Formation Damage and Its Impacts on Cuttings-Injection-Well Performance: A Risk- Based Approach on Waste-Containment Assurance

Abstract

With ever-tightening environmental regulations and the green initiatives of most operators, drilling waste disposal through downhole hydraulic fracturing often becomes the preferred waste management option. This is because the technology allows drilling wastes to be handled at the drilling site to achieve true zero discharge. However, formation damage due to solid-laden slurry injection can cause large uncertainties in injection well performance and waste containment assurance. Complicating the problems are the many formation damage mechanisms that are often difficult to model with confidence. A holistic approach based on Monte Carlo simulations has been developed for modeling formation damages and their competing contributions to injection well performance and waste containment assurance. This paper presents a probabilistic approach to modeling and evaluating associated uncertainties, particularly geology and formation damage, and their impacts on waste containment assurance and risk assessment in cuttings re-injection operations. Probabilistic results are important in designing cuttings injection operational procedures, and risk management, and in obtaining regulatory approval. Examples are given to illustrate how to model formation damage caused by intermittent slurry injections and its impacts on waste containment. Monitoring and validation procedures are given to increase quality assurance through operational data evaluation and risk-based modeling result validation. Introduction Oil and gas drilling and production produce wastes. For example, the principal waste product of the drilling process is often oil-contaminated drill cuttings. One of the preferred waste management options is downhole injections into a deep geological formation through hydraulic fracturing. This technology is often known as Cuttings Re-Injection (CRI) although other designations have been used, even if the injected material likewise includes production wastes such as produced sands, tank bottoms, produced waters and other by-products from oil/gas production.[1–11] Figure 1 is a schematic of a cuttings re-injection process. Very briefly, a drill cuttings injection operation involves the collection and transportation of solid waste from solids control equipment (in this case a shale shaker) 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 subsurface 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. Each batch injection may last from a few hours to several days or even longer, depending upon the batch volume and the injection rate.

Deep Underground Injection of Waste from Drilling Activities—An Overview

“…The selected batch volume for an intermittent process must be less than optimal, as well as the maximum batch volume, because of safety and operation limits. The duration of one batch injection can range from few hours to few days and depends on the batch volume and injection rate [32,57].…”

“…The main task of engineers is the characterization and visualization of the waste disposal domain, especially during waste injection above the fracture pressure of the disposal formation and prediction with some certainty of the fracture geometry (height and length). Disposal of the waste from drilling activities through a slurry injection into a suitable geological formation is very similar to the hydraulic fracturing as a method for reservoir stimulation, as well as to the lost circulation as a common problem during well drilling [57]. Due to that, for the purpose of numerical simulations and modeling of downhole waste slurry injection, commercial hydraulic fracture stimulators were used [25].…”

“…The required input information about the geological formation which is being considered for waste slurry injection can be obtained from wireline logs, well testing or through special petrophysical laboratory methods applied to core samples with reference to their reliability [39]. Guo et al [16] suggest the probabilistic approach for the assessment of the risk and containment of injected waste slurry, while Okorodudu et al [57] propose using a three-dimensional hydraulic simulator because of its capability to predict fracture geometry with more confidence through a simulation of single/multiple fractures, visualization capability, ability to achieve a more accurate result using less assumptions and assess risks based on a probabilistic model. Table 8.…”

“…The appearance of formation damage during the batch slurry waste injection and consequent change of the formation leak-off properties can directly affect the fracturing process and propagation of the disposal domain. If a new fracture or branch is created with each newly injected batch volume, the leak-off properties and the formation damage will depend on the proportion of the formation damage from previous injections [57]. Based on a large number of data from eight projects in the USA and Canada and the published information, Bruno et al concluded that, in large-scale injection projects, multiple fractures are likely to occur as a result of reduction in the fracture conductivity in combination with a stress increase within the waste pod.…”

General Principles of Well Barriers

“…Subsurface containment assurance is defined as the identification and mitigation of elements that could result in the potential loss of containment of subsurface fluids. The goal of subsurface containment assurance is to ensure no harm is caused to the environment and operated assets, or no impact on well operations due to the leakage of production or injection fluids from their intended zones [23,24]. One may claim that the subject of subsurface containment assurance fulfills the well integrity requirements, however, in fact it is more comprehensive than the well integrity.…”