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.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
4 February 2004
- Revised manuscript received:
3 May 2005
- Manuscript approved:
4 May 2005
- Version of record:
20 December 2007
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