A new workover method based on large-amplitude pressure pulsing is being used in Cold Heavy Oil Production (CHOP) wells in Alberta and Saskatchewan. The Pulsing Workover Tool (PWT) approach is based on prolonged highamplitude, low-frequency pressure pulsing of the wellbore liquid, while adding additional fluid at a controlled rate through the annulus. Typically, pulsing is continued for 8–12 hours, after which the well is placed back on production.
To date (March 15, 1999), over 25 workovers have been performed in the following fields: Lloydminster, Lindbergh, Morgan, Marsden, Luseland, Plover Lake, Bear Trap, Marwayne, and Wolf Lake. Several wells that had never produced much oil were improved to be economic producers. Blocked wells, where oil rate drops seemed to be associated with perforation plugging or stable sand arch formation, were substantially improved by the PWT approach. There have been many successful applications of PWT, and a few failures. Because candidate screening criteria are improving, the success ratio is increasing. The PWT approach will be useful for other applications as well in conventional oil well workovers.
Successful Cold Heavy Oil Production (CHOP) in the unconsolidated heavy oil sands of Alberta and Saskatchewan requires that sand continue to enter the wellbore to maintain economic oil production rates1. All sand exclusion devices such as gravel packs, screens, filters, and slotted liners greatly reduce or eliminate oil production.
A "typical" CHOP well has an average oil production history similar to that in Figure 1. The main features are2:
An extremely high sand rate (often in excess of 30% sand by volume of the dead produced fluids) that usually begins to drop off shortly after production begins.
A "stable" sand production period where sand influx rate is approximately constant ("stable" sand rate = f(Q, µ, + reservoir-specific parameters). Steady-state rates can be as high as 8–10% sand for high viscosity oils.
A liquid (oil + water) production rate that rises over a period of several months to a peak rate, then begins a gradual decline to an uneconomical oil rate, unless the well is plugged suddenly (e.g. by perforation blockages).
After a successful workover has been implemented in a CHOP well, a period of renewed economic production occurs, but rarely do the oil and sand rates reach the peaks they did in the first cycle, because of foamy oil drive depletion and reduced pressure gradients.
A number of cycles may ensue; several wells have sustained economial CHOP production for more than 10–12 years (5–8 years is most common for good wells).
After a number of these cycles, the well may become "exhausted", as the zone that has been affected by the sand yield and channeling is large, gradients are low, and the solution gas may have become depleted. Nevertheless, a good CHOP well may produce 50,000-80,000 m3 of oil and 1000–3000 m3 of sand in a 10-year life span.
Because sand influx is vital to the CHOP process (screens, slotted liners, and gravel packs are avoided), it follows that any natural reservoir process that stops or reduces sand ingress may negatively impact oil production rates.
