Abstract

The rapid expansion of cuttings and waste injection operations into major E&P regions throughout the world, and millions of barrels of drilling cuttings and waste injected in complex shale formations dictate understanding of uncertainties regarding its actual downhole placement and footprint in so-called disposal domain. The disposal domain is defined as area of influence created by hydraulically induced fractures that accumulate the bulk volume of cutting solids from repetitive cuttings slurry injections.

In spite of outstanding milestones achieved by cuttings and waste injection in recent years, the subsurface risks involved demand constant assessment to ensure thorough understanding of the injection and complex fracturing process. Limited understanding and characterization of the hydraulically induced fractures and waste domain footprint could potentially have a considerable environmental impact.

Understanding of past assessments of the cuttings and waste injection disposal domain provides alternative means of reevaluating shale gas complex hydraulic fracturing. This involves past assessments of multiple fracturing complex systems along the lines of the uniform disposal domain and a system of interconnected dendrite fractures with bifurcations from the flank of the fracture, and its evaluation in terms of minimum and maximum in-situ stress change in a localized near-wellbore fracture area, in distance perpendicular and along the preferred fracture plane.

This paper presents a comprehensive review of the historical waste disposal domain evolution from early developments -- the Mounds Drill Cuttings Experiment (MDCE) by Moschovidis et al. (2000) -- to current understanding derived from microseismic observations in the Barnett, Haynesville and Marcellus shale gas fracturing. Particular discussion is presented around the presence of fissures in shale formations and its impact on refracture initiation, extension and branching, together with an alternative assessment of the disposal domain from microseismic observations that took place during the MDCE. In addition, non-ideal bottom-hole pressure signatures reported in the past from observations during regular cuttings and waste injection are re-visited and re-aligned within a complex disposal domain model. Explanations and understanding of pressure anomalies are discussed.

The paper demonstrates that a true level of assurance and risk control can be achieved for cuttings and waste injection disposal (and also shale gas fracturing for stimulation) around mapping of the fracture - waste disposal - storage capacity via proactive integration of pressure monitoring and analysis with microseismic. This is the valuable information required by environmentalist and regulatory authorities to ensure that fracturing for disposal can be performed in an environmentally safe and technically sound manner.

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