SELECTIVELY FIRED, TUBING-CONVEYED PERFORATING GUNS SAVE RIG TIME

Phillip M. Snider
Marathon Oil Co.
Houston

Patrick L. Bond
Halliburton Energy Services
Dallas

Selective firing of tubing-conveyed perforating (TCP) guns during drill stem tests (DSTs) added flexibility and saved costs for Marathon Oil Co.

As an example, in the Garland field in Wyoming, the guns allowed perforating multiple zones in one trip. This saved 1 1/2-2 days/well in rig time and $23,000-30,000/well in electric wire line and DST tool charges. For international offshore operations, savings of $200,000/well appear possible.

Savings result not only from perforating multiple zones, but also from arbitrarily setting firing patterns with or without zone isolation.

The TCP guns are fired by applying a predetermined annulus pressure to actuate a delayed firing head. Then, pressure is bled off before the guns detonate. Next, gun detonation opens a chamber that allows the next firing head to "see" pressure. The process is repeated until all guns fire.

The firing order must be established and configuration set before running. Fig. 1 illustrates the operation of the selectively fired TCP guns.

Operational steps eliminated by the system include:

Killing the well
Pulling the test string and perforating guns
Rerunning the equipment
Correlating to depth
Re-establishing flow.

Halliburton Energy Services developed the system in response to Marathon's desire to save rig time by reducing gun trip time.

TESTING

A well, Utah Southern No. 67 (Fig. 2), in the Garland field was selected to test the equipment because Garland is a mature field with relatively tight, shallow, and layered pay intervals.

Nine zones, 119 ft, were perforated within a 400 ft overall interval at a depth of 4,000 ft. The pay zones were grouped together and selectively fired to evaluate the merits of the system.

To meet Marathon's test objectives, the perforating gun configuration was designed to actuate the firing with annulus pressure, fire top-down and nonsequentially, and provide no zone isolation.

To test the limits of the system, this configuration allowed shooting five zones nonsequentially with minimum shear pins in the firing head.

The nine separate pay zones were grouped into five intervals, totaling 119 ft of perforations (Table 1). Shot densities varied from 1 to 8 shots/ft, depending upon a zone's potential.

The assembly was loaded and spaced out so that all of the zones would be on depth when the packer was set. To align the control line away from the charges, gun loading required special attention to the phasing of the shots. A perforated control line would prevent firing the remaining guns and thereby abort the job.

The firing heads were pinned for a nominal surface actuating pressure of 1,988 psi. A bundle carrier and electronic gauges were run above the packer. Four gauges were run, two reading internal (tubing) pressure and two reading external (annulus) pressure.

Test personnel attended procedure and safety meetings before testing began. The Garland field has about 7% H2S in the gas and 195 ppm H2S in the oil.

A summary of the preparation for firing is as follows:

Verify depth of bottom hole assembly.
Swab to 2,000 ft to achieve an underbalanced condition (more pressure in the formation than in the pipe), and prove pressure integrity of tubing and packer.
Rig up surface shot detection equipment and surface pressure gauges, and pressure annulus to 1,000 psi to check for surface leaks and gauge accuracy.
Increase annulus pressure to preselected actuating pressure (2,230 psi) to actuate firing head.
Bleed annulus pressure to 320 psi, and shut down all nonessential equipment to eliminate background noise.

After about 7 min, the gun report was heard and recorded at the surface. Annulus pressure dropped about 100 psi, indicating that the select-fire system had been breached and the air chamber below it had filled with fluid from the annulus.

The annulus was repressured to 350 psi and held for 10 min. Because the system was shock-loaded during the first detonation, testers waited for reports from late-firing guns. No subsequent detonations were detected after this delay period. The intervals were then swab tested.

Subsequent detonations followed similar procedures.

INDIVIDUAL TOOLS

One criterion for designing the guns was to use as much existing, standard equipment as possible. This design reduced the lead time needed for selective fire jobs at remote international locations.

The following describes a typical system.

GUNS

Standard TCP guns are used in conjunction with an existing gun connector, or tandem. The connector can accommodate control-line clamps and centralizer buttons designed to prevent the control line from being damaged.

All other components of the guns are standard equipment.

FIRING HEADS

The system uses standard 2 1/2-in. time-delayed firing heads (TDFs) that are pressure actuated and provide a time delay before detonation.

This time delay allows actuating pressure to be relieved before the gun detonates, thereby protecting the next firing head in the select-fire assembly from seeing any excessive pressure before actuation.

The TDF remains sealed after detonation. This firing head has an extensive track record and is readily available at field locations.

CROSSOVER ASSEMBLY

The annulus-pressure crossover assembly transfers annulus pressure below the packer to actuate the firing heads.

This assembly is field proven and can be adapted to most retrievable packers.

PRESSURE ISOLATION SUB

The pressure isolation sub was originally built to snub TCP guns out of the hole. The sub allows the explosive train to be transferred below the gun to the select-fire sub, while providing pressure integrity.

Maintaining pressure integrity is important to confine the pressure and force it to act on the next firing head as opposed to relieving pressure through the newly perforated gun.

The tool has been thoroughly lab tested and field proven.

CONTROL LINE

The configuration for firing in a commingled zone requires a control line to maintain a closed pressure system.

On jobs where zone isolation is desired (i.e., the packer is moved after each detonation, the closed pressure system is not required; therefore, the control line can be eliminated.

A control-line sub provides a connection for the control line to enter or exit the assembly as needed.

SELECT-FIRE SUB

The select-fire sub is the only tool unique to the system. The sub provides a relatively simple means of isolating firing heads from pressure until the heads are ready to be fired.

To keep the applied pressure requirements constant, all of the firing heads can be pinned at the same actuating pressure.

The sub uses an explosive charge to breach a "T" and allow pressure to reach the next firing head.