Workover chemicals were placed in a group of seven CHOPS wells in the Sparky and GP zones of the Canadian Heavy Oil Belt. Rather than direct injection, the chemicals were placed using aggressive pressure pulsing over a period of 7–10 hours. A three-month postworkover assessment shows that each well is producing far more total liquids than before the workover, and that the oil-to-water ratio has improved greatly. For the seven wells, oil production rates three months after the workover were approximately 5 times the rates before the workover.
Chemical placement in these wells that co-produce sand appears to be much more successful if pulsing is used, and this is attributed to a much more efficient dispersion mechanism associated the dynamic excitation. Instead of channeling out into the formation and being recovered largely undiluted after production is reinitiated, it appears that the pulsing mixed the chemicals much more thoroughly in the formation and that channeling and chemical isolation were suppressed. Importantly, the new method provided payback within 60 days of the workover termination.
Aggressive pressure pulsing as a workover method for oilwells was introduced in the Canadian oil industry less than three years ago (Sept 1998) 1,2. The method was proposed based on a two-year long series of laboratory experiments that demonstrated flow rate increases during active pulsing.3 Assessment of the probable reservoir reasons for oil production decline in CHOPS (Cold Heavy Oil Production with Sand) wells led to the conclusion that pulsing workovers could help revitalize poor wells, and even turn non-producing wells into economic producers. The first 100 or so workovers in Canada were carried out without chemical placement, and an increasing success ratio was achieved, based on gradually better screening criteria and better understanding of the process effects in the reservoir.
Chemicals are added to wells to affect the conditions in the reservoir and achieve better production. In limestones, acidizing is widely used, and acids may also be used to dissolve clay minerals blocking sandstone pores in the near-wellbore region. In siliceous sandstones containing heavy oil, issues such as surface tension, permeability channeling, asphaltene and wax precipitation and wettability changes can often be addressed by chemical placement. However, in high viscosity oils in high porosity sandstones that have been exploited by CHOPS, chemical placement has in general not achieved a high success ratio. This can be attributed to a number of factors, discussed in more detail below.
Several years ago, it became apparent that placing chemicals with aggressive pulsing would likely achieve better results than simple injection. This judgment was based on the initial workovers carried out in 1999–2000. An opportunity to test this idea on a group of CHOPS wells in the Heavy Oil Belt presented itself in late 2000.
Using CHOPS implies maintaining continuous sand influx in order to sustain economical oil production.4