A new perforation drilling tool system has been introduced by Perf-Drill, Inc. of Shreveport, La. Using a unique patented articulated drill string, the device mills a casing window, then drills clean, 23/8-in. diameter perforation tunnels as much as 10 ft (3 m) out from the well bore at an angle of 30? from the casing axis (Fig. 1). The versatile tool can operate in any fluid, at any depth, and can be driven by a power swivel, topdrive or mud motor. It can even be conveyed and operated on coiled tubing (CT). Any number of perforations can be made on a single trip into the hole.

Presently, the tool is designed to operate in 41/2-in. and 51/2-in. casing, but the design is scalable, according to the company. The objective is to open a very large surface area for production or injection. This allows high conductivity flow with low drawdown or injection pressure. In its present configuration, the Perf-Drill tool can drill a 10-ft long, 23/8-in. diameter hole. This provides 895 sq in. of open perforation tunnel surface area, with only the portion passing through the cement sheath impermeable. Maximum radial penetration of 10sin30 equals 5 ft (1.5 m), well beyond typical damaged zones (Fig. 2).

Operation

First the tool is run to target depth, oriented and anchored. Depth control is achieved by running a casing collar locator. Driving a diamond mill/bit combination, the tool cuts a smooth window in 41/2-in., 10.5-lb, J55 casing in as little as 15 minutes (Fig 3). Completion of formation drilling is accomplished in about 45 minutes, depending on formation competence. Full circulation through the bit is provided to clean out the hole of cuttings. The casing cuttings produced by the diamond bit are small and granular in appearance, and are easily circulated out during drilling.

How it works

The bottomhole assembly consists of an anchor section, the 30-ft (9-m) long tool body, a 12-ft (3-m) upper drive shaft, and drill collars to provide weight-on-bit. This is connected to the work string, and three non-rotating centralizers are run on the bottom of the tubing. At surface, the Perf-Drill power feed system (called the Slow Hand) is attached to the top of the work string, followed by the power swivel. For CT deployment, the hydraulic injector head substitutes for the power feed system, and a downhole mud motor is inserted between the bottom of the CT string and the top of the Perf-Drill tool. The key to the system is its articulated drive shaft and goose-neck guide shoe (Fig 4). The gooseneck guides the drilling assembly from its initial position inside the tool body around a 30? bend and aligns it ready to exit the tool body and begin milling and drilling operations. The flexible drive shaft is constructed of interlocking elements that can flex axially while transmitting torque to the bit (Fig. 5). This allows the shaft to snake through the gooseneck guide shoe and be launched at its 30? angle as it exits the tool body. Once launched, the shaft elements lock into a straight alignment. Typical rotation speed is 125 rpm, but the Perf-Drill has been tested through a range from 80 rpm through 500 rpm. After the perforation tunnel is drilled, the shaft flexes as it is withdrawn back into the tool body. The anchor is unseated and the tool is ready to be moved to the next perforation level, if any. It is usual practice to make a slow rpm clean-out run of the perforation tunnel after drilling and before the drill shaft is completely withdrawn from the hole.

How it compares

Oil company research cites drawbacks often experienced by competing techniques. Jet perforating, by far the most common method, can produce perforation tunnels measuring from 1/4 in. to 1/2 in. in diameter. For example, a typical 1/2-in. x 36-in. perforation tunnel exposes a maximum of 56 sq in. of formation surface area. Tunnel length is a function of charge design and quality as well as formation compressive strength and composition. The problem is that jet perforators often create a crushed zone completely surrounding the perforation tunnel that can reduce local permeability by as much as 80%.

In naturally fractured formations it is particularly desirable to orient the perforation so that it penetrates as many fractures as possible, because the fractures serve as the conduits through which production flows to the well. Hydraulic fracturing has also been problematic, particularly if the perforation tunnels are misaligned with respect to the regional stress plane. In these cases, dramatic reduction in fracture efficiency and conductivity has occurred. Orienting perfs to align with natural fracture patterns or tectonic stressfields is non-trivial, and can be costly.

Production is significantly increased in sand control completions if a large perforation is made. This large hole improves the placement of sand in the perforation tunnels and helps reduce the pressure drop through each individual perforation tunnel for improved flow assurance and well integrity. This technique can be applied to Perf-Drill tunnels as well in cases where gravel-packing is recommended.

Applications

The Perf-Drill system can produce large surface area oriented perforation tunnels that can be used for production of gas or oil wells, for injection or for disposal of unwanted fluids or cuttings slurry. Operation is straightforward using a typical drilling or workover rig, a well-servicing unit or a coiled tubing unit. It can be deployed on land or offshore. If desired, the perforations can be stimulated, either by hydraulic fracturing or by matrix treatments using conventional methods.

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