Conducting the acidizing procedure

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. 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. 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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  Fig. 1—Acid treatment with poor rate control.

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  Fig. 2—Acid treatment with good rate control.

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, Paccaloni et al., and Prouvost and Economides 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 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.

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  Fig. 3—Matrix acid pressure chart with job evaluation.

Fig. 4 shows rates, pressures (both measured and simulated), and skin-factor change during acidizing, as presented by Prouvost and Economides. 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. McLeod and Coulter 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—Skin evolution during acid job.

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  Fig. 5-Injection pressure buildup with wellbore damage.

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  Fig. 6-Injection pressure buildup with damage removed.

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  Table 1

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  Table 2

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  Table 3

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  Table 4

Hill and Zhu advanced the monitoring of acidizing treatments, building on the earlier contributions of McLeod and Coulter, Paccaloni et al., and Prouvost and Economides. 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. proposed more active treatment monitoring into standard acidizing practice.

Zhu and Hill 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. 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. This was stimulated by the work of Bansal.

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.

A comprehensive HF acidizing radial flow model was modified by Gdanski and Schuchart 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.

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