THE INFLUENCE OF WATER-ROCK INTERACTIONS ON INORGANIC CONSTITUENTS IN PRODUCED WATERS FROM UNCONVENTIONAL PETROLEUM RESERVOIRS

The potential for waters associated with petroleum reservoirs to migrate in the subsurface or affect the surface environment has generated serious public concern. Despite the large volumes of water being produced by the development of unconventional petroleum resources, information on produced water compositions, and in particular on trace metals, is often scarce. The underlying controls on such compositions are also poorly understood, though they should reflect constituents inherited from formation waters or brines, water-rock interaction and any injected fluids. Although brines found in petroleum reservoirs sometimes have distinct chemical compositions, allowing them to be identified in shallow water systems, distinguishing brines from different reservoirs is challenging. Water-rock interactions should yield produced water compositions that reflect the chemistry and mineralogy of reservoir rock, imparting a geochemical fingerprint, particularly with respect to trace metals.

We conducted laboratory experiments using twelve petroleum-source rocks from nine U.S. basins that represent wide ranges of chemistry, mineralogy, thermal maturity and depositional environment. Source rocks were leached for 8 days under anoxic conditions at ~100 °C to simulate subsurface conditions. One experiment used deionized water, and a separate experiment used artificial brine with major element concentrations 2× seawater. A third experiment leached source rocks with 10% HCl for 1 hour. Compositions of leachates were compared to data available in the USGS Produced Waters database and to seawater. High temperature, deionized water leachates were comparable to produced waters concentrations for some elements but were low for many others. HCl leachates generated higher concentrations, but do not represent typical reservoir conditions. The artificial brine leaching solutions corresponded better to reservoir conditions and the resulting leachates compared most favorably to produced water trace element compositions due to enhanced trace element solubility. The results illustrate how water-rock interaction creates a geochemical fingerprint in produced waters that could potentially be used to trace produced water sources in the deep subsurface or in near-surface environments.