<br /> The results of laboratory block fracturing experiments are presented which investigate the dominant fracturing mechanisms associated with drilling-waste injection operations. The tests include injection into reactive and competent shales, high permeability sands, and multi-layered formations of varying permeability. Specific details of proposed fracture mechanism have been confirmed or, where found to be inappropriate, these have been refuted. The results of these experiments help confirm the adequacy of<br /><br />
<br /> Downhole injection provides a solution to the environmental problem of disposing oil contaminated solid and liquid wastes generated during drilling, thus allowing the continued use of oil based muds (Minton et al 1992). This is an accepted disposal technique in the Gulf of Mexico, Alaska and North Sea, and is now being implemented in other oil production areas elsewhere in the World.<br /><br /> In any disposal operation, containment of the injected waste must be assured. This requires the geometry of hydraulic fractures, expressed in terms of height and length, created by injection to be predicted with some confidence. Typically, these predictions have relied upon the use of numerical simulation codes and analysis techniques developed for hydraulic fracture stimulation of production wells. However, these models cannot simply be applied to waste disposal fracturing without caution as a number of important differences exist between the two injection and fracturing processes: 1. In drilling waste disposal operations, although the total injected volume may be larger than typical hydraulic fracture stimulations, injection is usually intermittent, injecting relatively small batches of fluid over a long period of time. 2. Increasingly, 'gumbo' shales at shallow depth are being targeted as disposal horizons in waste disposal operations (Willson et al 1993; Sirev?g & Bale 1993). These shales have a very low permeability, but they may undergo considerable plastic deformation and be susceptible to chemical reaction with the cuttings slurry.<br /> ?