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

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2013

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This document provides an overview of the geological characteristics of formations within the footprint of the South Texas Eagle Ford (EF) Shale play, with a particular focus on water. The EF play, spanning approximately 25 counties, has experienced significant development in recent years, with expansion into additional counties to the north. Despite its recent growth, the EF area has a long history of oil and gas exploration and production, with over 110,000 wells drilled in the past century, excluding the approximately 5,000 EF wells as of March 2013. The EF shale serves as a source rock, supplying oil and gas to reservoirs such as the Big Well, Pearsall fields, and the Giddings field. While predominantly rural, the EF area encompasses several large cities such as San Antonio and Laredo, which border its edges.

This document focuses on two key aspects of hydraulic fracturing (HF) in the EF play: water use and water disposal. The South Texas location of the play, coupled with its limited surface water resources, intensifies perceived conflicts with other water users.

The significant depth of the folded Paleozoic basement beneath the EF (exceeding 15,000 ft) allows for a thick sediment sequence of Jurassic and younger age. Positioned in the middle of this sequence (approximately 4,000 to 11,000 ft deep), the EF shale is separated from the ground surface by numerous formations, including the Midway Clay. This geological setup provides multiple horizons for fluid disposal. The thickness of the EF varies from approximately 100 ft east of Austin to over 500 ft at the Mexican border.

The sedimentary sequence above the basement initially comprises carbonate-rich formations such as the Edwards, Glenrose, and Austin Chalk formations, with the EF itself being a carbonate mudrock. Towards the end of the Cretaceous period, the succession transitions to siliciclastic formations characterized by alternating sandstones and claystones deposited in fluvial and/or deltaic environments. Some sand-rich intervals within this succession form freshwater aquifers in the EF footprint, including the Carrizo aquifer, as well as other aquifers of lesser water quality such as the Wilcox and Yegua-Jackson aquifers. Shallow subsurface water tends to be brackish outside of the aquifer outcrop areas.

In 2011, water use in the EF play amounted to approximately 24 thousand acre-feet (AF). The top HF users in the EF during that year were 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 due to operational changes, including a shift from gas to oil and condensate production and the use of gelled HF treatments instead of slick-water treatments. Currently, operators recycle minimal amounts of flowback/produced water, with brackish water accounting for approximately 20% of total water use. Recycling remains limited due to insufficient flowback volumes for subsequent HF operations, particularly in the early stages. Flowback/produced water is primarily disposed of in injection wells, with approximately 2,500 Class II injection wells active between 2008 and 2012, many of which are associated with waterflood operations rather than disposal. Preferred disposal horizons include formations of the Navarro-Taylor Groups in the Maverick Basin and the Wilcox and Edwards formations.

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