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Evaluation of acid treatments
Treatment evaluation leads to problem identification and to continuously improved treatments. The prime source of information on which to build an evaluation are the acid treatment report and the pressure and rate data during injection and falloff. The tasks of execution and evaluation go hand in hand. Proper execution, quality control, and record keeping are prerequisites to the task of accurate evaluation. Evaluation of unsatisfactory treatments is essential to recommending changes in chemicals and/or treating techniques and procedures that will provide the best treatment for acidizing wells in the future.
The evaluation process encompasses six major areas on which to focus when assessing job performance and acid treatment success:
- Injection rate and pressure.
- Final fall-off pressure record.
- Well production analysis (nodal analysis).
- Produced fluid samples.
- Post-treatment investigation concerning damage incurred during injection, acid removal of damage, post-treatment damage (precipitates), and verbal communications.
- Recommendations for continuous improvement.
Measuring productivity after treatment
The most important measure of the treatment is the productivity of the well after treatment. When the productivity stabilizes at the same production rate as before treatment, the flowing bottomhole pressure should be estimated from fluid levels or from measured flowing pressures. Static bottomhole pressure should be measured following any long shut-in periods. A well flow analysis should show whether the designed productivity was obtained. The pressure charts from the treatment, including both accurate injection rates and recorded injection pressures, can be analyzed using transient pressure analysis to determine when or if the wellbore damage was removed by the treatment.
An injectivity index can be calculated for the well both before and after the HF-acid stage.
The final overflush injection pressures and rates should give a fairly accurate measure of the well productivity before the well is ever returned to production. A useful source of information is the final pressure falloff after the treatment.
If the pressure exists at the wellhead, the falloff pressures should be recorded on site until the well goes on vacuum. If the well goes on vacuum too soon, fluid levels can be shot with a sonolog device until the level falls to near the static bottomhole pressure. These final falloff pressures can be used to estimate the wellbore condition after the acid treatment. If this analysis shows that the acid treatment removed all wellbore damage and later well production analysis shows that no post-acid precipitation occurred, the treatment is potentially successful. An example of this type analysis is shown in the example provided by McLeod. 
Underachieved or failed results
If the well injectivity or productivity (after the well returns to injection or production) is not close to that predicted by the falloff analysis, some damage probably occurred to the formation after the acid treatment ended. Subsequent damage after the treatment may be caused by precipitation of acid reaction products in the formation or by return of fines to the wellbore with internal pore plugging at or near perforations. This is especially true in gravel packed wells. Often, unsatisfactory performance results from imperfect coverage during the treatment. A change in the acid placement technique may be necessary for the next acid treatment in the field. If solid diverting agents were employed, changes in concentrations may be necessary, or perhaps another diversion technique would work better. Feasibility and economic analysis from the expected well production increase determine whether these changes are worthwhile. Usually, the damage during an acid treatment occurs at the time the first acid hits the formation. This first acid damage is usually caused by solids removed from the tubing walls prior to the acid reaching the formation. Also, acid may react adversely with some of the minerals in the formation, and perhaps a different acid or solvent (such as acetic acid or an aromatic solvent) should be used to first contact the formation. Many acid failures are caused by the elimination of needed wellbore preparations prior to the acid treatment.
Intended removal of damage
It is important to know that the treatment removed the damage in the wellbore during treatment as intended. If damage occurred after the treatment, steps can be taken to prevent that damage in a later treatment of that well or others in the reservoir by such steps as utilizing different additives to keep reaction products in solution, overflushing the reaction products deeper into the formation, using different acids or acid concentrations to prevent the excess precipitation of acid reaction products, or using stabilizers to prevent fines from returning to the wellbore and reducing productivity.
If the anticipated productivity was achieved, the acid treatment worked as designed. If not, the entire treatment should be reviewed to analyze the causes.
The engineer evaluating the treatment should individually discuss the treatment with the service company supervising engineer and the operating company supervisor. Their observations lead to future treatment improvements.
The acid treatment report and the pressure/rate treatment charts are the best sources of information. The engineer can observe and follow the injectivity during the entire process to see whether the injectivity decreased during the treatment. Plugging or reduced injectivity during the first injection into the wellbore can be traced to solids suspended in injected fluids at the beginning of the treatment. The condition of the well, well preparation, and QC sampling can reveal the source of these plugging solids.
Nitters et al. present a systematic approach for candidate selection, damage evaluation, and treatment selection and design using a recently developed integrated software package. They recognize the importance of evaluating skin factors from well tests to determine what could be improved. After identifying damage mechanisms, they used an expert system and geochemical simulator to select appropriate treatment fluids. They also developed software for the evaluation and design of acid placement.
Strategy for acidizing
Hashem et al. produced an excellent example of a complete strategy for acidizing. Well analysis and sampling identified the damage mechanisms that were removed by the appropriate acid systems and additives that were selected using:
- Formation mineralogy
- Extensive laboratory testing with core flood studies
- Acid and additive testing
Well preparation, job supervision, and on-site monitoring played key roles in the success of the acid treatments. Treatment evaluations were performed to identify problems with some acid treatments, which led to improved additive formulations and improved spent-acid cleanup procedures. These steps resulted in an 86% success rate in treatment of water-injection wells and significantly improved water injectivity.
- Acid cleaning of tubing
- Acid type and concentration designed for the mineralogy and the permeability of the formation
- Acids, additives, and solvent flush designed for proper acid/reservoir fluid compatibility
- Properly prepared wellbore and effective acid coverage
- Sufficient time provided for acid contact and penetration of all perforations
- Precipitation prevented or flushed away from the wellbore
- 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
- Nitters, G., Roodhart, L., Jongma, H. et al. 2000. Structured Approach to Advanced Candidate Selection and Treatment Design of Stimulation Treatments. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 1–4 October. SPE-63179-MS. http://dx.doi.org/10.2118/63179-MS
- Hashem, M.K., Nasr-El-Din, H.A., and Hopkins, J.A. 1999. An Experience in Acidizing Sandstone Reservoirs: A Scientific Approach. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1999. SPE-56528-MS. http://dx.doi.org/10.2118/56528-MS
Noteworthy papers in OnePetro
Paccaloni, G. and Tambini, M. 1993. Advances in Matrix Stimulation Technology. J Pet Technol 45 (3): 256-263. SPE-20623-PA. http://dx.doi.org/10.2118/20623-PA
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