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University of Texas - Bureau of Economic Geology (UT-BEG)


Geographical, Geological, and Hydrogeological Attributes
of Formations in the Footprint of the Eagle Ford Shale

An overview of mostly geological characteristics of formations in the footprint of the South Texas Eagle Ford (EF) Shale play with a focus on water was prepared. The ~25-county EF play has seen a dramatic development in the past few years and it keeps expanding to additional counties towards the north of the play. However, the EF area is not new to oil and gas exploration and production. At least 110,000 wells, not including the ~5000 EF wells (as of March 2013), has been drilled in the EF footprint during the past century, many of them still active. The EF shale is actually a source rock and has supplied oil and gas reservoirs such as the Big Well and Pearsall fields and the very large Giddings field. Despite the fact that most of the EF lies in rural areas, several large cities (San Antonio, Laredo) are located at its edges. The document focuses on those two key aspects of hydraulic fracturing (HF): water use and water disposal. The South Texas location of the play with its scarcity of surface water resources exacerbates perceived conflicts with other water users.

The large depth of the folded Paleozoic basement below the EF (>15,000 ft) allows for a thick sediment sequence of Jurassic and younger age. The EF shale is positioned towards the middle of the sequence (~4000 to 11,000 ft deep) leaving many formations between it and the ground surface, particularly the thick Midway Clay, and providing several horizons for disposal of fluids. Its thickness varies from ~100 ft East of Austin to >500 ft at the Mexican border. The sedimentary sequence is initially carbonate-rich with platform carbonate formations such as the Edwards or Glenrose formations or the Austin Chalk , including the EF which is a carbonate mudrock. Toward the end of the Cretaceous the succession turned siliciclastic with alternating sandstones and claystones deposited in mostly fluvial and/or deltaic environments. Some of the sand-rich intervals of the succession compose the fresh water aquifers in the EF footprint such as the Carrizo aquifer and other aquifers of lesser water quality such as the Wilcox and Yegua-Jackson aquifers. Shallow subsurface water tends to be brackish outside of the outcrop area and of the Carrizo aquifer.

Water use was ~24 thousand acre-feet (AF) in the EF play in 2011. In the same year, the top HF users in the EF consist of Webb (4.6 kAF), Karnes (3.9 kAF), Dimmit (3.7 kAF), and La Salle (2.9 kAF) counties. Although overall water use has increased, water use per well has decreased. The change is related to the switch by most operating companies from the gas to oil and condensate windows and the use of gelled rather than slick-water HF jobs. Currently operators recycle very little of the flowback / produced water but use brackish water (likely in the vicinity of 20% of total water use). One reason why recycling is minor to negligible is that flowback volumes are far from providing enough water for a subsequent HF operation, especially at early times. The median well produces ~25% of the injected volume after 6 months and ~40% and plateauing after 1 year. Flowback / produced water is actually disposed of in injection wells. Approximately 2500 Class II injection wells were active at least part of one year during the 2008-2012 period. Many are related to waterflood operations not disposal. Preferred horizons for disposal are the formations of the Navarro-Taylor Groups in the Maverick Basin, a multi-county area next to the Mexican border as well as the Wilcox and Edwards formations.

For a copy of this report, please contact awilcox@HARCresearch.org