Feeling the Pulse of Drill Cuttings Injection Wells - A Case Study of Simulation, Monitoring and Verification in Alaska

Summary 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 12.7×106 bbl of water, 30.9×106 bbl of slurry containing 2.0×106 tons or 2.2×106 cubic yards of excavated frozen reserve pit material and drilling solids, and 1.31×106 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. Introduction Early drill sites on the North Slope of Alaska were designed with reserve pits for surface storage of mud and cuttings from drilling operations. In 1993, the operator at the time agreed to remove the mud and cuttings from all reserve pits. Additionally, the practice of storing drilling mud and cuttings in surface reserve pits was discontinued. These waste streams are now managed as they are generated by way of injection, thus eliminating the need for surface reserve pits. The estimated total volume of reserve pit mud and cuttings to be managed by this process is over 5 million cubic yards (not including drilling mud and cuttings generated from ongoing drilling operations). After reviewing disposal options, slurry injection was selected as the preferred disposal technique to remediate the reserve pits. While drill cuttings injection projects have been operated worldwide since the early 1990s (Abou-Sayed et al. 1989; Malachosky et al. 1991; Sirevag and Bale 1993; Moschovidis et al. 1993). They were generally small in volume. Feasibility evaluation of large scale injection of oily waste injection in Alaska started in the late 1980s (Abou-Sayed et al. 1989). This field evaluation test also included a step-rate test, in-situ stress measurements, tiltmeter monitoring of ground surface deflections, and a wellbore hydraulic impedance test (Abou-Sayed et al. 1989). Approximately 2 million bbl of slurry, containing crude oil, unused frac sand, drilling muds, unset cement, and other elements, had been injected intermittently into this well at the time of the analysis. The injection rate varied from 500 to 4,000 B/D.