Investigating the Impact of Temperature and Flowrate on Scale Inhibitor Placement in Carbonate Reservoirs

During a squeeze treatment, due to the radial nature of the well, as the treatment progresses into the reservoir, the fluid velocity will drop. Additionally, the injected treatment will move faster than the thermal front, meaning the treatment will move through a temperature gradient, up to reservoir temperature. This paper summarises core flood experiments which mimic the transport of scale inhibitor (DETPMP) during a squeeze in a carbonate reservoir, to better understand inhibitor placement.

Core flooding experiments used an Indiana limestone substrate with DETPMP as the scale inhibitor in NSSW. Temperature and flowrate were varied to simulate the conditions at three areas of the reservoir. Namely, the near wellbore (cold/fast), passing through the thermal front and deeper in the reservoir (hot/slow). These results were compared to experiments carried out hot (95 °C) using the same flowrates, for comparison. Effluent compositional analysis and pH data were used for mass balance calculations of the main treatments, allowing determination of expected placement and the nature of the scale inhibitor retention (adsorption/precipitation) for each condition.

As the inhibitor moves from the near wellbore to deeper in the reservoir, it gets progressively hotter while simultaneously slowing down. The impact of this is that retention is lower (29.2%) in the colder region of the near wellbore (adsorption dominated). As the inhibitor transitions to deeper in the reservoir (increasing temperature/decreasing flowrate), retention significantly increases to 76.1% (precipitation dominated). Once at reservoir temperature, maximum retention is achieved at 89.6%. These results highlight the predicted increase in scale inhibitor retention with increasing temperature/decreasing flowrate, i.e. as the treatment progresses deeper into the reservoir. In comparison, at reservoir temperature and varying flowrate, the level of inhibitor retention is high (83.4 % - 89.6 %) and independent of flowrate used. The lower retention at near wellbore conditions can be attributed to the low temperature, suggesting greater dependence on temperature than flowrate for inhibitor retention for this system. Additionally, the three different retention isotherms identified, highlights the need to consider multiple regimes during a squeeze treatment to accurately determine scale inhibitor placement. These results show, that during DETPMP treatments in carbonates, the majority of inhibitor will be deposited beyond the thermal front with minimal retention in the near wellbore.

Using these findings, enables engineers to more precisely tune squeeze treatment volumes, including pre flush and overflush volumes to optimise scale inhibitor placement and subsequent treatment performance. This could be facilitated by development of existing treatment design models to better reflect the relationship between temperature, flowrate and location within the near wellbore.