Cyclic Gas/Steam Stimulation of Heavy-Oil Wells
Authors: Meldau Robert F. , Shipley Robert G. , Coats Keith H. 
Published: December 2007

Summary
Injecting air with steam nearly doubled oil production from three cyclic steam stimulations in a California reservoir producing 11 degrees API (O.99.g/cm3) oil. Computer model studies reveal several reasons for better oil recovery, including greater pressure drawdown, gas drive of heated oil near the well, and trapped residual gas deep in the reservoir.

Introduction
Cyclic steaming is a key tool to achieving economic production from reservoirs containing very viscous oil. Thus, there is an incentive for the development of better cyclic steaming techniques since heavy oil reservoirs in the U.S., Canada, and Venezuela contain a large amount of potential reserves.Adding gas to cyclic steam first was suggested by Pursley and Weinstein in 1974. They found oil increases of 50% or more from gas/steam stimulation compared with steam alone in physical and computer models of Cold Lake wells in Alberta and matched field results for steam stimulation with and without gas. Their work was the basis for trying air/steam stimulation of Paris Valley wells in California.The idea of cyclic gas injection to produce heavy oil wells is not new. Shelton reports the use of propane-rich natural gas with mixed results in the field, and Clark used exhaust gas with some success. Cyclic carbon dioxide has stimulated heavy oil wells successfully and is similar to gas/steam stimulation since both provide viscosity reduction, oil expansion, and reservoir energy.The Vapor Therm (TM) process has been used to recover heavy oil from several midcontinent reservoirs by cyclic gas/steam stimulation at oil/steam ratios of 1.6:1 to 2.2:1. In this process, water is mixed with hot combustion gas to produce a gas/steam mixture before injection. These stimulations used much less steam and much more gas than those reported here. Downhole steam generators under development also inject gas/steam mixtures.This paper briefly describes the Paris Valley thermal pilot and gives field results for cyclic steaming. Computer model results help explain the improved performance noted when gas is added to the steam.

Field Results
The wet combustion process used at Paris Valley consists of forming a heat pad by dry combustion and then switching to simultaneous air/water injection. A water/air ratio of 200 bbl/MMscf (1.12 x 10(-3) m3/m3) was chosen to move the heat forward at about the same calculated rate as the combustion front but maintain high combustion temperatures. Cyclic steaming was used in the producing wells before heat arrival from in-situ combustion. The pilot was terminated after 4 years in March 1979 because of high compressed air cost, low oil production rate, and controlled oil prices.Fig. 1 shows the pilot location. Figs. 2 through 4 and Table 1 describe the reservoir. The oil pay is underlain by a tilted oil/water contact. Note that the estimated tank oil viscosity of the Upper Lobe at a reservoir temperature of 87 degrees F (30.6 degrees C) is about 227,000 cp (or mPa-s) compared with 23,000 cp for the Lower Lobe. Typical primary production for an upstructure well is 5 BOPD (0.80 m3/d) and 40 BWPD (6.36 m3/d). Downstructure wells produce water with only a trace of oil.Each pilot pattern is about 5 acres (20 234 m3), and the distance between adjacent upstructure wells is about 150 ft (45.7 m) as shown in Fig. 2. The injectors were drilled off center to compensate for gravity effects on injected air.
JPT
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