Re-injection of oil contaminated drill cuttings is attracting considerable attention as a cost effective means of complying with environmental legislation concerning discharges of drilling wastes. Gyda is the first North Sea platform to adopt this method as the sole means of disposing of its drilling wastes. The ground cuttings slurry (ca. 15,000 bbls per well) is injected via the 13 3/8" × 9 5/8" casing annulus into hydraulic fractures of up to 213m radius created in the mudstone formation at 900m depth. Novel analyses using a 3D fracture simulator show that in the absence of stress or permeability barriers to prevent unconstrained height growth, fracture breakthrough to surface is theoretically possible if injected volumes exceed 90,000 bbls. The analyses further demonstrate that laterally extensive shallow sands do however provide effective permeability barriers to vertical height growth if injected volumes exceed 52,000 bbls.
The paper describes the techniques used in Prudhoe Bay to place small, highly conductive fractures in areas of the field where there is no fracture containment to prevent fracture growth into an aquifer.A reservoir study to investigate the effects of fracturing close to oil water contacts is presented, together with 3D fracture modelling of fractures with no downward containment. The paper highlights the importance of spurt losses in both frac designs and minifrac analyses, when fracturing high permeability reservoirs. A simple minifrac analysis, for use with complex fracture geometries, is provided.The fracture design methodology is presented, supported by field examples.
Not all unconventional plays are created equal, in a substantial number of regions around the world the tectonic environment is quite different from the typically relaxed and more passive states found widely in most, if not all, of the US unconventional plays. This is merely a function of the relative proximity of such plays to distinct geological features characterized by active tectonic plates and with dynamic margins and recent activity. The Nazca plate associated with the Andes, the Arabian plate linked with the Al-Hajar mountains and the Indian plate connected with the Himalayan mountain range are just a few examples of tectonically influenced regions, where potential hydrocarbon traps are subject to complex states of stress generated by convergent plates, subduction zones and associated faulting. This scenario often translates into severe strike-slip and reverse fault stress states. Additionally, the presence of both multi-layered and laminated formation geology as well as the presence of overpressure and pressure differentials, typical of tight gas and shale gas, can exacerbate this situation even further. This situation can result in an extremely challenging environment for the successful execution of hydraulic fracturing and the associated development of unconventional resources. This paper will demonstrate, that such complex stress-states will directly affect well completions and hydraulic fracturing in a multitude of ways, but that some of the most impactful consequences are often severe casing failures, production-liner restrictions and complex fracture initiation scenarios. Casing failures are responsible for increased intervention costs as well as higher costs for the upgraded and strengthened wells. Equally, such issues can severely impair efficient execution of the completion plan and create a bottle-neck to subsequent well production. Horizontal, complex and pancake fractures will typically develop in strike-slip / reverse fault stress states, often resulting in fracture conductivity and connectivity loss that will greatly impair the eventual well performance. Layer interface slippage and natural fault re-activation are dominant mechanisms for hydraulic fracture induced casing failures. Examples of micro-fracs, micro-seismic and other diagnostics will be presented aiming to document the practical difficulties encountered while completing wells in these complex environments. This paper will demonstrate that unconventional development in such environments requires a renewed focus on all aspects of well design and construction, from directional drilling and lateral placement to casing selection and lower completion design. All these considerations are made with the goal of enabling the competent delivery of a highly effective and conductive fracture network, to efficiently access and produce the hydrocarbon resource.
Since the inception of hydraulic fracturing, the industry has wrestled with the concept of over/under- flushing, and has always pitched this as a binary philosophy, attempting to determine/define that this is either a fundamentally good or a bad approach. This schism simply grew with the extensive development of unconventionals; the use of overflush being an inherent and fundamental requirement for an effective and economic unconventional completion sequence. This paper will demonstrate that the true answer, as any engineer would expect, is that a detailed assessment is what is required and on a case-by-case basis. The paper will describe and reference several fracturing case histories, in both the Conventional and Unconventional environments, where the application of an overflush, an underflush or an engineered approach have been assessed or applied. Rather than taking an easy headline grabbing route to perpetuate the myth that the process is a major paradigm, or simply either a good or a bad thing. This paper will discuss some of the key aspects that impact the suitability of one approach over another; and how engineering the implementation can lead to a broader range of applicability/suitability for the most economically effective outcome. This includes an appreciation of the production/economic profile and exposure to risk, which is hugely different in say an ultra-deep-water five well offshore development vs. a field development with some 100s – 1,000s of wells in a lower-cost onshore environment. The paper will describe and demonstrate some of the fundamental variables that need to be considered; attempting to elaborate on a few of the key parameters which can influence the effective outcome. The paper will also indicate that there are several different scenarios whereby any form of overflush can result in a detrimental impact on the production rate and EUR, and that these must be fully appreciated. Subtleties, related to reservoir characteristics and fracture geometry; should be examined and appreciated. Additional aspects such as how the production, drawdown and pore-pressure will be managed can also have an impact. All these considerations, and more, will be discussed, described, and referenced. While there is no doubt that the overflush debate will continue unabated, the intent of this paper is to reduce the damage (or uncertainty), one way or the other, that will result. It is an attempt, at a minimum, to ensure that the debate becomes solely a technical one related to the approaches to be taken rather than a black and white one of right or wrong. Ultimately, the paper will advise, and inform, that the approach should be fully considered, engineered in detail and tailored to each and every application and that as a result is no longer simply considered a binary question.
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