Basin Geochemical Evolution of the Eagle Ford and Effects on Trace Element Release to Matrix Acidizing Fluids

Abstract

Completion techniques such as matrix acidizing are used during production in source rock-reservoirs, which generate and trap hydrocarbons within organic-rich mudstones. However, high treatment costs are incurred to remove trace elements that have been released, before acidizing fluids can be reinjected or disposed of. One cost-effective solution in the production process could be to target chemofacies where trace elements are stored in more stable forms, thus leaving the trace elements in the formation by not mobilizing them. By identifying these chemofacies and the basin geochemical conditions leading to their deposition, it may be possible to improve the efficiency of unconventional production while simultaneously reducing environmental exposure. This research examines a section of core from the Late Cretaceous Eagle Ford Group, which was deposited in the southern Western Interior Seaway (WIS) under shifting redox conditions culminating in Ocean Anoxic Event II. In particular, I hypothesized that euxinic depositional conditions in the WIS enhanced burial of trace elements in relatively stable forms, which would reduce the mobility of trace elements to acidizing fluids during completion. Euxinic chemofacies were identified using XRF spectrometry, bulk organic analysis and detailed core description. The severity of euxinic conditions were identified using an Aryl Isoprenoid Index, and the residency of trace elements determined by sequential selective dissolution. Trace element mobility in euxinic chemofacies was analysed by leaching samples with HCl under reservoir conditions. Principal components analysis of experimental results indicate that up to 85% of trace element release from different chemofacies is controlled by bulk concentrations. However, between different euxinic chemofacies of the Eagle Ford, up to 75% of trace element release is controlled by the severity of reducing paleo-conditions. These reductions are comparable to existing treatment methods for acidizing fluids. Rate constants of trace element mobilization have also been estimated and used to calculate a kinetic model of metal release to acidizing fluids. I conclude that periodic euxinic conditions in the southern WIS enhanced the burial of trace elements in relatively stable chemofacies, from which trace elements are less easily mobilized. The application of kinetic models of rock-water interactions will allow operators to better identify sweet spots that promise lower treatment costs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015