There are several very elementary reasons why thermal-recovery processes cannot be used succesfully in some reservoirs. A re-examination of the fundamental flow equation will help understand why it is so. Fluid flow rate in a porous medium is directly proportional to the permeability. If the permeability is very low, flow rate will also be low. Since thermal-recovery processes require injection of steam or air at some predetermined rates, high thermal efficiency and economically attractive rates of flow cannot be acheived without a sufficiently high permeability. A second parameter effecting flow is the available pressure to move the extraneous fluids through the formation. In formations where the depth is too shallow, injection pressures have to be very low to prevent overburden lifting or fracturing the oil-bearing sands. These will destroy reservoir confinment and result in poor vertical sweep efficiencies. The resultant restriction of low injection pressure imposes the same limitations as low permeability. Finally, there is a third limitation that is not directly related to flow, but rather to the initial oil saturation throughout the reservoir. This is analogous to average ore grade in mining, and in conjuction with mineralized zone thickness, it is used to estimate the size of the resources in hte ground. It is unfortunate that most heavy oil accumulations do not have high oil saturation and massive thickness. Many oil deposits have less than 800 bbl/acre-ft in place, which is not a sufficiently high concentration to warrant economic justification for exploitation under the present cost and revenue structure. Thermal recovery will not be very succesful if any one of the above limitations characterize a given heavy oil deposit. Other concepts, such as mining should be investigated to determine economic potential for exploitation of such deposits on individual basis.
SURFACE MINING SYSTEMS
The number of generic surface mining systems available for oil mining is limited, although the possible variations on them by using different combinations of equipment is quite extensive. All of these are extractive methods in which the oil-bearing rock is removed and processed elsewhere. Any drainage of oil into the pit would be additional recovery over and above that recovered in the main processing plant. Potential surface mining systems can be divided into four categories.
This involves the removal of the overburden by an excavating machine that directly transports the material across the excavation and then dumps it in the area from which the ore has been removed. Strip mining is suitable for tabular deposits that are either flat or have a very shallow dip, of less than 10 degrees. The maximum overburden depth should be above 150 ft, and the overall pit depth should not exceed approximately 200 ft. The limits of strip mine operations are usually determined more by the capacity of the equipment than by the economics of the deposit.
This method involves the removal of the overburden by the excavating machines and the transporting of the materials around the pit on the level from which it is excavated to be dumped back into the pit in the area from which the ore has been removed. Terrace mining requires a reasonably tabular deposit with a large areal extent. Other than these two factors, the system is fairly flexible. The limits of terrace mining are generally set by the economic limiting stripping ratio.
Combination of Strip Mining and Terrace Mining
In this method, the upper overburden material is removed by terrace mining and the last cut to the ore is made by strip mining. This system is able to achieve greater depths than normal terrace mining due to the relatively low cost of stripping the last 100 ft in depth, the terrace mining system handles the extra material that the drag line cannot remove.
Open Pit Method
This system involves the removal of the waste material upgrades by truck, conveyer, train etc. for dumping outside the excavation limits. If the topography permits, level hauls to waste dumps can be achieved; however, usually an upgrade haul is encountered. The true open pit method has the greatest flexibility of all four systems, but also, in general, has the highest cost. In this case, the limits of operation will be determined directly by the economic stripping ratio.
UNDERGROUND MINING METHODS
Depending on the type of drainage, the underground mining methods may be classified into the following principle groups:
1) Drainage of oil by means of galleries driven directly into and through
the oil bearing rocks .
2) Drainage of oil through trenches or pits sunk into the oil bearing formation from the mine opening .
3) Drainage by means of vertical or slanting bore holes drilled into the oil bearing rock from openings driven either through the oil rock itself or outside the latter through the country rock above or below .
4) Drainage of oils by means of long horizontal or gently dipping bore holes drilled through the oil bearing rocks from openings driven directly through the oil bearing rock or from the country rock above the formation.
Drainage of oil through bore holes in the walls of galleries or even through trenches and drainage pits encounters difficulties of ventilation and requires extra precaution against the fire hazard, particularly in the case of light oils and possible gas blowouts. These methods have been applied in old oil fields: Pechelbronn (Alsace, France), Witze (Germany) and elsewhere.
The safest and therefore, the most widely used methods are those of the third group, in which bore holes are drilled from openings above or beneath the producing rocks. In particular, this is the principle method in the Ukhta fields (Komi, CIS).
The boreholes are usually built in clusters of 7 to 9 from special drill chambers located in drifts. Special drilling rigs permit the bore holes at any angle to the horizontal. This makes for uniform pattern of boreholes 12 to 18.5 metres apart. Bore hole diameters are 3 to 4 inches. When free flow ceases, they are usually operated by gas lift for which ½ inch, ¾ inch and 1 inch tubing is used. Most often the oil produced flows to the hoisting shaft by gravity as a film on the surface of water in the drainage ditches of underground workings.
Methods of drainage by means of horizontal boreholes are still in the experimental stage.
Extraction of drainable oil through mine openings does not ensure complete recovery. For complete recovery, it would be necessary to actually mine the oil reservoir rock, remove it to the surface for treatment by washing and then return it into the underground openings to prevent roof cave-ins. One of the methods used for the extraction of oil from the oil bearing rock is by thermal treatment.
Development and Exploitation of Oil and Gas Fields, Alexander Gurevich.
Enhanced Recovery of Residual and Heavy Oils, M.M. Schumacher.