Hydrogeology of Formations Used for Deep-Well Injection, Texas Gulf Coast


This research program was conducted to investigate fluid migration potential, fluid direction, and velocities in the regional hydrologic environment of the Texas Gulf Coast Tertiary formations in the context of deep-well injection of hazardous chemical wastes. The study focused on the Frio Formation due to its significant waste injection and the availability of a large database on formation pressures and water chemistry in the Frio.

Pressure data collected from drill-stem tests and bottomhole pressure measurements in onshore oil and gas wells were used to evaluate pressure regimes. Pressure-depth profiles and potentiometric surfaces constructed from this data revealed three hydrologic regimes: a shallow section with fresh to moderately saline water in the upper 3,000-4,000 ft, an essentially saline hydrostatic section 4,000-5,000 ft thick, and a deeper overpressured section with moderate to high salinities. The hydrologic complexity is further compounded by extensive depressurization in the 4,000-8,000 ft depth interval, likely resulting from the production of over 10 billion barrels of oil equivalent and associated brines over the past 50 years. It was not possible to construct a composite potentiometric surface of the entire Frio to determine "natural" flow gradients or "natural" points of discharge.

Potentiometric surfaces representing discrete depth intervals were mapped, and these values, along with flow gradients determined from potentiometric surfaces and published permeability and porosity data, were used to compute linear fluid flow velocities ranging from 0.01 ft/year to 105 ft/year in the lateral direction.

The potential for vertical fluid migration was investigated using equivalent environmental hydraulic head maps. The presence of widespread pockets of depressured formations significantly affects the direction and value of fluid gradients, as these depressured oil and gas fields may become potential sinks for the injected chemical wastes.

Published water chemistry data were supplemented by field sampling of waters from 32 oil fields. Active recharge of the Frio by continental waters does not seem to be occurring. All sampled waters appear to be in isotopic equilibrium with the rock matrix. Salt dome dissolution is the primary reaction controlling water chemistry in the northern section, while brines from the deeper geopressured section may be leaking into the hydrostatic section of the central and southern Gulf Coast Frio.

The absence of organic acids and the alteration of Frio oils from samples shallower than approximately 7,000 ft suggest biodegradation, which has useful implications for the degradation of injected chemical wastes.

A detailed analysis of the localized hydrodynamics in Victoria County, Texas, as a case study demonstrates the applicability of the developed techniques to injection facility siting and monitoring processes, particularly where depressurization was observed on a local, county-size scale.


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