referring now in more detail to the drawings , in which like numeral indicate like parts throughout the several views , fig1 illustrates the pulse wave generator 10 located in a well casing 12 at the production zone 14 of a well , with the pulse wave generator connected at its upper end to a gas supply conduit 16 that is the lower end of a coiled tubing unit at the surface of the well . openings 18 are formed in the well casing 12 for the ingress of oil from the production zone 14 . pulse wave generator 10 includes a housing 20 that is formed in three sections , the lower nose section 22 , the intermediate cylindrical section 24 , and the upper cylindrical section 26 . the intermediate cylindrical section 24 and upper cylindrical section 26 together form an elongated high pressure housing 28 which defines an elongated high pressure chamber 30 , while the lower nose section 22 defines internal low pressure chamber 32 which freely communicates with the interior of the well casing 12 through gas exhaust ports 34 . the upper end of upper cylindrical section 26 is reduced in diameter and includes annular sloped shoulder 36 and a cylindrical connector neck 38 . connector collar 40 is threadedly mounted at its lower end to connector neck 38 , and is threadedly mounted at its upper end to gas supply conduit 16 . strainer 42 is positioned in the internal passage 44 of the connector collar 40 . neck 38 of upper cylindrical section 26 of housing 20 defines a pilot valve opening 46 that communicates between internal passage 44 of connector collar 40 and elongated high pressure chamber 30 of the high pressure housing 28 . pilot valve 48 is slidably positioned in pilot valve opening 46 and is formed in two sections which are telescopically connected together , the upper section 50 and the lower section 52 . the upper and lower sections 50 and 52 are telescopically connected and together form a gas bleed passage 54 that freely communicates between gas supply conduit 16 and the internal passage 44 of collar 40 at its upper end , and high pressure chamber 30 at its lower end . the pilot valve 48 has larger effective pressure surfaces exposed to high pressure chamber 30 than to internal passage 44 of connector collar 40 . therefore , should the pressures on opposite sides of pilot valve 48 become equal , the pilot valve will be thrust upwardly to the position as illustrated in fig2 . further , these effective pressure surfaces facing upwardly and downwardly are proportioned so that if the pressure in the high pressure chamber 30 exceeds 80 % of the pressure in the gas supply conduit 16 and internal passage 44 of the connector collar 40 , the pilot valve 48 will be urged upwardly . elongated piston valve 56 is positioned in and is reciprocatable along the length of high pressure chamber 30 from the up , closed position shown in fig1 to the down , open position shown in fig2 . piston valve 56 defines an internal piston chamber 58 that opens through the upper end of piston 56 at pilot valve opening 60 . pilot valve seat 62 of piston valve 56 engages the lower end of pilot valve 48 and forms a seal between the pilot valve and the piston valve . o - ring seal 64 which is carried internally of piston valve 56 slidably seals against the lower end of pilot valve 48 , also forming a pilot valve seat . vent 66 extends from behind o - ring 64 to the atmosphere of well casing 12 , so as to allow freedom of movement of the pilot valve with respect to the piston valve . the external surfaces of the upper portion of the piston valve 56 are sized and shaped to slidably engage the internal surfaces of the high pressure housing 28 , guiding the piston valve along the length of the high pressure chamber 30 . the intermediate portion 68 of piston valve 56 is of reduced diameter , and gas ports 70 freely communicate between the internal piston chamber 58 and the high pressure chamber 30 of the housing . the lower end portion of the piston valve 56 includes a valve head 72 which is of larger breadth than the intermediate portion of the piston valve , and which slidably engages a cylindrical valve seat 74 that projects inwardly of the elongated high pressure housing 28 , between the high pressure chamber 30 and the lower pressure chamber 32 . when the piston valve 56 moves from its loaded position of fig1 to its discharge position of fig2 the valve head 72 moves down below its valve seat 74 so as to open communication between high pressure chamber 30 and low pressure chamber 32 , allowing the high pressure gases contained in the high pressure chamber 30 to be expelled out through the gas exhaust ports 34 of nose section 22 . high pressure passage 78 is formed in the sidewall of upper cylindrical section 26 of high pressure housing 28 , and communicates at its upper end with internal passage 44 of connector collar 40 and at its lower end with high pressure chamber 30 . at the point of communication of high pressure passage 78 with high pressure chamber 30 , the enlarged upper portion of piston valve 56 is undercut so as to form an annular space 80 between the piston valve and its annular shoulder 82 which is located above the point of entry of the high pressure passage 78 . a sliding seal 84 is formed between the smaller diameter portion of piston valve 56 and the high pressure housing 28 , so that the effect of the pressure in annular space 80 beneath the shoulder 82 of the piston valve 56 is to urge the piston valve upwardly toward its position as illustrated in fig1 . because the high pressure passage 78 is continuously in open communication with the high pressure of the gas supply conduit 16 , there will always be a high pressure that tends to urge piston valve 56 to its upward position during the continuous operation of the pulse generator . when the pulse wave generator ( the gas expulsion tool ) is to be used , the operator estimates the fracturing pressure at which the subterranean earth formation about the wellbore will fracture . this calculation will determine the pressure at which to operate the tool as well as some of the physical characteristics of the tool . a tool having the right dimensions will be selected for use at the wellbore sites . it is well known that the fracture gradient of most oil - bearing subterranean earth formations about a wellbore is approximately 0 . 7 psi per foot of depth . this is believed to be due to the fact that produceable oil bearing earth formations , such as porous rock , are mechanically similar , and will therefore have similar fracture gradients . when the pulse wave generator 10 is moved downwardly into the well casing , it is desirable that it be in the production zone 14 . once in position , high pressure gas is communicated to the pulse wave generator through gas supply conduit 16 . typically , the pressure for optimum operation would be at or above 50 % of the formation fracturing pressure . however , higher and lower gas supply pressures can be used , as may be available and desirable . the line gas moves through the collar 40 and its strainer 42 and then moves in parallel directions , one direction being through the pilot passage 54 that extends along the length of the pilot valve and which charges internal piston valve chamber 58 and high pressure chamber 30 of housing 28 . in the meantime , line pressure also communicates through high pressure passage 78 so as to bypass the pilot valve and to move beneath the annular shoulder 82 of piston valve 56 . thus , high pressure passage 78 tends to urge the piston valve 56 upwardly to maintain the piston valve in position as shown in fig1 . in the meantime , as the high pressure chamber 30 continues to be progressively charged with line pressure , and the pressure within the high pressure chamber continues to increase , the increasing pressure eventually will exceed approximately 80 % of the line pressure of gas supply conduit 16 , so that the 80 % pressure urges pilot valve 48 upwardly due to the larger effective pressure surface at the lower portion of the pilot valve . when the pilot valve moves upwardly , it unseats itself from its valve seat 62 and its o - ring valve seat 64 , which allows the high pressure to reach the upper end of piston valve 56 . since the lower end of the valve head 72 of piston valve 56 is in communication with the lower pressure of the well , the now higher pressure at the upper end of the piston valve abruptly forces piston valve 56 to surge downwardly through high pressure housing 28 , so that its lower valve head 72 moves beyond its valve seat 74 , to the position illustrated in fig2 . this allows the high pressure gas to surge downwardly from the high pressure chamber 30 , past the seal 74 into the lower pressure chamber 32 , and to surge outwardly through the gas exhaust ports 34 of nose section 22 . the surge of gas through the nose section 22 and through the openings 18 of the well casing 12 creates a shock wave inside the well casing , and through the openings 18 of the well casing out into the production zone 14 , where the liquid and solid material surrounding the well casing is shocked , disrupted , and displaced , causing the exterior material to open up and permit more flow of liquid . once the piston valve has been moved to its open position as shown in fig2 and the high pressure gas has been abruptly discharged , there is an absence of gas flow from the pulse wave generator so that the previous abrupt expansion of gases in the production zone will dissipate , so as to implode , which further tends to open up the production zone . after the gas shock wave has been emitted , the continuous application of line pressure through high pressure passage 78 results in a lifting force being applied to the annular shoulder 82 of the piston valve 56 , so that the piston valve is lifted from its open position of fig2 back to its closed position of fig1 . in the meantime , the line pressure is applied to the upper end of pilot valve 48 , urging pilot valve 48 from its upper , open position of fig2 toward its lower , closed position of fig1 where it registers with and closes against the upper end of piston valve 56 . the system is then ready for its next cycle of operation . the shape of the pulse of the gas &# 34 ; explosion &# 34 ; is important . the shape of the pulse determines the penetrating power of the pulse , and if the main frequency spectrum of the pulse is at or below 50 hz , the wave generated by the gas explosion can travel for long distances through the earth with minimum attenuation . further , it is important to note that this main frequency spectrum of the pulse is in the range of the natural frequencies of the materials which are plugging up the formation and impeding the oil flow . this desirable 50 hz frequency spectrum is determined by controlling the duration of the pulse , which is believed should be greater than 20 milliseconds . however , if the rise time of the pressure pulse is excessively long , there is no &# 34 ; shock wave &# 34 ; effect , only an oscillatory wave , which would not provide as effective cleaning . for this reason , the system is designed to provide a maximum rise time for the pressure pulse to be less than 18 milliseconds and a minimum pulse duration of 20 milliseconds . it should be understood the foregoing relates only to the preferred embodiments of the present invention , and that numerous changes and modifications may be made thereto without departing from the spirit and scope of the invention as set forth in the following claims .