Summary
The methane pressure-cycling (MPC) process is an enhanced-oil-recovery (EOR)
scheme intended for application in some heavy-oil reservoirs after termination
of either primary or waterflood production. The essence of the process is the
restoration of the solution-gas-drive mechanism. The restoration is
accomplished by reinjecting an appropriate amount of solution gas (mainly
methane) and then repressuring the gas back into solution by injecting water
until approximate original reservoir pressure is reached. This, aside from the
replacement of produced oil by water, recreates the primary-production
conditions. This novel recovery technique is being developed to target the
considerable portion of heavy-oil resources located in thin reservoirs. Primary
and secondary methods have managed to recover at best 10% of the initial oil in
place (IOIP). Heat losses to overburden and underburden or bottomwater zones
make thermal methods unsuitable for thin reservoirs.
Sandpack-flood tests in 30.5-cm (length) x 5.0-cm (diameter) sandpacks were
carried out for oils with a range of dead-oil viscosities from 1700 to 5400
mPa.s. The results showed that the pressure-cycling process could create a
favorable condition for recharged gas to contact the remaining oil in
reservoirs. This restores the situation whereby substantial amounts of gas are
in solution for further “primary” production. The effects on the efficiency of
the MPC process of cycle termination strategy, oil viscosity, and mobile-water
saturation were investigated. Simulations were conducted to investigate the MPC
process in three heavy-oil reservoirs in Saskatchewan, Canada. The effects on
the process of infill wells, oil viscosity, gas-injection rate, and the
presence of wormholes in reservoirs were studied.
Introduction
Heavy oil in thick-pay reservoirs (i.e., >10 m) is commonly produced
with thermal-recovery methods. These methods (steam injection and its variants)
are generally not suitable for thin reservoirs because of heat losses to
overburden and underburden or bottomwater zones (Fairfield and White 1982; Dyer
et al. 1994). The world’s large untapped oil resource remaining after recovery
by conventional technology offers potential for exploitation by a suitably
developed tertiary-recovery technique. For example, Saskatchewan accounts for
62% of Canada’s total heavy-oil resources (Bowers and Drummond 1997), including
1.7 billion m3 of proved reserves and 3.7 billion m3 of probable reserves
(Saskatchewan Energy and Mines 1998). Of the province’s proven initial heavy
oil in place, 97% is contained in reservoirs where the pay zone is less than 10
m, and 55% in reservoirs with a pay zone less than 5 m thick (Huang et al.
1987; Srivastava et al. 1993). Primary and secondary methods combined recover,
on average, only about 7% of the proven IOIP (Saskatchewan Energy and Mines
1998). The incentive is strong for the development of appropriate EOR
techniques that will maximize the recovery potential of and profitability from
these thin heavy-oil reservoirs.
Extensive literature is available on CO2, flue gas, and produced-gas
injection for heavy-oil recovery, including slug displacement, water
alternating gas (WAG), and cyclic (huff ‘n’ puff) processes (Huang et al. 1987;
Srivastava et al. 1993, 1994, 1999; Srivastava and Huang 1997; Ma and Youngren
1994; Issever et al. 1993; Olenick et al. 1992). A comparative study of the
oil-recovery behavior for a 14.1°API heavy oil with different injection gases
(CO2, flue gas, and produced gas) showed that CO2 was the best-suited gas for
EOR of heavy oils (Srivastava et al. 1999). Cyclic CO2 injection for heavy-oil
recovery was tested in the field, and field case histories indicated that oil
production was enhanced (Olenick et al. 1992). However, natural CO2 sources are
not available to most oil reservoirs. The cost of CO2 capture from flue gas and
other sources may range from U.S. $25 to $70/ton (Padamsey and Railton 1993).
Produced gas is available in large quantities at a much lower cost. With this
consideration, produced gas can be an economically effective agent for
heavy-oil recovery by the cyclic-injection process.
© 2006. Society of Petroleum Engineers
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