ABSTRACT:

This paper describes the data acquired during a comprehensive, joint-industry field experiment to improve the understanding of the mechanics and modeling of the processes involved in the downhole injection of drill cuttings. Disposal of {kill cuttings is of importance from both economic and environmental perspectives. The paper provides the field setting where the experiment was conducted, the formations selected for injection, the technical scope of work, the series of experiments conducted, and data acquired. The project consisted of three phases completed in 1998: drilling of the injection well and two observation wells, conducting about 20 intermittent cuttings-slurry injections into each of two disposal formations while imaging the created fractures from the observation wells, and coring through the imaged disposal zones by sidetracks from one of the observation wells.

INTRODUCTION

Reinjection of drilling waste has been implemented successfully over the past several years in many areas worldwide, e.g. the Gulf of Mexico, Alaska, North Sea, Azerbaijan, Venezuela, Trinidad, Bolivia, etc. It has been demonstrated to be a safe, environmental solution and (in the majority of cases) the most economical approach for disposal of solid wastes associated with drilling and production operations. The major issue is to assure that cuttings are permanently stored and contained in a small area of the depository formation and are not transported upward towards the surface, where aquifers may be present. Until recently, waste injection design was based on the conservative assumption that hydraulic fracturing creates a single fracture. Consequently, very large fractures have been predicted even for the relatively small scale disposal operations, such as annular injections. For example a penny shaped fracture of radius more than 700m is projected for injections of 20,000 m 3 of slurry. However, recent field experience has indicated that long-term repeated injections into shales and porous sandstone formations followed by shut-in periods do not result in a single fracture, but may result in waste storage in a system of multiple fractures contained within a relatively small area around the point of injection. New modeling approaches, such as the disposal domain concept, (Moschovidis et al, 1994) (Figure 1) or sand slurry mixing concept, have already been suggested (based on field observations) and recently have been verified experimentally in the laboratory by injections into medium and large blocks of sandstone and shale (Willson et al, 1999).

Based on theoretical arguments, the creation of a multiple-fracture disposal domain is expected when the horizontal in situ stresses are equalized due to accumulation of waste solids and possible shale swelling due to hydration. Intermittent injections with shut-in periods allow the fractures to dose and encourage creation of multiple fractures.

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