Experimental results on heavy-oil/CO2 systems with regard to phase behavior, PVT characteristics, dynamic extraction, and recovery performance are used to evaluate the effectiveness of CO2 as a heavy-oil mobilizing additive.
Under preselected pressure and temperature conditions, 360 SCF of CO2 were found to dissolve in one barrel of the investigated 10° API heavy oil. The resulting swelling factor was 1.12 and the oil viscosity was reduced by a factor of 100. In flood experiments, recovery rates on the order of 5-20% OOIP were achieved.
In general, up to 40 MMSCF of CO2 per day are needed for a field project. This quantity can be obtained from natural sources or from industrial processes. CO2 from natural sources is currently the cheapest. Extraction of CO2 from industrial process gases can be done by physical or chemical methods. Among the chemical methods, the monoethanol (MEA) and the ammonia (NH 3 ) processes are the cheapest.
A CO2 -steam injection process for heavy-oil recovery requires 200-500 SCF of CO2 per barrel of steam (water equivalent). The CO2 can be delivered to the field at 1-2/MSCF. For an added oil recovery factor of 5-10% OOIP, costs of CO2 may range between $1 and $8 per barrel of incremental oil. The economy and the specific demand on CO2 should be calculated for each individual field project.
Figures & Tables
Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.