The Eagle Ford Shale Field in the Gulf Coast Basin of South Texas, U.S.A.: A “Perfect” Unconventional Giant Oil Field

ABSTRACT The Eagle Ford shale Formation (Upper Cretaceous) in the Gulf Coast basin of south Texas was first commercially produced in 2008 and has since achieved production and reserve growth that is virtually unprecedented in the history of onshore North American oil and gas development. Through December 2014, the field had cumulative production of approximately 1.1 billion bbl of oil and condensate and 4.8 trillion cubic ft of natural gas (TCFG). Average daily production during 2014 was approximately 1.3 million bbl of oil and condensate per day and 4.9 billion cubic ft per day of natural gas (BCFGD). The horizontal rig count in October 2014 was approximately 200, resulting in approximately 300 wells drilled per month (RigData). While the entire resource potential of the Eagle Ford is still quite subjective, it has been estimated to be as high 25 to 30 billion bbl of oil equivalent (BBOE). The Eagle Ford shale covers a vast area that spans approximately 7 million ac (2,832,799 ha) of continuous prospective reservoir and therefore should be considered as one oil and gas accumulation, or field. As a result of the aerial extent of the field occurring at depths ranging from approximately 5000 ft (1524 m) to the north to approximately 13,000 ft (3962 m) to the south, the product mix covers the entire spectrum from low gravity–low gas oil-ratio oil to dry gas, and everything in between. The Eagle Ford Formation lies above the Buda limestone and beneath the Austin chalk over the entire field area. The formation varies in thickness from approximately 250 ft (76 m) to as much as 600 ft (183 m) and is composed of a variety of facies. The primary reservoir facies, herein referred to as the Hawkville facies, is the primary reservoir and is located near the base of the formation. The Hawkville facies, named for the field area located in LaSalle and McMullen Counties where the net reservoir thickness is found to be in excess of 300 ft (91 m), is a calcareous mudstone that was deposited in an anoxic environment as part of the Cretaceous seaway that traversed north to south through west-central United States and Canada. Long known as a source for oil and gas production from other Cretaceous reservoirs such as the Buda, Austin chalk, Olmos, and others, the Eagle Ford has moderate to high total organic content (TOC) ranging from 3 to 5% and thermal maturities based on vitronite reflectance ranging from approximately 0.7 to 1.3. Operationally, the field has proven to be one that has been relatively benign from a drilling perspective. The Tertiary is at the surface and is a sand-dominated section extending to as deep as approximately 8000 ft (2438 m) near the down dip limits of the field and results in excellent rates of penetration. The Cretaceous Midway, Taylor, and Austin formations also provide consistent drilling conditions. Wells in the field that are in the measured depth range of 15,000 to 18,000 ft (4572 to 5486 m) are commonly drilled from spud to total depth in 8 to 12 days or less. Completion operations are similarly advantaged. The reservoir is extremely brittle and is very receptive to the high-rate hydraulic fracturing processes that are necessary to establish commercial production, with a very low percentage of screened out fracture stimulation stages. Production operations vary widely based on product type, gas-to-oil ratio, and the volatility of the liquid. However, to date, there have not been material changes in the gas-to-oil ratio of the producing wells that would suggest degradation of ultimate recoveries based on bubble point or dew point effects. The regulatory and community effects have also been relatively benign in terms of impediments to the development of the field. The state of Texas and the oil and gas industry have a long history of working in a collaborative manner. Very early in the field’s history, the Eagle Task Force, led by railroad commissioner David Porter, was organized with members from the state bureaucracy, industry, and community with the intent of promoting economic activity, establishing best practices across the play, and reacting to issues that affected the constituencies within the group. One of the most significant issues is water usage, which is a common issue in all unconventional development that utilizes isolated multistage hydraulic fracturing. One of the benefits to the field is the presence of the Carrizo aquifer, which underlies a vast majority of the field. The Carrizo is a fresh-water aquifer that has proven to be an excellent source of water without experiencing material depletion, primarily because of its extremely large amount of available water and because it is actively replenished except in times of severe drought.

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