TESTS REVEAL PERFORATING CHARGE PERFORMANCE

Manual Jimenez Jr., Kem E. McCready, Russell E. Ott, Clarence W. White
Mobil Exploration & Production U.S. Inc.
Midland, Tex.

A series of independent tests, conducted by Mobil Exploration & Production U.S. Inc., has revealed that quality control by manufacturers of perforating gun charges can be relied upon during this interim period when API testing standards are undergoing changes.

At present, the performance of the charges is difficult to compare among manufacturers because of the lack of standardized testing procedures.

Standardized targets and methods will help decide which shaped charge to use for a specific perforation situation. Without such data, it is difficult to address field charge quality, an issue extensively publicized in the past.1

Unfortunately, the manufacturers have been reluctant over the past 2 or 3 years to initiate expensive testing programs. During this period, the API has been in the process of updating earlier API RP 43 Edition 4 procedures to the Edition 5 version, issued only in January 1991.

In effect, a moratorium on testing and issuing relevant data has been in place over this period of time, virtually up to the present.

Consequently, the industry has had to base decisions on the older and incomplete Edition 4 data-or production-line quality control (QC) data available from the manufacturers themselves.

However, in the case of the latter, no uniform standards exist and therefore no basis has been established for making meaningful comparisons.

API currently provides neither standards nor guidelines in the QC area. To alleviate this problem for the Mobil Midland division, an extensive perforating charge evaluation program was initiated.

The 4-in. guns/charges most commonly used in our completions were randomly selected from the field magazines.

These were tested in a common, controlled Berea target as well as in the manufacturer's QC target.

Results showed, in general, that:

A larger range in performance existed between 4-in. gun charges than was expected (about a 50% range); thus, some charges performed significantly poorer than expected.
The better performing charges behaved similarly, all performing in the upper 15% of the range.
Quality control of charge performance is being conducted by virtually all manufacturers, on an on-going basis, with traceability provided and records being maintained for client review as required.
Most charges tested performed in accordance with the manufacturer's own QC requirements. Results could be repeated reasonably well on retest (of field charges), in turn agreeing well with test data in the standardized, controlled Berea sandstone target.
Although little APT data have been issued recently, charge performance has continued to be controlled to the manufacturers' respective specified levels.
However, QC data alone cannot yet be used to randomly compare one manufacturer's charge to another due to differences in QC specifications among manufacturers-principally in targets utilized and the variable practice of setting minimum performance standards.

BACKGROUND

Serious questions of whether field charges perform in accordance with APT published data have been raised beginning in the early 1980s. In 1982, Schlumberger placed an advertisement in Oil & Gas Journal that presented test data suggesting that some commercial charges performed substantially below published APT data.

In 1984, an operator group administered by Exxon examined the allegations, confirming that some charges performed more than 40% below expectation. On retest in 1985, results were seen to improve, suggesting that more attention was being paid to quality control in manufacturing.

In 1987, Mobil devised a practical method of monitoring performance of field charges on an on-going basis, using production line QC data and emphasizing its relationship to the APT published values. This approach was adopted in 1988 by an industry performance monitoring group, the Program to Evaluate Performance of Perforating Equipment (Peppe). The group continued to encourage the manufacturing segment to maintain QC procedures and records on the charges that would relate to the APT published data.

More recently in 1990, another industry group, the Program to Evaluate Gun Systems (PEGS), has expanded the concept to include the entire gun system, with emphasis on performance at elevated temperatures and pressures, and over extended periods of time (longer aging periods).

AR of the foregoing have been aimed at aiding the perforating equipment manufacturer in developing and applying procedures for assuring better quality of equipment in the field. However, over this time frame, the manufacturers have been hampered by lack of a definitive industry standard for comparison.

The old API RP 43 Edition 4 perforating procedures (revised most recently in 1985) were considered outmoded and in need of substantial revisions. Only recently has the API committee concluded and published the new API RP 43 Edition 5 (January 1991).

That situation discouraged manufacturers from initiating expensive API testing and has resulted in today's paucity of data available in the field.

TEST PROGRAM

With this background, Mobil felt that an independent testing program was justified to meet ongoing business needs. Specifically, a test program was designed to:

Provide current comparative gun/charge performance data for field use pending issuance of new API data under Edition 5 procedures.
Confirm that quality control procedures encouraged by Peppe and PEGS were being adhered to during the interim period of changeover from API Edition 4 to Edition 5 procedures.
To ascertain that charges in the field were performing in accordance with the manufacturer's QC standards.
To identify the limitations concerning utility of QC data in comparing charge performance from one manufacturer to another.
The methods and procedures used followed those described by Sukup, et al.1
Production charges of interest were randomly selected from field magazines during unannounced visits by the authors.
Charge containers were identified as to production lot and date of manufacture, sealed, and shipped back to the manufacturer's facility for the tests.
The authors visited the manufacturer's plant and opened the sealed boxes; checked the QC records corresponding to the specific charges selected; and observed charge loading, test setup, shooting, and recovery of data.

To obtain reasonably accurate comparative data, it was essential that all test variables be controlled as closely as possible between different manufacturers. For instance, it was very important that a suitable target controlled as to physical properties and tested under common simulated gun and fluid/clearance conditions, be used for all tests.1

A test setup was adopted that represented one of the more common QC configurations utilized by the manufacturers, while maintaining specifications as close to API Edition 5 as practical.

For example, a 20.1% porosity Berea target was selected, midrange in terms of the API Edition 5 spec.

Multiple samples were taken from the same quarry stone and slab within the block to assure maximum consistency.

All targets were water-saturated for a minimum of 72 hr prior to testing.

The test configuration used is shown in Fig. 1.

Charges were fired from an actual gun section, using field mounting and alignment devices and port plugs.

Gun-to-target distance was controlled at 0.5-in. water clearance at ambient pressure and temperature conditions.

Up to six shots were individually fired and results averaged.

To ascertain whether charges performed according to the manufacturer's own QC requirements, tests were also conducted into their QC targets following their specifications and procedures.

Target configurations were very similar to that of Fig. 1, except that one or more of the following materials were employed:

Berea, at varying porosity specifications
Concrete, similar or identical to API Section 1, typically cured for 30 days
A special synthetic sandstone formulation.

RESULTS

Tables 1 and 2 provide average values of penetration and entrance hole diameters recorded and percentage differences, respectively.

Although all charges tested were of the same gun size (4-in. OD) and type (port plug), penetration performance in the controlled Berea ranged widely from 12.7 to 21.5 in., or 50% about the average of 18.0 in.

Similarly, entrance hole diameters varied from 0.33 to 0.53 in., with a range of 43% about the average of 0.46 in.

Two thirds of all charges performed within the upper 15% of the penetration range (Charges 1-5).

Average performance in the group was 20.0-in. penetration and 0.47-in. entrance hole size.

Significantly, the manufacturers' original QC values were in good agreement with their retest data in their own QC targets.

These in turn agreed reasonably well with data generated in the control Berea target, with a couple of exceptions.

However, minimum running averages (MRAs), those values below which performance becomes unacceptable and the production line is shut down to make corrections, were in over half the cases seen to be 15% or more lower than the observed original QC data.

This suggests that some companies tolerate a much wider range of performance variation than others, i.e., exercise looser production quality control.

For example, Charge No. 5 could have penetrated 23% less than the 18.2 in. recorded and still have been acceptable according to the manufacturer's QC requirement.

Of note for Charge No. 5, results obtained in the controlled Berea target were significantly poorer than either retest or original manufacturer's QC data. The reasons for this discrepancy remain undetermined.

Charge No. 6 seemed to be suffering from a "shelf life" problem that has since been corrected.

The poorest performing charge (No. 8) penetrated little more than half the distance recorded for the better charges.

However, the performance coincided closely to what one would expect from the QC specs.

In other words, the vendor did not represent it as a "barn burner," just an inexpensive "hole-in-the-pipe" product.

OTHER OBSERVATIONS

The comments we've made are based on a level of accuracy in the tests ranging between 10 and 15%, based on criteria as outlined by Sukup, et al.1

Data shown in the controlled Berea target are expected to relate to the new API RP 43 Edition 5 data to within about the same accuracy, except for Section 2 which should be about 20-25% lower because of differences in target stress and size.2

While one might prefer to see in place a uniform set of QC standards, these data suggest that present practices are working. Tighter target property controls would be required to improve reproducibility, but may not be considered cost effective.

The industry as a whole has made significant strides in implementing and maintaining QC procedures on perforating charges.

ACKNOWLEDGMENTS

Thanks are extended to to W. T. Bell, consultant, for his assistance in the design and conduct of the program, and to the service and manufacturing companies who participated in this testing, including Goex, Jet Research Center, Owen Tool, Schlumberger, Shaped Charge Speciality, Tolson Jet, and Western Atlas.

REFERENCES

Sukup, R.A., Ott, R.E., Robson, M.K., and Bell, W.T., "Technical Aspects of Controlling Field Performance of the Shaped Charge Perforator," Paper No. SPE 17172, Formation Damage Control Symposium, Bakersfield, Calif., Feb., 8-9, 1988.
Halleck, P.M., Saucier, R.J., Behrmann, L.A. and Ahrens, T., "Reduction in Jet Perforator Penetration in Rock Under Stress," Paper No. SPE 18245, 63rd Annual Technical Conference and Exhibition, Houston, Oct. 2-5, 1988.