Feeling the pulse of drill cuttings injection wells - A case study of simulation, monitoring and verification in Alaska

In April 1998, a program for continuous deep disposal of drill cuttings and open pit materials was initiated on the North Slope of Alaska. This ongoing injection project is commonly referred to as GNI, or Grind and Inject. Accumulated drilling cuttings and mud slurry are injected into a receptive Cretaceous soft sandstone in three wells: GNI-1, GNI-2, and GNI-3. Typical operations involve injecting slurry into one of the three wells continuously for a number of days and then switching injection to another well. The average injection rate is approximately 30,000 B/D. As of 30 September 2002, project injection has included 12.7 x 10(6) bbl of water, 30.9 x 10(6) bbl of slurry containing 2.0 x 10(6) tons or 2.2 x 10(6) cubic yards of excavated frozen reserve pit material and drilling solids, and 1.31 x 10(6) bbl of fluid from ongoing drilling operations. Knowledge of the fate of the drilling and open-pit materials during injection is paramount to assure the safe containment of the disposed materials without harm to the environment. Numerical modeling, well testing (including step-rate and pressure-falloff testing), and logging surveys were performed periodically to assess the operational integrity of the disposal wells and to ensure the safe containment of the disposed waste slurry. The high-volume capacity of these injectors highlighted the mechanisms for slurry being accepted by multiple and branched fractures-part of the slurry went to previous fractures during subsequent batch injections. This paper will detail how to integrate numerical simulations, well testing/monitoring, and operational data to estimate storage capacity and construct a clear representation of what was happening underground during this GNI operation. The work has implications on other large drilling-waste injection projects worldwide.