Completion system offers multizone stimulation in a single trip

A completion technology that promises economic and efficiency gains over conventional multiple-trip completion and stimulation is receiving a great deal of attention, onshore and off. In numerous field trials, BJ Services' ComPlete MST (multizone, single-trip) completion system has eliminated several operational steps compared with traditional multizone frac-pack/gravel-pack completions, providing at least a 20 to 40% reduction in completion costs and nonproductive time (NPT).

"The MST is part of BJ's ComPlete line of tools, all designed to help a client save rig time and money by minimizing the number of trips in and out of a hole," explained David Walker, Director of Completion Tools for BJ Services. "One tool replaces several different pieces of equipment, and the same ultimate goal is accomplished.

"The MST is used for single-trip perforating and gravel packing in many kinds of wells, including horizontal wells," Walker continued. "We designed the system to accomplish two purposes: to gravel pack multiple zones and to isolate the zones such that they could be selectively produced. The tool was aimed for those reservoirs with large gross-pay zones and numerous stress layers, which often present completion and stimulation challenges that single intervals do not."

Conventional gravel packs are often difficult and costly to perform in multilayer formations. Frac packs must be carefully planned and executed to avoid overstimulation situations, where the fracture breaches water zones, or understimulation, in which potential productive areas have poor reservoir-to-wellbore connectivity.

These stimulation challenges increase in reservoirs with large variations in formation properties. While mathematical modeling might provide some assistance in designing perforation or treatment scenarios that can circumvent these problems, the most effective method is to isolate and complete or stimulate each zone separately, according to Walker.

"Zonal isolation has been a major focus for us, historically. Ensuring that you only open a reservoir to treat it and then close it up again until you are ready to produce is good for several reasons: it prevents you from inadvertently putting damaging fluids in the formation, it prevents loss of fluids into the formation, and it keeps the well under control so that it doesn't surge in on you when you are busy doing something else."

Traditional methods for multizone treatment with zonal isolation call for the bottom zone to be treated first, followed by running bridge plugs into the hole and then repeating for successive zones uphole. At the end of the process, the bridge plugs are drilled out.

However, this can be a time-consuming process, requiring many trips into the well and increasing both NPT and expense. If the cost of the many downhole trips is not greatly exceeded by the expected production gains, the operator may have to make an economic decision on which zones to stimulate. "The result may be some combination of over- and understimulated zones, with other zones receiving no stimulation at all," Walker said.

The large costs associated with multiple trips downhole is significantly greater offshore, such as in the ultradeep Lower Tertiary play in the Gulf of Mexico (GOM), where 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.

According to Walker, these challenges were the basis for the development of the ComPlete MST system, which reportedly eliminates much of the NPT. "We had the original idea for this concept during a discussion with Total, who came to us with some problem formations in Indonesia. These formations had multiple stacked pays, all at the same pressure, but they wanted to do different things with the zones," Walker said. "They wanted to gravel pack two or three zones in the same well with two nongravel-packed zones, and they wanted to do all of this as economically as possible."

Novel installation, completion method attracts operator interest

BJ designed the MST to operate as follows. During installation, all sleeves are run in hole in the closed position to provide positive pretreatment zonal isolation. After the ComPlete MST assembly's upper gravel-pack / production packer is hydraulically set and tested, the work string and service tool are released from the completion assembly.

The service tool is then positioned so the sleeve-opening tool is above the lower production sleeve. Downward movement opens the sleeve, allowing communication through the screen. Next, the service tool sets the isolation packer, and then it opens the gravel-pack/frac-pack sleeve, allowing stimulation treatment fluid to be pumped down the work string and into the annulus. Pumping continues until sandout. Picking up the service tool allows excess slurry to be reversed out by pumping down the annulus and taking returns through the work string.

Pushing the sleeve-closing tool below the lowermost sleeve and then raising the tool through the zone closes all sleeves, positively isolating the treated zone. This is verified by applying pressure at surface to ensure integrity.

The process is then applied to subsequent zones uphole. After the last zone is completed, the work string and service tool are removed from the wellbore, leaving a large-bore, full-open ID completion with selective-flow capability. Production may be initiated by means of a run into the hole with coiled tubing, wireline, or tubing with a sleeve-opening tool.

"After the initial discussion with Total, we proposed this same technology to BP," said Walker. "They gave us the money to develop the tool, and they gave us two test wells.

"The first system we developed was a 7-in. tool for BP's smaller casing sizes. We tested two onshore wells in 2004 and 2005, where we fractured three zones on the first well, came back in and made some modifications, then went back out and fractured three zones on the second well. These were south Louisiana water zones; they were wells that depleted out and they just plugged them out, and we fractured three zones in one trip. The test went very well, we then made some minor modifications to the tool and commercialized it."

The tool's first commercial applications were in two two-zone wells offshore Indonesia, where all components of the MST system performed as designed, including multiple manipulations of the autolocator, sleeves, and valves as planned during the stimulation program.

To date, the functionality of the ComPlete MST system has been field-proven in three two-zone wells and one four-zone well offshore Indonesia and in the GOM. Planning is underway for 23 four-zone wells in Indonesia and several GOM deepwater wells over the next 10 to 20 months.

The system is currently available for use in 7- through 10 -in. casing sizes with no restrictions on zone length. Individual zones are produced through production sleeves incorporated in the well screen design below blank pipe and a gravel-pack or frac-pack sleeve with an isolation production packer. Up to five internal profiles are available with the current production sleeves to provide zonal selectivity for production.

No theoretical limit to number of zones

Walker indicated that there should be no theoretical limit to the number of zones the MST can isolate in a given trip, but there are practical limitations in terms of material durability. "Depending on overall length of the system, the only limit we have right now is the erosion resistance of the service tools and equipment. In some of these wells, particularly Lower Tertiary wells, you might pump fracturing treatments on the order of 500,000 lbm per zone for five zones. You'll be pumping 3-4 million lbm through your service tool, and the wear on the equipment internals will take a toll.

"However, we're pretty limitless in terms of the length of service we can do," Walker confirmed. "We've run a system as long as 3,000 ft into the wellbore already, and while there are some challenges at these lengths, such as temperature, pressure, even tubing movement, it is not impossible."

Cost savings significant, and growing

In terms of cost and NPT savings, BJ reported that results from the numerous field trials showed that the MST system afforded a 20 to 40% effective reduction in completion-cycle time and cost compared to standard multizone, multitrip completion technologies. "Depending on the number of zones, the savings might even be greater than that," Walker said. "For some of the jobs I've been involved in, I've seen a 25% savings for two zones, and it could be 50-60% for four to five zones easily. For the multizone wells in Indonesia, they were talking about saving 60% on completions, and cutting the total time from 9 days to 4."

The company further estimated that by eliminating one-third of a typical 12-day completion schedule on a multizone deepwater well, an operator can save as much as USD1 million on the total operation cost even with a conservative USD250,000 day rate for the rig.

"The speed with which the industry has been accepting this technology is incredible," Walker concluded. "After being technically selected by Total and BP, we've tested it for China National Offshore Oil Corporation, and Chevron wants to use this for a deepwater GOM project. All this activity has taken place in the last 2 to 3 years."

To learn more about the ComPlete MST contact Walker by email.

 

Ted Moon is the Technology Editor of JPT Online. He brings information on emerging technologies, R&D successes, new field applications, updates from SPE papers about recent innovations, and more. If you have a question or suggestion for future article topics, email Ted at teched@spe.org.