Depending on well depth, tripping time can account for a significant amount of non-productive time (NPT) and make a major impact on overall completion cost. Minimizing NPT is critical to economic success. A new, multizone, single-trip gravel- and frac-packing system has changed the cost-benefit equation for completing deep and deepwater wells with multiple, varied production zones.

Producing many zones
A reservoir with a large gross pay zone and numerous stress layers is difficult to complete and stimulate effectively as a single interval. Frac packs, in particular, require careful planning to avoid over-stimulating (fracture breaches water zones) or under-stimulating (potentially productive area has poor wellbore-to-reservoir connectivity). But even ordinary gravel packs can be difficult and expensive to achieve in multi-layer formations.

Large contrasts in formation properties exacerbate the problem. For example, a

Figure 1. Sand control screens, sliding sleeves and isolation packers are the foundation of the ComPlete MST (multizone, single-trip) completion system. (Images courtesy of BJ Services)
high-permeability layer will preferentially accept completion/stimulation fluids and gravels compared with a lower-permeability layer. Mathematical modeling can provide some assistance in designing perforation and treatment diversion designs to improve the likelihood of successfully treating many formation layers, but the best method is to isolate each zone during treating operations. In some formations with highly variable layers and extremely long pay zones, multizone treatment with zonal isolation is essential.

This is traditionally done by perforating, stimulating and isolating each zone individually. This is a time-consuming process, requiring many trips and therefore much time and expense. Thus, the expected production must be able to far exceed the cost of these many trips if the well is to meet its economics.

In extremely deep water, like the ultradeep Lower Tertiary play in the Gulf of Mexico, daily rig costs are higher and tripping times are longer because of the extreme water and well depths. To meet economic targets, operators need either exceptionally prolific reservoirs or new technologies that can safely and reliably enable completions, sand control and stimulation in fewer trips, substantially improving development economics.

Minimizing NPT
To accomplish this, the new ComPlete MST (multizone, single-trip) system facilitates gravel- or frac-packed completions across multiple production intervals in a single trip. Its design allows complete flexibility in sand placement techniques, including positive, selective isolation of all zones during completion, stimulation and production operations. An operator can even elect to complete some zones and return later to complete others without a rig (Figure 1).

The result is an effective reduction in completion cycle time and cost by 25 to 60%, depending on the number of zones the operator chooses to complete. Eliminating one-third of a typical 12-day completion schedule on a multizone deepwater well can save an operator as much as US $1 million on the total operation cost, even with a conservative $250,000 day-rate for the rig. (Deepwater and ultra-deepwater rig rates are approaching two to three times that figure now, making the savings even more dramatic.) The savings makes it economical for an operator to selectively and optimally stimulate additional marginal pay zones, increasing overall field recovery.

The system eliminates a number of operational steps compared with traditional multizone frac-pack and gravel-pack completions:
• Multiple perforating trips; packer plug retrieval runs;
• Multiple gravel pack assembly runs; and
• Stimulation trip runs on workstring.
Benefits to the operator include:
• Individual zonal isolation while treating;
• Zonal isolation after treating;
• Selective access for production;
• Optimal flow through large bore;
• Support for large-bore service tools for optimal treating rates; and
• Quicker realization of production initiation.

Case histories
To date, the system has been proven in three two-zone wells and one four-zone well offshore Indonesia and in the Gulf of Mexico. Planning is underway for 23 four-zone wells offshore Indonesia, plus several more in the Indian Ocean and Gulf of Mexico over the next 10 to 20 months.

In the first commercial application of the MST system in April 2007, the system was run in the Banuwati field offshore Indonesia (Figure 2). The well comprised two zones around 7,000 ft (2,135 m) depth. The operator chose the system because of its selective isolation and production capabilities and its ability to significantly reduce completion cycle time.

In the operation, both zones were perforated, and the MST bottomhole assembly was run into the hole to provide selective stimulation treatment of both zones. The versatility of the
Figure 2. In the first commercial use offshore Indonesia, the multizone single-trip system operated as designed, including multiple manipulations of the autolocator, sleeves and valves.
system was further demonstrated when pre-stimulation testing on the upper zone indicated a total change in treatment design was necessary. The decision was made to temporarily suspend operations on the well until further evaluation of the pretreatment data could be performed. The tool string was successfully retrieved from the well, leaving both zones completely isolated from the well bore. A week later, the service tool was again run into the well bore to complete the upper zone with a stimulation treatment designed to optimize the reservoir production. Production was commingled from both zones and exceeded the operator’s expectation.

Although the operation was not completed in a single trip, the system operated as designed. Therefore, the operator used the system for a two-zone, high-rate water pack in a nearby well in May. This time, the well did not provide any surprises, and the completion was successful in a single trip.

Between the two wells, the system saved the operator more than four days of rig time compared to the expected duration of a normal dual completion:
• Single trip for tubing-conveyed perforating (TCP) (saved 24 hrs per well);
• Single trip for packer plug retrieval (saved 12 hrs per well); and
• Single trip running in and locating the frac pack assembly (saved eight hrs per well).
In July, the system was run into a well in the Bekapai field offshore Indonesia. For this well, high-rate water packs were completed on four zones of interest in 35 hours of rig time, with about 3,000 lb of 20/40 gravel placed in each zone.

In the first Gulf of Mexico application later in July, perforation guns were run below the MST tool in an effort to further decrease NPT. The operation was successful but required two runs: After perforating, well conditions dictated that the bottomhole assembly be retrieved from the well and a deburring run be made across the perforated intervals. The system was redressed and the well successfully completed. After the system was installed, it performed as anticipated. One zone was frac packed with 21,000 lb and one with 25,000 lb of gravel as planned. Well production also met operator expectations.

Tool options
The system is currently available for use in 7- through 10 3/4-in. casing sizes with no restrictions on zone length or zone spacing. The number of zones to be treated in a single trip is limited only by the erosional resistance of the service tool. For example, the limit on the 7 5/8-in. system exceeds 1 million lbs of proppant pumped at 35-bbl/min treatment rates.
Each zone contains isolated gravel-pack screens with integral production sliding sleeves, a frac pack/gravel pack sleeve for placing sand (gravel) or proppant, and an isolation packer.
Individual zones are produced through selective operation of the production sleeves incorporated in the well screen design. Up to five internal profiles are available with the current production sleeves to provide zonal selectivity for production.

Additional applications for the system include non-sand control multizone stimulation treatments and openhole multizone applications.