Electric resistance heating devices – Heating devices – Borehole type
Reexamination Certificate
2000-10-26
2002-03-05
Jeffery, John A. (Department: 3742)
Electric resistance heating devices
Heating devices
Borehole type
C166S060000, C166S302000, C219S635000, C219S643000
Reexamination Certificate
active
06353706
ABSTRACT:
BACKGROUND OF THE INVENTION
Major problems exist in producing oil in heavy-oil reservoirs because of the high viscosity of the oil. Because of this high viscosity oil, a very high pressure gradient builds up around the wellbore, thereby utilizing almost two-thirds of the reservoir pressure in the immediate vicinity of the wellbore. Furthermore, as the heavy oils progress inwardly to the wellbore, gas in solution evolves more rapidly into the wellbore. Since the dissolved gas reduces the viscosity, this evolution further increases the viscosity of the oils in the immediate vicinity of the wellbore. Such viscosity effects, especially near the wellbore, greatly impede production, and the resulting wasteful use of reservoir pressure can reduce the overall primary recovery from such reservoirs.
Similarly, in light-oil deposits, dissolved paraffin in the oil tends to accumulate around the wellbore, particularly in the screens and perforations and within the deposit up to a few feet from the wellbore. This precipitation effect is caused by the evolution of gases and volatiles as the oil progresses into the vicinity of the wellbore, thereby decreasing the solubility of paraffin and causing it to precipitate. Also, the evolution of gases causes an auto-refrigeration effect which reduces the temperature, thereby decreasing the solubility of the paraffins. Similar to paraffin, other condensable constituents can also plug up, coagulate, or precipitate near the wellbore. These include gas hydrates, asphaltenes, and sulfur. In the case of certain gas wells, liquid distillates can accumulate in the immediate vicinity of the wellbore. Such accumulation reduces the relative permeability near the wellbore. In all such cases, such near wellbore accumulations reduce production rates and reduce ultimate primary recoveries.
Electrical resistance heating has been employed to heat the reservoir in the immediate vicinity of the wellbore. This has been the subject of recent pilot tests. Basic systems are described in Bridges U.S. Pat. No. 4,524,827 and in Bridges et al., U.S. Pat. No. 4,821,798. Such systems are applicable largely for new wells. Prior to installation, some modifications of casing near the wellbore are usually needed to permit electrical resistance heating in the reservoir near the wellbore. For a cased-hole completion, the electrode which is in the reservoir must be isolated from the casing by fiberglass tubing above and below the electrode as discussed in Bridges et al., U.S. Pat. No. 4,821,798.
In the case of open-hole completions, considerable modification of the downhole screen and near reservoir casing and tubing is required. For existing wells, the old gravel pack and screens must be removed and a new gravel pack and screen system installed so that an electrically isolated electrode can be positioned in the deposit. Such electrode may be part of the gravel pack and screening system.
Such near wellbore heating systems have been demonstrated to massively heat the reservoir just outside the wellbore and to reduce or eliminate many of the aforementioned thermally responsive flow impediments. Such elimination can result in demonstrated flow increases of 200 to 400%. These procedures are used primarily in new well installations for cased-hole completions, but can be also used for either new open-hole completions or to retrofit existing wells with open-hole completions.
However, open-hole modifications are largely limited to either new wells or existing wells that have a very high flow rate, because the cost of installing either a new well or repacking an existing open-hole completed well with a new electrode assembly and gravel pack system is large.
What is desired, then, is a method of retrofitting old wells, either cased or open-hole completions, which is inexpensive and yet heats some of the reservoir in the immediate vicinity of the wellbore adjacent to the formation as well as within the wellbore itself. One method of doing this has been attempted before with a mixed degree of success. This technique employs the use of cylindrical resistance heaters which are coaxially situated in the wellbore and are positioned in the wellbore immediately adjacent to the reservoir. The earliest patent in the literature on this subject matter was issued in July of 1865 in U.S. Pat. No. 48,584 which is described as an electric oil well heater. Since then, numerous patents have been issued which have covered this type of inside wellbore heating. Such past art includes Pershing U.S. Pat. No. 1,464,618, Stegemeier U.S. Pat. No. 2,932,352, McCarthy U.S. Pat. No. 3,114,417, Williams U.S. Pat. No. 3,207,220 and Van Egman et al., U.S. Pat. No. 4,704,514. Such systems, heating inside the wellbore, received considerable attention in the 1950's and early 1960's, with some improvements reported in some reservoirs and other reservoirs showing mixed results. One principal difficulty encountered with such heaters was that they burned out at intervals so frequent that their use could not be justified. Though some of the causes of the failure of these resistors were due to poor designs, some fundamental problems also exist which contributed to the burn-out problem.
The useful heat supplied by the cylindrical resistor flows out of the wellbore and into the formation by thermal conduction. At the same time, unavoidably, the flow of fluids inwardly into the wellbore removes, via convection, transfers heat transferred by convection from the formation toward the producing well. In the wellbore itself, the heat is further unavoidably removed from the annular space between the heater and the screen or casing, via convection caused by the upward flow of oil in the well. Therefore, in order to achieve a noticeable increase in temperature just outside of the wellbore, very high heater temperatures were required. Such higher heater temperatures may also be accompanied by the deposition of scale or products of low temperature pyrolysis on the heater. This further thermally isolates the heater, thereby causing requirements for even higher resistor temperatures, which further compounds the problem. As a consequence of this fundamental counter flow heat problem between outward thermal diffusion and inward thermal convection, such an approach would be effective only in slowly producing wells and would become decreasingly less effective as the flow rate was increased much above a few tens of barrels per day for typical installations.
One method to mitigate the aforementioned problem would be to create a situation such that the casing itself, in the completed zone, would provide the heat. Alternatively, for an open-hole completion, the screen and/or gravel pack might preferably provide the heat rather than a small diameter cylindrical resistor element coaxially located within the wellbore next to the producing zone. By so doing, the radius of the heat producing element or resistor could be extended from approximately 1 in. to about 8 in., depending on the diameter of the wellbore or screen in the completed zone. Such an arrangement would give at least a fourfold improvement in the amount of heat which could be transferred based on a given temperature of the heated element. In addition, such an arrangement would eliminate in the annulus convection heat losses in the annulus due to the upward thermal convection of the fluids once they entered into the wellbore itself.
Earlier techniques have been ineffectively addressed in two U.S. patents; 1) by A. W. Marr in U.S. Pat. No. 4,319,632 and 2) by S. D. Sprong in U.S. Pat. No. 2,472,445. In either case, no system is adequately described which embodies the use of such casing heating systems and which is combined with an efficient downhole power delivery and control system. For example, in the case of Marr, the electrical heating system had one electrical contact with the casing at the surface and the other contact in the producing zone. As a consequence, current flowed from the bottom of the casing up along the entire surface, thereby heating the entire casing string and adjace
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Jeffery John A.
Uentech International Corporation
LandOfFree
Optimum oil-well casing heating does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optimum oil-well casing heating, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optimum oil-well casing heating will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2890303