A study on chemical interactions between waste fluid, formation water, and host rock during deep well injection

A new disposal well was drilled in the vicinity of an injection well that had been in operation for 12 years. The drilling activities provided an opportunity to assess the fate and transport of waste products injected in the nearby well, and the impact, if any, on the host geologic formation. The origin of the fluid collected while drilling the new well and the interaction between injected waste and the formation were investigated using analyses of formation waters, waste, and formation minerals, by applying traditional graphical methods and sophisticated numerical models. This approach can be used to solve a wide range of geochemical problems related to deep well injection of waste. Trilinear Piper diagrams, Stiff diagrams, and correlation plots show that the chemical characteristics of recovered fluid at the new well are similar to those of formation water. The concentrations of most major constituents in the fluid appear diluted when compared to formation water sampled at other locations. This could be explained by mixing with waste, which is less saline than formation water. However, the waste injected near the new well consists primarily of ammonia and sulfate, and these waste constituents are not found at concentrations elevated enough to suggest that significant mixing of formation water with waste has occurred. To determine whether chemical interactions between injected waste and formation could explain the chemistry of fluid recovered from the new well, we simulated the chemical reaction between waste, formation water, and the formation rock by numerical modeling. Initial modeling calculations were done using a multicomponent geochemical reaction-path model to simulate fresh waste reacting with the formation. A more complex simulation coupling flow, transport, and reaction was then run using a multicomponent geochemical reactive transport model. These numerical simulations were carried out to calculate porosity changes and evaluate chemical processes resulting in mineral precipitation and dissolution. The simulations indicate that the waste injected in the vicinity of the new well is mildly reactive with the formation, and that there is a small porosity increase upon injection (approximately one to two percent) in the near-wellbore region. The simulations predict the precipitation of anhydrite, ammonium clay, and ammonium feldspar minerals. However, the precipitation of these minerals is not sufficient to account for the lower-than-expected sulfate and ammonia concentrations away from the injection well. This modeling work is preliminary in nature, and provides an example of the application of sophisticated modeling tools to problems involving deep-well injection of waste