It is not uncommon in the oil producing industry to encounter liquid hydrocarbons which do not flow at a rate sufficient to be of commercial interest. This is generally caused by a high viscosity of the oil at formation temperature. In order to lower the viscosity of such oil, it is a well known technique to increase the temperature of the formation. The reduction of the viscosity of the oil has two important effects. First, it allows the oil to flow easier within the formation and reduces pumping power required to bring it to the surface. Secondly, the reduction in oil viscosity also increases the oil relative mobility and reduces the water relative mobility. The latter effect thus reduces the water production.
Another important application for heat treatment is the prevention or removal of waxes or asphaltenes buildup in the wellbore and near-wellbore region. Other benefits resulting from thermal treatments include clay dehydration, thermal fracturing at high temperatures, prevention of thermal fracturing in water zones at low temperatures and sand consolidation in unconsolidated formations. In water flooding situations, injection well looses its injectivity due to various problems including clay swelling, and therefore thermal treatment can improve the injectivity. In the case of downhole electrical heating, some of the current may be diverted to prevent the corrosion of tubing, casing, pump rods and other downhole components and to prevent buildup of corrosion products.
White et al. in J. Petrol Technol, 1965, 1007 discloses the use of a downhole electric heater to ignite the fuel in situ. The heater is removed and air is supplied to maintain a combustion front. The process managed to improve oil production to four times the precombustion rate while reducing the water cut to 8%. The oil continued to produce at twice the normal rate for several months after the treatment.
U.S. Pat. No. 5,070,533 describes a downhole heater design which uses the casing or tubing as electrodes. One electrode is aligned with the pay zone. The opposite electrode is located outside the pay zone and preferably at least three times the diameter of the hole away from the first electrode. In order to pass from one electrode to the other, the current must pass through the pay zone. The current is carried either by a conductive formation or by the water in the formation. The high resistance to current flow results in localized heating, and the system is preferably operated only while the well is producing. A major problem with this procedure is the potential for accelerated corrosion at the interface of the anode.
U.S. Pat. No. 4,285,401 teaches the combination of a downhole heater with a water pump. If the heater is powered then pressurized water is directed through the heater and to the formation where it will penetrate at the rock formation and thermally stimulate the well If the heater is not activated, then the pressurized water is to turn a turbine and assist in the downhole pumping of production fluids. The use of pressurized water also prevents the heater from overheating and burning out the elements. The method is said to prevent heat losses along the pipe from pumping steam from the surface.
U.S. Pat. No. 4,951,748 is concerned with a technique of heating based on supplying electrical power at the thermal harmonic frequency of the formation. Three-phase AC power is converted to DC and then chopped to single phase AC at the harmonic frequency. The harmonic frequency heating occurs in addition to the normal ohmic heating. The harmonic frequency of the rock or fluid is determined in the laboratory prior to application in the well. This frequency may be adjusted during well heating as the harmonic frequency may fluctuate with temperature and pressure.
U.S. Pat. No. 5,020,596 describes a downhole heating process which begins by flooding the reservoir with water from an injection well to a desired pressure. A fuel-fired downhole radiant heater in the injection well is ignited and heats the formation and water. The heat radiates along the entire length of the heater to keep the isothermal patterns close to vertical and provide a good sweep. The heater consists of three concentric cylindrical tubes. A burner within the innermost tube ignites, and burns a source of fuel and air. Apertures are sized and positioned to develop laminar flow of the combustion products from the burner such that the heat transfer is effective along its entire length. The combustion products are removed from the annular space between the two outer tubes. The design of the heater minimizes local hot spots and should heat the reservoir evenly. The temperature which can be reached in the reservoir is dependent upon the pressure of the reservoir. However, the use of a long radiant heater such as the above implies important losses of heat in an effort to achieve equal flow over the entire height of the reservoir.
U.S. Pat. No. 5,120,935 describes a downhole packed-bed electric heater comprising two electrodes which are displaced from each other. The gap is filled with conductive balls. Resistive heating occurs when current is passed through the heater. The multiple paths of current flow through the heater prevent failure of the heater due to element burnout. The heater provides a large surface area for heating while maintaining a low pressure drop between the inlet and outlet of the heater. The length and diameter can be adjusted to satisfy well design and heating requirements. Formation heating is achieved by passing a solvent through the heater which is heated up, passes into the formation and transfers the heat to the formation.
U.S. Pat. No. 4,694,907 uses a downhole electric heater to convert hot water to steam. Instead of producing steam on the surface and pumping it downhole, it is suggested to heat water on the surface, pump it downhole where an electric heater converts the hot water to steam. The electric heater is a series of U-tubes disposed circumferentially around the water injection tube. Each U-tube can be individually controlled. The injection tube is closed at the bottom with orifices displaced radially. Water flows out the injection tube and past the heater tubes where it is vaporized. Electric power is supplied via a three-phase grounded neutral "Y" system with one end of each heater element being common and neutral. The system also supplied DC current to the heater.
U.S. Pat. No. 5,060,287 is concerned with a copper-nickel alloy core cable for downhole heating. The cable is capable of withstanding temperatures to 1000.degree. C. and utilizing voltages to 1000 volts. The cable is especially useful for heating long intervals. U.S. Pat. No. 5,065,818 describes a heater using this material which is cemented into an uncased borehole. The heater can provide heat to about 250 watts per foot of length.
U.S. Pat. No. 1,681,523 discloses a heater comprising two concentric tubes. The inner tube acts as a conductor and the heating coils are wrapped at various locations along the whole length of the conductor. The other conductor is an insulated cable that runs parallel to the conductor tube all the way to the surface. Both tubes, along with multiple heating elements, are housed in a larger casing. Air is circulated downward through the inner pipe and upward through the annular space between the inner and outer pipes. At the surface, a pump is used to recirculate the air. In this manner, the whole length of the pipe is heated, and the air circulation distributes the heat. The purpose of such heating is to keep the entire production line heated to prevent paraffin deposition. Heated air never comes out of the system. Further, the temperature of heating and the electrical connections, power and temperature requirements are not entertained. Such a heating system is not suitable for hot-fluid injection in a formation, since for such use, an end of the heater must be open. Also, the multiple connections of the heating elements with the conductors will render the heating system inoperable in the presence of formation fluids, for example, like salt water. It is likely that the temperature applied with this system are not particularly high (the melting point of paraffin is lower than 60.degree. C.), since the multiple electrical connections would not sustain prolonged exposure to high temperature.