Interrogating Flowback Chemistry for Damage Markers in the Eagle Ford
Water produced during flowback and production operations may be one of the largest sources of untapped knowledge in hydraulic fracturing. This water, once in intimate contact with the reservoir, proppant pack, and tubulars, may contain a fingerprint reflective of damage related to polymer cleanup, propped pathway integrity, fines release, mineral scale precipitation, and bacterial activity. In cooperation with the operator, Lonestar Resources, of three Eagle Ford shale wells, we developed a workflow to characterize flowback water for indications of damage.
In this study, 50 produced water samples per well were collected for a three-well pad in Dimmit County, Texas, over the course of a year. Select samples were analyzed for up to 50 parameters. From this comprehensive laboratory survey, we determined 1) which methods yielded data least corrupted by interferences, 2) which practices preserved time-sensitive markers, and 3) which sample frequencies captured sufficient resolution of the downhole effects of interest. From these learnings, we developed a workflow to generate an appropriately scoped, high-quality dataset to facilitate interpretation of subsurface phenomena.
Several findings involved the measurement and analysis of guar, solids, salinity, scaling ions, and trace minerals. For example, although guar is present in high concentration initially, the concentration for all three wells declined to near-detection limits after several months, while water production continued. We hypothesize that the recovery of guar and its molecular weight distribution is indicative of the degree of cleanup from the proppant pack and may be useful for optimizing breaker design and flowback strategy for subsequent wells. Another finding was that salinity could be readily measured by a variety of simple and robust measurement techniques (i.e., conductivity, specific gravity, refractive index, etc.). Salinity, measured on a continuous basis through one of these robust techniques, may provide a unique way to observe shifts in producing zones due to failures related to pinch points. With a suite of chemical markers and the workflow to exploit them, produced water could be a valuable tool with which to monitor subsurface behavior, identify damage mechanisms, and ultimately improve completion practices.