EFFECTS OF INJECTED WASTE WATER ON A HETEROGENEOUS SYSTEM

The San Andres, located in the Midland Basin (Texas, USA), is a predominantly carbonate formation. Due to different flow regimes and a complicated diagenetic history, the prevalence of dolomite and the degree of porosity and permeability vary greatly throughout the formation. This heterogeneity is compounded by the re-injection of produced water (waste water from oil production), changing the formation chemistry and flow dynamics. As a result, the movement of fluids, both formation and injected, can be highly unpredictable. This study hypothesizes that unique isotopic^1,2 and geochemical² signatures between formation and produced waters will allow observation of flow paths, fluid-rock interaction, and fluid changes over time.

Previous studies have shown that fluids with moderate² to high salinities can persist even after fluid is no longer being introduced^3,4,5. This can result in a complex system of diagenesis from one flow event³ or repeated short lived events^3,5. Areas of higher permeability can focus flow⁵ or act as conduits to more reactive sediments⁴, so an understanding of formation characteristics is necessary to model more realistic fluid flow. Thin section analysis and d¹⁸O and d¹³C values from the cores provide porosity, dissolution, and precipitation information.

Preliminary mixing models indicate that the resulting fluid will have characteristics that are between those of the formation water and injected fluid. This high salinity fluid provides reactants for dolomitization to occur, and as products are removed, evaporites may form down the flow path, with the potential to be dissolved as fluid continues to be injected⁴. As dolomitization occurs, porosity and permeability may either increase or decrease⁴. Due to the highly heterogeneous nature of the formation, models will indicate the predicted outcome of fluid flow through intervals with varying characteristics. This work will inform decisions of ideal intervals for fluid injection and will be broadly applicable to other petroleum fields where fracking and fluid injection occur in altered carbonate systems.

1. Engle et al., 2016

2. Saller and Stueber, 2018

3. Jones et al., 2002

4. Al-Helal et al., 2012

5. Garcia-Fresca et al., 2012