| At present, low pressure and low permeability gas fields account for 80% of the proved gas reservoir , with the large reserves and wide-ranging distribution. As is well known, if the porosity, permeability and pressure are all low, it would become hard to improve the gas productivity and thereby increases economic benefit to the whole gas industry. In order to improve the yield of gas and solve the "gas hunger" problem, the perforating and fracturing method was put forward. The conventional methods have disadvantages of shallow perforating depth, small diameter of eyelet and time-consuming. Moreover, the compacted zone damage, lower permeability of gas reservoir, poor effect of yield will be caused. In view of the reasons mentioned above, a new idea-using pulsed abrasive water jet(PAWJ) perforation to improve the permeability of low permeability gas reservoirs and the yield of gas has been put forward.The study of this thesis included the generation mechanism and dynamic characteristics of the PAWJ, yield-increasing mechanism of pulsed abrasive water jet perforation. Field testing and application for this new improved technique had been carried out in Sichuan Basin gas well.The conclusions of this thesis are as follows:Firstly, large eddy simulation (LES) turbulence model was used to investigate the flow behavior in the chamber of self-oscillation nozzle, the change law of dynamic pressure, velocity vector and turbulence peremeters in both internal and external flow field. The formation mechanism of self-excited oscillation pulsed abrasive water jet were revealed.And the formation mechanisms of PAWJ were revealed binding the abrasive effected by pulsing and bubble collapse in jet and distribution law of solid abrasive in pulsed water jet.And also, design criteria of self-oscillation abrasive nozzle suitable for yield-increasing mechanism of perforation in low permeability gas reservoir was establishmented binding calculation of LES and experimental results, that is: when the chamber length/diameter ratio(L/d1)is 2.3~3.3, the diameter ratio (d2/d1) of anterior nozzle to posterior nozzle is 1.2~1.3. Test system of perforation underground of PAWJ was developmented, the system was composed of gnerating device of pulsed abrasive jet, test device of fluctuating pressure, test system of fluctuating pressure using for PIV and simulation experiment system device of erosion underground, pressure oscillation characteristics, pulsed cavitation characteristics and high energy poly characteristics of pulsed abrasive jet were researched, the results showed that: the outlet peak pressure of self-oscillation nozzle is 2.5 times higher than common continuous jet, and the wave crest and the wave trough are not completely symmetric. The length of bubble cloud increases at first then decreases with the length of oscillation chamber increases, with pump pressure increases, the length of bubble cloud increases, and with confining pressure increases, the length of bubble cloud increases and then decreases. The diameter and depth of pulsed abrasive jet erosion specimen (8mm plate + 30mm sandstone) are 1.9 and 4.7 times higher than common jets.Thirdly, yield-increasing mechanism of pulsed abrasive jet perforation was researched according to development and occurrence characteristics of low permeability gas reservoir, combining own characteristics of pulsed abrasive jet. Pulsed abrasive water jet could improve the area and depth of perforation, reduce perforation pressure, secondary pollution caused by the shot perforation should be avoided, mechanical vibration communicates in elastic medium as the form of ultrasonic wave, causes to sound pressure variation, eliminates gas resistance, shock wave forms at the moment of bubble collapse, expands the pore radius of rocks, enhances the effect of microfissure, it’s benefit to the effect of follow-up filling sand fracturing, local high temperature is produced at the moment of bubble collapse, and thermal effect is produced by sound radiation, reduces viscosity of natural gas, and improves the seepage velocity.Last but not least, self-oscillating abrasive water jet nozzle suitable for low permeability gas reservoir perforation was designed originally, and field experiment of perforation yield-increasing was carried out in one gas well in the Sichuan Basin, and the results showes that: the method could erode out sanitary seepage channel and relieve the reservoir pollution of near borehole zones. Compared with perforating and fracturing of conventional nozzle, the method could reduce fracture pressure by 10-20 MPa, daily output increases to 35 times after construction and is 1.5 times higher than conventional perforating and fracturing of adjacent well, the method improves yield of the gas well effectively and has a good effect on increasing production.The main creative points of the research is: generation mechanism of self-excited oscillation pulsed abrasive water jet and pulsed abrasive jet were revealed through using LES model. Design criteria of self-oscillation abrasive nozzle suitable for yield-increasing mechanism of perforation in low permeability gas reservoir was establishmented, test system of perforation underground of PAWJ was developmented. Yield-increasing mechanism of pulsed abrasive water jet perforation was revealed. Research conclusions established the theoretical and experimental basis for the application of pulsed abrasive water jet in low permeability gas reservoir perforating and fracturing yield-increase reform. |
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