Recently it has been shown that the presence of residual oil in a formation can have a considerable influence on the trapping mechanisms for particles present in re-injected produced water (Ali 2007, Ali et al 2005, 2007, 2009). This article reports on a further set of extensive coreflow experiments which confirm and extend these results. The tests were conducted in a CT-scanner, allowing direct observation of the build-up of particle deposition along the core.
These experiments are relevant to operational issues associated with PWRI (Produced Water Re-Injection). In many cases, produced water is injected into formations containing oil, so reduced oil saturation is achieved rapidly in the area around the well. Even if the well is outside the oil zone, trapped oil droplets are always present in produced water, and a residual oil zone will gradually build-up around the well.
Major differences are found between the deposition profiles for fully water saturated cores and the cores having residual oil saturation. In particular, particles penetrate deeper into the core with residual oil saturation and considerably more particles pass completely through the core without being trapped. The X-ray technique allows direct observation during the experiment of the deposition process inside the core, eliminating the complicating effect of any external filter cake. As a result, an analysis can be performed of the deposition parameters relevant inside the core, using Deep-bed Filtration Theory, and the results of this analysis are presented. In particular, it is shown that the values of the filtration function determined from the CT-scan (X-ray) data are consistent with those obtained from analysis of the effluent concentration. Moreover, both methods of analysis find quite clearly that the filtration coefficient increases with decreasing flowrate.
The results indicate that formation damage near a wellbore during water injection will be reduced by the presence of residual oil, and that particles will penetrate deeper into the formation. The result is also relevant to injection under fracturing conditions, since particle deposition in the wall of the fracture (where residual oil may be present) is one of the mechanisms governing fracture growth.
Produced water is usually treated before reinjection to remove as much oil as possible. Nevertheless, some oil will always remain in the injection water. Over time, oil will accumulate around the wellbore, and form an oil-bank in which there is residual oil saturation. Moreover, many injection wells are located in the oil zone of the reservoir. Over time, the initial oil saturation will be reduced to residual oil saturation around the well. Coleman & McLelland (1994) emphasized the importance of water injection at residual oil saturation rather than at full brine saturation. They stated that even after years of service, a water injection well would still have residual oil saturation in the near wellbore region. It is very relevant, therefore, to discuss the effect of this residual oil on the injectivity of the well.