Prognosis for Safe Water-Disposal-Well Operations and Practices That Are Based on Reservoir Flow Modeling and Real-Time Performance Analysis
Authors: Gogri M. P. , Rohleder J. M. , Kabir C. S. , Pranter M. J. , Reza Z. A. 
Published: April 2018

Oklahoma has been at center stage of induced seismicity. Water-disposal activities have been associated with triggering the increasing number of seismic events. The objective of the study is to provide a simple diagnostics method and procedure for safe water-disposal operations. A comprehensive suite of scenarios and parameters has been analyzed that affect water disposal. On the basis of this study, prognosis will lead to safe water-disposal operation without the adverse effect.

A suite of reservoir models involving water injection helped understand disposal-well performance. The well operational limits correspond to disposal-zone fracture gradient. The modified Hall analysis is used to ascertain the point of departure from normal injection behavior. Limiting cumulative injected volumes are determined and investigated for various scenarios from simple to increasingly complex subsurface conditions. This investigation includes studying the effects of disposal-zone porosity, compartment size, conductivity, formation compressibility, heterogeneity, and natural fractures. In addition, we explored the effects of communication with overlying producing zone, communication through completion anomaly, seal integrity, and fluid complexities.

This study illuminates an overall understanding of disposal-well performance through various scenario analyses. A relationship between disposal-zone fracture gradient and limiting cumulative injection volume is established. For a fracture gradient of 0.7 psi/ft, this limiting pore-volume (PV) injection is less than 2%, which corresponds well with the conventional wisdom learned from carbon dioxide (CO2) injection-well performance. The relationship of disposal-zone compartment size, established with rate-transient analysis (RTA), with limiting cumulative injection volume is formulated. Analyses from the various statistical design of experiments (DoEs) reveal the important variables that may affect disposal-well performance. The disposal-well operation can be devised in real time with the modified Hall analysis that can reveal the departure from normal injection-well behavior. Factors accentuating the departure from normal behavior include disposal-zone porosity, formation compressibility, and seal integrity. Situations in which pressure release through leaks or communication with an adjacent formation takes place will naturally accommodate a larger volume of disposal water. Also, we learned that limiting cumulative injection volume and not injection rate (assuming injection pressure gradient is less than the fracture gradient) triggers a departure from normal injection behavior.

Using a suite of numerical reservoir models and the reservoir-monitoring tools involving modified. Hall analysis and RTA led to a comprehensive understanding of disposal-well performance. This study found a relationship of fracture gradient with limiting cumulative injection volume, and identified key variables affecting the disposal-well behavior. These findings led to a smart and safe disposal-well monitoring scheme, which will help disposal-well management become more economical and environmentally friendly.