Cold production is a recovery process used in unconsolidated heavy oil reservoirs in Alberta and Saskatchewan, Canada. In this process, sand and oil are produced together under primary conditions. Oil production rates can typically increase by one order of magnitude when sand is produced. The production of sand into a perforation was modeled using a horizontal sand pack flooded with live oil. Previous sand production experiments were performed using dead oil. The pack was scanned with an X-ray computed tomography (CT) scanner. A wormhole (high porosity channel) developed within the sand pack starting at the production end as soon as the back pressure was decreased suddenly from 780 psi (5.38 MPa) to 500 psi (3.40 MPa). The wormhole was stable to collapse when the production pressure was decreased from 780 psi (5.38 MPa) to 500 psi (3.4 MPa) and maintained at that pressure for 3 hours. The wormhole developed within the high porosity region (lowest cohesive strength) of the sand pack indicating that worn-holes in the field will likely develop in the weakest sands which are normally the sands with little cementation and therefore more oil. Under this rapid depressurization, gas did not come out of solution while the back pressure was maintained at 500 psi (3.4 MPa). The wormhole collapsed when the production pressure was decreased to atmospheric pressure. This indicates that a sudden decrease in the bottom hole pressure in a well may lead to wormhole collapse.
Cold production has been used with commercial success to recover heavy oil in the Lloydminster area of Alberta, Canada. High production rates have been reported for heavy oil fields under primary recovery1to11 when large quantities of sand are produced with the oil. Several authors have attributed this high oil production rates to the formation of high permeability cavities1,2,12,13, channels (wormholes) 5to10 or both11,12,13,16,17.
Solution gas drive is generally considered to be the main drive mechanism1,2,3. In many cases, solution gas drive by itself is not sufficient to explain the enhanced oil recovery. Significant increases in oil production occurred only when large quantities of sand were produced. Experiments in sand packs with live oil (without sand production) have shown that the permeability of the sand pack is not increased when gas bubbles are generated3.
Tracer experiments between injection and production wells have been performed in the field by several operators5to11 to measure the travel time between wells after considerable sand production has taken place. In general, this time was at least one order of magnitude shorter than that normally predicted for unaltered formations. These anomalously short times were explained by the formation of either fractures or wormholes.
Theoretical models of cold production, which assume that a radial zones of dilated sand develops from a wellbore when sand is produced into a well, have been developed14,15,16. The assumption of a large dilated zone around a wellbore differs from our observation of a high permeability channel (wormhole).
The Clearwater sand used in the experiment was obtained from the collection tanks at Suncor's former Burnt Lake pilot project4.