Heavy oil viscosity reduction and production

This invention provides an apparatus and process for producing heavy crude oil from a subterranean formation penetrated by a well bore. In accordance with the process, an aqueous alkaline chemical solution is introduced into or formed in the well bore penetrating the formation. The aqueous alkaline chemical solution mixes and reacts with produced heavy crude oil in the well bore and ultrasonic waves are emitted into the mixture whereby an emulsion is formed. The viscosity of the formed emulsion is less than that of the crude oil or the crude oil and water mixture flowing into the well bore which allows the oil to be more efficiently pumped to the surface and transported for further processing.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to apparatus and methods for reducing the viscosity
 of crude oil produced from a subterranean formation in order to facilitate
 pumping and/or transporting the oil.
 2. Description of the Prior Art
 The production of crude oil from an oil reservoir is generally assisted to
 a great extent by naturally occurring forces associated with the
 reservoir. These naturally occurring forces include the expanding force of
 natural gas, the buoyant force of approaching water and the force of
 gravity. Primary recovery techniques utilize these forces to cause the oil
 to migrate from the formation into the well bore. Unfortunately, the
 natural forces are typically only sufficient to allow a small percentage
 of the total oil in the reservoir to be produced.
 Secondary recovery techniques are generally employed to recover more of the
 oil in the formation. These techniques utilize extraneous energy forces to
 supplement the naturally occurring forces in the formation and force the
 oil from the formation into the well bore. The extraneous forces can be
 generated from a large variety of sources including gas injection, steam
 injection and water injection. Secondary recovery techniques are typically
 initiated even before the primary forces of the reservoir are exhausted.
 Water flooding is one example of a secondary recovery technique that has
 been successfully employed in different types of formations. Generally, in
 accordance with water flooding techniques, one or more injection wells and
 one or more production wells are utilized. An aqueous solution is injected
 through the injection well(s) in order to drive the oil to the production
 well(s) where it can be produced. Many modifications to basic water
 flooding techniques have been developed. These modifications include the
 use of certain chemicals and materials in the injection water to help
 displace the oil from the formation. For example, thickening agents are
 often employed to thicken the water and thereby increase its efficiency in
 driving the oil to the producing well(s). Surfactants have been employed
 to reduce the surface tension of the oil in the formation and thereby
 facilitate its production.
 Aqueous alkaline solutions, e.g., caustic solutions, have been successfully
 utilized for flooding certain types of reservoirs. For example, alkali
 metal hydroxides such as sodium hydroxide react with organic acids present
 in the oil and depress the interfacial tension between the oil and the
 water resulting in emulsification of the oil. The emulsified oil is more
 easily displaced from the formation. This type of secondary recovery
 technique is often referred to as caustic flooding.
 Another secondary recovery technique that has been employed to increase the
 recovery of oil in certain situations involves the use of sonic energy.
 For example, sonic stimulation has been utilized in Russia to improve oil
 production in depleted water flooded and water-dry oil reservoirs. The
 sound waves generally function to heat and reduce the viscosity of the
 oil, increase the permeability of the formation and generally induce
 migration of the oil to the well bore.
 Secondary recovery techniques involving heavy and highly viscous crude oil
 ("heavy crude oil") are especially challenging. In order to efficiently
 produce heavy crude oil, the viscosity of the oil must be substantially
 reduced. Transportation of heavy crude oil (e.g., by pipeline) can also be
 difficult to accomplish in an efficient manner unless the viscosity of the
 oil is first reduced. Numerous techniques have been employed to reduce the
 viscosity of heavy oil. For example, U.S. Pat. No. 3,823,776 to Holmes
 discloses a process for increasing the recovery of heavy oil having a low
 acid value whereby an oxygen-containing gas is injected into the formation
 to oxidize the oil and establish an in situ combustion zone in the
 formation. An aqueous caustic solution is then injected into the well to
 quench the in situ combustion zone and react with organic acids present in
 the oil to facilitate production of the oil. U.S. Pat. No. 2,670,801 to
 Sherborne discloses that ultrasonic energy (10 to 3,000 kHz) facilitates
 recovery of heavy oil by in situ heating of the oil droplets and
 emulsification of the droplets to a water phase saturated with gas.
 Unfortunately, the techniques utilized heretofore to facilitate recovery of
 heavy oil from subterranean formations are often not very successful. The
 cost of reducing the viscosity of heavy oil to a level whereby the oil can
 be lifted out of the formation and transported for further processing
 often exceeds the potential gain to be realized by producing the oil.
 Accordingly, there is a need for an improved apparatus and corresponding
 process for treating heavy crude oil produced from a petroleum reservoir
 whereby the viscosity of the oil can be substantially reduced and the oil
 can be produced and transported for further processing in an economical
 and efficient manner.
 SUMMARY OF THE INVENTION
 It has been discovered that the viscosity of viscous and often heavy crude
 oil can be dramatically reduced by converting the oil to a stable
 microemulsion. The microemulsion is formed by combining alkaline chemicals
 with the oil and subjecting it to ultrasonic energy. The reduction in the
 viscosity of the oil allows it to be efficiently pumped out of the well
 bore and transported from the well site for further processing, i.e., the
 lifting costs and pipeline transportation costs are dramatically reduced.
 In one aspect, the present invention provides apparatus for increasing the
 recovery of heavy crude oil from a subterranean oil bearing formation
 penetrated by at least one well bore. The apparatus includes storage means
 positioned on the surface for containing an alkaline chemical or aqueous
 alkaline chemical solution (e.g., one or more storage tanks on the drill
 site), conduit means extending from the storage means through the well
 bore to the formation for conducting the alkaline chemical or aqueous
 alkaline chemical solution from the storage means to the formation, and
 ultrasonic stimulation means positioned within the well bore for emitting
 ultrasonic waves into heavy oil-water-alkaline chemical mixture formed in
 the well bore. The ultrasonic stimulation means includes a transducer
 positioned in the well bore for emitting ultrasonic waves into the
 oil-water-alkaline chemical mixture in the formation whereby the oil and
 water are converted to a lower viscosity emulsion, and electric power
 means operably connected to the transducer for providing energy to the
 transducer. The transducer preferably includes an electric powered
 magnetostrictive actuator, more preferably an electric powered
 magnetostrictive actuator comprised of a drive rod formed of a terfenol
 alloy.
 In another aspect, the present invention provides a process for producing
 heavy crude oil from a subterranean oil bearing formation penetrated by at
 least one well bore. In accordance with the process, an alkaline chemical
 or aqueous alkaline chemical solution is introduced into the well bore
 into which heavy oil and water or heavy oil alone is produced. The
 alkaline chemical or aqueous alkaline solution is introduced into the well
 bore in an amount sufficient to mix with the heavy crude oil and water or
 the heavy crude oil alone in the well bore. Simultaneously with the
 introduction of the alkaline chemical or aqueous solution thereof into the
 well bore, the resulting mixture of oil, water and alkaline chemical is
 subjected to ultrasonic stimulation by emitting ultrasonic waves therein
 which converts the mixture into a lower viscosity emulsion. The emulsion
 is then produced from the formation through the well bore and transported
 by pipeline to a point of further processing.
 The procedure by which the viscosity reduction of the heavy crude oil is
 achieved includes the use of water or brine with an alkaline chemical
 additive such as sodium hydroxide, calcium hydroxide, sodium silicates and
 other strong bases. The water (or brine) used to make up the alkaline
 solution can either be supplied from an external source or in part or in
 total from water (or brine) produced with the oil. When the resulting
 water (or brine) and alkaline chemical are mixed with the heavy crude oil
 in the presence of ultrasonic stimulation, a semi-stable to stable
 emulsion is rapidly formed which has a dramatically lower viscosity than
 the untreated viscous oil.
 It is, therefore, an object of the present invention to provide an
 apparatus and process whereby the effective viscosity of heavy crude oil
 produced into a well bore is substantially reduced thereby allowing the
 oil to be produced and transported from the well in an economical and
 efficient manner.
 Additional objects, features and advantages of the invention will be
 readily apparent to those skilled in the art upon a reading of the
 detailed description of preferred embodiments of the invention which
 follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
 By the present invention, an apparatus and process for producing heavy
 crude oil from a subterranean oil bearing formation penetrated by a well
 bore are provided. The apparatus and process can be used in the bottom of
 the well bore as described herein and/or at the entrance of a surface or
 subsea pipeline or other location where it is desirable to reduce the
 viscosity of oil. As used herein and in the appended claims, the term
 "heavy crude oil" means crude oil having an API gravity of less than about
 20. Such heavy oils typically have viscosities in excess of 1,000
 centipoises at ambient conditions of temperature and pressure.
 The application of ultrasonic energy to heavy crude oil, water or brine and
 an alkaline chemical makes it possible to generate stable microemulsions
 having low viscosities. A key to implementation of this technique is to
 start with the viscosity of the oil in a range where it can participate in
 emulsion forming mechanisms with water or brine. For heavy crude oil that
 is extremely viscous, it may be necessary to heat the oil to reduce the
 viscosity such that it falls in a range where emulsions can be formed. The
 ultrasonic stimulation process contributes to the heating of the oil.
 For oil that is extremely viscous, it is sometimes more effective to
 initially lower the viscosity of the oil before ultrasonic treatment of
 the mixture of oil, water or brine and alkaline chemical. Laboratory
 experiments indicate that there is a relationship between the initial
 viscosity of an oil prior to ultrasonic treatment and the viscosity of the
 emulsion formed. If the initial viscosity of the oil is extremely high,
 the viscosity of the resultant emulsion may still be higher than desired
 to obtain a fluid with good flow characteristics. However, by heating
 extremely viscous oil prior to ultrasonic treatment, a lower viscosity
 microemulsion can be obtained. This heating of the oil can be achieved in
 various ways such as by placing a heating apparatus in the well bore,
 injecting steam in the well bore and the like.
 Referring now to the drawing, a preferred embodiment of the inventive heavy
 oil recovery apparatus, generally designated by the numeral 10, is
 described. As schematically illustrated, a well bore 12 extends from the
 surface 14 and penetrates a heavy oil producing subterranean formation 16.
 A cemented casing 18 extends around the perimeter of the well bore 12. A
 plurality of perforations 20 extend through the cemented casing 18 into
 the formation 16 and establish fluid communication between the well bore
 12 and the formation 16. A string of production tubing 24 extends through
 the well bore 12 from the surface 14 to a point in the well bore within
 the formation 16 and adjacent to the perforations 20. The tubing 24
 conducts oil from the formation 16 to the surface 14. A submersible
 electric pump 30 having a motor 32, inlet 34 and electric wireline 36 are
 attached to the production tubing 24. The pump 30 pumps oil through the
 tubing 24 to the surface 14. The exact structures of the casing 18,
 perforations 20, tubing 24, pump 30 and associated equipment (e.g., guide
 apparatus, centralizers and so forth) are not critical to the present
 invention and have been generally described only to the extent necessary
 to illustrate the invention. The nature and operation of such equipment
 are well known to those skilled in the art.
 The apparatus 10 includes storage means generally designated by the numeral
 40 positioned on the surface 14 for containing an alkaline chemical or the
 components of an aqueous alkaline chemical solution. Conduit means 42
 extend from the container means 40 through the well bore 12 to the
 formation 16 for conducting the alkaline chemical or aqueous alkaline
 chemical solution from the storage means to near the bottom of the well
 bore 12 within the producing formation 16. Ultrasonic stimulation means 45
 are positioned within the well bore 12 for imparting ultrasonic wave
 energy to a mixture 46 of heavy crude oil, water and alkaline chemical
 therein.
 The storage means 40 includes one or more conventional mixing tanks (not
 shown). The conduit means 42 includes at least one capillary or other
 relatively small diameter tube 43 that extends through the well bore
 between the outside of the production tubing 24 and the inside of the
 casing 18. Tube 43 can include a plurality of injection nozzles 48 that
 inject an alkaline chemical or aqueous alkaline chemical solution into the
 well bore 12 whereby the alkaline chemical or solution contacts and mixes
 with heavy crude oil or heavy crude oil and water therein.
 The alkaline chemical or aqueous alkaline chemical solution is pumped from
 the storage means 40 into the tube 43. The solution can be batch mixed in
 the storage means or, alternatively, the components can be individually
 conducted or conveyed from separate tanks and mixed on the fly as they are
 pumped into the tube 43.
 The ultrasonic stimulation means 45 includes one or more transducers 50
 positioned in the well bore for emitting ultrasonic wave energy into the
 well bore and into the mixture of heavy crude oil, water and alkaline
 chemical therein and an electric power means 52 operably connected to the
 transducer(s) 50. As used herein and in the appended claims, "positioned
 in the well bore" means positioned at a point in the well bore such that
 the ultrasonic waves emitted by the transducer(s) 50 contact the mixture
 of heavy crude oil, water and alkaline chemical in the general vicinity of
 where the oil enters the well bore. For example, the transducer(s) 50 can
 be positioned in the well bore 12 slightly above, slightly below or within
 the portion of the well bore actually penetrating the heavy oil producing
 formation 16. Preferably, the transducer(s) 50 are submerged in the fluid
 mixture 46 in the bottom of the well bore 12.
 The transducer(s) 50 can be mounted directly on the pump 30 or other
 portion of the work string. Alternatively, as shown in the drawing, the
 transducer(s) 50 can be suspended by a cable 56 below the pump 30. In some
 cases, it is advantageous to employ a plurality of transducers 50 in
 regularly spaced positions along the perforated portion of the casing 18.
 In addition to assuring that the heavy crude oil and other components
 mixed therewith in the well bore 12 are contacted by ultrasonic waves, the
 use of multiple transducers strategically placed in the oil flow path
 ensures that the viscosity of the oil is reduced and maintained at a
 sufficiently low level prior to when the oil is pumped by the pump 30. The
 intensity of the energy imparted by each transducer 50 as well as the
 exact number of transducers that should be used will vary depending on
 several factors including the ultrasonic wave exposure time required to
 reduce the viscosity of the oil to a sufficient level and the overall
 production rate of the well.
 Each transducer 50 that is employed preferably includes an electric powered
 magnetostrictive actuator, most preferably a magnetostrictive actuator
 comprised of a drive rod formed of a terfenol alloy. The terfenol alloy is
 composed of the metals terbium, dysprosium and iron. Each transducer 50
 directly transforms electrical energy into mechanical action. In one
 embodiment, a terfenol rod is attached to a radiating bar or other
 element. Referring to the energy transducer generally designated by the
 numeral 2 in FIG. 2, a coil 4 surrounding the terfenol rod 6 creates an
 alternating magnetic field in the rod 6 which causes the rod 6 to extend
 and contract resulting in a corresponding displacement of the attached bar
 or other element 8. The excitation of the attached bar or other element 8
 imparts the ultrasonic waves to the mixture of heavy crude oil, water and
 alkaline chemical in the well bore 12. Particularly preferred transducer
 actuators for use in accordance with this invention include
 Terfenol-D.RTM. drive rods and are commercially available from Extrema
 Products, Inc. of Ames, Iowa.
 The power means 52 of the ultrasonic stimulation means 45 includes an
 electric control unit 60 positioned on the surface 14, a signal
 conditioning unit 62 located at the surface 14 or located in the well bore
 12 between the control unit and the transducer(s) 50, and the electric
 wireline 36 extending and transmitting electric power from the control
 unit 60 to the signal conditioning unit 62 and then to the transducer(s)
 50.
 The use of transducers having magnetostrictive actuators including terfenol
 alloy drive rods to impart sonic energy to the heavy crude oil is very
 advantageous. The terfenol alloy drive rod is a great improvement compared
 to prior art actuators including sucker rods or pizeo crystals for a
 variety of reasons. First, actuators including terfenol drive rods are
 more durable than other types of actuators and they do not fatigue as
 easily. Actuators with terfenol rods are also more energy efficient than,
 for example, pizeo crystal actuators. A greater amount of electricity is
 converted into sonic waves by actuators with terfenol drive rods. Also,
 actuators with terfenol drive rods are highly tunable allowing resonant
 frequency levels to be established.
 In carrying out the inventive process, it may first be necessary to reduce
 the viscosity of the heavy crude oil in the well bore by heating the oil.
 That is, when the heavy crude oil produced into the well bore has a very
 high initial viscosity, i.e., a viscosity above about 10,000 centipoises,
 the viscosity of the emulsion produced may not be at a low enough level.
 While the ultrasonic wave energy imparted to the oil heats it to some
 extent, it may be necessary to install a heater 70 such as an electric
 powered heater in the well bore (shown in dashed lines in the drawing) to
 heat the oil and lower its viscosity to a level below about 10,000
 centipoises, preferably to a range of from about 1,000 to about 8,000
 centipoises and most preferably to from about 2,500 to about 4,000
 centipoises. Other techniques of heating the oil can also be utilized such
 as injecting steam into the formation and the like.
 As mentioned above, the water or brine required to form a microemulsion
 with the heavy crude oil in the well bore 12 can be water produced with
 the oil whereby only the alkaline chemical must be pumped from the storage
 means 40 on the surface 14. If little or no water is produced with the
 heavy crude oil, the required water can be mixed with the alkaline
 chemical on the surface 14 and pumped into the well bore 12 as an alkaline
 chemical solution.
 The alkaline chemical or aqueous alkaline chemical solution used is pumped
 from the storage means 40 into the tube 43 and through the nozzles 48 into
 the well bore 12 adjacent to the formation 16. Upon entering the well bore
 12, the alkaline chemical or aqueous alkaline chemical solution contacts
 and mixes with the heavy crude oil and water or the heavy crude oil alone
 therein. The alkaline chemical reacts with naphthenic and other acids
 present in the crude oil to form large "soap-like" molecules having a low
 interfacial tension. As the alkaline chemical contacts and reacts with the
 heavy crude oil, the crude oil is bombarded with ultrasonic waves emitted
 from the ultrasonic transducer(s) 50. The combined use of an alkaline
 chemical and ultrasonic energy in the presence of water and oil results in
 the rapid formation of a semi-stable to stable emulsion, generally a
 microemulsion. As stated above, in this emulsified state, the crude oil
 has a significantly lower viscosity than the viscosity of the crude oil
 alone or the crude oil mixed with water.
 The aqueous alkaline solution that is pumped into the well bore 12 or
 formed therein has a pH of at least about 8 and the chemical or solution
 is introduced into the formation at a rate sufficient to form a
 microemulsion with the rate of heavy crude oil flowing into the well bore.
 Preferably, the aqueous alkaline solution has a pH in the range of from
 about 10 to about 13, more preferably in the range of from about 12 to
 about 13. The solution contains the alkaline chemical in a concentration
 in the range of from about 0.001 to about 10 molar, more preferably in the
 range of from about 0.01 to about 8 molar.
 The alkaline chemical used is preferably selected from the group consisting
 of sodium hydroxide, calcium hydroxide, sodium silicate compounds, sodium
 bicarbonate, magnesium hydroxide and mixtures thereof. More preferably,
 the alkaline chemical is selected from the group consisting of sodium
 hydroxide and calcium hydroxide. Most preferably, the alkali metal
 hydroxide is sodium hydroxide. The specific rate of aqueous alkaline
 solution introduced into or formed in the well bore 12 will vary depending
 upon various factors including the production rate of the heavy crude oil
 into the well bore 12, the initial viscosity of the heavy crude oil and
 the production rate of water, if any. Generally, the aqueous alkaline
 chemical solution is introduced into or formed in the well bore whereby
 the volume ratio of the aqueous alkaline chemical solution to heavy crude
 oil is in the range of from about 1:10 to about 10:1, more preferably from
 about 1:3 to about 3:1; most preferably about 1:2.
 The ultrasonic waves produced by the transducer(s) 50 are emitted in the
 well bore 12 at a frequency sufficient to enhance the formation of a
 stable emulsion between the water therein and the reaction product of the
 alkaline chemical with the heavy crude oil therein. The exact frequency
 and energy intensity of the emitted ultrasonic waves is dependent on
 various characteristics of the oil such as its initial viscosity,
 production rate and the like. Generally, the ultrasonic waves emitted into
 the well bore by the ultrasonic transducer(s) 50 are at a frequency of at
 least about 15 kilohertz, more preferably at a frequency in the range of
 from about 15 kilohertz to about 25 kilohertz and most preferably at a
 frequency of 20 kilohertz. At a frequency of approximately 20 kilohertz,
 the corresponding energy intensity level is particularly effective in
 achieving the objects of the present invention. An ultrasonic transducer
 having a magnetostrictive actuator including a terfenol drive rod can be
 used to achieve energy intensities at the transducer of from about 0.1 to
 about 100 watts per square centimeter.
 The time period for which the crude oil should be subjected to the
 ultrasonic energy to achieve the desired emulsification and viscosity
 reduction will vary from a few seconds to several minutes. In a preferred
 embodiment, the crude oil is continuously subjected to sonic stimulation
 while production is ongoing.
 The following examples are provided to further illustrate the invention.
 EXAMPLE 1
 Tests were conducted on heavy crude oil from the Hamaca reservoir in
 Venezuela having an API gravity of approximately 8. Test samples of the
 oil were mixed with aqueous sodium hydroxide solutions at the temperatures
 and in the amounts given in Table I below. A number of the mixtures were
 insonicated (bombarded) with ultrasonic waves for the times given and
 producing the results shown in Table I below.
 TABLE I
 Aqueous Sodium Sodium
 Hydroxide Hydroxide
 Insonication.sup.1 Solution Solution
 Time, Temperature, Amount.sup.2, % by Concentration,
 Viscosity.sup.3,
 min. .degree. C. volume molar cp
 No insonication 23 No additive No additive 785,600
 No insonication 50 No additive No additive 29,200
 1 23 33 0.1 Did not

emulsification.sup.4
 .sup.1 All insonication was conducted at approximately 20 kHz.
 .sup.2 The percent by volume of the NaOH solution was based on the volume
 of the NaOH solution divided by the total volume of the crude oil and
 NaOH solution.
 .sup.3 The viscosities of the samples were measured using a Brookfield
 viscosimeter.
 .sup.4 The sample was not mixed well enough to give an accurate viscosity
 reading.
 In a second series of tests, the temperatures employed were raised to some
 extent. The results of these tests are as follows:
 TABLE II
 Aqueous Sodium Sodium
 Hydroxide Hydroxide
 Insonication.sup.1 Solution Solution
 Time, Temperature, Amount.sup.2, % by Concentration,
 Viscosity.sup.3,
 min. .degree. C. volume molar cp
 No insonication 60 No additive No additive 9880
 No insonication 70 No additive No additive 4448
 No insonication 75 No additive No additive 2832
 1 75 33 0.1 9.90.sup.4
 3 75 33 0.1 6.60.sup.4
 .sup.1 All insonication was conducted at approximately 20 kHz.
 .sup.2 The percent by volume of the NaOH solution was based on the volume
 of the NaOH solution divided by the total volume of the crude oil
 and NaOH solution.
 .sup.3 The viscosities of the samples were measured using a Brookfield
 viscosimeter.
 .sup.4 These samples formed stable microemulsions and had very low
 viscosities even after cooling to room temperature.
 From the results given in Table II, it can be seen that the process of the
 present invention achieves very significant heavy crude oil viscosity
 reduction.
 Thus, the present invention is well adapted to carry out the objects and
 attain the ends and advantages mentioned as well as those which are
 inherent therein. While numerous changes may be made by those skilled in
 the art, such changes are encompassed within the spirit of this invention
 as defined by the appended claims.