Prognosis for Safe Water-Disposal-Well Operations and Practices 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: October 2017

Oklahoma has been at the center stage of induced seismicity. Water-disposal activities have been attributed to trigger 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. Prognosis based on this study 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 employed 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 storativity, compartment size, conductivity, formation compressibility, heterogeneity, and natural fractures. Additionally, 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 of disposal zone fracture gradient and limiting cumulative injection volume is established. For a fracture gradient of 0.7 psi/ft, this limiting pore-volume injection is less than 2%, which corresponds well with the conventional wisdom learned from CO2 injection-well performance. The relationship of disposal-zone compartment size, established with rate-transient analysis, with limiting cumulative injection volume is formulated. Analyses from the various statistical design of experiments reveal the important variables that may affect disposal-well performance. The disposal-well operation can be devised in real time using the modified-Hall analysis that can reveal the departure from normal injection-well behavior. Factors accentuating the departure from normal behavior include disposal-zone storativity, formation compressibility, and seal integrity. Situations, where 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 and rate-transient analyses 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 economic and environmentally friendly.