Arthur J. Pyron
Pyron Consulting
Pottstown, Pa.
http://www.pennenergy.com/index/petroleum/display/1405276813/articles/oil-gas-journal/volume-109/issue-23/exploration-development/marcellus-shale-gas-hydrogeology-and-the-truth.html
Theissue of hydraulic fracturing as a development method for shale gasreservoirs such as the Marcellus has gone from an intraindustrydiscussion to a politically motivated issue de jour in the media and innongovernmental organizations.
These latter groups haveformulated the opinion that fracing destroys overlying freshwateraquifers by the introduction of “poisons” into the aquifer. In no areahas this “discussion” been more overly promoted by the media andoverscrutinized by politicians than in the Appalachian basin, andspecifically, in the development of the Marcellus shale in Pennsylvaniaand New York.
Introduction
The discussion of the development of the Marcellus shalehas gone from industry-based economics to citations of legal precedentsand legislative initiatives. As all of this information has accruedwhat has been lost is the science associated with the Marcellus, andmore importantly, the interrelationship between Marcellus developmentand groundwater hydrology. In this article, the author will present somebasic concepts.
When Europeans settled the northeasternUS, they found a land that was similar to the countries from which theycame. They also noted that abundant surface water supplies wereavailable, and that this was consistent with what they previously knew.
Asa result, these new settlers developed their new homes in areas whereabundant fresh water was available at the surface (i.e., rivers,streams, lakes, and springs).
The native peoples of thisland also developed their communities in areas of surface freshwateravailability. These settlements grew into larger communities and citiesprimarily because fresh water was easily obtained.
Communitieswithout available surface water supplies had to rely on drilling waterwells for their freshwater supply. The process of drilling water wellsis an art unto itself. In northeastern and north central Pennsylvaniaand in adjacent New York, these wells are drilled in glacial sediments.
Hardrock aquifers are also present in the region and are locallysignificant but not widespread. The size of these communities is oftenrestricted because the availability of fresh water is limited.
Glacial aquifers
Glacialsediment covers most of New York and northern Pennsylvania; thissediment was deposited during the Wisconsinan glacial period (70,000 to7,000 years ago). These sediments have a thickness ranging from 100 to300 ft. Typically, the glacial deposits consist of interbedded silt andclay and occasionally a medium to fine sand.
Wells drilledin the sand can yield water flows in excess of 200 gpm. Wells drilled inthe silt or clay often yield less than 2 gpm.
Glacialaquifer wells are recharged by infiltration of rainwater and snowmelt.Soils in the area have an abundance of humus and organic debris, and thepercolating water has ample amounts of dissolved solids and naturallyoccurring organic compounds.
High concentrations of iron,manganese, and nitrates are often found in the water; nitrates usuallycome from the breakdown of agricultural fertilizers and havehistorically been associated with chronic and short-term disease. Thequality of the water is often poor, requiring filtration. In addition,seasonal changes in the quality of the water often occur.
Simplified Marcellus geology
TheMarcellus shale is the lower member of the Hamilton Group, aninterbedded shale and siltstone of Middle Devonian age. The Tullyformation, an impermeable limestone also of Middle Devonian age, liesconformably above the Hamilton. The Cherry Valley, a limestone andshale(?), lies below the Marcellus. The Upper Devonian, interbeddedsiltstone and shale, lies unconformably above the Hamilton Group. TheLower Devonian lies unconformably below the Cherry Valley and consistsof limestone, dolomite, and sandstone.
The Lower Devonianprobably represents a marginal carbonate ramp environment; localizedpatch reefs or mounds are found in the Onondaga member of the LowerDevonian. The Middle and Upper Devonian represent a prograding deltaenvironment, thicker nearer the Adirondacks and thinning farther west.
Mississippianand Pennsylvanian rocks covered the Devonian rocks. Erosional processesand glaciation removed both the Mississippian and Pennsylvanian in mostof northern Pennsylvania and New York; the only exception to thisextreme is northwestern Pennsylvania and adjacent New York and areassouth of the maximum extent of the Wisconsinan glaciation.
Inthe areas of Pennsylvania and New York in which the Hamilton-Marcellusexists, it is accurate to say that the top of the formation is foundbetween 1,000 and 5,000 ft beneath the surface topography. The top ofthe Marcellus member can be found at a depth of 2,000 and 6,000 ft belowthe surface. The nominal 1000 ft plus of rock and sediment includes theTully lime, the Upper Devonian, and younger stratigraphic section, andglacial sediments and alluvium.
The Tully lime forms amostly impermeable “seal” above the Hamilton Group. There is littleevidence of large-scale seepage of natural gas from the Hamilton Group,although there is some microseepage (and this can be a tool useful inexploration). There is no evidence of infiltration of surface watersinto the Hamilton Group, nor is there much coproduced formation waterfrom the Hamilton Group, as the resource is a thermogenic or dry gas.
Marcellus horizontal drilling
TheHamilton (Marcellus) is a three-dimensional horizon that lies at depthsin excess of 1,000 ft below the surface. The aquifer horizon isisolated from the Marcellus by layers of mostly impervious rock. The gasfrom the Hamilton Group is thermogenic and contains little to nocoproduced water.
Many operators choose to drill horizontalwells to produce the Hamilton Marcellus resource. This means that thehorizontal component of the well is drilled within the reservoir rock atsome predetermined depth beneath the surface essentially parallel tothe surface trend.
After drilling and logging the well,many operators choose to use hydraulic fracturing techniques to enhanceproduction of the well. This includes the introduction of a watersolution and sand under pressure to enhance the fracturing of thereservoir rock.
To the best of the author’s knowledge, nocompany offering hydraulic fracturing services claims that its fracturespropagate more than 100 ft from the wellbore.
Wells inwhich the Hamilton Marcellus is found at depths of 1,000 to 3,000 ft areusually drilled vertically. The economics of drilling horizontal wellsat this depth often isn’t viable.
Environmental considerations
Thegeometry of a horizontal well drilled into the Hamilton (Marcellus)involves drilling the horizontal leg at a depth of 4,000 to 6,000 ftbeneath the surface. Presuming optimum fracing of 100 ft, the depth tothe upper fractures is 3,900 to 5,900 ft beneath the surface and 3,600to 5,600 ft beneath the glacial freshwater aquifers.
Evenif one presumes that microfractures and other fractures are present inthe overlying rock, the impermeable nature of the Tully limestone actsas a “seal” and prevents most vertical migration of natural gas from theHamilton Marcellus. Geologically, it is highly unlikely that thesegases could intermingle with shallow groundwater.
Theprocess of drilling a well, like almost every other human activity,involves an interaction between the environment and humans.
Thestates of New York and Pennsylvania have environmental regulations,including waste water discharge, that preclude the discharge of producedwater and frac fluids. Fines, loss of permits, and legal processes arethe 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
Anexisting infrastructure of permitting, site inspection, andenvironmental compliance is in place to protect the environmental.Violations of these regulations are discussed in the permitting processand during site inspections. Violators of the regulations are dealt withby state and federal agencies.
The recent expansion ofMarcellus drilling in Pennsylvania and New York has brought an increaseof claims of environmental damage to aquifers from releases of producedwater and drilling fluids.
The author has shown that thesubsurface geometry in the Hamilton (Marcellus) precludes the upwardmovement (seepage) of natural gas or produced water from the reservoirto the overlying aquifer(s). The only possible exception to thisstatement are operational problems, such as split casing or bad cementjobs; since both of these are harmful to operation of the well, theoperator usually addresses them immediately.
Surface waterdischarge should be prevented by enforcement of the existing state andfederal regulations. If damage occurs to shallow groundwater aquifers,it is likely that it is caused by infiltration of naturally occurringcompounds or the makeup of the aquifer itself. As permeability of theglacial sediments decreases, infiltration of new water slows and waterremaining in the aquifer has a tendency to get “stale.”
Toblame deterioration of fresh water in aquifers on the drilling ofMarcellus wells is not scientifically valid. It is a violation ofscientific reality, which involves the interpretation of data using thescientific method. It is not an exercise in consensus building orpolitical 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 (pyronconsulting@yahoo.com)is sole proprietor of Pyron Consulting, Pottstown, Pa. Pyron has workedfor Fortune 500 companies, small business, municipalities, and privateindividuals in a variety of projects. Pyron Consulting is dedicated toproviding short-term, project related technical and management supportfor clients with unique project needs. With 32 years of professionalexperience, Pyron Consulting has developed both geological and businessexpertise and has project management experience ranging from sitereviews to million-dollar drilling programs. Pyron has an MS in geologyfrom the University of Texas at El Paso and has completed more than 15hr of postgraduate short course work in advanced geological and economictopics.
