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

Abstract

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, standing for Grind and Inject. Accumulated drilling cuttings and mud slurry is 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 barrels per day. As of September 30, 2002, project injection has included 12.7×10⁶ barrels of water, 30.9×10⁶ barrels 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 barrels 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 sequestration of the materials without harm to the environment. Numerical modeling, well testing (including step rate and pressure falloff testing) as well as logging surveys were performed periodically to assess the disposal wells' operational integrity and to ensure the safe containment of the disposed waste slurry. The great 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. The current paper will emphasize on how to integrate numerical simulations, well testing/monitoring and operational data to estimate storage capacity and to construct a clear representation of what was happening underground during this grind and injection 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, ARCO agreed to remove the mud and cuttings from all reserve pits. Additionally, ARCO and BP discontinued the practice of storing drilling mud and cuttings in surface reserve pits. These waste streams are now managed as they are generated by way of injection, 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. Drill cuttings injection projects have been operated worldwide since early 1990s.^1-5 However, these projects were generally small in volume. Feasibility evaluation of large scale injection of oily waste injection in Alaska started in late 1980s.¹ This field evaluation test also included a step-rate test, in-situ stress measurements, tilt-meter monitoring of ground surface deflections and a wellbore hydraulic impedance test.¹ Approximately 2 million barrels of slurry, containing crude oil, unused frac sands, drilling muds, unset cement and others, had been injected intermittently into this well at the time of the analysis. The injection rate varied from 500 bpd to 4,000 bpd.

The first large scale slurry injection project for the reserve pit closure started in March 1995 on the North Slope of Alaska. A total of 8 million barrels of slurry were injected into this well (well DS 4-19) over two years at approximately 30,000 bpd.⁶ Injected slurry contained a total of 413,000 cubic yards or 340,00 tons of excavated frozen mud and drill cuttings. This project demonstrated that Grind and Injection (GNI) is an environmentally safe and cost effective method for disposing the mud and drill cuttings resulted from pit closure on the North Slope of Alaska. It should be noted that the terms of DCI (Drill Cuttings Injection) and GNI are used interchangeably throughout this paper.