Drilling waste disposal through downhole hydraulic fracturing is often the preferred waste management option because it can achieve green operation and often has favorable economics. Most field situation comprise of injection in either dedicated well or in the annulus of an existing well. Containment of the disposed waste must be ensured and one of the questions in drilling waste injection operations is what the capacity of a disposal well or annular scheme is? The answer to this question depends on downhole waste storage mechanisms. It is evident from laboratory simulation studies and field operation experience that multiple fractures are created in drill cuttings injection (DCI) operations and the capacity of a disposal well is much larger than that estimated from single fracture simulations. More importantly, as more solids are injected into the disposal formation, the local stress is modified. Because of this change in local stress, fracture shapes and extents at the beginning of a DCI operation can be significantly different from the fracture shapes and extents at the end the operation. Modeling of this fracturing evolution process is necessary and essential to ensure the safe containment of the disposed waste and to estimate accurately the disposal capacity of a drilling waste disposal well.
This paper presents a numerical algorithm for modeling the multiple fracturing and fracture evolution process during drill cuttings injection operations. Case studies show that the modeling results based on multiple fracturing have significant impacts on DCI operations engineering such as injection pressure requirement and disposal capacity. The results also provide insight into best practices for the containment of disposed waste, when injection can continue into a previous zone and when is there a need to inject into a different zone or when a new disposal well should be drilled. For the purpose of brevity, "disposal well" will be used to designate either a dedicated injector well or an annular injection scheme.
Figure 1 shows an overview of a drill cuttings injection (DCI) scheme. Briefly, a drill cuttings injection operation involves collection and transportation of solid waste from control equipment on the rig to a slurrification unit, where the cuttings are ground (if needed) to small particles in the presence of water to make a slurry. The slurry is then transferred to a slurry holding tank for final slurry rheology conditioning. The conditioned slurry is pumped through a casing annulus or a tubular into sub-surface fractures created by injecting the slurry under high pressure into the disposal formation. The waste slurry is often injected intermittently in batches into the disposal horizon. The batch process consists of injection of roughly the same volumes of slurry and shutting-in the well after each injection. This allows the disposal fracture to close onto the cuttings and to dissipate any build-up of pressure in the disposal formation. This fracture closure and local fracture closure pressure increase due to the presence of the injected solids promote new fracture creation from next batch injection. The new batch fracture will not be aligned with the azimuths of previous existing fractures. Each batch injection may last from a few hours to several days or even longer, depending upon the batch volume and the injection rate.