Arthur J. Pyron
Pyron Consulting
Pottstown, Pa.
The issue of hydraulic fracturing as a development method for shale gas reservoirs such as the Marcellus has gone from an intraindustry discussion to a politically motivated issue de jour in the media and in nongovernmental organizations.
These latter groups have formulated the opinion that fracing destroys overlying freshwater aquifers by the introduction of “poisons” into the aquifer. In no area has this “discussion” been more overly promoted by the media and overscrutinized by politicians than in the Appalachian basin, and specifically, in the development of the Marcellus shale in Pennsylvania and New York.
Introduction
The discussion of the development of the Marcellus shale has gone from industry-based economics to citations of legal precedents and legislative initiatives. As all of this information has accrued what has been lost is the science associated with the Marcellus, and more importantly, the interrelationship between Marcellus development and groundwater hydrology. In this article, the author will present some basic concepts.
When Europeans settled the northeastern US, they found a land that was similar to the countries from which they came. They also noted that abundant surface water supplies were available, and that this was consistent with what they previously knew.
As a result, these new settlers developed their new homes in areas where abundant fresh water was available at the surface (i.e., rivers, streams, lakes, and springs).
The native peoples of this land also developed their communities in areas of surface freshwater availability. These settlements grew into larger communities and cities primarily because fresh water was easily obtained.
Communities without available surface water supplies had to rely on drilling water wells for their freshwater supply. The process of drilling water wells is an art unto itself. In northeastern and north central Pennsylvania and in adjacent New York, these wells are drilled in glacial sediments.
Hard rock aquifers are also present in the region and are locally significant but not widespread. The size of these communities is often restricted because the availability of fresh water is limited.
Glacial aquifers
Glacial sediment covers most of New York and northern Pennsylvania; this sediment was deposited during the Wisconsinan glacial period (70,000 to 7,000 years ago). These sediments have a thickness ranging from 100 to 300 ft. Typically, the glacial deposits consist of interbedded silt and clay and occasionally a medium to fine sand.
Wells drilled in the sand can yield water flows in excess of 200 gpm. Wells drilled in the silt or clay often yield less than 2 gpm.
Glacial aquifer wells are recharged by infiltration of rainwater and snowmelt. Soils in the area have an abundance of humus and organic debris, and the percolating water has ample amounts of dissolved solids and naturally occurring organic compounds.
High concentrations of iron, manganese, and nitrates are often found in the water; nitrates usually come from the breakdown of agricultural fertilizers and have historically been associated with chronic and short-term disease. The quality of the water is often poor, requiring filtration. In addition, seasonal changes in the quality of the water often occur.
Simplified Marcellus geology
The Marcellus shale is the lower member of the Hamilton Group, an interbedded shale and siltstone of Middle Devonian age. The Tully formation, an impermeable limestone also of Middle Devonian age, lies conformably above the Hamilton. The Cherry Valley, a limestone and shale(?), lies below the Marcellus. The Upper Devonian, interbedded siltstone and shale, lies unconformably above the Hamilton Group. The Lower Devonian lies unconformably below the Cherry Valley and consists of limestone, dolomite, and sandstone.
The Lower Devonian probably represents a marginal carbonate ramp environment; localized patch reefs or mounds are found in the Onondaga member of the Lower Devonian. The Middle and Upper Devonian represent a prograding delta environment, thicker nearer the Adirondacks and thinning farther west.
Mississippian and Pennsylvanian rocks covered the Devonian rocks. Erosional processes and glaciation removed both the Mississippian and Pennsylvanian in most of northern Pennsylvania and New York; the only exception to this extreme is northwestern Pennsylvania and adjacent New York and areas south of the maximum extent of the Wisconsinan glaciation.
In the areas of Pennsylvania and New York in which the Hamilton-Marcellus exists, it is accurate to say that the top of the formation is found between 1,000 and 5,000 ft beneath the surface topography. The top of the Marcellus member can be found at a depth of 2,000 and 6,000 ft below the surface. The nominal 1000 ft plus of rock and sediment includes the Tully lime, the Upper Devonian, and younger stratigraphic section, and glacial sediments and alluvium.
The Tully lime forms a mostly impermeable “seal” above the Hamilton Group. There is little evidence of large-scale seepage of natural gas from the Hamilton Group, although there is some microseepage (and this can be a tool useful in exploration). There is no evidence of infiltration of surface waters into the Hamilton Group, nor is there much coproduced formation water from the Hamilton Group, as the resource is a thermogenic or dry gas.
Marcellus horizontal drilling
The Hamilton (Marcellus) is a three-dimensional horizon that lies at depths in excess of 1,000 ft below the surface. The aquifer horizon is isolated from the Marcellus by layers of mostly impervious rock. The gas from the Hamilton Group is thermogenic and contains little to no coproduced water.
Many operators choose to drill horizontal wells to produce the Hamilton Marcellus resource. This means that the horizontal component of the well is drilled within the reservoir rock at some predetermined depth beneath the surface essentially parallel to the surface trend.
After drilling and logging the well, many operators choose to use hydraulic fracturing techniques to enhance production of the well. This includes the introduction of a water solution and sand under pressure to enhance the fracturing of the reservoir rock.
To the best of the author’s knowledge, no company offering hydraulic fracturing services claims that its fractures propagate more than 100 ft from the wellbore.
Wells in which the Hamilton Marcellus is found at depths of 1,000 to 3,000 ft are usually drilled vertically. The economics of drilling horizontal wells at this depth often isn’t viable.
Environmental considerations
The geometry of a horizontal well drilled into the Hamilton (Marcellus) involves drilling the horizontal leg at a depth of 4,000 to 6,000 ft beneath the surface. Presuming optimum fracing of 100 ft, the depth to the upper fractures is 3,900 to 5,900 ft beneath the surface and 3,600 to 5,600 ft beneath the glacial freshwater aquifers.
Even if one presumes that microfractures and other fractures are present in the overlying rock, the impermeable nature of the Tully limestone acts as a “seal” and prevents most vertical migration of natural gas from the Hamilton Marcellus. Geologically, it is highly unlikely that these gases could intermingle with shallow groundwater.
The process of drilling a well, like almost every other human activity, involves an interaction between the environment and humans.
The states of New York and Pennsylvania have environmental regulations, including waste water discharge, that preclude the discharge of produced water and frac fluids. Fines, loss of permits, and legal processes are the response of government to violations of these regulations. Sometimes, if discharge of drilling water to surface water is suspected, governments will issue fines under clean water regulations.
Industry heavily regulated
An existing infrastructure of permitting, site inspection, and environmental compliance is in place to protect the environmental. Violations of these regulations are discussed in the permitting process and during site inspections. Violators of the regulations are dealt with by state and federal agencies.
The recent expansion of Marcellus drilling in Pennsylvania and New York has brought an increase of claims of environmental damage to aquifers from releases of produced water and drilling fluids.
The author has shown that the subsurface geometry in the Hamilton (Marcellus) precludes the upward movement (seepage) of natural gas or produced water from the reservoir to the overlying aquifer(s). The only possible exception to this statement are operational problems, such as split casing or bad cement jobs; since both of these are harmful to operation of the well, the operator usually addresses them immediately.
Surface water discharge should be prevented by enforcement of the existing state and federal regulations. If damage occurs to shallow groundwater aquifers, it is likely that it is caused by infiltration of naturally occurring compounds or the makeup of the aquifer itself. As permeability of the glacial sediments decreases, infiltration of new water slows and water remaining in the aquifer has a tendency to get “stale.”
To blame deterioration of fresh water in aquifers on the drilling of Marcellus wells is not scientifically valid. It is a violation of scientific reality, which involves the interpretation of data using the scientific method. It is not an exercise in consensus building or political correctness, and it is not a tool to advance political goals. The geologic facts are the truth, if we care to interpret them.
The author
Arthur J. Pyron ([email protected]) is sole proprietor of Pyron Consulting, Pottstown, Pa. Pyron has worked for Fortune 500 companies, small business, municipalities, and private individuals in a variety of projects. Pyron Consulting is dedicated to providing short-term, project related technical and management support for clients with unique project needs. With 32 years of professional experience, Pyron Consulting has developed both geological and business expertise and has project management experience ranging from site reviews to million-dollar drilling programs. Pyron has an MS in geology from the University of Texas at El Paso and has completed more than 15 hr of postgraduate short course work in advanced geological and economic topics.
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