The History of Multi-Stage Fracturing and the Future Ahead
The technology surrounding hydraulic fracturing isn’t particularly new. In fact, its origins can be traced back more than a century. Just before the beginning of the American Civil War, an adventurous oil driller in New York named Preston Barmore lowered gunpowder into a well and dropped a red-hot iron down a tube, resulting in an explosion which fractured the reservoir rock and increased hydrocarbon flow from the well. But just like other evolving technologies with long lasting legacies such as the telephone or automobile, hydraulic fracturing has innovated and advanced itself as time has progressed.
The recent oil and gas shale boom has propelled hydraulic fracturing into the public eye more prominently than ever before, but the technology that kickstarted the shale boom was decades in the making. Dating back to the 1940s, hydraulic fracturing had been used to stimulate conventional formations in vertical wells, but the modern sense of shale fracking didn’t truly begin until the 1990s saw George P. Mitchell develop a technique (after years of trial and error) that allowed hydraulic fracturing to be successfully implemented in horizontal wells in the Texas Barnett shale.
Some have dubbed Mitchell the “Father of Fracking” for his contributions, but it would be more historically accurate to credit him as the father of the US shale boom.
Mitchell Energy did not invent any new technology, but they persevered where others had failed to combine technologies into a commercially viable method to extract gas from the tight Texas Barnett Shale formation. This incredible success was not kept a trade secret for long and a new ‘land rush’ soon followed as operators began to lease mineral rights in shale regions, which sparked a new drilling boom that has continued on until the present.
Birth of the Shale Revolution
Geologists mapped some of the oil and gas shale basins in the Northeastern region of the United States as early as the 1920s but the formations were not considered productive due to extremely low permeability. In fact, operators used caution when drilling through these shales to reach deeper pay zones because the gas which migrated into the drill hole could lighten mud weights and cause issues for well control.
Once fracking was combined with horizontal drilling and other burgeoning technologies, such as 3D seismic imaging, it became a viable option for extracting oil and gas from unconventional shale formations. The ingenious pairing meant that horizontal wells could greatly extend the wellbore contact within a shale formation for thousands of feet outwards as opposed to only contacting the vertical thickness of the reservoir (approximately 200 feet in many formations).
The rock could then be fractured by injecting a high pressure mixture of water, chemicals and sand into the horizontal section. Millions of gallons of water and millions of pounds of sand are typically pumped into a shale well during hydraulic fracturing. The pressurized water and sand mixture creates new fractures and interconnects existing natural fractures in the shale. The sand fills the newly opened spaces to act as a ‘proppant’ to hold the fractures open after the pressure is released. This process frees some of the trapped oil or gas and provides increased conductive paths to the wellbore.
With the advent of new found ways to tap into shale plays with emerging fracking technologies, the commercial viability of shale completions would never be the same again.
The Evolution of Shale Completions
As operators began implementing hydraulic fracturing in horizontal shale formations they experimented with various tools, perforation techniques, length of perforation intervals, and types of fracture treatments. A common practice called Plug and Perf (PnP) was initially adopted which is also referred to as a Limited Entry technique.
The process involves pumping a frac plug downhole along with e-line perforating guns. The guns are fired to perforate a series of spaced perforation clusters within each lateral stage and then pulled from the hole. A ball is pumped downhole to seal against the top of the plug which had been set and the hydraulic fracture treatment can then be pumped through the exposed perforations above the plug. The procedure is repeated over and over again with new stages until the entire length of the horizontal section is treated. The plugs are then drilled out using coiled tubing or jointed pipe. Various lengths of perforated stages were attempted and a range of 200 feet to 500 feet was typically adopted.
The basic operating assumption had been that all perforation clusters would receive equal stimulation regardless of formation characteristics. Inconsistencies in production results and post stimulation fracture analysis have led to the conclusion that formation factors play a large part in the overall efficiency of PnP completions.
Formation heterogeneity can result in natural stress fractures and variable in situ fracture gradients along the lateral section. This can cause stimulation treatments to concentrate at perforation clusters near natural fractures or at clusters near the lowest formation fracture gradients rather than achieving equal distribution. Consequently some clusters can be overstimulated while others may be pinched off and receive little or no treatment within a single stage.
The Development of Single-Point Entry Completions
Engineers took notice of the potential for inconsistent stimulation of clusters in PnP completions and sought various alternative completion technologies. One such alternative is Single-point Entry fracturing which allows for stimulation of each desired treatment location individually. This type of completion typically utilizes a coiled tubing activated sleeve or a ball drop system with sleeves having graduated ball seat IDs. The sleeve is integral to the casing string or liner and is prepositioned at the desired fracture location within each stage to be pumped. The sleeve is opened mechanically via coiled tubing or by hydraulic means if a ball is dropped onto a seat in the sleeve.
The fracture position is controlled since treatment enters the formation only through ports in the sleeve. This assures that each fracture is being stimulated and better control can be maintained by allowing pumping design changes to be implemented in real time. Stage Completions offers an alternative Single-point Entry system called the SC Bowhead II which utilizes a collet which is pumped downhole to activate a prepositioned sleeve. This system eliminates the need for coiled tubing and does not incorporate graduated ball seats which result in ID restrictions for longer laterals.
The Future of Shale Fracking
Recent trends in shale fracking point to a future of larger fracture treatments and longer horizontal lateral lengths. 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. The SC Bowhead II system lends itself to extended reach applications as it utilizes a collet that is pumped downhole and does not require coiled tubing for mechanical activation nor does it use graduated ID ball seats. The SC Bowhead II systems also ensures that each fracture placement receives stimulation and larger hydraulic treatments can be pumped with confidence.
Forward-Thinking Industry Leaders Recognize the Advantages of Single-Point Entry Technologies
The data is clear that single-point entry completion technology offers decisive technical advantages for drilling operators eyeing the future. No other technology on the horizon is geared toward planning and optimizing wells with the kind of control and predictability Single-point entry practices can offer in terms of efficiency.
The reduced horsepower requirement that Stage Completions systems provide has resulted in potential cost reductions for fracturing operations. A number of operators have experienced better real time control during pumping operations, and more accurate treatment modeling has translated itself into improved production results across the board.
The trending emphasis on choosing the best completion practice for each well application should encourage the industry to collectively give serious consideration to these systems.
To date, the industry has been resilient in responding to new technical challenges. And engineering a way past this latest set of market driven obstacles will have very tangible applications for how the industry as a whole tackles shale plays.
As the global demand for fracking continues to climb, operators will have to innovate to meet new challenges in the field and to the bottom line of doing business as they emerge. Stage Completions seeks to be an innovator on global terms, able to impact the industry and sustainability of our world as a whole with forward thinking initiatives that have the potential to become industry standards.
About this Blog
In this blog, we hope to engage the oil and gas community with information about multistage fracturing. We think analyzing and offering our expertise about the ways our industry is evolving will help others seek the most innovative technologies and practices as they become available. If there’s a subject you’d like to know more about, please let us know. Thank you for joining us in the conversation.