Patent Publication Number: US-2015083388-A1

Title: Steam-impulse pressure generator for the treatment of oil wells

Description:
FIELD OF THE INVENTION 
     The present invention relates to the oil and gas industry and in particular to devices for increasing well productivity through the processing of the bottom-hole formation zone (BFZ). 
     BACKGROUND OF THE INVENTION 
     In the field of intensification of oil and gas production, there are many problems in improving the filtration properties of the bottom-hole formation zone and increasing the productivity of wells, where the oil contains high concentration of clogging impurities, especially in old oil fields. There are many different devices that address this problem, but they demand great expense or are not effective enough or both. 
     Many different patented devices are intended to influence the bottom-hole formation zone by means of an appropriate treatment of oil wells. See, for example, Reference book on the extraction of oil, ed. SH. K. Gimatudinov, M M.: Nedra, 1974; A. A. Popov, Implosion in the processes of oil production. M.: Nedra, 1996; M. Suchkov, Temperature modes of working wells and thermal methods of oil production. M.: IKI, 2007; V. P. Dyblenko, Wave methods of influence on oil layers with hard extracting reserves. M.: JSC “VNIIOENG”, 2008. All of these devices are grouped around a number of well-known methods of influence (or impact) on the bottom-hole formation zone. The most well-known methods can be categorized as: (1) shock-wave, (2) chemical, (3) thermal, (4) pressure, (5) implosion, or (6) cavitation, as well as their various combinations. 
     RU2110677 concerns a known method and device for treating oil well by means of thermo-gas-chemical influence. Blocks of solid propellant with an igniting block attached to them are put into the well and burnt in the perforation zone. High volumes of gases are generated during a fast (1-5 sec) combustion of a large charge (30-150 kg). These gases influence the bottom-hole formation zone by means of thermal, chemical and mechanical influence. Very often this provoked various disturbances in bottom-hole formation zone, and it was one of the reasons for the refusal from wide use of this method. 
     RU2127362 concerns a known method and device for treating oil wells by means of thermokinetic chemical processing carried out by consecutive ignition of solid propellant charge and checker. In this method, a pod with solid propellant charge is lowered inside the well, filled with fluid, and burned after being set in the perforation zone. This method differs in the way that the pod is lowered with production piping inside the well, and the hard fuel checker is used as a cap to hermetically seal the empty tubing. The ignition of the checker is delayed after the combustion the main propellant charge. 
     The foregoing methods have several shortcomings: first, powder charges, with all shortcomings inherent to explosive substances, are used; second, burning products—gases with very high temperature (possibility of carbonizing paraffin, resins, etc. instead of melting them) and with high toxicity; and third, before device descent, all pump equipment and production tubing have to be removed from the well. 
     With respect to a purely pressure approach, the most used highly effective application has been the hydro-fracturing method, described in different patents such as U.S. Pat. No. 2,896,717, RU2170818, and RU2183739. After hydro-fracturing the oil-bearing layer, the well productivity often increases several times. However, this method requires the use of powerful and quite complex oil-field equipment that ensures the ramming of large volumes of liquid at pressures exceeding the rock pressure. Of all methods, this one is the most expensive. 
     Also known in the art are a number of devices and methods based on them for treatment of oil wells. These influence the bottom-hole formation zone with gas-pulse process using pulse pressure generators (PPG). See SU1089348, RU2147337, and RU2334873. 
     The technology of gas-pulse treatment of bottom-hole formation zone, based on application of pulse pressure generators, belongs to type of physical-mechanical methods of processing the bottom-hole formation zone and is intended for recovery and improvement of filtration characteristics of oil-bearing layer, which were affected over time during operation or drilling of the wells. Recovery of filtration characteristics allows one to intensify inflow and to increase the time between maintenance periods of operation of wells. 
     All devices of this class operate on the basis that a closed volume with output nozzles is filled with propellant mixture, which combustion sharply increases pressure in the chamber and through output nozzles combustion gases impacts the bottom-hole formation zone. A variant of this design is the use of submersible pressure generators filled with gaseous nitrogen. 
     A known pulse pressure generator (SU1089348) contains inside its enclosure a working agent, representing a powder charge, an igniter of this powder charge, and hermetically sealed nozzles distributed all over the enclosure. The efficiency of treatment of the bottom-hole formation zone using this generator is very low, due to the fact that it is very difficult to perform simultaneous depressurization of all nozzle openings. In case of depressurization of one part of the nozzle openings, the pressure of powder gases in the enclosure sharply falls, and the remaining part of the nozzle openings can be not depressurized. The result is a shortcoming wherein only a part of the bottom-hole formation zone is treated. This naturally reduces overall performance of this device. 
     RU2147337 is another known method and device for processing of the bottom-hole formation zone of oil wells. It involves an immersion pulse pressure generator, made in the form of housing, in which is placed a working agent, a tank for storage of gaseous environment, received as a result of the combustion of working agent (for example, the reaction of combustion azide alkali metal (Na2N3) and metal oxide (Fe2O3)), as well as elements for ignition of working agent, in particular, an pyro device (squib) and igniter. The housing has detachable nozzles installed that interconnect the interior of the device with the well&#39;s media. The construction includes a piston type valve and rupturing element, both responsible for opening the nozzles at the same time, thus allowing passage through those nozzles of a generated volume of gas at the same pressure, ensuring uniform conditions of impact on the bottom-hole formation zone. Also included is a dampening element that reduces the speed of the piston movement, thus protecting the body of the device. The main disadvantages of this device are:
         1. It uses as the working agent a detonation-capable mixture, especially under strong impact, which represents danger to personnel and requires extra care during handling and transportation;   2. There is no way to safely release high pressure gas in case of malfunction of the valve;   3. present mechanical elements reduce the reliability of the device;   4. its application requires costly work over rigs to pull the production tubing out of the well, and   5. constant damage to the geophysical cable head and cable itself occurs due to lack of adequate protection elements.       

     Another known device is taught in RU2334873, disclosing an immersion pulse pressure generator, consisting of a housing with a working agent and initiation element made as a single unit, rupturing element and nozzle holes. As a working agent, this device uses composite material on the basis of granulated ammonium nitrate and epoxy compound with sufficient mass to produce more than 800 l/kg of gaseous products, with impulses magnitude in the range of 1.10-1.35 times the rock pressure of the treated layer, as well as pulse duration of up to one minute and pulse frequency for this time—not less than 14-15 pulses. 
     One or several of the pulse pressure generators of this type are lowered on the cable into the hole, in the interval to be treated. A special remote starter device generates an electrical pulse which operates one, several, or all of the elements of initiation of working agents, placed inside each generator. As a result of ignition and combustion of working agent, only gaseous products are formed. Gas pressure in the pulse pressure generator housing at the end of the combustion of working agent reaches specific value, at which point membrane breaks and releases hot gases under high pressure through nozzle holes into the bottom-hole zone, creating multiple pulsatile pressure and heat disturbances in the bottom-hole formation zone. The main advantages of this prior art are:
         1. high gas-generating capacity;   2. high parameters and duration of impulse impact, created by the working agent;   3. working agent does not belong to a category of explosive substances;   4. working agent and its initiation element are made as a single unit; and   5. the uniform operation of nozzles depends on only one breakable membrane.
 
However, drawbacks of this prior art are:
   1. reduced effectiveness of the impact on bottom-hole formation zones with high viscosity heavy oils, typical of many of the currently existing fields. This is because the resinous-asphaltene depositions of such oils have high adhesion to the surfaces of the channels and pores in the rock formation. Due to this, even those pulsatile pressure amplitude values and its duration, that are implemented in this method, do not provide sufficient removal of such deposits in the pores and channels of the rock formation, reducing the efficiency of the process;   2. design features of the membrane reduce the reliability of operation of the device;   3. its application requires costly work over rigs to pull the production tubing out of the well;   4. constant damage to the geophysical cable head and cable itself occurs due to lack of adequate protection elements; and   5. even though the working agent is not explosive, it is classified as a toxic substance.       

     All above-mentioned devices have significant shortcomings that limit their application, including a lack of efficiency, complexity of design, and high labor-intensiveness of the process. Their use involves danger both for the wells, operating personnel, as well as for the environment, caused by the fact that almost all used substance or components that are explosive or toxic. 
     The object of the present invention is to improve apparatus used for influencing the bottom-hole formation zone and provide operational reliability and environmental safety of the device. 
     SUMMARY OF THE INVENTION 
     The present invention involves a steam-impulse pressure generator, sometimes referred to as a “SIPG.” It has no direct identical analogs. The physical and mechanical influence on the bottom-hole formation zone can be attributed to the class of devices under the common name of impulse pressure generators (“IPG”). But unlike the latter, the preferred implementation of the invention consists of two main functional parts, separated by a burnable wall (membrane); a heat-generating device, which is an hermetic enclosure, which cavity is filled with a working agent preferably in the form of heat-generating blocks; and a steam-kinetic chamber which is not hermetic and preferably or as much as possible an open enclosure due to holes (nozzles) evenly distributed over preferably its entire surface, thus providing minimal dynamic resistance for an exit of the vapor-gas mixture, while offering as much interference as possible against the melted products of combustion exiting into the well. In the process of the direct contact of high temperature melt (coming from the main body, with a temperature of preferably not less than 2350 K) with the borehole fluid (water), there is a hydrodynamic disturbance of high-intensity in the form of series of steam-pressure pulses. The disturbance penetrates into the bottom-hole formation zone and improves the filtration properties of the rock formation not only and not so much by the removal of the wax, paraffin and resin depositions, but rather because of layer micro-ruptures in the bottom-hole formation zone. 
     Preferably a device according to aspects of the invention operates according to a controlled process of explosion like steam generation, which allows the maximum expression of high-energy barometric effects and achieves very high intensity of heat transfer from the high-temperature melted products of combustion of high-energy fuel compositions to well fluids (water). Preferably, an embodiment of the invention uses powder composite compounds with a controllable rate of burning. It is also preferred that the compounds do not require the presence of a gaseous oxidizer, and they preferably provide heat in the range of 7.5 MJ/kg. As the products of combustion are not gaseous constituents, the barometric effect is achieved solely through the phenomena inherent to violent steam generation resulting from the interaction of the combustion products in the molten state with the borehole fluid (water). 
     Instant steam generation occurs under certain conditions of direct contact of high-temperature (not below 2350 K) melted combustion products with the borehole fluid (water). In this case, the impulse processes of thermophysical interaction between the contacting media, accompanied by intense vaporization of an explosive nature. The latter, in turn, is accompanied by the emergence and spread in the environment of the wellbore fluid of hydrodynamic disturbances in the form of power pulses of high intensity and millisecond duration, which penetrate through the almost transparent steam-kinetic chamber SKC and into bottom-hole formation zone porous medium, causing a rapid expansion of existing cracks and the formation of new cracks. 
     The energy impulse of the steam-water mixture includes thermal and kinetic components, the first of which is responsible for the decrease in the viscosity of oil (this is a short-term effect) and melt the asphalt-resin-paraffin deposits. A second component is responsible for the expansion of oil flow channels (cracks in the bottom-hole formation zone rocks) (this is a long-term effect). The cause of the second component of the impact on the bottom-hole formation zone is the contact of cold water with hot enclosure&#39;s wall of the steam-impulse pressure generator, that causes a massive pressure jump which vector is directed away from the axis of the device to the periphery and which pushes the steam-water mixture under pressure 1.1-1.3 times the downhole pressure through perforations in the casing (or open hole walls) in the bottom-hole formation zone. 
     Cooling of steam bubbles in the fluid during intense heat-mass transfer processes with the formation rock causes the formation of the low-pressure zone near the bottom-hole well walls, this effect being similar to implosion, which, by virtue of the continuity of the liquid medium, stimulates the inflow of fluid from the reservoir. Cooling of the steam-water mixture under certain conditions can trigger cavitation effects due to the collapse of vapor bubbles, which generates high-frequency acoustic and shock waves that increase unclogging of channels of the bottom-hole zone with the removal of contaminants into the internal cavity of the well, and the expansion of oil flow channels that, in general, reduces, the hydraulic resistance of the collector. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be better understood by reference to the accompany  FIGS. 1-4  representing illustrative embodiments of a steam-impulse pressure generator. In the drawings: 
         FIGS. 1A and 1B  show general schematic views of steam-impulse pressure generator devices embodying various aspects of the present invention.  FIG. 1A  shows a generator designed for single use in the well.  FIG. 1B  shows a generator designed for serial connection with additional generators for bigger perforation zone treatment. 
         FIGS. 2A and 2B  show different versions of a universal adapter that provide compatibility of a steam-impulse pressure generator with any kind of geophysical connector. 
         FIGS. 3A and 3B  show protective guiding units (“PGU”) for supplying melted product of combustion into steam-kinetic chamber (SKC) that protects the fuel blocks inside the steam-impulse pressure generator housing from moisture, as well as providing a standardized, optimal delivery of the melt into the steam-kinetic chamber SKC and protects the main steam-impulse pressure generator body from burn-through. 
         FIG. 4  shows a basic view of a steam-kinetic chamber (“SKC”) attached to a threaded version of a protective guiding unit. 
         FIGS. 5A ,  5 B, and  5 C show types of hull cross sections of the steam kinetic chamber.  FIG. 5A  shows straight windows (nozzles);  FIG. 5B  shows slanted windows (nozzles); and  FIG. 5C  shows an arrangement having two metal hulls or pipes with respective slanted windows (nozzles) in opposite directions. 
         FIG. 6  (A-H) shows eight different illustrative versions of the steam-kinetic chamber windows (or nozzles) which provide a maximum degree of “transparency”, screening capacity, with a minimum degree of deformability. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are represented in the drawings.  FIG. 1A  shows a schematic view of a steam-impulse pressure generator device intended for single use in a well. Proceeding from top to bottom, the generator consists of a geophysical connector  1 - 1 ; a universal adapter (“UA”)  1 - 2 ; an enclosure  1 - 3 ; a protective guiding unit (“PGU”),  1 - 4  for melted product of combustion; and a steam-kinetic chamber (“SKC”)  1 - 5 . 
     The geophysical connector  1 - 1  is a standard geophysical device well-known and used by different international companies. 
     The universal adapter  1 - 2  is a transitional device intended to provide reliable hermetic sealing, mechanical and electrical connection of the main equipment of wire logging unit with the steam-impulse pressure generator; provide protection for geophysical connector  1 - 1  and geophysical cable from sharp changes in temperature and pressure encountered when using steam-impulse pressure generator; and ignite the mixture in the main body of the steam-impulse pressure generator. 
     Enclosure  1 - 3  of the steam-impulse pressure generator is a hollow cylinder made preferably of heat-resistant steel. Enclosure  1 - 3  contains a working agent, preferably in the form of compressed blocks of fuel with controllable rate of combustion, with heat release preferably of 7.5 MJ/kg, the combustion of which forms molten products at a temperature of preferably not lower than 2350 K. 
     The resulting melt is supplied to a protective guiding unit  1 - 4 , made of heat-resistant steel and burns through a membrane, penetrating into steam kinetic chamber  1 - 5 . Preferably the working agent comprises powder composite formulations that do not require the presence of gaseous oxidizer and in the combustion products themselves there are no gaseous components. The agent and its products of burning are ecologically safe for environment. 
     Steam-kinetic chamber  1 - 5  is a hollow metal cylinder. A plurality of windows (nozzle)  4 - 4  are located on its surface. At the bottom of chamber  1 - 5  is an internal thread to which is screwed a metallic threaded cap  4 - 5  which acts as a barrier against solidified fractions of the melt exiting the generator and entering the well. 
     A series of steam-impulse pressure generators may be provided.  FIG. 1B  shows a steam-impulse pressure generator designed for serial communication with other such devices. It includes a node connection  1 - 6  at the bottom of the  FIG. 1B  device connected to the universal adapter  1 - 2  of the next steam-impulse pressure generator in the series. Node  1 - 6  is identical to the geophysical connector  1 - 1  ( FIG. 1A ) in size and electrical characteristics. 
       FIGS. 2A and 2B  show cross sections of two different versions of universal adapters. The universal adapter consists of a main body  2 - 1  ( 1 - 2  in  FIG. 1 ), at the top and the bottom of which is a thread for connecting geophysical connectors  1 - 1 ,  FIG. 1  and the enclosure  1 - 3 ,  FIG. 1 . The  FIG. 2A  version combines the ignition system in a single package whereas the  FIG. 2B  version shows the universal adaptor with an ignition (initiation) system as a separate enclosure. 
     Located in the lower part of the universal adapter of  FIGS. 2A and 2B  is a system of initiation, which includes a case  2 - 15 . Case  2 - 15  is combined with (integral with) the main body of the universal adaptor in the version of  FIG. 2A . Case  2 - 15  is not combined with the main body of the universal adaptor in the version of  FIG. 2B . The initiation system also comprises an initiation mixture  2 - 16 , shields  2 - 17 , a discharge module  2 - 14  made in form of two metal plates separated from each other by an insulating gasket, connected at one end to the main body of the universal adapter. The other end is secured through a mounting screw  2 - 13  to a central metal rod  2 - 2  of the device. The plates of discharge module  2 - 14  receive a high voltage electrical pulse from a pulse generator, through the geophysical cable and geophysical connector ( FIG. 1   a ,  1 - 1 ). As a result of the high voltage, an electrical arc forms between those plates, with temperatures superior to 2500° K, which ignites the initiation mixture  2 - 16  at a temperature of ignition in the range of 1280-1380° K. This in turn ignites the main mixture inside the enclosure  1 - 3  of the steam-impulse pressure generator (see  FIG. 1A ,  1 B). 
     Protection of geophysical connector ( FIG. 1A ,  1 - 1 ) from sharp jumps in pressure is provided by means of the assembly consisting of Teflon sleeve  2 - 9 ; composite epoxy materials sleeves  2 - 8 ,  2 - 10 ; nut and washer  2 - 5 ,  2 - 6 , and the central metal rod  2 - 2 . With the help of which, by screwing the nut  2 - 5  on the rod  2 - 2 , creates a normalized compressive force of Teflon bushing  2 - 9 , in the range of 70-80 MPa, which is superior to the downhole pressure that is in the range of 10-50 MPa. 
     Protection of the geophysical connector ( FIG. 1A ,  1 - 1 ) from abrupt jumps of temperature is provided by means of the assembly consisting of insulating sleeves of composite epoxy materials  2 - 10 ,  2 - 12  and the air gap between those sleeves. Additionally, the heat is being effectively dissipated by heat conducting body  2 - 1  of the universal adapter, being surrounded by the wellbore fluid and further dissipating heat released during the combustion of working agent. 
     A mounting screw  2 - 13  fastens the discharge module  2 - 14  to the central metal rod  2 - 2  and the body  2 - 1  of universal adapter and places them in electrical contact. 
     The top of  FIG. 2  shows the unit providing electrical contact of the universal adapter with geophysical connector&#39;s center conductor through the metallic pin  2 - 3 , guide nut  2 - 5  and with member  2 - 4  and hold-up (push-up) spring  2 - 7 . 
       FIG. 3  shows details of alternate illustrative protective guiding units shown in  FIG. 1  as member  1 - 4 . These operate to guide melted combustion product into the steam-kinetic chamber  1 - 5  of  FIG. 1 . Both versions comprise a solid cylinder made of heat-resistant steel having a calibrated, burnable membrane  3 - 3  barrier in the middle, illustratively 1-1.5 mm thick. Each further comprises a calibrated inner conduct or conduit  3 - 2  for melted product to flow into the steam-kinetic chamber. 
     Two illustrative ways of connecting the protecting guiding unit with the steam-impulse pressure generator enclosure  1 - 3  are presented in  FIGS. 3A and 3B .  FIG. 3A  represents a “welded” version of connection with the steam-impulse pressure generator, while  FIG. 3B  represents a threaded connection option with the steam-impulse pressure generator enclosure using protective rubber rings  4 - 2  shown in  FIG. 4 . In this figure, the position of  3 - 1  shows the connection with the steam-impulse pressure generator&#39;s enclosure. Position  3 - 2  shows a calibrated diameter channel for melted combustion product flow into the steam-kinetic chamber. Position  3 - 3  shows a melting (that is, meltable) membrane, and  3 - 4  shows the location of threaded connection of the protective guiding unit with steam-kinetic chamber. See  1 - 5  in  FIG. 1 . 
       FIG. 4  schematically shows a steam-kinetic chamber  1 - 5  ( FIG. 1 ) with a threaded protective guiding unit. In  FIG. 4  thus shows a protective guiding unit  4 - 1 , protective rubber rings  4 - 2 , steam-kinetic chamber  4 - 3 , windows (nozzles)  4 - 4  around the outside of the chamber  4 - 3 , and a metallic threaded cap  4 - 5  at the base of the assembly. Protective rubber rings  4 - 2  are intended for making hermetically sealed connection with the enclosure. 
     Steam-kinetic chamber  4 - 3  preferably consists of a hollow non-hermetic cylinder made of heat resistant steel, the top end of which is attached to protective guiding unit  4 - 1 , while the bottom end is sealed with metallic threaded cap  4 - 5 , intended to provide a barrier to prevent solidified fractions of the melt falling into the well. A plurality of evenly distributed windows (nozzles)  4 - 4  are located on the entire outside surface and intended as conducts (conduits) to release generated overheated steam under high pressure. Preferably the diameter or width, d, of those windows  4 - 4  must be 2-3 times smaller than the wall thickness, s, of the steam-kinetic chamber, so that melted combustion products will be unable to exit the chamber. 
       FIGS. 5A ,  5 B, and  5 C show three illustrative horizontal cross sections of the hull of steam-kinetic chamber  4 - 3 .  FIG. 5A  represents a cross-section of the chamber hull where the windows (nozzles)  4 - 4  are radially oriented around the circumference of the chamber.  FIG. 5B  shows a hull cross-section with slanted windows (nozzles)  4 - 4  around the circumference. As can be seen, the windows (nozzles) are not radially oriented.  FIG. 5C  shows a cross-section where the chamber uses two concentric hulls, each with slanted holes (nozzles). The outer hull illustratively has the same orientation of non-radial windows (nozzles) as  FIG. 5B , while the inner hull has non-radial windows (nozzles) slanted in the direction opposite to the slant of those in the outer hull. That is, one may be slanted “left” while the nozzle it is in communication with is slanted “right.” The windows (nozzles) are in communication with one another to permit the exit of steam. One may note that the communicating pair of windows/nozzles of the inner and outer hulls as seen in  FIG. 5C  take on a chevron shape (in cross section) so that a plurality of vertical openings which in cross section are chevron-shaped extend along the outside surface of the steam kinetic chamber. This provides the maximum degree of “Transparency &gt;&gt;α=0.5-0.6 and shielding ability at the minimum degree of deformability or the loss of dimensional stability at elevated temperatures. 
     In  FIGS. 4 and 5 , the windows (nozzles) are generally vertical and parallel to the length of the steam-kinetic chamber. Multiple variations can be used, and various configurations of the windows (nozzles) are shown in  FIG. 6 . For example,  FIG. 6A  shows rows of vertical windows.  FIG. 6B  shows interleaved vertical windows. Various patters of square, diamond, and circular windows are shown in  FIGS. 6C ,  6 D,  6 E, and  6 F. Horizontal windows, perpendicular to the length of the steam-kinetic chamber can also be used and are represented in  FIGS. 6G and 6H . In all cases, preferably the windows are evenly distributed around the entire outside surface of the steam-kinetic chamber. Other shapes can be adopted within the scope of the present invention. 
     Recapitulating, embodiments of the invented arrangement as described herein include some of the following aspects:
         1. The working agent consists of compressed high-energy powder mixture in the form of separate blocks with raised thermal emission.   2. The working agent is housed in a heat-generating device, with a heat-resistant steel hermetic enclosure.   3. A depressurization system of the case by its melting, calculated on the predetermined pressure in the well and executed in the form of circular metallic membrane of calibrated thickness.   4. A protective assembly supplies melted combustion product into the steam-kinetic chamber, with a burnable membrane (depressurization system).   5. A steam-kinetic chamber is a non-hermetic and, as much as possible, open enclosure, with windows (nozzles), evenly distributed over the entire surface, with the minimum dynamic resistance against exit of the steam-water mixture, while offering as much interference as possible against the solid material exiting into the well, providing an optimum combination of indicators of “transparency” and shielding ability.   6. A universal adapter, compatible with any geophysical connector (cable head), with a calibrated protection system of well logging cable and geophysical connector from sharp pressure and temperature jumps, and also preferably includes an electronic switch, for switching several lowered into the well steam-impulse pressure generator and initiation system;   7. Ability of serial connection of multiple steam-impulse pressure generators on one logging cable.       

     The main aspects or features that are directly responsible for the creation of conditions and achievement of the regimes of controlled series of steam-pressure pulses are features 1, 2, 3, 4, and 5. Supporting role is played by features 6, 7. 
     Persons skilled in the art will appreciate that the particular choice of various design options and materials described above depends on the characteristics of the treated wells. It will be understood by persons skilled in the art that the foregoing description of preferred embodiments is illustrative and that the scope of the present invention is intended to be set forth in the following claims.