Hydraulic fracking technology, widely used worldwide for several decades, involves the high-pressure injection of a liquid mixture, causing fissures to be formed in the strata through which oil is able to run to the bottom of the well.
A proppant agent is then used to keep these fissures open.
Recent trends in shale fracking point to a future of larger fracture treatments and longer horizontal lateral lengths.
Shales and other low-permeability formations require multistage completions, hydraulic fracturing, and horizontal wells to produce at economic rates.
Larger sand placements will put more emphasis on control of the individual treatments to ensure that each is implemented as designed and Single-point Entry systems have inherent advantages in this respect.
Lateral lengths of 5,000 feet to 10,000 feet have routinely been deployed and a few laterals in excess of 18,000 feet have recently been completed.
Longer laterals yield better economics in theory as more horizontal contact with the formation results in increased EUR (Estimated Ultimate Recovery) and higher initial production rates for each vertical wellbore that is already capitalized. However, technical challenges arise with extended lateral lengths as coiled tubing reach is limited by friction against the casing or liner wall and graduated ball seat diameters eventually become a restriction to production.
With the development of horizontal drilling, multi-stage fracking technology, involving the direct creation of fissures throughout several sections of a well, has become widely used.
Today this tends to be used in conjunction with other technologies in enhanced oil recovery, although it is being constantly improved.
Multi stage fracturing is a technological breakthrough that is helping to change how fracking is applied in unconventional resources development.
As drilling technology exploits more complex and unconventional reservoirs, completion technology is being designed and developed to effectively fracture and stimulate multiple stages along a horizontal wellbore.
The growth in multi-stage fracturing has increased due to completion technology that can effectively place fractures in specific places in the wellbore
By placing the fracture in specific places in the horizontal wellbore, there is a greater ability to increase the cumulative production in a shorter time frame.
Limiting technologies in the completion of horizontal wells have slowed that growth in some reservoir applications (e.g. reservoirs that require specific fracturing treatments at certain intervals to make them economic to produce).
In developing complex low-permeability reservoirs the inadequacy of standard multi-stage fracking has become obvious insofar as each newly-fracked zone has to be kept separate from the preceding one by a metal or composite ball.
The diameter of these balls is reduced from zone to zone, with the result that the way these wells are constructed makes more than 10 fracking operations impossible.
New spaced-perforation technology has no such limitations.
Multi-stage fracking through spaced perforation involves multi-use compacted “cushions”-packers which expand under pressure to isolate those areas in which fracking has been completed.
Once complete, the cushion deflates to its normal size and the equipment is transported to the next port.
Open hole Multi Stage Fracturing Method
Before the development of the special designed system, the only options for completing an open-hole horizontal well was barefoot or using slotted or perforated liners
The development of a system to set in open hole, provide mechanical diversion and allow multiple fractures to be performed along the entire horizontal wellbore, with the benefit of cost and time savings
A mechanical open-hole packer system is capable of resisting high differential pressures, with fracturing ports located between the packers
OHMSs use hydraulically set mechanical packers instead of cement to isolate sections of the wellbore.
These packers have elastomer elements that extrude to seal against the wellbore and do not need to be removed or milled out to produce the well, and they provide isolation throughout the life of the well.
Series of packers could be run simultaneously in the well on a liner, and the fractures could be pumped in continuous succession.
When the system reaches total depth, the packers can be set, instead of using wireline and perforating the casing to allow fracturing, these systems have fracturing ports to create openings between the packers.
The major advantage of OHMS is that all the fracture treatments can be performed in a single, continuous pumping operation without the need for CT or wireline, saving time and costs and reducing high-risk health, safety, and environmental operations.
Once the stimulation treatment is complete, the well can be flowed back immediately and production brought on line.
Cemented-Liner Multistage-Fracturing Method
This type of completion involves cementing production casing in the horizontal wellbore and plug-and perforation/stimulation.
The mechanical isolation in the liner is achieved by setting bridge plugs using pump down wireline or coiled tubing (CT), followed by perforating and then fracturing the well to provide access to the reservoir.
The cement is able to provide the mechanical diversion in the annulus, while the bridge plug provides the mechanical diversion inside the liner
This process then is repeated for the number of stimulations desired for the horizontal wellbore. After all stages have been completed, CT is used to drill out the composite plugs, thus re-establishing access to the toe of the horizontal wellbore.
Production using this method also can be limiting because cementing the wellbore closes many of the natural fractures and fissures that would otherwise contribute to overall production.
Microseismic monitoring of hydraulic fracturing operations has proven particularly valuable in locating and attempting to characterize the very small-magnitude events generated during the fracturing process in unconventional resources plays, where effective stimulation is critical to well performance.
Microseismic characteristics include: depth of target, rock type, and physical size of the slip surface.
Using microseismic monitoring, geoscientists try to determine fracture height, width, azimuth, and some estimate of stimulated rock volume.
They also are interested in the local geology of these events, including properties such as source types, implosive (closing) and explosive (opening) events, compensated linear vector dipole, double-coupling, and dip, strike and rake characteristics.
In turn, producers are using these attributes to help determine well spacing and orientation, the number and placement of stages, and forward prediction through discrete fracture network modeling.
Horizontal drilling has had a major impact on microseismic acquisition geometry.
The combination of horizontal drilling and hydraulic fracturing allows additional “contact” with the reservoir, without which many of these plays would be uneconomic.
3 dominant microseismic field acquisition geometries:
- borehole monitoring
- true surface monitoring
- shallow buried grids
Spaced-perforation technology not only increases oil recovery, but also allows geophysical investigations to be undertaken from within the well as well as repetitive fracking.
In repeated multi-stage fracking (re-fracking) a special chemical compound is used to isolate fissures created in previous fracking operations.
Gazprom Neft’s first multi-stage re-fracking operations were undertaken by Gazpromneft-Noyabrskneftegaz at the Vyngapurovskoye field in the Yamalo-Nenets Autonomous Okrug.
Records held by Schlumberger show this to be the first time such technology has been used at a traditional reservoir, not just in Russia but worldwide.