposited on downhole equipment, tubulars and perforations can lead to production decline and eventually cause well abandonment. Proactive approach of preventing scale precipitation applying scale inhibitor “squeeze” treatments is recognised to be the one of the most economically and technically favourable option for scale management in most oilfields. One of the factors determining the success of the squeeze treatment is good retention of the scale inhibitor within the formation rock, which leads to an extended squeeze lifetime. The two main retention mechanisms are adsorption and precipitation. Field cases have shown that implementing precipitation squeezes may result in an extended squeeze lifetime, compared to pure adsorption treatments, with some additional benefits for production. Thus, the current thesis is on the topic of precipitation squeeze treatments. In the first part of this research, we examine the factors that govern the retention and subsequent release of phosphonate scale inhibitors in precipitation squeezes: viz. equilibrium solubility and the dissolution rate. The equilibrium solubility diagrams of 3 common phosphonate SI/Ca/Mg precipitates were obtained as a function of both temperature and brine Mg/Ca molar ratio, in static solubility tests. Subsequently, the flow rate effect and dissolution rates were measured in non-equilibrium sand pack flooding tests. In addition, we define other parameters that must be considered in the dissolution model, in order to accurately predict scale inhibitor returns in precipitation treatments. Conclusions derived from the study allowed a qualitative dissolution model to be developed by Flow Assurance and Scale Team (Heriot-Watt University) that describes phosphonate SIs release in precipitation treatments. All the data obtained in this work can be directly used to calculate the dissolution rates under different flow rates and numerically model the dissolution behaviour of the phosphonate SI/Ca complexes. Once the data and model are incorporated into the squeeze design software, more accurate predictions of the inhibitor return in precipitation squeeze operations can be obtained. The second part of the thesis presents a comprehensive study on the precipitation behaviour and performance of phosphate esters, again in the context of precipitation squeeze treatments. It is shown, that this class of scale inhibitors can effectively mitigate scale formation at lower temperatures which represent the most severe thermodynamic conditions for sulphate scale inhibition. FTIR and NMR analytical techniques have been applied to explain the inhibition efficiency data versus temperature variation, showing the structural changes in the phosphate ester solutions over the temperature range 20-95oC. Finally, the mechanism of phosphate ester performance is discussed and compared to those of conventional phosphonate scale inhibitors. The results obtained in this part of thesis are of practical significance for the effective design of lower temperature phosphate esters squeeze treatments, as this chemistry represents (i) a more environmentally friendly alternative to phosphonate scale inhibitors, and (ii) a chemical that is significantly easier to detect within produced brine (by ICP) than many polymers currently used by the industry.