ERLE C. DONALDSON, WAQI M. ALAM and NASRIN BEGUM, Tetrahedron, Inc.
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| ESG’s downhole monitoring service was used to acquire and process microseismic events (colored by fracture stage) during a 143-stage hydraulic fracture operation in the Horn River basin. Courtesy of ESG Solutions and Nexen Inc. |
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The single greatest technological development that has made the production of oil and gas from shale economically feasible is the ability to deviate vertical boreholes into horizontal laterals, extending 10,000 ft or more. Numerous problems were overcome to allow cementing, perforating and radial distribution of multiple horizontal laterals from a single vertical well. The resulting innovations that have made the greatest impact on production from shale and tight sandstone are 1) “slick-water” fracture fluid for deep, brittle reservoirs; 2) multi-stage hydraulic fracturing of horizontal wells; and 3) monitoring of microseismic signals emanating from the initiation and propagation of fractures.
Slick-water fracturing. The development of slick-water fracturing followed the realization that synthetic polymers used for viscosity control, such as polyacrylamines, inhibited turbulence in fluids pumped through tubing at high velocities. Less turbulence reduced the friction of the fluids moving in the tubing and, consequently, eliminated a significant amount of pressure loss from transport of the fluids from the surface to the bottom of the well. At the bottom, high pressure is required to initiate and propagate fractures, and retain sufficient fluid velocity for transport and placement of proppants. The proppants hold the fractures open against the natural formation closure stress after completion of a fracture treatment and reduction of fluid pressure, as production is initiated.
The realization that a small amount of polyacrylamines with high molecular weight (0.01 to 0.1%) allowed pumping rates of 100 bbl/min in 5½-in. tubing led to applications for fracturing deep (>6,000 ft), hard and brittle shale formations. The low viscosity (<5 cP) limits the proppant carrying capacity to about 2.5 lb/gal of 100-mesh sand. The sand drops out immediately after entering the fracture and builds up as a sand dune to a height that depends largely on the velocity of the fluid. Sand moving across the top of the dune is entrained in the fluid and carried further into the fracture.
Slick water consists of fresh or saline water containing the polymer and other additives for specific effects: surfactants to enhance proppant carrying ability, and a biocide to inhibit slime-producing bacteria and control sulfate reducers that can sour a reservoir. Gelled slick water is used in ductile shales and high-permeability rocks.
Multi-stage fracturing. Several methods have been developed, using packers and jet fracturing tools, to place hydraulic fractures at specific locations in long horizontal open- and cased-hole wells. One method that has made a considerable impact on fracture treatment of shale beds is the multi-stage, ball-and-seat technology, which allows multiple fractures, placed at specific locations, with continuous pressure pumping. The technology is continually evolving: multiple ports, with sliding sleeve that open two to five sections, allow placement of 50 to 60 fractures simultaneously; more effective placement of proppants; use of smaller fluid volumes and less proppants per stage.
Microseismic monitoring. A third technology that is improving the precision of horizontal well hydraulic fracturing is microseismic monitoring of borehole events taking place during the development of fractures. Seismic events generate P- and S-waves that travel at different velocities, thus the difference in time between the arrivals of the waves at a distant location is proportional to the distance traveled. Assuming that the velocities of the waves are known, geophones placed at three depths in a triangular pattern, with the event to be recorded approximately in the center of the pattern, will locate the event that can be imaged on a computer. Furthermore, the amplitude of the waves recorded by the geophones is a function of the relative size of the fracture. Waves representing the fractures are mapped in real time to show the azimuth growth trends. Real-time analysis of the fracture dynamics allows rapid decisions with respect to the pumping schedule, number and locations of fracture placement, and proppant schedules. The technology allows the operators to maximize the fracture volume in zones that are most productive, leading to greater efficiency in production.
Environmental impact. The greatest environmental concern of shale gas development is the risk of vertical fracture growth into a subsurface, freshwater aquifer. However, rapidly improving microseismic monitoring of fracture growth is eliminating the guesswork of fracture placement. In addition, fracture growth is limited by the leak-off rate of the injected fluid that stops fracture growth, when the leak-off rate is equal to the rate of injection. Consequently, experience with the fracturing of thousands of wells under freshwater aquifers has shown that fractured zones that are separated by 500 ft or more from other formations are not impacted by the fracture treatment. This has been confirmed by the microseismic surveys and migration of tracer chemicals added to the frac fluids.
Another environmental factor is the treatment and disposal of flow-back frac fluid. This fluid is either disposed in Class II injection wells or treated with emerging technologies for re-use, thus saving large quantities of water, chemicals and cost. The escape of produced methane is still another area of concern because methane captures 20 times more heat in the atmosphere than carbon dioxide. Care has to be taken to minimize such leakage from natural gas production equipment and pipeline equipment through improved monitoring and efficient maintenance. 
EDITOR’S NOTE: Drs. Donaldson, Alam and Begum are authors of a technical book, Hydraulic Fracturing, scheduled for release from Gulf Publishing Co. in Winter 2012.
THE AUTHORS
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ERLE C. DONALDSON is senior consulting engineer for Tetrahedron, an international engineering consulting firm. Previously, he was on the faculty of the School of Petroleum and Geological Engineering at the University of Oklahoma. Since retiring in 1990, he has consulted for various oil companies, universities and governmental agencies. Dr. Donaldson earned a BSc in chemistry from The Citadel, BSc in chemical engineering from the University of Houston and PhD in chemical/petroleum engineering from the University of Tulsa.
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WAQI ALAM is president of Tetrahedron. Dr. Alam has been providing consulting services to the petroleum and environmental industries for more than 22 years. He received his PhD in petroleum engineering from the University of Oklahoma, and has an MS in engineering management and BS in chemical engineering.
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NASRIN BEGUM is currently a program manager for Tetrahedron. Dr. Begum is an environmental health scientist who has evaluated environmental risks associated with various chemicals, including chemicals related to the petroleum industry. She has a PhD in environmental health from Colorado State University and MS in Plant Pathology from Oklahoma State University.
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