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Difference between revisions of "Conducting the acidizing procedure"

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You've decided that your well is a [[Acidizing candidates|good candidate for acidizing]], [[Formation evaluation for acidizing|assessed the formation]], [[Acid treatment design|designed the treatment]], [[Acidizing preparations|prepared the well and equipment]], so now you're ready to conduct the treatment. This page describes both the process and things you should be doing during and immediately after the treatment.
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You've decided that your well is a [[Acidizing_candidates|good candidate for acidizing]], [[Formation_evaluation_for_acidizing|assessed the formation]], [[Acid_treatment_design|designed the treatment]], [[Acidizing_preparations|prepared the well and equipment]], so now you're ready to conduct the treatment. This page describes both the process and things you should be doing during and immediately after the treatment.
  
==Injection-rate control and monitoring==
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== Injection-rate control and monitoring ==
The main acid job should be circulated in place with HCl acid placed across the formation before the packer is set or before the bypass valve is closed. All perforations should be covered by acid before injection starts. Injection should start at a predetermined injection rate and the pressure observed to determine the condition of the wellbore. If the pressure rises close to the pressure limit, the rate should be cut in half until the pressure stabilizes at a level below the formation fracturing pressure. When the HF acid stage reaches the formation, a pressure drop is normally observed. The rate should not be changed as long as a positive pressure is observed at the wellhead. If the well goes on vacuum, the rate should be instantly raised until a positive pressure is observed at the wellhead. Hold the new rate steady as the acid is injected. Nevertheless, the constant injection rate of HF acid into the wellbore should not exceed an optimum 1/2 bbl/min unless the perforated interval is greater than 25 ft. If the formation is very thick, the rate can be 0.02 bbl/min per foot of net pay. Other authors have different opinions on allowable injection rates, as discussed later.
 
  
==Pressure behavior during acid injection==
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The main acid job should be circulated in place with HCl acid placed across the formation before the packer is set or before the bypass valve is closed. All perforations should be covered by acid before injection starts. Injection should start at a predetermined injection rate and the pressure observed to determine the condition of the wellbore. If the pressure rises close to the pressure limit, the rate should be cut in half until the pressure stabilizes at a level below the formation fracturing pressure. When the HF acid stage reaches the formation, a pressure drop is normally observed. The rate should not be changed as long as a positive pressure is observed at the wellhead. If the well goes on vacuum, the rate should be instantly raised until a positive pressure is observed at the wellhead. Hold the new rate steady as the acid is injected. Nevertheless, the constant injection rate of HF acid into the wellbore should not exceed an optimum 1/2 bbl/min unless the perforated interval is greater than 25 ft. If the formation is very thick, the rate can be 0.02 bbl/min per foot of net pay. Other authors have different opinions on allowable injection rates, as discussed later.
Two pressure responses are often observed during acid treatment. '''Fig. 1''' shows one response. <ref name="r1" /> In this well, when acid hit the formation, pressure dropped immediately. As the pressure dropped, the rate was increased; then the pressure began to rise. The rate was reduced, and then the well was shut in while another batch of acid was mixed on site. Injection was restarted at a rate of 2 bbl/min, then cut back to 1.5 bbl/min and stabilized at 2 bbl/min for the final injection of overflush. Rate should be held constant for a period of time at least until the pressure stabilizes. Haphazard changes in rate make it impossible to determine on site what the quantitative response of the well is to the acid treatment, unless newer computer models and monitoring equipment are available, as discussed later. A better-controlled acid treatment is shown in '''Fig. 2'''. <ref name="r1" /> Here, the rate is stabilized at 0.55 bbl/min. When the HF acid stage entered the formation, the pressure slowly declined but stayed above 0 psi. This rate was continued as long as the pressure was observed and is the type of response that one should observe when a well is treated to remove wellbore damage.  
 
  
When the overflush reaches the formation, the rate may be increased as fast as allowed, as long as the pressure stays below the fracturing pressure. The faster overflush rate will push the spent acid deeper into the formation and overdisplace the spent acid reaction products more efficiently away from the wellbore. This safely finishes the treatment and allows the spent acid to be produced back sooner. The well should be flowed immediately, unloaded with nitrogen, swabbed back, or put on artificial lift.
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== Pressure behavior during acid injection ==
  
<gallery widths=300px heights=200px>
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Two pressure responses are often observed during acid treatment. '''Fig. 1''' shows one response. <ref name="r1">McLeod Jr., H.O., Ledlow, L.B., and Till, M.V. 1983. The Planning, Execution, and Evaluation of Acid Treatments in Sandstone Formations. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October 1983. SPE-11931-MS. http://dx.doi.org/10.2118/11931-MS.</ref> In this well, when acid hit the formation, pressure dropped immediately. As the pressure dropped, the rate was increased; then the pressure began to rise. The rate was reduced, and then the well was shut in while another batch of acid was mixed on site. Injection was restarted at a rate of 2 bbl/min, then cut back to 1.5 bbl/min and stabilized at 2 bbl/min for the final injection of overflush. Rate should be held constant for a period of time at least until the pressure stabilizes. Haphazard changes in rate make it impossible to determine on site what the quantitative response of the well is to the acid treatment, unless newer computer models and monitoring equipment are available, as discussed later. A better-controlled acid treatment is shown in '''Fig. 2'''. <ref name="r1">McLeod Jr., H.O., Ledlow, L.B., and Till, M.V. 1983. The Planning, Execution, and Evaluation of Acid Treatments in Sandstone Formations. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October 1983. SPE-11931-MS. http://dx.doi.org/10.2118/11931-MS.</ref> Here, the rate is stabilized at 0.55 bbl/min. When the HF acid stage entered the formation, the pressure slowly declined but stayed above 0 psi. This rate was continued as long as the pressure was observed and is the type of response that one should observe when a well is treated to remove wellbore damage.
 +
 
 +
When the overflush reaches the formation, the rate may be increased as fast as allowed, as long as the pressure stays below the fracturing pressure. The faster overflush rate will push the spent acid deeper into the formation and overdisplace the spent acid reaction products more efficiently away from the wellbore. This safely finishes the treatment and allows the spent acid to be produced back sooner. The well should be flowed immediately, unloaded with nitrogen, swabbed back, or put on artificial lift.
 +
 
 +
<gallery widths="300px" heights="200px">
 
File:Vol4 Page 308 Image 0001.png|'''Fig. 1—Acid treatment with poor rate control.'''<ref name="r1" />
 
File:Vol4 Page 308 Image 0001.png|'''Fig. 1—Acid treatment with poor rate control.'''<ref name="r1" />
  
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</gallery>
 
</gallery>
  
When the overflush reaches the formation, the rate may be increased as fast as allowed, as long as the pressure stays below the fracturing pressure. The faster overflush rate will push the spent acid deeper into the formation and overdisplace the spent acid reaction products more efficiently away from the wellbore. This safely finishes the treatment and allows the spent acid to be produced back sooner. The well should be flowed immediately, unloaded with nitrogen, swabbed back, or put on artificial lift.  
+
When the overflush reaches the formation, the rate may be increased as fast as allowed, as long as the pressure stays below the fracturing pressure. The faster overflush rate will push the spent acid deeper into the formation and overdisplace the spent acid reaction products more efficiently away from the wellbore. This safely finishes the treatment and allows the spent acid to be produced back sooner. The well should be flowed immediately, unloaded with nitrogen, swabbed back, or put on artificial lift.
  
 
== On-site evaluation of acid treatment effectiveness ==
 
== On-site evaluation of acid treatment effectiveness ==
The pressure and rate chart of the acid treatment show the effect of acid volume on the formation as the acid treatment proceeds. The papers of McLeod and Coulter, <ref name="r2" /> Paccaloni ''et al.'', <ref name="r3" /><ref name="r16"><ref name="r17"> and Prouvost and Economides<ref name="r4" /> are significant to the on-site evaluation of acidizing treatments. On-site data monitoring follows and evaluates the progress of damage removal by acid. '''Fig. 3'''<ref name="r3" /> shows injection rate and pressure plotted on a precalculated chart of pressure vs. rate and crossplotted with a family of skin-factor curves based on steady-state injection. The successive points clearly show the reduction in skin factor. These plots may be somewhat misleading because pressure transients are ignored after rate changes; however, no on-site computer is required.
 
  
<gallery widths=300px heights=200px>
+
The pressure and rate chart of the acid treatment show the effect of acid volume on the formation as the acid treatment proceeds. The papers of McLeod and Coulter, <ref name="r2">McLeod, H.O.J. and Coulter, A.W.J. 1969. The Stimulation Treatment Pressure Record—An Overlooked Formation Evaluation Tool. J Pet Technol 21 (8): 952–960. SPE-2287-PA. http://dx.doi.org/10.2118/2287-PA.</ref> Paccaloni ''et al.'', <ref name="r3">Paccaloni, G., Tambini, M., and Galoppini, M. 1988. Key Factors for Enhanced Results of Matrix Stimulation Treatments. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17154-MS. http://dx.doi.org/10.2118/17154-MS._</ref><ref name="r16">Paccaloni, G. 1979. New Method Proves Value of Stimulation Planning. Oil & Gas J. 19 Nov. pp. 155-60</ref><ref name="r17">Paccaloni, G. 1979. Field History Verifies Control, Evaluation. Oil & Gas J. 26 Nov. pp. 61-65</ref> and Prouvost and Economides<ref name="r4">Prouvost, L.P. and Economides, M.J. 1989. Applications of Real-Time Matrix-Acidizing Evaluation Method. SPE Prod Eng 4 (4): 401–407. http://dx.doi.org/10.2118/17156-PA.</ref> are significant to the on-site evaluation of acidizing treatments. On-site data monitoring follows and evaluates the progress of damage removal by acid. '''Fig. 3'''<ref name="r3">Paccaloni, G., Tambini, M., and Galoppini, M. 1988. Key Factors for Enhanced Results of Matrix Stimulation Treatments. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17154-MS. http://dx.doi.org/10.2118/17154-MS.</ref> shows injection rate and pressure plotted on a precalculated chart of pressure vs. rate and crossplotted with a family of skin-factor curves based on steady-state injection. The successive points clearly show the reduction in skin factor. These plots may be somewhat misleading because pressure transients are ignored after rate changes; however, no on-site computer is required.
 +
 
 +
<gallery widths="300px" heights="200px">
 
File:Vol4 Page 310 Image 0001.png|'''Fig. 3—Matrix acid pressure chart with job evaluation.'''<ref name="r3" />
 
File:Vol4 Page 310 Image 0001.png|'''Fig. 3—Matrix acid pressure chart with job evaluation.'''<ref name="r3" />
 
</gallery>
 
</gallery>
  
'''Fig. 4'''<ref name="r4" /> shows rates, pressures (both measured and simulated), and skin-factor change during acidizing, as presented by Prouvost and Economides. <ref name="r4" /> This method requires an onsite computer but considers pressure transient effects when rate is changed. Such plots are a tremendous help in analyzing on-site acidizing performance and in follow-up well analysis. More information is also available in the excellent text on stimulation by Economides and Nolte. <ref name="r5" /> McLeod and Coulter<ref name="r2" /> presented the first example of injection pressure buildup analysis before and after acidizing cleanup in '''Figs. 5 and 6'''. Calculation to obtain formation permeability before and after acidizing are shown with data in '''Tables 1 through 4'''.  
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'''Fig. 4'''<ref name="r4">Prouvost, L.P. and Economides, M.J. 1989. Applications of Real-Time Matrix-Acidizing Evaluation Method. SPE Prod Eng 4 (4): 401–407. http://dx.doi.org/10.2118/17156-PA.</ref> shows rates, pressures (both measured and simulated), and skin-factor change during acidizing, as presented by Prouvost and Economides. <ref name="r4">Prouvost, L.P. and Economides, M.J. 1989. Applications of Real-Time Matrix-Acidizing Evaluation Method. SPE Prod Eng 4 (4): 401–407. http://dx.doi.org/10.2118/17156-PA.</ref> This method requires an onsite computer but considers pressure transient effects when rate is changed. Such plots are a tremendous help in analyzing on-site acidizing performance and in follow-up well analysis. More information is also available in the excellent text on stimulation by Economides and Nolte. <ref name="r5">Economides, M.J. and Nolte, K.G. 2000. Reservoir Stimulation, 20-4–20-8, third edition. West Sussex, England: John Wiley and Sons, Ltd.</ref> McLeod and Coulter<ref name="r2">McLeod, H.O.J. and Coulter, A.W.J. 1969. The Stimulation Treatment Pressure Record—An Overlooked Formation Evaluation Tool. J Pet Technol 21 (8): 952–960. SPE-2287-PA. http://dx.doi.org/10.2118/2287-PA.</ref> presented the first example of injection pressure buildup analysis before and after acidizing cleanup in '''Figs. 5<ref name="r6">McLeod, H.O.J. 1984. Matrix Acidizing. J Pet Technol 36 (12): 2055–2069. SPE-13752-PA. http://dx.doi.org/10.2118/13752-PA.</ref> and 6'''. Calculation to obtain formation permeability before and after acidizing are shown with data in '''Tables 1 through 4'''.
  
<gallery widths=300px heights=200px>
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<gallery widths="300px" heights="200px">
 
File:Vol4 Page 311 Image 0001.png|'''Fig. 4—Skin evolution during acid job.'''<ref name="r4" />
 
File:Vol4 Page 311 Image 0001.png|'''Fig. 4—Skin evolution during acid job.'''<ref name="r4" />
  
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</gallery>
 
</gallery>
  
Hill and Zhu<ref name="r7" /> advanced the monitoring of acidizing treatments, building on the earlier contributions of McLeod and Coulter, <ref name="r2" /> Paccaloni ''et al.'', <ref name="r3" /> and Prouvost and Economides. <ref name="r4" /> The use of the inverse injectivity diagnostic plot permits the real-time evaluation of treatments and further assists in post-treatment evaluations. Montgomery ''et al.''<ref name="r8" /> proposed more active treatment monitoring into standard acidizing practice.  
+
Hill and Zhu<ref name="r7">Hill, A.D. and Zhu, D. 1996. Real-Time Monitoring of Matrix Acidizing Including the Effects of Diverting Agents. SPE Prod & Oper 11 (2): 95-101. SPE-28548-PA. http://dx.doi.org/10.2118/28548-PA.</ref> advanced the monitoring of acidizing treatments, building on the earlier contributions of McLeod and Coulter, <ref name="r2">McLeod, H.O.J. and Coulter, A.W.J. 1969. The Stimulation Treatment Pressure Record—An Overlooked Formation Evaluation Tool. J Pet Technol 21 (8): 952–960. SPE-2287-PA. http://dx.doi.org/10.2118/2287-PA.</ref> Paccaloni ''et al.'', <ref name="r3">Paccaloni, G., Tambini, M., and Galoppini, M. 1988. Key Factors for Enhanced Results of Matrix Stimulation Treatments. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17154-MS. http://dx.doi.org/10.2118/17154-MS.</ref> and Prouvost and Economides. <ref name="r4">Prouvost, L.P. and Economides, M.J. 1989. Applications of Real-Time Matrix-Acidizing Evaluation Method. SPE Prod Eng 4 (4): 401–407. http://dx.doi.org/10.2118/17156-PA.</ref> The use of the inverse injectivity diagnostic plot permits the real-time evaluation of treatments and further assists in post-treatment evaluations. Montgomery ''et al.''<ref name="r8">Montgomery, C.T., Jan, Y.-M., and Niemeyer, B.L. 1995. Development of a Matrix-Acidizing Stimulation Treatment Evaluation and Recording System. SPE Prod & Oper 10 (4): 219-224. SPE-26579-PA. http://dx.doi.org/10.2118/26579-PA.</ref> proposed more active treatment monitoring into standard acidizing practice.
  
Zhu and Hill<ref name="r9" /><ref name="r10" /> provide good field examples of monitoring acid treatments with concurrent skin evolution for both diverted and nondiverted treatments. Monitored on-site evaluation was later confirmed with well-flow analysis of post-treatment well performance. However, more work is needed on evaluating causes of treatment failures or skin increases.
+
Zhu and Hill<ref name="r9">D. Zhu, Hill, A.D., and Morgenthaler, L.N. 1999. Assessment of Matrix Acidizing Treatment Responses in Gulf of Mexico Wells. Presented at the SPE Mid-Continent Operations Symposium, Oklahoma City, Oklahoma, 28-31 March 1999. SPE-52166-MS. http://dx.doi.org/10.2118/52166-MS.</ref><ref name="r10">Hill, A.D. and Zhu, D. 1996. Real-Time Monitoring of Matrix Acidizing Including the Effects of Diverting Agents. SPE Prod & Oper 11 (2): 95-101. SPE-28548-PA. http://dx.doi.org/10.2118/28548-PA.</ref> provide good field examples of monitoring acid treatments with concurrent skin evolution for both diverted and nondiverted treatments. Monitored on-site evaluation was later confirmed with well-flow analysis of post-treatment well performance. However, more work is needed on evaluating causes of treatment failures or skin increases.
  
 
== Spent acid production control ==
 
== Spent acid production control ==
The well should be produced first at the same rate before acidizing. As soon as the well has cleaned up and all spent acid has been recovered or reduced to zero water cut, the producing rate may be increased. In formations with moveable fines, the rate should be increased once each week to finally reach the optimum producing rate for that well. Increasing the rate gradually helps the return of any dispersed solids and prolongs the improvement for the acid treatment.
 
  
Ali ''et al.''<ref name="r11" /> discussed a method to minimize production facility upsets offshore by special handling of the returned acids at the surface. The cost of fluid handling is further reduced by optimizing use of additives with improved laboratory testing procedures. <ref name="r12" /> This was stimulated by the work of Bansal. <ref name="r13" />
+
The well should be produced first at the same rate before acidizing. As soon as the well has cleaned up and all spent acid has been recovered or reduced to zero water cut, the producing rate may be increased. In formations with moveable fines, the rate should be increased once each week to finally reach the optimum producing rate for that well. Increasing the rate gradually helps the return of any dispersed solids and prolongs the improvement for the acid treatment.
  
==Produced fluid sampling==
+
Ali ''et al.''<ref name="r11">Ali, S.A. et al. 1997. Process Optimized Acidizing Reduces Production Facility Upsets. Oil & Gas J. 95 (6): 44.</ref> discussed a method to minimize production facility upsets offshore by special handling of the returned acids at the surface. The cost of fluid handling is further reduced by optimizing use of additives with improved laboratory testing procedures. <ref name="r12">Ali, S.A., Durham, D.K., and Elphingstone, E.A. 1994. Testing Identifies Acidizing Fluid/Crude Compatibility Problems. Oil & Gas J. 92 (13): 47.</ref> This was stimulated by the work of Bansal. <ref name="r13">Bansal, K.M. 1993. Effect of Nonproduced Fluids on Produced Water Treatment Equipment Efficiency. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, Louisiana, 2-5 March 1993. SPE-25199-MS. http://dx.doi.org/10.2118/25199-MS.</ref>
Spent acid samples should be collected at the surface to properly analyze the response of the well to the acid treatment. These samples should be analyzed for pH immediately and kept in airtight containers for chemical analysis. Chemical analysis of these samples can provide information for use in evaluating why a well did not respond to acid treatment. If precipitates or emulsions are a problem, the return samples will show the reason. Whatever solids are precipitating to cause possible damage to the rock around the wellbore may be present in these collected samples. Steps can be taken to reduce precipitation by changes in acid concentration, preflush fluids, and/or additives in the next scheduled treatment.  
 
  
Analysis of well flowback may indicate problems and concerns not readily evident otherwise. Such problems may arise from acid or spent acid mixing with lost completion brines and/or formation water, significant dissolution of carbonates, and total consumption of acid. The insight obtained helps to design optimum formulations for future treatments. <ref name="r14" />
+
== Produced fluid sampling ==
  
A comprehensive HF acidizing radial flow model was modified by Gdanski and Schuchart<ref name="r15" /> to account for deep-matrix mixing and back production of sandstone-acidizing treating fluids. Deep matrix mixing may require back production of at least two treatment volumes of aqueous fluid to recover the spent injected acids. Matching the ionic return profiles can provide information about formation mineralogy and excess precipitation.  
+
Spent acid samples should be collected at the surface to properly analyze the response of the well to the acid treatment. These samples should be analyzed for pH immediately and kept in airtight containers for chemical analysis. Chemical analysis of these samples can provide information for use in evaluating why a well did not respond to acid treatment. If precipitates or emulsions are a problem, the return samples will show the reason. Whatever solids are precipitating to cause possible damage to the rock around the wellbore may be present in these collected samples. Steps can be taken to reduce precipitation by changes in acid concentration, preflush fluids, and/or additives in the next scheduled treatment.
  
To summarize, on-site supervision of acid treatments is critical to successful acidizing. Long treatments can best be controlled by two persons—one to coordinate the acid schedule and rate and pressure control, and the other to check materials; titrate acid; and monitor volumes, rates, and pressures. The engineer who recommended and designed the job and the supervisor who prepared the well for acidizing make a good combination. Good data and record keeping greatly help the job of evaluating acidizing results.
+
Analysis of well flowback may indicate problems and concerns not readily evident otherwise. Such problems may arise from acid or spent acid mixing with lost completion brines and/or formation water, significant dissolution of carbonates, and total consumption of acid. The insight obtained helps to design optimum formulations for future treatments. <ref name="r14">Al-Dahlan, M.N. and Nasr-El-Din, H.A. 2000. A New Technique to Evaluate Matrix Acid Treatments in Carbonate Reservoirs. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58714-MS. http://dx.doi.org/10.2118/58714-MS.</ref>
  
==References==
+
A comprehensive HF acidizing radial flow model was modified by Gdanski and Schuchart<ref name="r15">Gdanski, R.D. and Shuchart, C.E. 1998. Advanced Sandstone-Acidizing Designs With Improved Radial Models. SPE Prod & Fac 13 (4): 272–278. SPE-52397-PA. http://dx.doi.org/10.2118/52397-PA.</ref> to account for deep-matrix mixing and back production of sandstone-acidizing treating fluids. Deep matrix mixing may require back production of at least two treatment volumes of aqueous fluid to recover the spent injected acids. Matching the ionic return profiles can provide information about formation mineralogy and excess precipitation.
<references>
 
<ref name="r1" >McLeod Jr., H.O., Ledlow, L.B., and Till, M.V. 1983. The Planning, Execution, and Evaluation of Acid Treatments in Sandstone Formations. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October 1983. SPE-11931-MS. http://dx.doi.org/10.2118/11931-MS. </ref>
 
  
<ref name="r2" >McLeod, H.O.J. and Coulter, A.W.J. 1969. The Stimulation Treatment Pressure Record—An Overlooked Formation Evaluation Tool. ''J Pet Technol'' '''21''' (8): 952–960. SPE-2287-PA. http://dx.doi.org/10.2118/2287-PA.</ref>
+
To summarize, on-site supervision of acid treatments is critical to successful acidizing. Long treatments can best be controlled by two persons—one to coordinate the acid schedule and rate and pressure control, and the other to check materials; titrate acid; and monitor volumes, rates, and pressures. The engineer who recommended and designed the job and the supervisor who prepared the well for acidizing make a good combination. Good data and record keeping greatly help the job of evaluating acidizing results.
  
<ref name="r3" >Paccaloni, G., Tambini, M., and  Galoppini, M. 1988. Key Factors for Enhanced Results of Matrix Stimulation Treatments. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17154-MS. http://dx.doi.org/10.2118/17154-MS. </ref>
+
== References ==
  
<ref name="r4" >Prouvost, L.P. and Economides, M.J. 1989. Applications of Real-Time Matrix-Acidizing Evaluation Method. SPE Prod Eng 4 (4): 401–407. http://dx.doi.org/10.2118/17156-PA. </ref>
+
<references />
  
<ref name="r5" >Economides, M.J. and Nolte, K.G. 2000. ''Reservoir Stimulation'', 20-4–20-8, third edition. West Sussex, England: John Wiley and Sons, Ltd.</ref>
+
== Noteworthy papers in OnePetro ==
  
<ref name="r6" >McLeod, H.O.J. 1984. Matrix Acidizing. ''J Pet Technol'' '''36''' (12): 2055–2069. SPE-13752-PA. http://dx.doi.org/10.2118/13752-PA.</ref>
+
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
  
<ref name="r7" >Hill, A.D. and Zhu, D. 1996. Real-Time Monitoring of Matrix Acidizing Including the Effects of Diverting Agents. ''SPE Prod & Oper'' '''11''' (2): 95-101. SPE-28548-PA. http://dx.doi.org/10.2118/28548-PA. </ref>
+
== External links ==
  
<ref name="r8" >Montgomery, C.T., Jan, Y.-M., and Niemeyer, B.L. 1995. Development of a Matrix-Acidizing Stimulation Treatment Evaluation and Recording System. ''SPE Prod & Oper'' '''10''' (4): 219-224. SPE-26579-PA. http://dx.doi.org/10.2118/26579-PA.</ref>
+
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
 
 
<ref name="r9" >D. Zhu, Hill, A.D., and  Morgenthaler, L.N. 1999. Assessment of Matrix Acidizing Treatment Responses in Gulf of Mexico Wells. Presented at the SPE Mid-Continent Operations Symposium, Oklahoma City, Oklahoma, 28-31 March 1999. SPE-52166-MS. http://dx.doi.org/10.2118/52166-MS. </ref>
 
 
 
<ref name="r10" >Hill, A.D. and Zhu, D. 1996. Real-Time Monitoring of Matrix Acidizing Including the Effects of Diverting Agents. ''SPE Prod & Oper'' '''11''' (2): 95-101. SPE-28548-PA. http://dx.doi.org/10.2118/28548-PA.</ref>
 
 
 
<ref name="r11" >Ali, S.A. et al. 1997. Process Optimized Acidizing Reduces Production Facility Upsets. ''Oil & Gas J.'' '''95''' (6): 44. </ref>
 
  
<ref name="r12" >Ali, S.A., Durham, D.K., and Elphingstone, E.A. 1994. Testing Identifies Acidizing Fluid/Crude Compatibility Problems. ''Oil & Gas J.'' '''92''' (13): 47.</ref>
+
== See also ==
  
<ref name="r13" >Bansal, K.M. 1993. Effect of Nonproduced Fluids on Produced Water Treatment Equipment Efficiency. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, Louisiana, 2-5 March 1993. SPE-25199-MS. http://dx.doi.org/10.2118/25199-MS. </ref>
+
[[Evaluation_of_acid_treatments|Evaluation of acid treatments]]
 
 
<ref name="r14" >Al-Dahlan, M.N. and Nasr-El-Din, H.A. 2000. A New Technique to Evaluate Matrix Acid Treatments in Carbonate Reservoirs. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58714-MS. http://dx.doi.org/10.2118/58714-MS.  </ref>
 
 
 
<ref name="r15" >Gdanski, R.D. and Shuchart, C.E. 1998. Advanced Sandstone-Acidizing Designs With Improved Radial Models. ''SPE Prod & Fac'' '''13''' (4): 272–278. SPE-52397-PA. http://dx.doi.org/10.2118/52397-PA. </ref>
 
 
 
<ref name="r16" >Paccaloni, G. 1979. New Method Proves Value of Stimulation Planning. ''Oil & Gas J.'' 19 Nov. pp. 155-60 <ref />
 
 
 
<ref name="r17" >Paccaloni, G. 1979. Field History Verifies Control, Evaluation. ''Oil & Gas J.'' 26 Nov. pp. 61-65 <ref />
 
</references>
 
 
 
==Noteworthy papers in OnePetro==
 
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
 
 
 
==External links==
 
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
 
  
==See also==
+
[[Acid_treatment_design|Acid treatment design]]
[[Evaluation of acid treatments]]
 
  
[[Acid treatment design]]
+
[[Acidizing_preparations|Acidizing preparations]]
  
[[Acidizing preparations]]
+
[[Acidizing_safety_and_environmental_protection|Acidizing safety and environmental protection]]
  
[[Acidizing safety and environmental protection]]
+
[[Matrix_acidizing|Matrix acidizing]]
  
[[Matrix acidizing]]
+
[[PEH:Matrix_Acidizing]]
  
[[PEH:Matrix Acidizing]]
+
== Category ==
 +
[[Category:3.2.4 Acidizing]] [[Category:YR]]

Latest revision as of 13:49, 1 July 2015

You've decided that your well is a good candidate for acidizing, assessed the formation, designed the treatment, prepared the well and equipment, so now you're ready to conduct the treatment. This page describes both the process and things you should be doing during and immediately after the treatment.

Injection-rate control and monitoring

The main acid job should be circulated in place with HCl acid placed across the formation before the packer is set or before the bypass valve is closed. All perforations should be covered by acid before injection starts. Injection should start at a predetermined injection rate and the pressure observed to determine the condition of the wellbore. If the pressure rises close to the pressure limit, the rate should be cut in half until the pressure stabilizes at a level below the formation fracturing pressure. When the HF acid stage reaches the formation, a pressure drop is normally observed. The rate should not be changed as long as a positive pressure is observed at the wellhead. If the well goes on vacuum, the rate should be instantly raised until a positive pressure is observed at the wellhead. Hold the new rate steady as the acid is injected. Nevertheless, the constant injection rate of HF acid into the wellbore should not exceed an optimum 1/2 bbl/min unless the perforated interval is greater than 25 ft. If the formation is very thick, the rate can be 0.02 bbl/min per foot of net pay. Other authors have different opinions on allowable injection rates, as discussed later.

Pressure behavior during acid injection

Two pressure responses are often observed during acid treatment. Fig. 1 shows one response. [1] In this well, when acid hit the formation, pressure dropped immediately. As the pressure dropped, the rate was increased; then the pressure began to rise. The rate was reduced, and then the well was shut in while another batch of acid was mixed on site. Injection was restarted at a rate of 2 bbl/min, then cut back to 1.5 bbl/min and stabilized at 2 bbl/min for the final injection of overflush. Rate should be held constant for a period of time at least until the pressure stabilizes. Haphazard changes in rate make it impossible to determine on site what the quantitative response of the well is to the acid treatment, unless newer computer models and monitoring equipment are available, as discussed later. A better-controlled acid treatment is shown in Fig. 2. [1] Here, the rate is stabilized at 0.55 bbl/min. When the HF acid stage entered the formation, the pressure slowly declined but stayed above 0 psi. This rate was continued as long as the pressure was observed and is the type of response that one should observe when a well is treated to remove wellbore damage.

When the overflush reaches the formation, the rate may be increased as fast as allowed, as long as the pressure stays below the fracturing pressure. The faster overflush rate will push the spent acid deeper into the formation and overdisplace the spent acid reaction products more efficiently away from the wellbore. This safely finishes the treatment and allows the spent acid to be produced back sooner. The well should be flowed immediately, unloaded with nitrogen, swabbed back, or put on artificial lift.

When the overflush reaches the formation, the rate may be increased as fast as allowed, as long as the pressure stays below the fracturing pressure. The faster overflush rate will push the spent acid deeper into the formation and overdisplace the spent acid reaction products more efficiently away from the wellbore. This safely finishes the treatment and allows the spent acid to be produced back sooner. The well should be flowed immediately, unloaded with nitrogen, swabbed back, or put on artificial lift.

On-site evaluation of acid treatment effectiveness

The pressure and rate chart of the acid treatment show the effect of acid volume on the formation as the acid treatment proceeds. The papers of McLeod and Coulter, [2] Paccaloni et al., [3][4][5] and Prouvost and Economides[6] are significant to the on-site evaluation of acidizing treatments. On-site data monitoring follows and evaluates the progress of damage removal by acid. Fig. 3[3] shows injection rate and pressure plotted on a precalculated chart of pressure vs. rate and crossplotted with a family of skin-factor curves based on steady-state injection. The successive points clearly show the reduction in skin factor. These plots may be somewhat misleading because pressure transients are ignored after rate changes; however, no on-site computer is required.

Fig. 4[6] shows rates, pressures (both measured and simulated), and skin-factor change during acidizing, as presented by Prouvost and Economides. [6] This method requires an onsite computer but considers pressure transient effects when rate is changed. Such plots are a tremendous help in analyzing on-site acidizing performance and in follow-up well analysis. More information is also available in the excellent text on stimulation by Economides and Nolte. [7] McLeod and Coulter[2] presented the first example of injection pressure buildup analysis before and after acidizing cleanup in Figs. 5[8] and 6. Calculation to obtain formation permeability before and after acidizing are shown with data in Tables 1 through 4.

Hill and Zhu[9] advanced the monitoring of acidizing treatments, building on the earlier contributions of McLeod and Coulter, [2] Paccaloni et al., [3] and Prouvost and Economides. [6] The use of the inverse injectivity diagnostic plot permits the real-time evaluation of treatments and further assists in post-treatment evaluations. Montgomery et al.[10] proposed more active treatment monitoring into standard acidizing practice.

Zhu and Hill[11][12] provide good field examples of monitoring acid treatments with concurrent skin evolution for both diverted and nondiverted treatments. Monitored on-site evaluation was later confirmed with well-flow analysis of post-treatment well performance. However, more work is needed on evaluating causes of treatment failures or skin increases.

Spent acid production control

The well should be produced first at the same rate before acidizing. As soon as the well has cleaned up and all spent acid has been recovered or reduced to zero water cut, the producing rate may be increased. In formations with moveable fines, the rate should be increased once each week to finally reach the optimum producing rate for that well. Increasing the rate gradually helps the return of any dispersed solids and prolongs the improvement for the acid treatment.

Ali et al.[13] discussed a method to minimize production facility upsets offshore by special handling of the returned acids at the surface. The cost of fluid handling is further reduced by optimizing use of additives with improved laboratory testing procedures. [14] This was stimulated by the work of Bansal. [15]

Produced fluid sampling

Spent acid samples should be collected at the surface to properly analyze the response of the well to the acid treatment. These samples should be analyzed for pH immediately and kept in airtight containers for chemical analysis. Chemical analysis of these samples can provide information for use in evaluating why a well did not respond to acid treatment. If precipitates or emulsions are a problem, the return samples will show the reason. Whatever solids are precipitating to cause possible damage to the rock around the wellbore may be present in these collected samples. Steps can be taken to reduce precipitation by changes in acid concentration, preflush fluids, and/or additives in the next scheduled treatment.

Analysis of well flowback may indicate problems and concerns not readily evident otherwise. Such problems may arise from acid or spent acid mixing with lost completion brines and/or formation water, significant dissolution of carbonates, and total consumption of acid. The insight obtained helps to design optimum formulations for future treatments. [16]

A comprehensive HF acidizing radial flow model was modified by Gdanski and Schuchart[17] to account for deep-matrix mixing and back production of sandstone-acidizing treating fluids. Deep matrix mixing may require back production of at least two treatment volumes of aqueous fluid to recover the spent injected acids. Matching the ionic return profiles can provide information about formation mineralogy and excess precipitation.

To summarize, on-site supervision of acid treatments is critical to successful acidizing. Long treatments can best be controlled by two persons—one to coordinate the acid schedule and rate and pressure control, and the other to check materials; titrate acid; and monitor volumes, rates, and pressures. The engineer who recommended and designed the job and the supervisor who prepared the well for acidizing make a good combination. Good data and record keeping greatly help the job of evaluating acidizing results.

References

  1. 1.0 1.1 1.2 1.3 McLeod Jr., H.O., Ledlow, L.B., and Till, M.V. 1983. The Planning, Execution, and Evaluation of Acid Treatments in Sandstone Formations. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October 1983. SPE-11931-MS. http://dx.doi.org/10.2118/11931-MS.
  2. 2.0 2.1 2.2 McLeod, H.O.J. and Coulter, A.W.J. 1969. The Stimulation Treatment Pressure Record—An Overlooked Formation Evaluation Tool. J Pet Technol 21 (8): 952–960. SPE-2287-PA. http://dx.doi.org/10.2118/2287-PA.
  3. 3.0 3.1 3.2 3.3 Paccaloni, G., Tambini, M., and Galoppini, M. 1988. Key Factors for Enhanced Results of Matrix Stimulation Treatments. Presented at the SPE Formation Damage Control Symposium, Bakersfield, California, 8-9 February 1988. SPE-17154-MS. http://dx.doi.org/10.2118/17154-MS._ Cite error: Invalid <ref> tag; name "r3" defined multiple times with different content Cite error: Invalid <ref> tag; name "r3" defined multiple times with different content
  4. Paccaloni, G. 1979. New Method Proves Value of Stimulation Planning. Oil & Gas J. 19 Nov. pp. 155-60
  5. Paccaloni, G. 1979. Field History Verifies Control, Evaluation. Oil & Gas J. 26 Nov. pp. 61-65
  6. 6.0 6.1 6.2 6.3 6.4 Prouvost, L.P. and Economides, M.J. 1989. Applications of Real-Time Matrix-Acidizing Evaluation Method. SPE Prod Eng 4 (4): 401–407. http://dx.doi.org/10.2118/17156-PA.
  7. Economides, M.J. and Nolte, K.G. 2000. Reservoir Stimulation, 20-4–20-8, third edition. West Sussex, England: John Wiley and Sons, Ltd.
  8. 8.0 8.1 8.2 McLeod, H.O.J. 1984. Matrix Acidizing. J Pet Technol 36 (12): 2055–2069. SPE-13752-PA. http://dx.doi.org/10.2118/13752-PA.
  9. Hill, A.D. and Zhu, D. 1996. Real-Time Monitoring of Matrix Acidizing Including the Effects of Diverting Agents. SPE Prod & Oper 11 (2): 95-101. SPE-28548-PA. http://dx.doi.org/10.2118/28548-PA.
  10. Montgomery, C.T., Jan, Y.-M., and Niemeyer, B.L. 1995. Development of a Matrix-Acidizing Stimulation Treatment Evaluation and Recording System. SPE Prod & Oper 10 (4): 219-224. SPE-26579-PA. http://dx.doi.org/10.2118/26579-PA.
  11. D. Zhu, Hill, A.D., and Morgenthaler, L.N. 1999. Assessment of Matrix Acidizing Treatment Responses in Gulf of Mexico Wells. Presented at the SPE Mid-Continent Operations Symposium, Oklahoma City, Oklahoma, 28-31 March 1999. SPE-52166-MS. http://dx.doi.org/10.2118/52166-MS.
  12. Hill, A.D. and Zhu, D. 1996. Real-Time Monitoring of Matrix Acidizing Including the Effects of Diverting Agents. SPE Prod & Oper 11 (2): 95-101. SPE-28548-PA. http://dx.doi.org/10.2118/28548-PA.
  13. Ali, S.A. et al. 1997. Process Optimized Acidizing Reduces Production Facility Upsets. Oil & Gas J. 95 (6): 44.
  14. Ali, S.A., Durham, D.K., and Elphingstone, E.A. 1994. Testing Identifies Acidizing Fluid/Crude Compatibility Problems. Oil & Gas J. 92 (13): 47.
  15. Bansal, K.M. 1993. Effect of Nonproduced Fluids on Produced Water Treatment Equipment Efficiency. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, Louisiana, 2-5 March 1993. SPE-25199-MS. http://dx.doi.org/10.2118/25199-MS.
  16. Al-Dahlan, M.N. and Nasr-El-Din, H.A. 2000. A New Technique to Evaluate Matrix Acid Treatments in Carbonate Reservoirs. Presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 23-24 February 2000. SPE-58714-MS. http://dx.doi.org/10.2118/58714-MS.
  17. Gdanski, R.D. and Shuchart, C.E. 1998. Advanced Sandstone-Acidizing Designs With Improved Radial Models. SPE Prod & Fac 13 (4): 272–278. SPE-52397-PA. http://dx.doi.org/10.2118/52397-PA.

Noteworthy papers in OnePetro

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External links

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See also

Evaluation of acid treatments

Acid treatment design

Acidizing preparations

Acidizing safety and environmental protection

Matrix acidizing

PEH:Matrix_Acidizing

Category