FIELD OF THE INVENTION This invention relates broadly to the extraction of crude oil from oil-bearing geological formations, and particularly concerns both a method and apparatus for stimulating crude oil flows in an established oil well that has been drilled into an oil-bearing geological formation of particular interest.

BACKGROUND OF THE INVENTION It is well-known that, with utilization time, the productivity of a conventional well that extracts crude oil from an oil-bearing geological formation will decline, and that the principal reasons for such production diminishment most often are reductions of natural pressures in the formation crude oil reservoir resulting from preceding reservoir depletion, reductions in formation permeability near the well bore due to deposits of scales and precipitates that inhibit the flow of crude oil toward the well casing, or a combination of these specific causes.

Numerous different methods have been utilized to enhance the recovery of crude oil from wells exhibiting reduced flows. Such methods have included increasing reservoir pressures in the oil bearing formation by high-pressure injection of a process fluid such as water, steam. or gas.

Also, such recovery enhancement methods have included increasing formation permeability by removing localized deposits through mechanical scraping, injection of solvents or reactive acids, acoustic vibration, and even hydraulic fracturing.

Such known oil well productivity enhancement methods may, in the most successful cases, function to increase oil recovery by up to 50% to 70% of the total crude oil in place, but

also have a number of disadvantages. Such disadvantages include limited applicability, low probabilities of success (e. g., 40% to 50% for the more successful methods), and possible negative side-effects such as irreversible damage resulting from acidifying a clay-containing formation. Also, certain of the methods are extremely costly to apply to an oil well and may range to as much as $500,000 per well as in the case of great well depth.

We have discovered a novel apparatus and method which may be utilized, especially with respect to an established oil well that is either non-producing or marginally producing, to stimulate the flow of crude oil from the oil-bearing geological formation into which the well has been drilled and toward and into the interior of the casing of the well thereby enhancing well productivity. The novel method and apparatus obtain numerous crude oil recovery advantages in comparison to the known oil well productivity enhancement methods summarized above.

Other advantages associated with the invention will become apparent from consideration of the descriptions, drawings, and claims which follow.

SUMMARY OF THE INVENTION The apparatus aspect of the present invention basically is comprised of a reservoir of process fluid such as water or crude oil, a high-pressure process fluid pump, a jet generator head assembly having multiple, properly-oriented injector elements, and inter-connected, high-pressure tube sections which join the reservoir, pump, and jet generator head assembly into a functional oil well stimulator system. The vertically oriented high-pressure tube sections and the joined jet generator head assembly of the present invention are typically vertically reciprocable and are inserted into the production casing of an established oil well with the jet generator head assembly being positioned adjacent the oil-bearing geological formation from which oil flows are to be stimulated.

The method aspect of the present invention comprises the basic steps of providing a high-pressure flow of a process fluid to the perforation zone of an oil well casing where crude oil is first received from a nearby oil-bearing geological formation, converting that high-pressure flow of process fluid at the oil well casing perforation zone into multiple outwardly and downwardly directed high-velocity jets of process fluid that each have combined liquid and vapor phases, and flowing the multiple, high-velocity jets of process fluid throughout the oil well casing perforation zone and into the nearby oil-bearing geological formation. The process fluid may be either water or crude oil ; the use of an additive or additives with the water or crude oil process fluid is not necessary.

The multiple, high-velocity jets of multi-phase process fluid are caused to reciprocate over the extent of the oil well casing perforation zone, as by vertical reciprocation of the apparatus that generates the multiple, high-velocity process fluid jets, for an extended period of time, e. g. one hour. Afterwards, the jet generator apparatus and its connected tubing are withdrawn from within the oil well production casing and oil well production resumed in the preferred manner.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevation view, partially in section, of a preferred embodiment of the apparatus of the present invention installed in part within the production casing of a conventional oil well; FIG. 2 is a partial section view of the apparatus of the present invention taken at line 2-2 of FIG. 1; FIG. 3 is a section view taken at line 3-3 of FIG. 2;

FIG. 4 is an elevation view of the apparatus flow director core element illustrated in section in FIG. 2; FIG. 5 is a section view taken at line 5-5 of FIG. 4; and FIG. 6 is an enlarged section view of one of the apparatus injector elements shown in FIGS. 2 and 3.

DETAILED DESCRIPTION The present invention, from both apparatus and method standpoints, involves the generation and utilization of multiple, high-velocity process fluid jets. The process fluid is preferably either water or crude oil, and the emitted fluid jets typically have a velocity in the range of from 200 meters per second to 500 meters per second. Also, and because of their method of generation, such multiple emitted jets contain liquid process fluid with entrained vapor-filled cavities and are also referred to herein as cavitating jets.

As to utilization of the multiple cavitating process fluid jets, in the case of oil wells that are open-hole completed wherein the well production casing is set on top of the oil-bearing reservoir the emitted multiple high-velocity process fluid jets pass through the open bottom of the casing directly into the reservoir. In the case of oil wells that are completed with a production casing having a perforation zone embedded in the oil bearing geological formation reservoir, the emitted multiple high velocity process fluid jets both impinge upon and scan the interior surface of the production casing perforation zone, and after passing though the casing perforations indirectly enter the oil bearing geological formation reservoir.

A preferred embodiment of suitable apparatus for generating and properly utilizing the multiple, high-velocity process fluid jets necessary to the practice of the present invention is illustrated schematically in FIGS. 1 though 6 of the drawings. FIG. 1 schematically illustrates the

invention apparatus, designated generally as stimulator system 10, functionally combined with a typical production casing 12 of a conventional oil well. The stimulator system is basically comprised of a source 14 of high-pressure process fluid, a process fluid jet generator assembly 16 positioned in the lower extreme of production casing 12, joined high-pressure fluid flow tube sections 18,20,22 and 24, and system accessories that functionally interconnect process fluid source 14 and process fluid jet generator assembly 16.

Source 14 is typically comprised of a liquid pump 26 capable of pumping process fluid from an internal fluid reservoir 28 to connected pump outlet tube sections 18, to flexibly joined supply flow tube sections 20, and to reciprocable vertical tube sections 22 at an elevated fluid pressure that typically ranges between approximately 200 and 350 atmospheres (i. e., 2,900 and 5,100 pounds per square inch). Joined return tube sections 24 are provided to conduct the return flow of process fluid from the uppermost interior region of production casing 12 to process fluid reservoir 28. Reservoir 28 may advantageously include a capability for filtering out returned geological formation fines, particles, and other debris brought up from the bottom of production casing 12 from the return flow of process fluid to minimize operating wear in pump 26.

As previously indicated, the system process fluid is preferably selected from the fluids group comprised of water and crude oil. Tube sections 20 are preferably joined together by high-pressure swivel joints to facilitate the reciprocable upward and downward movement of the string of joined vertical tube sections 22. An upper vertical tube section 22 may be provided with benchmark markings and positioned in the region of wellhead cap 23, which cap may include a well blowout preventer, for use in obtaining close or rather precision control of the position of jet generator subassembly 16 within the bottom of production casing 12. Stimulator system 12 is essentially completed with the inclusion of various accessories such as conventional quick-acting shut-off valves 30, pressure gauges 32, and a lift-ring and crane-connected lift cable combination

34,36 attached to the uppermost end of the vertical string of tube sections 22. During operation of stimulator system 10, crane-connected lift cable 36 is normally reciprocated vertically at a velocity of approximately 1.5 meters per minute, and with jet generator assembly 16 in its desired depth position, over a total operating time of about one hour per cycle of apparatus use.

Adequate rotational scanning of the casing 12 interior surface by the output process fluid jets of jet generator assembly 16 occurs as a natural consequence of the random torsional oscillation of the string of vertical tube sections 22 during system operation. However, the present invention also contemplates use of torsional oscillations intentionally induced on vertical tube sections 22.

The directions of process fluid flow in-stimulator system 10 is shown by arrow notations "A"provided in FIGS. 1 and 2. Arrow notation"B"is also provided in FIG. 1 to indicate the reciprocating movement of the installed string of vertical tube sections 22 and attached jet generator assembly 16 during scanning of an oil well production casing zone of perforations.

FIG. 2 essentially illustrates system jet generator assembly 16 and its installation in a section of oil well production casing 12 having perforations or channels 38, in greater detail. The generator assembly 16 is comprised of a tubular filter element 40, a coupler/housing element 42 which, along with filter element 40, is threadably connected to the lower extremity of the vertical tube section 22, and a jet generator head subassembly 44 that is threadably connected to coupler/housing element 42. Although threadable connections are described, other types of connections have application in alternative embodiments not shown in the various figures.

Positioned within coupler/housing element 42 and jet generator head subassembly 44 is an internal core element comprised of upper and lower core sections 46 and 47 having integral flutes or channels 48. The internal core element upper and lower sections 46 and 47 are assembled into a unitary structure, along with jet generator head subassembly 44, using a threaded fastener 52 that cooperates with a threaded hole 50 centrally provided in core element

upper body section 46 and using alignment pin element 54 and key element 56 to retain the assembly parts in their proper relative positions.

Also, upper core element body section 46 is provided with a centrally positioned integral top protuberance 49 that serves to gradually turn the process fluid flow from tube section 22 away from the tube section longitudinal axis and thereby prevent or minimize flow stagnation and flow recirculation prior to process fluid flow entry into core element channels 48. Also, although not shown in FIG. 2, it should be noted from FIG. 4 that the channels 48 preferably swirl or are spirally oriented along the length of core body sections 46 and 47, and that the channel 48 cross sectional areas decrease in the direction of fluid flow along their length. Thus, the fluid flow in each channel 48 is accelerated and provided with axial and radial velocity components of a tangential flow with minimum hydraulic losses prior to entry into annular chamber 58 of jet generator head subassembly 16.

An annular groove or recess 59 is provided in the exterior of jet generator head element 44 in the region of the exit openings of injector elements 60. The recess 59 functions to effectively partially block or impede the flow of fluid bounced off the near production casing wall. The recess 59 operates to keep the cavitating process fluid within the region to be stimulated for a considerably longer period of time than would otherwise be possible without the recess 59. Also, process fluid flow preferably occurs at a flow rate in the range of approximately 3 to 30 liters per second.

Particular attention should be given to both FIGS. 2 and 3 for their disclosure of a preferred, compound-angle positioning of injector elements 60 within jet generator head subassembly 44. Injector elements 60 function much like Venturi tubes because they operate to accelerate and decrease the pressure of the liquid process fluid. As shown in FIG. 2, each injector element 60 is tilted downwardly in the preferred direction of process fluid flow, preferably to an

angle of approximately 8° from a cross section plane which is orthogonal to the longitudinal axis of annular chamber 58. This preferred angle maximizes the cavitation effect and therefore the efficiency of the invention. As shown in FIG. 3, each injector element 60 is also oriented at an angle of approximately 14° with respect to an intersecting radius so that the high-velocity process fluid jets generated therein also have a clockwise direction. The tangential and downwardly injection of the multiple, high-velocity process fluid jets is in a direction opposite the thread-tightening direction of the threads that join elements 22,40,42, and 44 so that the force reaction to the issuing multiple jets operates to continuously urge the jet generator assembly 16 components into a tightened condition. Additionally, while contributing to improved resonance, the angles also impart a downwardly swirling or vortex action to the process fluid which improves the circulation of the process fluid about the sump at the bottom of the well. The improved circulation contributes to enhanced cleaning action by increasing contact of fresh process fluid with the well bore perforations and surrounding structure and by energetically mixing the removed debris and deposits with the process fluid for further abrasive effect and for removal from the well.

Attention should be given to details of a preferred injector element. First, and although not illustrated in the drawings, the external cylindrical surface of each injector element 60 is preferably threaded and cooperates with a matching thread that is provided in each cooperating angled cylindrical bore in jet generator head element 44. Such screw threads or other types of equally effective connecting features enable subsequent and convenient replacement of worn injector elements 60 with new elements, and are important because the operating life of each injector element is typically only about 10 hours. An alternative embodiment, not shown, of the jet generator head element 44 is formed with integral injector elements 60 and is configured to be replaceable as a single unit to improve injector maintenance efficiency. With this alternative

embodiment, the annular recess 59 is also easily replaced if it becomes worn. Additionally, recess 59 can thereby be replaced with a recess, not shown, of different size and configuration for use in applications requiring more or less process fluid flow blocking action.

Referring to FIG. 6, each injector element 60 has a conventional conical or parabolic convergent or reducing section 62, a conventional cylindrical throat section 64 joining the convergent section 62, and preferably a stepped diffuser section 66 joined to the throat section.

The injector convergent section 62 and throat section 64 diameters are preferably sized so that the process fluid flow therein accelerates to a velocity in the range of approximately between 200 and 500 meters per second and is accompanied by a static pressure drop of about 100 atmospheres.

The inner diameter of the throat section 64 is preferably, approximately between 1 and 10 millimeters, and more preferably between approximately 1.2 to 3.2 millimeters, and more preferably approximately 2 millimeters. The smaller diameters have been found to be more preferable for shallower wells, while the larger diameters are more preferable for deeper wells.

Also and preferably, the large-angled conical surface of divergent section 66 of each injector element 60 is provided with integrally-formed steps 68 which function to trip the flow boundary layer and trigger a transition to turbulent fluid flow. Preferably, approximately between 4 to 5 steps 68 are integrally formed in divergent section 68 each preferably sized to be approximately 0.0625 inches in height and depth and each being preferably spaced apart approximately 0.03125 inches. The plurality of steps also tends to focus the jet stream of process fluid into a more coherent stream of fluid. Step size controls the scale of turbulence vortices. The resulting turbulent process fluid flow has enhanced internal flow pressure oscillations, becomes separated from the wall of diffuser section 66, and is injected into the oil well as a two-phase, liquid-vapor jet stream whose diameter is mainly controlled by the diameter of injector throat section 64.

Because the issuing jet stream of process fluid is comprised of a liquid phase and entrained cavities filled with a vapor phase, it properly may be referred to as a cavitating jet stream.

Practice of the method aspect of the invention in a production oil well involves: (i) cleaning the well production casing perforation holes 38 and nearby well bore formation by highly-dynamic, high velocity process fluid jets, and (ii) advantageously affecting the oil-bearing geological formation permeability by means of high frequency, large-amplitude pressure oscillations or shock waves that propagate deep into the well bore.

The multiple jet generator assembly 16 of the present invention is attached to the lower end of the string of joined high-pressure tube sections 22 and is gradually lowered into the perforation interval of an oil well production casing 12. If the well is open-hole completed, which means that the production casing 12 is set on top of the oil bearing formation reservoir, the multiple jet generator assembly 16 is delivered to the bottom of the well. The highly-pressurized process fluid is filtered, accelerated to high velocities in a spiral flow, and subsequently injected into the oil well. The process of fluid flow acceleration and injection is controlled by design to ensure fluid pressure drop in the injector elements 60 to a pressure below atmospheric pressure.

This results in spontaneous"boiling"of the process fluid at a fairly low temperature and the formation of numerous vapor-filled cavities in the jet flow, and the fluid flow is injected into the well as a two-phase mixture of process fluid liquid and entrained cavities filled with process fluid vapor.

As the multiple, high-velocity jets of two-phase process fluid impinge upon the oil well production casing 12, the entrained vapor-filled cavities collapse due to a rise in fluid pressure, and each such cavity collapse results in a rapid increase in local pressure (so called hydraulic impact). The combined effect of a large number of such implosions occurring near the production casing 12 solid surfaces leads to an up to an order of magnitude increase of total

dynamic pressure of the jet impinging on the casing surface and additionally creates high-frequency and large-amplitude pressure oscillations near the casing interior surface. The estimated pressure amplitude in the produced waves is in the range of approximately 100 to 100,000 newtons per square meter and the estimated frequency range is between approximately 10 kilohertz to 100 megahertz.

First, the combined effect allows efficient cleaning of the perforations 38 or perforation channels in the well casing 12 thereby removing scales and precipitates from the nearby well bore formed during previous well production. Second, the high-velocity cavitating jets of process fluid penetrating the well casing perforations or perforation channels 38 operate as efficient drills, enlarging the perforation channels 38 and substantially increasing their effective surface areas.

Third, the numerous vapor implosions and consequent liquid impacts make possible rapid grinding and subsequent removal of mud, sand, clay, salt, or other debris that may have precipitated near the well bottom during prior well production and sometimes blocking casing perforations 38. Lastly, and most importantly, large-amplitude fluid pressure oscillations are transmitted through the production casing perforation channels and/or perforation holes 38 and penetrate deep into the oil-bearing geological formation reservoir as intense high-frequency vibrations. These vibrations, which are damped quite slowly due to their high initial energy (proportional to squared amplitude and squared frequency), produce microstructural changes in the oil-bearing formation and thereby change porosity and permeability as well as crude oil mobility. This in turn results in an increase of the crude oil flow toward the well bore.

Most of the process fluid returns to the well head by way of the annular space formed between the exterior surface of vertical tube sections 22 and the interior surface of production casing 12. After ground solid waste or debris contained in the returned process fluid is separated out, the process liquid (water or crude oil) is stored in reservoir 28 for subsequent reuse.

Although the above described invention is described only with respect to use in oil well production stimulation, one having skill in the art of well construction and stimulation will recognize that the invention is also suitable for stimulating production of other types of natural resource wells including water, gas, and thermal wells. Various changes may be made to the shapes, sizes, and apparatus constructions illustrated in the drawings or described above without departing from the meaning, scope, or intent of the claims which follow.