Mechanism and numerical simulation of pressure stagnation during water jetting perforation

52Pet.Sci.(2008)5:52-55DOI 10.1007/s12182-008-0008-2Mechanism and numerical simulation of pressurestagnation during water jetting perforationHuang Zhongwei, Li Gensheng*, Tian Shouceng, Shen Zhonghou andLuo HongbinKey Laboratory of Petroleum Engineering, Ministry of Education, China University of Petroleum, Bejing 102249, ChinaAbstract: When perforating with an abrasive water jet, it is possible that the pressure in the hole(perforation) will be higher than that in the annulus because of water jet blasting against the hole wall,which also is the theoretical basis for the technology of hydro-jet fracturing. This paper analyzes themechanism of generating pressure stagnation in water jet hole, and puts forward a new concept of hydro-seal. Then, the distribution of pressure in the hole was simulated with the finite element method. Thesimulation results showed that the pressure in the hole was higher than that in the annulus. Also, the lowerthe annular pressure (confining pressure) and the higher the blasting pressure, the greater the pressuredifference. An experiment indicated that the cement sample was lifted up under the pressure stagnation inthe hole, which proved the finite element simulation results obviously.Key words: Water jet, perforation, pressure stagnation, hydro-seal, mechanism1 Introductionresearch, the AWJ can form a spindle-shaped hole (Fig. 1)in the formation when the stand-off distance is smaller thanWhen perforating with an abrasive water jet (AW,), the 10 mm (Li et al, 2002). When the high speed AWJ passesAWJ will be pesurized through the tubing to the downhole the casing wall and cement, and impacts the hole end, it hastool's nozzle, exiting with high velocity to penetrate the to returm from the same path to the annulus. The hole in thecasing and form a hole (perforation) in the formation.casing wall, normally 10 mm diameter, not only is the way ofCompared with the conventional perforation, it can avoidthe entering jet (red part in Fig. 1), but also provides the pathdecreasing the formation permeablity because of the crushed for the returning fuid (green part in Fig. 1), which seeminglyzone existing in bullet perforation (Li et al, 2000, enhancing acts as a“seal" to increase the pressure in the hole (pressurethe oil well production and fracturing efficiency. Some stagnation), and consequentially fractures the formation.laboratory experiments have been done to optimize the fluid The finite element method is a practical way to calculate theand the abrasive parameters in the AWJ perforation (Li et pressure rise.al, 2002; 2004; 2006). Combining the AWJ perforation withfracturing, becomes a new technology, hydro-jet fracturing,3 Finite element simulationin which packer can be eliminated, different intervals canbe fractured, avoiding the risk of fracturing failure due toThe effect of perforation parameters on fracturing is verythe packer sealing problem (East et al, 2005; McDaniel et complex (Li et al, 2050. The peroration cnfigration in thisal, 2004; Hoch et al, 2003; Rees et al, 2001; Surjatmadja et simulation was established, according to the experimentalal, 2007). The critical issue in this technology is that, at theresults (Li et al, 2002), as follows: Perforation depth 500 mm,blasting position, the pressure in the hole should be highermaximum diameter 50 mm, the diameter of hole in the casingthan that in the annulus (confining pressure). In the authors'wall 10 mm, and the perforation is spindle-shaped. Drawingviewpoint, it is the“hydro-seal”that brings about the pressurethe half part of the perforation (Fig. 2), we calculated theenhancement (pressure stagnation) in the hole to ensurepressure distribution when changing the nozzle inlet pressurethe success of hydro-jet fracturing. In this paper, the finitebut fixing confining pressure and changing confining pressureelement method and experimental data were used to analyze but fxing nozle inlet pessre, rspectivelethe pressure stagnation.3.1 Distribution of pressure in the hole when2 Mechanism of pressure stagnationchanging nozzle inlet pressure but fixing confiningpressureIn blasting perforation, the AWJ is generally used toThe confining pressure was set at 5 MPa, while the inletpenetrate casing and formation. According to the previouspressure was set at 30, 35, 40, 45 and 50 MPa, respectively.Fig. 3 depicts th中国煤化工1ozzle exit when* Corresponding author. email: ligs@cup.edu.cnthe inlet pressurReceived October 15, 2007YHCNMHG.Pet.Sci.(2008)5:52-5553CasingCementPerforationFormationNozzleAWJReturning fluidFig.1 Schematic view of the AWJ holeHole in casing wallEnd of the holeFig. 2 Half profile of the hole1. 98e+01 BOe+001.79e+021.69e+021.59e+021.49e+021.39e+021.29e+021.19e+021.09e+029.92e+018.93e+017.94e+016.95e+015.95e+014.96e+013.97e+011 .98e+019,93e+001.93e-03Fig. 3 Velocity vector near the nozzle exitjet velocity attenuates rapidly after the jet exits the nozzle,distance of0.1 m (25 times as great as the nozzle diameter),decreasing from 198 m/s to approximately 60 m/s at the the pressure becomes stable, while at the end of the hole, thedistance of 60 mm. This phenomenon is different from value goes up slightly. At an inlet pressure of 50 MPa, theconventional free jet (blasting into the air), which has a pressure in the hole is approximately 12 MPa, 7 MPa higherlonger constant velocity area, with axial distance 5-50 times than the confining pressure.as long as the nozzle diameter. However in this calculated3.2 Distribution of pressure in the hole whenregion, as shearing action exists between the entering andreturning fluids, enery exchange will occur at the contact changing confining pressure but fixing nozzle inletlayer, resulting in rapid velocity attenuation.pressureFig. 4 shows the distribution of pressure in the hole atWhen the inlet pressure was set at 35 MPa, the distributiondifferent inlet pressures when the confining pressure was of pressure in中国煤化工t the confiningconstant at 5 MPa. This indicates that after the stand-offpressures of 5,YHCN M H Gely. Fig. 5 shows.54Pet.Sci.(2008)5:52-556CInlet pressure-30MPa5MPag40MPa30一45MP- 50MP0.10.30.c0.50.6Hole length, mFig. 4 Pressure dstribution in the hole at different inlet pressuresFig. 6 Experiment of pesure stagnation in the perforation50-40Confining pressurestagnation) in the perforation, which was predicted previously-5MPa- 15MPaby infinite element method.-20MPa4 Conclusions201) During water jet blasting, the occurrence of pressurestagnation in the perforation was due to the formation of“hydro-seal", which also is the theoretical basis to realizehydro-jet fracturing.0.).3.5.62) The simulated results show that the lower the confiningpressure and the higher the blasting pressure, the greater thepressure difference between perforation and annulus.Fig. 5 Distribution of pressure in the perforation at different3) Higher confining pressure is unfavorable to enhanceconfining pressurespressure stagnation in the perforation and to perform hydro-that the pressure in the hole would increase with the confiningjet fracturing in deeper wells.4) In the experiment, the cement sample was lifted up onpressure. When the confining pressure was 5.0 MPa, theone side, which proved the occurrence of pressure stagnationpressure in the hole was 9.4 MPa, 4.4 MPa higher than thein the perforation.confining pressure. Similarly, the pressure difference valueswere 3.7, 2.9 and 2.2 MPa when the combinations were Acknowledgements10.0/13.7, 15.0/17.9 and 20.0/22.2 MPa, respectively. Theabove results show that, due to the formation of“hydro-seal",This work was financially supported by National Naturalthe pessue in the hole was always higher than the confining Science Foundation of China (No. 50774089) and High-pressure. However, the higher the confining pressure, the tech Research and Development Program of China (No.smaller the difference, this is unfavorable to zone isolation 2007AA09Z315). The authors would like to thank them forduring hydro-jet fracturing.their approval to publish.3.3 Experiment of pressure rise in the holeReferencesFig. 6 is the picture of the experiment performed to prove East L, Rosato J, Farabee M, et al. New multiple interval fracturethe pressure enhancement in the hole. The experimentalstimulation technique without packers. International Petroleumconditions are as follows: a cement sample, with a diameterTechnology Conference held in Doha, Qatar, 21-23 November, 2005of 2.0 m, a length of 1.2 m, was enclosed with a thin steel(SPE paper 10549)sheet. A 5 1/2”casing was cemented in the cement sample.Hoch O, Stromquist M, Love G, et al. Multiple precision hydraulicA blasting tool, with a diameter of 108 mm and four nozzlesfractures of low-permeability horizontal openhole sandstone wells.SPE Annual Technical Conference and Exhibition held in Denver,(diameter being 4, 5, 6 and 8 mm, respectively) was used.Colorado, 5-8 October, 2003 (SPE paper 84163)Pump pressure was 45-50 MPa, and corresponding flow rateLi D C, Tang G H, Sun X B, et al. A study on perforation-crushed zone.was 2.2 m/min. During the experiment, the annulus betweenPetroleum Exploration and Development. 2000. 27(5): 112-117 (intubing and casing was open. From this picture, we could findthat one side of the sample was lifted up by the high pressure Li GS, Huang zv中国煤化工e of the influence ofin the perforation while blasting. This visual phenomenongeostress and Pfracturing. 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