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Background image from http://www.arabiaoilandgas.com (accessed March, 2013)Background image from http://www.arabiaoilandgas.com (accessed March, 2013)
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KerrEdwards
UvaldeKinney
Kimble
Medina
TravisSutton
Real
Mave
Hays
Gillespie
Llano
Blanco
Bandera
MasonWilliamson
Comal
Kendall
Burnet
Guadal
Cal
MenardSchleicher
Bell
Austin
San Antonio
-2000
Eagle Ford Petroleum Windows (Petrohawk, EOG, DI)
Oil
Wet Gas/Condensate
Dry Gas
Eagle Ford Shale- Austin Chalk Outcrops (TNRIS)(NW limit of Eagle Ford-Austin Chalk presence)
Aquifers
Gulf Coast Aquifer
Carrizo-Wilcox Aquifer
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Houston
Austin
Laredo
San Antonio
Corpus Christi
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Houston
Webb
Duval
Frio
KerrEdwards
Uvalde
Bee
r
Zavala
Kinney
Dimmit La Salle
Kimble
Medina
Matagorda
Travis
LeeSutton
Nueces
Real
Maverick
DeWitt
Kleberg
Lavaca
Calhoun
Hays
Goliad
Wharton
Fayette
Gillespie
Llano
Live Oak
McMullen
Wilson
Victoria
Bastrop
Harris
Gonzales
Colorado
Karnes
Austin
Refugio
Jackson
Blanco
Grimes
Bandera
MasonWilliamson
Jim Wells
Comal
Kendall
Fort Bend
Waller
Milam
Burleson
Aransas
Burnet
Brazoria
Guadalupe
Caldwell
Brazos
San Patricio
Washington Montgomery
WalkerMenardSchleicher
Jim Hogg
RobertsonBell
Austin
Laredo
San Antonio
Corpus Christi
100
200
300
100200
100
200
200
100
100
200
300
-8000
-6000-4000
-1200
0
-1000
0
-2000
-14000
0 25 50
Miles
±
Eagle Ford Shale Play,Western Gulf Basin,
South Texas
Map Date :May 29, 2010
Mexico
TX
NMLA
AROK
Western Gulf
Basin
Mexico
Eagle Ford Producing Wells (HPDI)! OIL
! GAS
Eagle Ford Petroleum Windows (Petrohawk, EOG, DI)
Oil
Wet Gas/Condensate
Dry Gas
Top Eagle Ford Subsea Depth Structure, Ft (Petrohawk)
Eagle Ford Shale Thickness, Ft (EOG)
Eagle Ford Shale- Austin Chalk Outcrops (TNRIS)(NW limit of Eagle Ford-Austin Chalk presence)
A
A’
A A’
A
B
C
Eagle Ford shale
Private well
Municipal well
D
Carrizo - Wilcox aquifer
Gulf Coastaquifer
San Antonio
A
A’
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Houston
Austin
Laredo
San Antoni
Co hristi
- 8000
- 60004000
- 1000
0
-14000
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Houston
Austin
Laredo
San Antoni
Co hristi
-
-
- 5000
- 7000
- 9000
- 11000
Legend0 -2000 ft 2000-3000 ft 3000-4000 ft 4000-5000 ft
5000-6000 ft
6000-7000 ft
7000-8000 ft
8000-9000 ft 9000-10000 ft
Eagle Forddepth contours
Carrizo-Wilcox depth
contours
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Mary Kate Stewart and Benjamin Surpless Department of Geosciences, Trinity University, San Antonio, TX
II. Oil and Gas Formation and the Eagle Ford Shale
1. Aquifer Fluid Mechanics
IV. Aquifers over the Eagle Ford shale
The Eagle Ford Shale Play
VI. Depth AnalysisI. Introduction
2. The Carrizo-Wilcox Aquifer
V. Methods
VII. Results
VIII. Future study
Resources
After a significant vertical drill (see A, right), the drilling is steered to horizontal (see B, right). Horizontal drilling allows oil companies to retrieve a greater amount of oil compared to vertical oil drilling because the horizontal drilling pipes run parallel to the formation that contains the target resources. The primary goal of the fracking pro-cess is to increase the permeability of the target formation (the Eagle Ford) adjacent to this horizontally - drilled section. The upper portions of the well are cased in steel and concrete to both strengthen the well for the fracking process and to prevent groundwater contamination (see C, right).
Although fracking production methods vary, fractures in the rock are usually initiated by two steps: plugging and initiating new fractures with a perforation gun; and propagating both natural and initiated fractures by the injec-tion of high-P fracking fluids. A horizontal section of the horizontal pipe is isolated using a plug, and the perforat-ing (perf ) gun is inserted into that section. The perf gun then uses small explosives to initiate new fractures in the volume of rock surroundng the horizontal well (see B and D, right). Fracking fluid (pressurized water mixed with bactericides, buffers, stabilizers, fluid-loss additives, and soap) is then pumped through the pipe to generate exter-nal stress on the shale in order to propagate both natural and new (created by perf gun) fractures. Once the propa-gation process is complete, proppants (sand or ceramic particles) in the fluid remain behind and hold open the fractures to retrieve the oil/gas and flowback during production (see red arrows in D, right) (Dobb, 2013).
Oil and gas are formed through three stages: diagenesis, cata-genesis and metagenesis . Diagenesis occurs in an anoxic envi-ronment as a result from pressure created by layers of sedi-ment that have buried and compressed organic mud. This pressure releases carbon dioxide, methane, and water to create a hydrocarbon called kerogen. As depth increases during burial, so does temperature and pressure.
Catagenesis occurs as kerogen reaches greater depths and temperatures during burial. The depths and temperatures at which catagenesis occurs are called the oil and gas windows (Fig. 1). The oil window lies between about 3.5-6.5 km depth, with temperatures ranging from 90° to 150°C. Although natu-
ral gas is formed in the same range of temperatures and pres-sures as oil, the gas window extends far below the greatest depths and temperatures of the oil window. (Fig. 1).
In general, chemical reactions caused by increased pressure and temperature promote a decrease in the molecular com-plexity of hydrocarbons, which in turn decreases viscosity until only the simplest molecular structure is left, creating methane. This final disinegration of kerogen into methane is termed metagenesis (Marshak, 2008). The distribution of oil and gas production in the Eagle Ford shale is controlled by these pro-cesses, so the depth of the shale determines what can be pro-duced at each well location.
The process of hydraulic fracturing (fracking) has come under intense media focus as the number of wells established by the method has grown exponentially over the last 5 to 10 years. Much of the information provided to the public is anecdotal, without ref-erence to any rigorous scientific studies. In addition, little attention has been paid to the relationship between the depth of the hydraulic fracturing process and the depth of the aquifer systems that overlie the formations targeted for the withdrawal of oil and/or natural gas. The goal of our research is to investigate one aspect of the hydrau-lic fracturing process in the Eagle Ford shale in south Texas, focusing on whether the hydraulic fracturing process itself is likely to open fluid pathways to overlying aquifers, thus permitting groundwater contamination in the region where oil and gas are cur-rently produced from the Eagle Ford shale, in south Texas.
Oil and Gas Windows
We used three different maps to compare the depth differences between the top of the Eagle Ford shale and the base of aquifers that overlie the regions of active production, assuming that the likelihood of drinking water being directly contaminated by the hydraulic fracturing process can be related to the thickness of rock separating the fracking operation from regional aquifers.
These maps include: 1) a map displaying depths to the top of the Eagle Ford shale as well as the regions where oil and gas are currently produced (Fig. 1); 2) a map showing the distribution of aquifers that would be affected by a contamination event, primarily the Carrizo-Wilcox (Fig. 5); and 3) a map showing depths to the base of the aquifers that overlie the Eagle Ford shale (Fig. 7).
We developed a new map that shows the vertical separation between the top of the Eagle Ford shale and the base of the aquifer system. Figure 8 was created by overlaying contour maps of the depth to the Carrizo-Wilcox and the depths to the top of the Eagle Ford shale. To improve resolu-tion, we estimated the positions of contour lines at 1000 foot intervals between existing 2000 foot intervals. We used the intersections of these contour lines and spatial estimation to develop the contoured depth map shown in Section VI.
Figure 8 (below) displays the distribution of depth differences between the base of the Carrizo-Wilcox aquifer and the top of the Eagle Ford shale. The dia-gram includes the published contour lines from both maps (at 2000 foot inter-vals) as well as the estimated positions of 1000 foot intervals between the pub-lished intervals. While the map does show a general increase in vertical separa-tion between the aquifers and the Eagle Ford toward the southeast, irregulari-ties in the base of the Carrizo - Wilcox aquifer result in less predictable separa-tion values. The smallest vertical separation between the units (where petro-leum is currently produced) is approximately 2000 feet, near the northern limit of production.
As the map above shows, there is a significant but variable vertical separation between the base of the Carrizo-Wilcox aquifer and the top of the Eagle Ford shale at most locations. Thus, it would be unlikely for direct groundwater con-tamination to occur from the fracking process itself. Furthermore, if oil or gas were to escape from a leaking well casing within the horizontal section of the system, it would have to travel a minimum of 2000 feet upward, and in most cases significantly further, to reach the base of the aquifer.
Our results suggest that it is unlikely that the hydraulic fracturing process within the Eagle Ford shale is likely to directly contaminate groundwater resources. However, we think surface and near surface operations pose signficantly greater risks to groundwater contamination events.
In addition, new drilling in formations above the Eagle Ford, including the Austin Chalk and the Olmos Formation, are currently being evaluated (Hiller, 2012). Although not evaluated here, production in those formations would be of higher risk to a groundwater contamination event because production would be taking place with less total vertical separation relative to the base of the overlying aquifer.
More importantly, groundwater contamination is more likely to occur in surface operations, with simple spills or leaks leading to the direct infiltration of con-taminants downward into the aquifer system. Another potential contamination risk is related to well casing failures, especially during times of increased pres-sure during fracking operations. This is especially true where the well passes through the aquifer itself. Although well casings have several layers of cement and steel casing, cement is permeable and fractures as it ages. It is clear that a greater number of scientifically rigorous studies is required to more completely evaulate the risk of groundwater contamination in fracking operations.
Davidson, S., and Mace, R., 2005, Aquifers of the gulf coast of Texas: an overview: Texas Water Development Board Report. Dobb, E., 2013, The new oil landscape: National Geographic. DOE (Department of Energy), web, graphic based on "Shale Gas: Applying Technology to Solve America's Energy Challenges," as posted on http://www.netl.doe.gov/. Accessed March, 2013.Eagle Ford Shale Play, Western Gulf Basin, South Texas, 2010, EIA: http://www.eia.gov/oil_gas/rpd/shaleusa9.pdf (accessed Sept 2012).Eagle Ford Shale Geology, 2012: http://eaglefordshale.com/geology/Edwards Aquifer, 2011, Houston Advanced Research Center: http://gulfcoast.harc.edu/WaterResources/KarstAquifers/EdwardsAquifer/tabid/2240/Default.aspxGeologic Cross Section of atascosa Country, Texas, Troell, The Champion Group: www.championgroup.comRyder, P., 1996, Ground Water Atlas of the United States, Oklahoma, Texas: Coastal Lowlands Aquifer System, USGS: USGS Hydrogeologic Atlas 730-E.Heath, R., and Trainer, F., 1968, Introduction to ground-water hydrology: New York, John Wiley and Sons, Inc., p. 5-30.HARC (Houston Advanced Research Center), web, The Major Aquifers in Texas, as posted on gulfcoast.harc.edu. Accessed March, 2013.Hiller, J., 2012, Like the layers of a crazy cake: Houston Chronicle, p. D1. Horizontal-Directional Oil & Gas Well Drilling, Horizontaldrilling.org: http://www.horizontaldrilling.org/ (accessed Sept 2012).Hydraulic frarcturing-is it safe?, 2011, Institute for Energy Research: http://www.instituteforenergyresearch.org/2011/05/03/hydraulic-fracturing-is-it-safe/Marshak, S., 2008, Earth: portrait of a planet, 3rd ed.: New York, W.W. Norton & Company, Inc., p. 489-495.Tarbuck, E., Lutgens, F., and Tasa, D., 2010, Earth: an introduction to physical geology, 10/E: Prentice Hall Publishing, 744 p.Troell, A., 2013 (web), Geologic cross-section of Atascosa County, as posted on the Champion group website. Accessed March, 2013.
III. The Hydraulic Fracturing Process
Porosity and permeability are the leading controls on the rate and di-rection of fluid flow in the subsurface. Porosity is defined as the ratio of the volume of void space to the total rock volume, and permeability is the capacity of the rock to transmit fluid under a pressure gradient. Grain size and shape have a direct control on the porosity and there-fore permeability of the rock. If the majority of grains are the same size and shape and are regularly distributed throughout the rock, this in-creases porosity and permeability. This is known as homogenous po-rosity and is common among sands (e.g., Heath and Trainer, 1968). If the aquifers that overlie the Eagle Ford are near the oil and gas win-dows, then the oil and gas have the potential to infilitrate the perme-able sands of the aquifer and contaminate the groundwater. Figure 5 (right) displays the major aquifers of Texas.
The Carrizo-Wilcox aquifer and a small portion of the Gulf Coast aquifer overlie the Eagle Ford formation, but only the Carrizo-Wilcox has the potential of contaminated groundwa-ter from fracking operations (Figs. 5 and 6). The northernmost extent of the Gulf Coast aquifer does lie above the region of dry gas production, but it lies above the Carrizo-Wilcox aquifer in that area (Fig. 6). The Carrizo-Wilcox consists of two regions: the central region which is fed by the Brazos, Trinity and Colo-rado Rivers; and the southern region, fed by the Rio Grande, San Antonio, Nueces, Guadalupe, Colorado, and Lavaca rivers (Ryder, 1996).
Contact: [email protected]
Research Questions:
1. How does the distribution of gas and oil production in the Eagle Ford shale relate to Texas aquifers?
2. What is the vertical separation between the base of these aquifers and the upper contact of the Eagle Ford shale?
3. Based on this vertical separation, are there areas in Texas where the fracking process itself poses significant risk to groundwater resources?
Figure 1. Distribution of oil and gas windows. Source: Tarbuck et al. (2010)
Figure 2. Map showing the depth to the top of the Eagle For shale, the oil and gas win-dows, and drilling sites within the Eagle Ford Shale Play in south Texas (EIA, 2010). the cross-section for line of section A-A’ is shown in Figure 3.
Figure 3. Simplified geologic cross section of Atascosa County (A-A’ on Fig. 2). Cross-section reveals the formations above and below the Eagle Ford as well as the dip of all units toward the southeast. Source: Troell (2013)
The Eagle Ford shale and other Mesozoic and younger units dip gently toward the Gulf of Mexico, to the southeast. (Figs. 2 and 3). The relatively impermeable Eagle Ford Formation is underlain by the Buda Formation and overlain by the Austin Chalk (Fig. 3). The Buda Formation is made up of relatively permeable limestone, but the gas and oil are trapped once they reach the impermeable Eagle Ford shale. Since the depth to the top of the Eagle Ford increases to the southeast, different types of petro-leum are produced at different locations (Fig. 2). (Eagle Ford Shale Geology, 2012)
The Eagle Ford Shale Play produces three primary products (Fig. 2): oil, wet gas, and dry gas. The oil window (green shading) is located between 4000-12000 feet in depth, and the gas window between 6000-14000 feet in depth (wet gas = orange; dry gas = red). Because the Eagle Ford is relatively impermeable, standard drilling methods cannot access the oil or gas within the formation. In-stead, horizontal drilling that uti-lizes fracking technologies is neces-sary for production.
Figure 4. Generalized diagram of the hydraulic fracturing process. See text for discussion. Significantly modified from DOE (2013).
Figure 5. Map showing the major aquifers of Texas. Image from HARC (2013).
Figure 6. Map of the Carrizo - Wilcox and Gulf Coast aquifers in relation to the Eagle Ford production areas. Modified from EIA (2010) and HARC (2013).
Figure 7. Depth to the base of the Carrizo - Wilcox aquifer system. Modified from Ryder, 1996.
The Carrizo - Wilcox aquifer is composed of both the Carrizo For-mation and the underlying Wilcox group. These units are com-posed primarily of permeable sands, although textural and com-positional variability within the aquifer results in significant vari-ability in hydraulic conductivity. The base of the aquifer ranges from the surface to 8000 feet below sea level (Fig. 7, right). The water from this aquifer is used for public water supply, rural do-mestic use, and manufacturing (Ryder, 1996).
Although the formations that make up the aquifer do dip gently toward the Gulf of Mexico (southeast), as shown in cross section A - A’ (Fig. 3; Section II), the generalized cross-section does not show the irregular depth distributions of the base the aquifer. In addition, these formations thicken toward to southeast. The ir-regular distribution of depths to the base of the system (Fig. 7) is important for any analysis of groundwater contamination by fracking operations in the underlying Eagle Ford shale.
3. Base of the Carrizo-Wilcox aquifer
Assessing the likelihood of groundwater contamination from hydraulic fracturing in the Eagle Ford shale: a map-based analysis of the Eagle Ford and overlying aquifers
Figure 8. Map showing the vertical separation between the top of the Eagle Ford shale and the base of the Carrizo-Wilcox aquifer system.
Figure 8. Map showing the vertical separation between the top of the Eagle Ford shale and the base of the Carrizo-Wilcox aquifer system.
Approximate northern limit ofproduction in Eagle Ford shale
