A general trend toward higher pump rates, larger proppant volumes and higher stage counts has been effective in helping oil and gas operators increase production, but it comes with a new set of challenges. The increased intensity of hydraulic fracturing treatments has required completion technology companies to come up with innovative solutions to deal with fluid friction and erosion.
As the oil and gas industry drives farther into the realm of unconventional reservoir exploitation, it finds itself challenged to reduce the distance oil and gas has to travel to the wellbore for recovery. Traditionally, in a high-permeability reservoir fluids can travel thousands of feet through the rock in several years, draining the reservoir quickly enough to produce at economic rates. However, in modern tight oil and shale plays the rock’s permeability is so low that fluids can take years to travel inches or feet through the rock matrix, meaning petroleum is recovered far below economic rates, if at all.
The shale revolution was built on the backbone of a developer’s ability to create multiple hydraulic fracture superhighways from horizontal wellbores so that the fluids only need to travel a short distance through the natural matrix before jumping on the hydraulic fracture autobahn to production. As developers reach the lower limits of reservoir permeability, more fractures are necessary to effectively drain the reservoir, typically meaning more fracture stages are necessary in a lateral.
In the early 2000s lack of technology and expertise in horizontal multistage fracturing typically limited these treatments to five or fewer stages, with 152.4 m (500 ft) or more between fractures —far too wide to optimally drain many low-permeability reservoirs. Early plug-and-perf (PNP) methods were inefficient, unreliable and expensive, meaning an operator simply couldn’t afford greater numbers of fracture stages. Sliding sleeve systems could do this work efficiently and inexpensively, but there was a general limitation on stages because of the need for the incremental increase of the size of the ball seats. First-generation versions of the technology increased by a quarter inch at each stage.
Both technologies have evolved significantly, resulting in higher stage counts and efficiencies. PNP has become a more viable option for high stage counts through development of wireline simultaneous operations and limited entry techniques, but as service prices begin to recover from the downturn of 2014, the time and cost of the method is coming back into focus.
Because of its ability to treat a well without flushing proppant or shutting down, the reigning champion in efficiency remains the ball-activated sliding sleeve. However, the technology must overcome specific engineering challenges to compete with the stage counts possible from PNP.
The major challenge in increasing the stage count in a ball-activated sliding sleeve system is the need to gradually increment the ball seats and ball sizes from toe to heel.
If these increments are too small, there will be insufficient material interference between the ball and seat to provide pressure isolation from the stage below.
Improvements in ball-and-seat design, materials and quality control have allowed the implementation of smaller 1⁄16-in. ball seat increments, resulting in 35 to 40 stages. However, less efficient completions methods would need to be supplemented to accommodate the 50 or more stages many operators now desire.
A second challenge faced in increasing the stage count in ball-and-sleeve style completions is fluid friction. As fluid passes through a sliding sleeve ball seat, it enters a brief period of increased turbulence, causing a pressure drop. Though each individual ball seat generates only a small additional friction pressure contribution, a completion string of 35 to 40 sliding sleeves can contribute thousands of psi of additional friction and is a source of operational concern.
Fortunately, through the use of computational fluid dynamics modeling, sliding sleeve systems can be redesigned to reduce this friction contribution by more than 60%.
A recent development in ball-seat technology provides a step change in stage count limitations by providing a simple and reliable solution with much smaller seat increments. The new StackFRAC HD-X system is capable of more than 80 single-point entry stages. This is accomplished by addressing previously discussed issues of friction and erosion. Friction is dramatically reduced through a simulation-driven reshaping of the internal fluid pathway. During a fracture treatment, a ball-and-seat combination must hold back thousands of psi to effectively isolate one fracturing stage from another with only 1⁄16-in. diametric difference between the ball and seat. In other words, on one edge only 1⁄32 in. of material overlaps to hold back differential pressure during fracturing.
Advanced quality-control techniques such as laser micrometry allow a level of precision necessary to accomplish 1⁄16-in. and even 1⁄32-in. ball-seat increments with sufficient pressure rating for fracturing. With modern completions trends that use high-rate and high-proppant volumes the material overlap can be expected to erode by half or more when using conventional metallurgy. This leaves the ball actuators highly prone to bypassing seats and makes effective stimulation of high-stage-count completions impossible.
The StackFRAC HD-X system uses a new generation of erosion-resistant ball seats that greatly reduce erosion even in high-volume completions. Its effectiveness was proven recently for one operator in North Dakota completing the Bakken Formation.
The treatment employed a 65-stage 4½-in. StackFRAC HD-X system to a measured depth of roughly 6,400 m (21,000 ft) and a true vertical depth of 3,291 m (10,800 ft). Stage spacing averaged 47.2 m (150 ft). Oil-swellable SwellPLUS packers were used for annular isolation.
All stimulation stages were successfully placed as designed using a crosslinked fluid at a maximum pump rate of 30 bbl/min. The proppant total for the well was 4.5 million pounds, or roughly 70,000 lb/stage.
During treatment, each stage registered a strong positive indication of ball seat and sleeve shift as supported by a suite of high-fidelity real-time pressure and acoustic instruments called ePLUS Retina deployed on the surface.
Laboratory testing indicates that if more than 0.015 in. of material had been lost on the smaller ball seats, the ball sealers should have blown through the sleeves they were designed to stop in, a blow-through failure. Similarly, if more than 0.01 in. of material had eroded on the larger diameter seats, blow-through failure should have occurred. Because the ball seat and sleeve shift confirmations were provided by the Retina system, it was concluded that erosion of the new seats was minimal.
As horizontal completion techniques continue to evolve in the new shale economy, a general trend toward higher pump rates, larger proppant volumes and higher stage counts is helping to more effectively drain a reservoir. As the industry rises from the ashes of the most recent downturn, operators are beginning to suffer the effects of increasing costs for pumping services without a commensurate increase in oil price to pay for such increases. Fortunately for those operators, the manufacturers of sliding sleeve systems have continued to innovate and are ready to meet the challenges of the larger, more aggressive stimulation treatments in vogue.