ABSTRACT:

The ongoing Prudhoe Bay Grind and Inject (GNI) program began in the mid 1990?s. State and Federal Environmental regulations require the operator to assure the injected material not be allowed to migrate to groundwater aquifer zones or to the surface. Sensitive fracture modeling and annual surveillance activities have been conducted to ensure the integrity of the process. A recent extensive technical review of the field?s collected information, injection simulation results and well testing data was conducted to evaluate the containment of the injected slurry. The current paper discusses the results of engineering simulation and field monitoring efforts within the perspective of a geomechanics review to verify the injected slurry extent. Novel interpretations of pressure records along with new simulation results are presented. Analysis of the waste domain simulation, based on fracture propagation in compactable (soft) rocks and well testing data, confirm that the fracture domain is confined in the intended injection zone. This paper will also compare the results of successive fracture domain simulations to the results of annual well tests over the last 5 years. Finally, the applicability of other potential means of disposal domain monitoring and diagnostic tools such as surface and downhole tiltmeters, micro seismic diagnostic, and tracers, are assessed from a geomechanics perspective.

INTRODUCTION

The ongoing operation of Prudhoe Bay Grind and Inject (GNI) project started in 1998. The project involves injecting slurry of drilling-related waste into a soft target formation in three wells, GNI-1, GNI-2, and GNI-3. Typical operations involve injecting slurry into one of the three wells continuously for 10 to 14 days, and then switching injection to another well. That is, at one time, only one injector is active while the other two wells are shut-in. The average shut-in period for each well is 24 days. The average injection rate is 19 ? 23 barrels per minute (bpm). The project is by far the largest drill cutting injection in terms of slurry volume (including fluid and solid volume). Cuttings are delivered to the grinding facility from closeout operations in existing reserve pits and from ongoing drilling operations. As of September 30, 2004, total injection includes 20.0×106 barrels of water, 44.6×106 barrels of slurry containing 2.9×106 tons or 3.2 ×106 cubic yards of excavated frozen reserve pit material and drilling solids and 2.3×106 barrels of fluid from ongoing drilling operations. As the project continues, the involved technicians, engineers, and scientists have worked diligently to maintain all aspects of the GNI project facilities in good working condition. Their efforts have resulted in the safe and successful injection of a large amount of waste into the target formation.

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