The Mounds Drill-Cuttings Injection Field Experiment: Final Results and Conclusions

Abstract

Abstract This paper summarizes the results obtained from a comprehensive, joint-industry field experiment designed to improve the understanding of the mechanics and modeling of the processes involved in the down hole injection of drill cuttings. The project was executed in three phases: drilling of an injection well and two observation wells (Phase 1); conducting more than 20 intermittent cuttings-slurry injections into each of two disposal formations while imaging the created fractures with surface and down hole tiltmeters and down hole accelerometers (Phase 2); and verifying the imaged fracture geometry with comprehensive deviated-well (4) coring and logging programs through the hydraulically fractured intervals (Phase 3). Drill cuttings disposal by down hole injection is an economic and environmentally friendly solution for oil and gas operations under zero-discharge requirements. Disposal injections have been applied in several areas around the world and at significant depths where they will not interfere with surface and subsurface potable water sources. The critical issue associated with this technology is the assurance that the cuttings are permanently and safely isolated in a cost-effective manner. The paper presents results that show that intermittent injections (allowing the fracture to close between injections) create multiple fractures within a disposal domain of limited extent. The paper also includes the conclusions of the project and an operational approach to promote the creation of a cuttings disposal domain. The approach introduces fundamental changes in the design of disposal injections, which until recently was based upon the design assumption that a large, single storage fracture was created by cuttings injections. Introduction Fracture geometry has a predominant influence on the design of disposal injections, economic feasibility, waste containment and permitting. Until recently, waste-injection design was based on the conservative assumption that a single hydraulic fracture is created. Consequently, very large fractures have been predicted even for the relatively small-scale disposal operations, such as annular injections. However, field experiences indicate that long-term injections into shales and sands, interrupted by shut-in periods (either by design, or dictated by mechanical failures, or by other field operations) 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. The disposal-domain concept1 (Figure 1) has been proposed based on injection-pressure behaviour of disposal injections in the Valhall Field and has been recently verified experimentally in the laboratory by injections into medium and large blocks of sandstone and shale2.