An eight-zone completion, with each interval stimulated individually, resulted in savings of 30% over conventional completion costs. The secret: thinking outside the casing.

Anew perforating and stimulation technology jointly developed by Marathon, BJ Services and the Expro Group features perforating guns run on the outside of the casing string.
The system allows selective stimulation of multiple production intervals without the need to set isolation plugs. This is accomplished through the use of integral isolation valves run below each of the perforating gun modules. It has proved beneficial to several customers, not only from a cost savings point of view (multiple zone completions can be accomplished in hours rather than days) but also from the enhanced productivity gained by more accurate placement of fracturing treatments in the formations being stimulated.
In September 2001, operators used the system to complete a well for BP in southern Alberta. BP and Marathon jointly operate the Callum 16-12 well near an area known as the Whaleback, where development plans call for several wells to be drilled directionally from pads due to the region's environmental sensitivity. Overthrust sheets in this geologically complex area have created a subsurface environment where the formation package of multiple sands can be encountered as many as three times in a single wellbore.
Because each interval of these formations must be stimulated to be commercial and because the production from the multiple zones can be commingled in a single production string, the project lends itself well to the technology. The Callum 16-12 well also is interesting due to its complexity; five control lines were run from the surface, including a methanol injection sub and a subsurface safety valve. In addition, operators directionally drilled the well, and in places the hole was in very poor condition. The completion marked the first time the system was run with a tapered string of casing.
System design
Developers designed the EXcape system to allow operators to individually perforate and stimulate many intervals in a minimum amount of time. It eliminates wireline runs to add perforations and set isolation plugs, reducing the time between individual interval frac jobs from hours or days to minutes.
The system's fundamental element is a "Y" block module that holds the firing head and perforating gun assembly (Figure 1). Each of the modules is positioned in the string across a zone to be perforated. Below each gun is an isolation valve actuated by pressure transmitted from the perforating gun. The system uses control lines to transmit pressure from the surface to fire the gun modules. The modules are prepared in advance and transported to location; the maximum length of each individual gun section is limited to 22 ft (6.7 m) to keep the entire assembly manageable on the rig.
Pressure from a control line actuates each of the perforating modules (Figure 2). The guns perforate the formation and both walls of the casing simultaneously. As the gun fires, pressure is transmitted to a hydraulic reservoir that, in turn, operates a flapper valve, effectively isolating the zone just perforated from the previously perforated interval downhole. The zone then is stimulated and the action repeated as required for each subsequent interval, moving from the bottom of the well upward. After the zones are stimulated, the breakable flapper valves are removed with wireline or coiled tubing.
Completion
The operators decided to use the EXcape system for the Callum 16-12 well because a reduction in completion costs was necessary to make the Callum wells achieve the maximum profit. Previous wells drilled in the area had employed composite bridge plugs to isolate each zone for fracture stimulation. The bridge plugs then were drilled out using coiled tubing. With the large number of intervals, this approach was costly.
In the 16-12 well, the Expro team identified only eight zones of interest after reaching total depth. The team selected the appropriate modules and began to pick up equipment. Although the modules were run in the hole in less than 8 hours, a key seat just below the surface casing required the string to be pulled back. After removing the modules, the team made a conditioning trip to total depth with drill collars and a blank EXcape module. The team added drill collars below the first module to centralize and add weight to the string in the hope that it would help maneuver through the key seat. On the next attempt, the crew ran the entire assembly to a depth of 10,500 ft (3,200 m) in 18 hours, including installation of an injection sub, safety valve and related equipment.
A gamma ray correlation log run to determine the actual position of the gun modules relative to the formations found that in more than 9,843 ft (3,000 m), calculations were off by less than 8 in. (0.2 m) across the entire length of the EXcape assembly. The equipment was cemented in hole, the rig moved off the well and the fracturing treatment scheduled.
Perforating and fracturing
Expro designed the completion such that a braided line would be cut as each gun was fired, giving a positive indication of gun detonation in addition to the acoustic method normally used. As each of the modules fired, a surface recorder would measure minute changes in resistance in the line and change those readings into a depth value. At the same time an acoustic receiver on the wellhead would send a wireless signal to a laptop in the control van for analysis of whether the gun had fired. The unit's heightened sensitivity enabled the team to monitor footsteps several hundred feet from the receiver.
The team applied pressure to the control line to fire the first gun in the system and then monitored the well for buildup. The crew recorded a slight blow as well as a change in resistance and line length from the braided wireline. The first fracture treatment was pumped away in about 45 minutes.
The team shut down the pumps and flowed back the well while the equipment and materials were prepared for the second treatment. The second interval was successfully perforated, and the second fracture treatment was pumped in about 1 hour. The team carried out the same procedure to perforate the third zone, although it did not require stimulation. The operator previously decided that only the first three of the eight zones would be completed that day, allowing the well to be evaluated. The remaining intervals were treated the following week. All five modules were fired and four zones fracture-stimulated in a single day.
Significant savings
Crews carried out the entire pumping program in less than 96 hours - a significant time reduction compared to the conventional completion technique in this area, where 35 days typically were allocated for completing a well. Based on the two previous wells BP drilled in the area, the average cost per zone was US $275,000 (C $420,000). The Callum 16-12 completion cost, on average, $190,000 ($290,000) per zone - a savings of about $511,000 ($780,000). The operator realized this through reduced wireline runs, decreased fracture equipment setup time, less time on location, lower well-testing equipment costs and an overall increase in the efficiency of the completion operations.
Risk mitigation also was a benefit. By reducing the number of times wireline, perforation guns or tools were run in the hole, the risk of problems occurring downhole decreased.
Track record
The Expro Group has completed nine wells employing the EXcape system, including more than 108 individual perforating modules. The average well depths have been 12,000 ft (3,658 m), with nine to 12 modules per well. Favorable client feedback includes reports of increased production realized in addition to significant savings in rig time. New technologies soon will eliminate the control lines from the system, saving additional time and money. In the meantime, the system has proven to be a beneficial and cost-effective method for stimulating wells with multiple reservoirs. The use of the system will be considered for other BP operations where multiple-zone completions are required.

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