A typical 3-zone completion assembly is latched onto the sump packer once on depth.
For a number of years, the search for an efficient and cost-effective method of completing multi-layered or very thick reservoirs that require sand control has been a high priority for offshore operators seeking to maximize completion efficiency and reduce rig time. Recently, Baker Oil Tools' Mini-Beta One-Trip Multi-Zone Completion System was credited with helping China's Bohai Oil Corporation reduce completion time per well by up to 66%, and total development cost by more than 60% in the JZ93 Bohai Bay field.
This success and others achieved with the new system are the result of a technological evolution that has been ongoing for almost two decades. The new system holds promise for deepwater and other offshore operations, based on its ability to optimize technology vessel logistics.
By eliminating the need to run completion equipment between each pumping operation, this system allows pumping operations for all of the zones of a multi-zone completion to be carried out without standby time. This, in turn, allows the entire operation to be completed with a single mobilization of equipment and eliminates the problem of trying to schedule mobilization of pumping equipment for each zone of the completion.
Early one-trip systems
The first one-trip multi-zone gravel pack system was used in the Beta Field development offshore California in the early 1980s. Although the system introduced for that application provided some of the desired functionality, it was not applicable for all reservoirs.
The original one-trip multi-zone gravel pack system was twice modified in subsequent years - once for a specific shallow application and once for use in wells in the Green Canyon area of the Gulf of Mexico. All of these systems however, had limitations that hindered their acceptance for use as general-purpose gravel pack systems. The foremost limitations of these systems were:
- Equipment design and service tool space-out problems made it difficult to isolate upper zones while gravel packing lower zones.
- Zones that were located close together could not be treated because overall assembly length dictated that the minimum distance between zones be at least 45 ft.
- Small inner string diameters limited treatment rates possible through the equipment and precluded fracturing.
The 1990s witnessed a rapid increase in the number of gravel pack completions pumped above fracturing pressures in unconsolidated sands. Using tip screen-out frac designs, it was no longer unusual to have 100,000 lb or more of gravel placed outside the casing of a single gravel pack zone. Tip screen-out fracturing drove the development of gravel pack equipment cap-able of operating with higher pump rates, higher pressures, and ever-increasing vol- umes of sand. These criteria placed even great-er restrictions on the use of one-trip multi-zone gravel pack systems.
Ironically, during this same time, the concept of completing three or more zones within the same wellbore flourished as a means of keeping costs down and maximizing hydrocarbon recovery. These zones were sometimes produced separately and sometimes commingled. The process implemented for these multi-zone completions became known as stack-pack completions.
In a stack-pack, completion operations for each zone are usually carried out sequentially. The lowest zone is perforated, completed, and isolated. Then the operations are repeated for the next higher zone, and so forth. Stack-pack completions can become quite time consuming because they require a number of different trips into the wellbore.
To remove limitations of previous one-trip multi-zone gravel pack systems and offer an efficient alternative to stack packing, a new system was developed. Design criteria addressed the rate, pressure, and spacing problems that had been encountered previously to expand the range of applicable completions that could be delivered. The new system can be used with multiple zones of varying length and with gravel placement treatments that can be pumped either above or below fracture pressures with large volumes of sand. It also allows for treatments to be pumped at higher rates usually associated with fracturing.
Earlier systems featured a relatively small concentric string of tubing that ext-ended from the crossover port to the surface. The crossover port is the portion of the service tool string that allows a sand slurry to be carried down the tubing string and redirected into the casing/screen an-nulus below the gravel pack packer. The function of the "extension string" was to carry sand down to the crossover port and still allow returns back to the surface.
In the first modification of the system, this "extension string" was eliminated by placing a tubing swivel at the top of the service string just above the gravel pack packer. In the new system, the tubing swivel at the top of the service tool string has been modified to allow the concentric string to have a larger inner diameter, thus reducing friction pressures at higher pump rates.
A more robust crossover port in the new system accommodates the higher pump rates and sand slurry concentrations. Guidelines for the expected life of the crossover port were generated with a combination of laboratory testing and mathematical simulation of erosional rates. They are used during operations to monitor the amount of sand that can be placed through a crossover port before the service tools must be pulled from the well and replaced.
A third design aspect to optimize operation of the system was the addition of a seal bore directly below the isolation packers. This seal bore allows the isolation packers to be set without manipulating any equipment in the gravel pack extension. The crossover port is located across from the isolation packer. This places a seal both above and below the packer. The packer is then set with applied pressure.
In the new system, the overall length of the assembly around the isolation packers has been shortened so that the minimum distance between zones is now 28 ft, compared to 45 ft in previous systems. This modification, which was accomplished by eliminating the squeeze position from the tool design, extends the benefits of one-trip multi-zone completions to layered reservoirs with intervals of varying lengths.
Finally, in the new design, the gravel pack is performed with the tool in the circulating position, which is located mechanically. If it becomes necessary to squeeze fluids into the formation, the annulus is closed to eliminate fluid returns.
After the well has been perforated and a gauge ring run through the perforations, a sump packer is set on depth, either with wireline or on drill pipe. The entire multi-zone assembly is then picked up and run into the well. The assembly is latched into the sump packer once it is on depth.
The top gravel pack packer is then set hydraulically. After testing the top packer, the service tools are released from the assembly, moved uphole, and positioned mechanically (with weight indication), where each isolation packer is set hydraulically and tested.
Next, the service tools are lowered to the lowest zone, positioned mechanically, and the gravel placement treatment is pumped for that zone, either below or above fracture pressure. Since there is not a squeeze position, the treatment is pumped in circulate position with the annulus closed. As a result, annulus pressure can be monitored throug-hout the entire treatment.
As soon as a sand screen-out has occurred, the service tools are positioned in reverse and the excess sand is reverse-circulated from the wellbore. This places the crossover port and an evacuation port between a single set of seals and allows the excess sand slurry to be reversed from the tubing string without placing any pressure on zones either below or above the zone that is being treated.
With each zone totally isolated from all other zones, it is possible to pump each zone either above or below fracture pressure and rate, without consideration of the other zones in the well. This means that any combination of treatments can be used in the completion of the multiple zones.
Among completions performed in China with this system, it is typical to pump one or two zones as a circulating gravel pack with the remaining two or three zones pumped as a frac pack. Once all zones have been treated, the service tools are pulled from the well, the upper completion string is run, and the well is placed on production.
Individual, zonal isolation allows for a combination of treatment alternatives above, or below fracture pressures with no effect on adjacent zones.
Several reservoir conditions must be satisfied before the one-trip multi-zone gravel pack system can be used. First, to perforate all the different production intervals at the same time, each zone should be of similar pore pressure so as to minimize the chance of cross-flow between zones prior to their mechanical isolation with the completion equipment. Next, the 28-ft length of the tool assembly dictates that the distance between adjacent zones (the distance from the top perforation of a lower zone to the bottom perforation of the next higher zone) be 28 ft or greater.
Another consideration is the pressure differences between adjacent zones throughout the life of the completion. These differences are limited by the pressure differential across the isolation packers placed between zones. With an isolation packer set in 9-5/8-in. casing, the maximum pressure between zones is limited to 5,000 psi during the life of the completion. With a 7-in. or a 7-5/8-in. completion, this pressure difference increases to 10,000 psi.
Experience reducing time
Within the past 18 months, 270 zones in 81 wells have been completed using the most recently designed one-trip multi-zone gravel pack system These completions have been equally split between treatments pumped above fracture pressure and those pumped below fracture pressure.
A comparison of the one-trip system to the more standard stack-pack type of completion emphasizes the efficiency that can be achieved from running all of the equipment at one time. Previous experience has shown that stack-pack completions typically require 2.0 to 2.5 days per zone. To date, this completion system has shown an average time requirement of 23 hours per zone for either a 3- or a 4-zone completion.
In the recent Bohai Bay project, Bohai Oil Corp. was able to individually frac pack or gravel pack four zones in as few as 2.4 days, compared to 7.77 days previously required for a four-zone stack-pack completion. Even with the relatively inexpensive costs in the Bohai Bay area (rig time valued at an average of $40,000 per day), the operator estimates that use of the system resulted in savings of more than $2.5 million U.S. in rig time.