The phenomenon of wettability shift to an oil-wet behavior and reduced relativepermeability to oil at high temperatures due to steam injection in the cyclicsteam (CS) process was identified as one of the factors responsible for theless than expected performance at the Elk Point Cummings formation. This paperreports on the results of a laboratory investigation and on the discovery of amethod as well as its field implementation to prevent wettability shifts atelevated temperatures. The contact angle experiments revealed that awettability transition to an oil-wet state occurred at elevated temperatures inthe rock-fluids system consisting of a quartz crystal surface, Lindbergh crudeoil and synthetic brine that represented a mixture of steam and formationwater. A mechanism is proposed to explain temperature dependence of reservoirwettability by applying a Zisman-type correlation and thin wetting filmstability considerations.
It was discovered during this experimental investigation, that the wettabilityreversal at elevated temperatures could be prevented. As the temperature in thecontact angle cell was increased. calcium carbonate (3), due to itsdecreasing solubility at higher temperatures, precipitated onto the quartzcrystal surface. This deposition of calcite particles on the crystal surfaceimparted an oil resisting nature and a strongly water-wet behavior to thesurface even at elevated temperatures. The effectiveness of this discovery onimproved oil recovery was further confirmed through core floods with andwithout3 precipitation. The discovery was then field tested in anElk Point cyclic steam pilot well. This paper reports for the first time, boththe laboratory and field results of the wettability control technology forproductivity enhancement in thermal enhanced oil recovery.
Considering the enormous resource base of 425 × 109 m3 ofheavy oil and bitumen in western Canada, Millet (1994) concludes that the oilproduction business will eventually be working mainly with heavy oils andbitumens. Lloydminster heavy oil belt alone is estimated to contain 3.1 ×109 m3 of oil-in-place with API gravity ranging from 16to 24 ° and viscosity in the range of 100 to 400 mPa.s at reservoir conditions(Polikar et al., 1993). Cummings is one of tbe extensive formations in the Lloydminster heavy oil belt and contains a 13 ° API heavy oil with a stock-tankoil viscosity of about 10,000 mPa.s at reservoir temperature of 24 °C. Theimmense magnitude of the original oil-in-place makes the Cummings reservoirsprime candidates for improved oil recovery processes with potential forsignificant economic benefit through incremental oil recovery over primaryproduction.
PanCanadian's experience with good primary production as well as the reservoirresponse to cyclic steam (CS) pilot tests at the Elk Point Cummings formationhave been recently presented (Karyampudi, 1995). Some of the productionperformance characteristics observed during these CS pilot tests at Elk Pointwere the absence of a peak in oil production rates (that is normally expectedearly in the production phase) and the unexpectedly high water cuts during thefirst and subsequent cycles. The main reason for these trends was initiallysuspected to be a shift in wettability of the near-wellbore region to anoil-wet behavior at elevated temperatures encountered during the steaminjection phase.