February 29, 2008

Multi-stage fracturing of horizontal wells in the Bakken


Packers Plus Energy Services Inc., Calgary-based company, says its unique technology can deliver precisely-controlled fracs along a horizontal wellbore at an affordable price.
"Without our StackFrac system, the Bakken formation in southeastern Saskatchewan would still be uneconomic for the most part," says Dan Themig, president of the seven-year-old private firm.

Producers had already attempted straightforward stimulation of open hole horizontal well bores, sometimes dubbed "Hail Mary" fracs. That tactic sometimes worked but more often the formation would crack mainly at its weakest point rather than along the entire wellbore. Overstimulation at one point not only limits the increase in oil or gas production but sometimes triggers water incursion into the well.



Horizontal well with controlled fractures along the length


An alternative approach was attempted on horizontal wells which had been cased with a production liner and then cemented. The wellbore would be segmented with bridge plugs, then stimulated through perforations in the steel liner. This technique required multiple coiled tubing trips. Fracing each segment of the wellbore involved rig up and rig down of the stimulation equipment. This type of operation may take weeks and generates expenses that frequently prove uneconomic. Although horizontal drilling was well established by the turn of the century, the oilpatch still hungered for a satisfactory horizontal stimulation method - quick, consistently cost-effective and repeatable on a large scale.

A StackFrac operation begins with the insertion of a steel liner into the well. The liner is segmented with tire-shaped rubber seals called packers, capable of sustaining differential pressure ratings of 10,000 psi at 400 degree Fahrenheit. (See the drawing above.) Between each pair of packers are one or more ports. Each port includes two features, an aperture and a specific diameter. The aperture, when
opened, permits frac fluid to flow into the annulus - the space between the liner and rock formation. Also, the internal diameter of each port is smaller than its neighbour, with the smallest diameter at the end of the liner (the importance of this feature will become clear in a moment).

Once the liner is fully in place, frac fluid is pumped through it into the well. After full circulation is achieved, the rubber packers are expanded. They can increase in size by 40% and will conform to hole irregularities like ovalities and washouts. A small ball is then inserted into the frac fluid and is pumped along the liner until it seats itself within the last port. (For example, the ball may be two
inches in diameter, the diameter of the final port a half inch smaller.) As pressure rises against the seated ball, the adjacent port aperture opens and frac fluid flows into that "stage" or segment of the wellbore. The fracturing or rock cracking process is sometimes tracked with microseismic gear to ensure that it's effective.

When the bottom segment of the well has been fraced, the crew will inject a slightly larger ball into the well, which will seal and open the next port. The process will be repeated until the entire wellbore has been stimulated. Acidizing can also be handled through the ports. Well sections not worth stimulating can be passed by. Each stage can be production-tested individually if desired. Frac design can be
tailored to avoid overstressing any section of the wellbore, greatly reducing the risk of water incursion. If water does invade a portion of the wellbore, that stage can be sealed off. The liner is left in place and, if appropriate, can be designed for use in further downhole operations.

To date, the record TVD (true vertical depth) is more than 15,000 feet and the deepest well has been more than 25,000 feet MD (measured depth). Themig is confident that 30,000 feet will be manageable within six months.

Despite mounting successes across the Western Sedimentary Basin as well as the United States, the novel technology didn't make much splash until two years ago. That's when Petrobank began applying it in the Saskatchewan Bakken. Unstimulated, a Bakken horizontal well typically makes 10 to 30 barrels per day, hardly an economic return for an expenditure of $1.2 million. When stimulated using earlier
technologies, however, water cuts routinely jumped from near nothing to 70% of total production. StackFrac enabled Petrobank to stimulate oil flow with minimal additional water, which transformed the Bakken into Canada's hottest oil play.


FURTHER READING
Powerpoint presentation on the Stackfrac drilling technique

Cased hole StackFrac

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