Coiled tubing injector head and blowout preventers are rigged up on the wellhead, supported by a crane, while pumping equipment and frac tanks are located away from the wellhead.

Completing a well with multiple producing zones used to be a time-consuming and expensive operation. Traditionally, each zone had to be isolated and treated individually if there was to be any chance of optimal stimulation. This meant numerous trips into the well with perforating guns and temporary bridge plugs along with the associated lengthy clean-up afterwards as plugs were milled up and the well was flowed back.

Often, operators would elect to bypass a less-promising zone just to simplify the operation. Or they might try to treat two adjacent zones simultaneously to save time and money. Usually, that technique meant that neither zone received the optimum treatment but rather some sort of average treatment. In extreme cases, a high-conductivity zone would get all the treatment and the other, tighter zone would get none at all.

These thoughts were on the minds of Apache engineers as they planned the completion of one of their South Texas wells in the aptly named Gunpoint field. Previous experience in the area required two or more weeks to complete each well. The company consulted Schlumberger, which offers a wide variety of multizone completion options, each with distinct advantages for specific applications.

One technique fit the South Texas situation best. The AbrasiFRAC stimulation service allows the operator to perforate and treat each zone individually in a single rigless field operation (Figure 1).

How it worked

Elegant in its simplicity, the technique was nonetheless effective and highly efficient. A CT unit was rigged up on the well and conducted all facets of the operation (Figure 2). First, a permanent bridge plug was set below the lowermost zone to be used as a depth reference, and the first zone was conventionally perforated. The coil was then run into the well and marked, followed by the well being fracture-stimulated in the first zone. A specially designed hydraulic jetting nozzle, which was attached to the end of the CT, was then placed across the next zone to be completed.

Pumping an abrasive slurry consisting of frac sand, a high-velocity jet was formed at the tip of the tool that quickly perforated the casing and cement sheath and initiated a perforation tunnel into the formation. The second fracture treatment was then pumped down the CT/casing annulus. This process was again repeated for a third fracture treatment. In all, more than 500,000 lbs. of 20/40 mesh proppant was pumped. This technique also enabled a direct measurement of bottomhole pressure, which improved the post-job evaluation compared to what was possible with surface pressure measurements.

Since the abrasive slurry is granular, it does not damage the formation in the region of the perforation tunnel. As soon as the jetting phase is complete, a high-rate treatment is pumped down the CT/casing annulus to create the fracture. Traditional real-time treatment monitoring is provided in the surface control cabin so the designed pumping schedule can be implemented and any necessary changes made on the spot. When implemented in conjunction with Schlumberger live downhole CT services, critical formation measurements can be made before, during, and after treatment to help ensure maximum performance. These measurements are useful in various ways; for example, distributed temperature survey measurements offer a qualitative guide to fluid distribution and monitoring of the interval flow profile before and just after the stimulation treatment.

The treatment slurry can be custom-designed to suit the formation and can contain any proppant, additives, or diverters deemed appropriate to ensure the fracture is completed as planned. The last step is to pump a sand plug that is tailed-in behind the treatment to isolate the zone. If the sand plug sets too high, the CT jet can be used to quickly wash down the top using reverse circulation to adjust plug-back depth to the desired level. All of these steps are accomplished on the same CT trip into the well.

After the sand plug has been placed, the jet gun is positioned opposite the next zone to be treated, and the operating sequence is repeated. This can be done for as many zones as desired without having to trip the CT out of the well. The final step is to wash out the sand plugs and circulate them out of the hole. The well can be flowed back and cleaned up while accurate flow and pressure readings are taken to enable evaluation of the efficacy of the treatment.

Results tell the tale

In the South Texas well, Apache saved an estimated US $750,000 in completion costs. This figure reflects time saved as well as spread costs saved by not having to mobilize additional service units to accomplish some tasks. The CT unit, crane truck, and associated stimulation spread are capable of performing all steps of the completion operation. Many CT units have an integral crane, further reducing the number of vehicles required on location. Most importantly, the well performance was equal to that of offset wells in the area that were treated using traditional techniques that required an average two weeks to complete.

This stimulation technique is being considered for treating additional multizone wells in the Gunpoint field due to the considerable cost savings realized by the company.

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