Ultimate Capacity of a Disposal Well in Drilling Waste Injection Operations

Abstract

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. Introduction 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.

Feeling the Pulse of Drill Cuttings Injection Wells—A Case Study of Simulation, Monitoring, and Verification in Alaska

“…Multiple fractures from slurry injection have been observed both in laboratory-simulated slurry injection tests and field trials (Abou-Sayed et al 2002;Abou-Sayed et al 2003;Guo et al 2000;Keck 2002;Willson et al 1999;Guo and Abou-Sayed 2003). These factors were considered in the present analyses.…”

“…With any E&P waste slurry injection operations, release of disposed waste into the environment must be avoided and waste containment assured. Assurance of waste containment and operation integrity requires careful engineering, operational data collection and evaluation, and a welldesigned well testing and monitoring program (Abou-Sayed and Guo 2001;Abou-Sayed et al 2002;Abou-Sayed et al 2003;Guo et al 2000).…”

Colorado Drill-Cuttings-Injection Pilot Results

“…Full-scale field tests (Moschovidis et al 2000;Abou-Sayed et al 2003) have shown that intermittent injection promotes multiple fractures that limit fracture extension and increases the capacity of the disposal domain.…”

Increased Assurance of Drill Cuttings Re-Injection - Challenges, Recent Advances and Case Studies

“…20 One of the consistent elements of these programs is that multiple fractures are created from repeated slurry injections and they characteristically occupy an evolving region (sometimes called the disposal domain). 5,[23][24][25][26] The reason that new fractures are created from repeated slurry injections is that shut-in periods between injections allow the disposal fractures to close onto the cuttings and dissipate any build-up of pressure in the disposal formation. The presence of the injected cuttings causes the re-distribution of the local stresses resulting in new fractures being produced with new injections.…”