Hydraulic fracturing or “fracking” is widely employed in the petroleum industry to release natural gas and oil from compact rocks such as shale and “tight” sandstones. This technology has blossomed into a major issue in some regions of the country due to the efforts of environmental organizations to demonize the practice. The process of fracking involves pumping massive amounts of water (up to a few million gallons) under high-pressure into the objective rock formation effectively shattering it to release the bound up gas and/or oil in the rock. These rocks have substantial porosity or storage capacity for hydrocarbons, but the pore spaces are too small and lack the interconnectivity to allow the hydrocarbons to flow freely through the rock so that it can be recovered through a well bore.
The fluid used in fracking is about 99.5% water with chemical additives making up the remainder. The chemicals suppress adverse reactions between the injected fluid and the rock formation itself, such as to prevent iron scale build-up and clay minerals from swelling and obstructing migration pathways for the oil and gas. Though a small percentage, the volume of chemicals may be significant due to the huge volume of water employed in the fracking process.
A propping agent, such as coarse sand, is also injected into the zone of interest with the water to “prop” open the fractures and prevent them from closing up once the pressure is relieved. Some of the water and chemicals ultimately return to the surface along with the oil and natural gas that is later produced. This water must be separated from the hydrocarbons and disposed of in an environmentally prescribed manner under terms of numerous federal and state laws and regulations.
Hydraulic fracturing has been used in the petroleum industry since the 1940s to enhance oil production in conventional oil reservoirs, but its use has evolved through time in both technological sophistication and application to enable oil and gas recovery from compact rocks that was previously unachievable.
The bulk of the concerns raised by environmental groups revolve around alleged groundwater contamination by chemicals used in the fracking process and fractures they claim could allow natural gas to invade water wells. Most people are unaware of the precautions taken by oil and gas operators to prevent such environmental consequences. Some are dictated by federal and state laws and others are dictated by good oil field practices that enhance the safety and operational efficiencies of the drilling process.
Protecting Freshwater Zones During Drilling
Freshwater bearing rocks are generally limited to as few as tens of feet to as much as a few hundred feet below the surface, even less in some coastal areas. At greater depth, rocks are saturated with saltwater. The reason is that the vast majority of sedimentary rocks were originally deposited as sediments in an ocean, just as all streams flowing into the oceans today are depositing their sediment loads to build up on the ocean floor. Fresh water is limited to a relatively shallow band below the surface where the saltwater has been flushed by percolating groundwater from rainfall as part of the hydrologic cycle.
Freshwater zones are protected by numerous laws and regulations during and following the drilling of an oil or gas well. Multiple string of casing are set in the hole and cemented into place to prevent any possible contamination by fluids moving up from saltwater or hydrocarbon bearing zones below.
The Telescoped casing configuration is illustrated in the adjacent schematic.
Typically, a large diameter pipe is driven to the top of bedrock, a few to several tens of feet depending on soil thickness. This is followed by a somewhat smaller diameter string of conductor casing as drilling proceeds to perhaps a thousand feet, then a smaller string of surface casing is run to two thousand feet or so to seal off all near surface problems including possible contamination of freshwater zones and the potential for caving of poorly consolidated rocks into the hole. All the casing strings are individually cemented in place by filling the annulus between the pipe and the edge of the hole and/or any overlaps with previous casing runs with cement. Usually, two or more additional strings of casing are run in the hole and cemented in place to isolate the hydrocarbon bearing zone before fracking operations can begin as a prelude to final completion of the well for actual gas production.
A second technological innovation has been combined with fracking to open up a whole new frontier for new oil and gas reserves. The significance of horizontal drilling to the development of oil and gas from “tight” rocks cannot be overstated. Drillers have long had the capability of drilling directional or slanted holes at various angles. But, true horizontal drilling has come about within the past 20 years or so with the development of drilling technology that includes an electronic sensors inserted into the drill string near
the bit that transmits real-time data on the location of the bit and enables the driller to steer the bit along a pre-determined path in the subsurface. The well is drilled vertically for some thousands of feet, depending on the depth of the objective horizon, to a planned “kick-off” point. The angle of the hole is then gradually increased from vertical to achieve a horizontal path (near 90 degrees from vertical) by the time it reaches the objective horizon. The well bore can then be extended up to a thousand feet or more within the objective zone.
Afterwards, the well is cased and production tubing installed and cemented in place. A perforating gun is lowered into the hole and bullets fired through the tubing and casing into the objective zone to be followed by the previously described fracking process that is confined to the hydrocarbon bearing zone, usually several thousand feet below the surface. Fracking in essence creates artificial permeability that releases isolated pockets of trapped oil and/or gas so that it can move through the fracture system to the well bore for recovery at the surface.
Advantages of Hydraulic Fracturing and Horizontal Drilling
Until the advent of massive hydraulic fracturing technology and horizontal drilling, the natural gas and oil being recovered today from shale formations extending from south Texas to the Canadian border and east to the Appalachians would not be technologically or economically possible.
Within an objective rock section that may be only 50 feet thick, horizontal drilling can open up a thousand feet or more of the hydrocarbon bearing zone to the well bore dramatically increasing the production and recovery from a single well. This impacts both the economics of exploring for and developing new oil and gas resources while limiting the operational footprint and environmental impact on the surface to a small fraction of the number of wells that would otherwise be required to recover the same amount of hydrocarbon.
As is often the case, technological advances soon catch on and are exported worldwide. Major oil and gas companies such as Exxon Mobil, Chevron, Conoco, Shell and others are placing large bets on the future of natural gas as a good investment. They are securing access to potential unconventional oil and gas resources by buying smaller companies that own these resources and negotiating access to other such resources internationally.
Shale Gas Potential in Moore County, North Carolina
A potential shale gas play has opened up in North Carolina, a state with almost no history of oil and gas development. The North Carolina Geological Survey recently completed a favorable assessment of the shale gas potential of the Sanford basin falling mainly in northeastern Moore and Lee counties in the southeastern part of the state. As a result, a number of gas companies have sent agents into the state to acquire leases on
promising acreage with optimism that the State of North Carolina will develop the necessary Oil and Gas Law, regulations and environmental safeguards to allow drilling operations in the future. Fracking would be an essential part of any development of these potential gas reserves, but the state’s governor recently vetoed a bill that would allow these operations to take place in the state.
The Sanford basin is actually part of a much larger Triassic graben trend of many individual basins east of the Appalachian Mountains extending northward roughly parallel to the Atlantic coast from the southeastern US through North Carolina all the way to southeastern New York State. The Triassic grabens are fault bounded basins within the earth’s crust resulting from the tensional stresses associated with the break-up of the super continent Pangea and the opening of the Atlantic Ocean beginning in the early part of the Triassic period about 250 million years ago. As basin subsidence proceeded, streams flowing into the basins from the highlands to the west kept the basins filled gradually building up thick wedges of alternating layers of sandstone and shale. The dark shale units of the sedimentary sequences are the targets for potential shale gas exploitation.
The concern by many that hydraulic fracturing operations will damage freshwater aquifers is not well founded. Tens of thousands of successful fracking operations have been conducted in virtually every oil and gas basin in the U.S. Both the technology and the operating techniques are well established and reliable.
The vast majority of fracking operations take place many thousands of feet below the surface, far below any freshwater zones that are additionally protected from contamination by multiple strings of casing cemented in place. It is a relatively simple engineering calculation to determine the fracture strength of the rocks in question based on the overburden pressure and the nature of the rock itself. This determines the optimum pressure required to achieve the intended fracking result. There is close to zero chance that the fractures could run amok and extend through thousands of feet of overlying rock layers of varied composition reaching the surface to interfere with freshwater aquifers.
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Charles Holbrook is a geologist retired from the petroleum industry. He is co-owner, President and Chief Operating Officer for The Voice of North Carolina, Ltd. Click on his name in the left side bar for a biographical sketch and a list of articles on energy and the environment.