Deep Mix A Stimulating Thought

Fracs Observed in Real Time

Gee, it just reeks of Old Economy, and yet folks these days are clamoring for it at whatever price the market dictates.

We're talking natural gas, the fuel-of-choice for domestic electricity generation and staying warm during this frigid winter. Storage supplies are dwindling, production levels are nothing to write home about and prices have skyrocketed for this hot commodity.

So what could be more timely than the debut of a new technique that could lead to an improved, safer and much lower-cost way to pull additional natural gas out of marginally-producing fields?

A U.S. Department of Energy (DOE)-sponsored project with RealTimeZone Inc. (RTZ) in New Mexico has successfully demonstrated such a technique, which uses a new method of mixing the fluids used to fracture, or frac, gas-bearing formations.

The natural gas industry spends more than $1 billion per year to fracture these reservoir rocks to release more gas, according to Gary Covatch at the DOE's National Energy Technology Laboratory (NETL). The frac treatments allow increased reservoir contact, with the fracture-creating fluids moving from the well to as much as 1,000 feet or so into the formation to create channels that let more gas move into the wellbore.

Traditionally, the fracture stimulation fluids are mixed at the surface. RTZ, however, has developed a downhole-mixing technique designed to give the operator more control over the fracturing process.

The company has applied the stimulation technique in the Permian Basin, where it used the treatment to restore nearly 300,000 cubic feet/day (cfd) of natural gas production from a 12,300-foot natural gas well scheduled for plugging.

The tab was half the price of a traditional frac job.

The precursor to the DOE-RTZ project dates back several years when RTZ developed a real time stimulation diagnostic system in conjunction with Halliburton, with some assistance from Schlumberger, according to George Scott, one of the principals at RTZ.

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Gee, it just reeks of Old Economy, and yet folks these days are clamoring for it at whatever price the market dictates.

We're talking natural gas, the fuel-of-choice for domestic electricity generation and staying warm during this frigid winter. Storage supplies are dwindling, production levels are nothing to write home about and prices have skyrocketed for this hot commodity.

So what could be more timely than the debut of a new technique that could lead to an improved, safer and much lower-cost way to pull additional natural gas out of marginally-producing fields?

A U.S. Department of Energy (DOE)-sponsored project with RealTimeZone Inc. (RTZ) in New Mexico has successfully demonstrated such a technique, which uses a new method of mixing the fluids used to fracture, or frac, gas-bearing formations.

The natural gas industry spends more than $1 billion per year to fracture these reservoir rocks to release more gas, according to Gary Covatch at the DOE's National Energy Technology Laboratory (NETL). The frac treatments allow increased reservoir contact, with the fracture-creating fluids moving from the well to as much as 1,000 feet or so into the formation to create channels that let more gas move into the wellbore.

Traditionally, the fracture stimulation fluids are mixed at the surface. RTZ, however, has developed a downhole-mixing technique designed to give the operator more control over the fracturing process.

The company has applied the stimulation technique in the Permian Basin, where it used the treatment to restore nearly 300,000 cubic feet/day (cfd) of natural gas production from a 12,300-foot natural gas well scheduled for plugging.

The tab was half the price of a traditional frac job.

The precursor to the DOE-RTZ project dates back several years when RTZ developed a real time stimulation diagnostic system in conjunction with Halliburton, with some assistance from Schlumberger, according to George Scott, one of the principals at RTZ.

Reservoir stimulation treatment monitoring has long been practiced by completion engineers using radioactive tracers. It was kind of an after-the-fact approach; however, where the gamma ray tool was used after the treatment was pumped to detect tracers behind pipe to discern where the treatment ended up, or the vertical height of the treatment.

"The DOE was familiar with the patent we co-authored with Halliburton for real time stimulation monitoring and contacted us a little over two years ago," Scott said, "and they basically wanted to fund continued development of the system."

All Mixed Up

DOE's National Energy Technology Laboratory (NETL) is working with RTZ on the project, which is valued at $1.3 million with the federal government contributing $922,000.

Scheduled for completion in June 2002, the effort is in its last two phases:

  • Field testing the downhole mixing technique.
  • Real-time monitoring of the fracture as it is created.

"Once you observe a frac treatment in real time and see what it's doing, you realize it would be nice to tweak a few things here and there to modify and control the stimulation treatment to maximize and optimize it," Scott explained.

For example, it's common to want to avoid excessive fracture height while acquiring as much fracture length as possible. Engineers also want to observe proppant placement occurring in the reservoir.

"If you don't have a stimulation system to go hand-in-hand with real-time monitoring," Scott said, "then what do you do with it?

"For instance, in the past when you're fracing a well and the sand is concentrated too high, it'll pack off in the fracture," he said, "and in a matter of seconds you'll have a screenout and pressure that gets way high, way fast, and you must shut down instantly or blow something up.

"If you tried to lower the concentration of sand in the fluid at the surface it would be too late, because it would take a half-hour to get it pumped and reach the perfs two miles down."

Enter downhole-mixing, where different components are pumped -- some down tubing and some down casing -- that mix down the wellbore.

When a pressure increase is detected, the completion engineer can dilute the concentration of sand going into the formation in real time by instantly increasing the tubing volume, likely avoiding a premature screenout or early abort of the frac job.

Changes in stimulation pressures observed at the surface allow the operator to know if the fracture is being created as planned. Altering the fluid mixture can ensure the fracture goes in its intended direction, and fracture length can be optimized with minimized fracture height.

In other words, this real time system lets the operator make adjustments "on the fly" to enhance the stimulation process for greatly improved production results.

Fertile Ground

The Permian Basin, where RTZ plies its trade, is fertile territory for implementation of the downhole-mixing system technique.

Many producing zones occur with water zones in close proximity, according to Scott, and a big frac job can easily treat right into the water, causing irreversible damage.

In deep wells such as the Morrow, which is part of the focus of the DOE-RTZ test project, the engineer is quite restricted on what can be pumped as a function of pressure. So any method whereby the operator can decrease the treating pressure is significant.

While real time stimulation diagnostic data typically are most appreciated by the completion engineer, there's a lesson here for reservoir geologists, according to Scott, who cautions they need to be more cognizant of the entire completion process.

"The geologist might take the brunt of putting together a bad prospect when, in fact, it was just stimulated out-of-zone," he said.

"It only takes about 30 minutes for a bad completion to ruin a good well."

Scott noted the experimental Morrow frac job at 12,300 feet was accomplished at half the pressure of a typical frac treatment -- 5,000 lbs/square inch versus about 10,000 -- and sported a price tag of $40,000 versus $80,000 for a traditional fracture stimulation procedure.

"The zones we went into wouldn't have warranted a substantial amount of money spent on a frac job," he said, "but since we were able to do an experimental poor boy approach, we make a commercial well with the technique.

"If we can go into wells about to be plugged or actual dry holes -- which we plan to do -- and make any kind of production, especially commercial production, it speaks volumes for how this technology can be applied under even more significant circumstances."

The fluid used in the test was comprised of bauxite mixed with a methanol gel at the surface that was blended with liquid carbon dioxide (CO2) down the wellbore. The bauxite serves as a proppant to keep the fracture open, and the gel and CO2 create the fracture, penetrating deep into the reservoir rock.

After the fracture is formed, the miscible CO2 becomes gaseous and moves out of the formation, allowing the fracture fluid to be removed from the rock at a faster rate and enabling the well to produce gas sooner.

If RTZ's technology can be used on even 20 percent of the fracture stimulation treatments implemented by domestic gas producers, it could save the industry more than $100 million per year, according to Covatch at DOE-NETL.

He noted this could perhaps reduce natural gas prices and allow companies to apply additional resources to locate and produce more gas.

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