A Coupled Reaction and Transport Model for Assessing the Injection, Migration and Fate of Waste Fluids
Authors: Liu X. , Ortoleva P. 
Published: December 2007

The use of reaction-transport modeling for reservoir assessment and management in the context of deep well waste injection is evaluated. The study is based on CIRF. A (Chemical Interaction of Rock and Fluid), a fully coupled multiphase flow, contaminant transport, and fluid and mineral reaction model. Although SWIFT (Sandia Waste-Isolation Flow and Transport Model) is often the numerical model of choice, it can not account for chemical reactions involving rock, wastes, and formation fluids and their effects on contaminant transport, rock permeability and porosity, and the integrity of the reservoir and confining units. CIRF. A can simulate all these processes.

Two field cases of waste injection were simulated by CIRF.A. Both observation data and simulation results show mineral precipitation in one case and rock dissolution in another case. Precipitation and dissolution change rock porosity and permeability, and hence the pattern of fluid migration. The model is shown to be invaluable in analyzing near borehole and reservoir-scale effects during waste injection and predicting the 10,000 year fate of the waste plume. The benefits of using underpressured compartments as waste repositories were also demonstrated by CIRF. A simulations.

Introduction

The disposal of fluid waste by deep well injection is an environmental issue of great concern. According to the USEPA regulations, five classes of injection wells have been established: Class I-injection of municipal or industrial waste (including hazardous waste) below the deepest underground sources of drinking water (USDW), Class II-injection related to oil and gas production, Class III- injection for mineral extraction and recovery, Class IV-injection of hazardous or radioactive waste into or above a USDW, and Class V-all other wells not included above that inject fluids into or above a USDW. Class I hazardous waste injection well operators must submit a No-Migration petition to the USEPA to obtain an exemption from the "land ban" restrictions on the disposal of hazardous wastes. Modeling of waste injection is usually required for such No-Migration petition.

No-Migration Petition modeling is currently performed to calculate the configuration of the injected waste plume during operation and the period of 10,000 years. Models such as the Sandia Waste-Isolation Flow and Transport Model-SWIFT currently used in support of No-Migration Petition applications include the effects of groundwater flow and contaminant transport. However, they do not account for the coupling of reaction and transport and its impact on the waste plume composition, geometry and extent nor do they account for the possible destruction of confining units due to waste fluid-rock interactions. The CIRF. A (Chemical Interaction of Rock and Fluid) model can simulate all these processes. The objective of this study is to investigate the use of the coupled reaction-transport model to address the critical issues related to waste injection and storage.

Injection of waste fluids into a reservoir can activate a host of complex chemical reactions and material or energy transport phenomena. A key element of waste repository management is a quantitative model for predicting the fate of injected waste fluids. This requires a knowledge of the chemical evolution of the waste fluid and the viability of the repository to either contain waste fluids over an acceptable period of time or render them neutral through fluid/rock interaction or thermal decomposition (as for organic waste). The injected fluid could lose or diminish its toxicity and could remain sequestered on the 10,000 year time scale and beyond. As in situ experiments are usually not feasible nor even desirable, a reliable predictive model is essential. A predictive model can be used in a number of ways: P. 661