At the end of 2015 the U.S. held 5.6% or approximately 369 Tcf of worldwide conventional natural gas proved reserves (British Petroleum Company, 2016, “BP Statistical Review of World Energy June 2016,” British Petroleum Co., London). If unconventional gas sources are considered, natural gas reserves rise steeply to 2276 Tcf. Shale gas alone accounts for approximately 750 Tcf of the technically recoverable gas reserves in the U.S. (U.S. Energy Information Administration, 2011, “Review of Emerging Resources: U.S. Shale Gas and Shale Oil plays,” U.S. Department of Energy, Washington, DC). However, this represents only a very small fraction of the gas associated with shale formations and is indicative of current technological limits. This manuscript addresses the question of recovery efficiency/recovery factor (RF) in fractured gas shales. Predictions of gas RF in fractured shale gas reservoirs are presented as a function of operating conditions, non-Darcy flow, gas slippage, proppant crushing, and proppant diagenesis. Recovery factors are simulated using a fully implicit, three-dimensional, two-phase, dual-porosity finite difference model that was developed specifically for this purpose. The results presented in this article provide clear insight into the range of recovery factors one can expect from a fractured shale gas formation, the impact that operation procedures and other phenomena have on these recovery factors, and the efficiency or inefficiency of contemporary shale gas production technology.
Summary Horizontal wells combined with successful multistage-hydraulic-fracture treatments are currently the most-established method for effectively stimulating and enabling economic development of gas-bearing organic-rich shale formations. Fracture cleanup in the stimulated reservoir volume (SRV) is critical to stimulation effectiveness and long-term well performance. However, fluid cleanup is often hampered by formation damage, and post-fracture well performance frequently falls to less than expectations. A systematic study of the factors that hinder fracture-fluid cleanup in shale formations can help optimize fracture treatments and better quantify long-term volumes of produced water and gas. Fracture-fluid cleanup is a complex process influenced by multiphase flow through porous media (relative permeability hysteresis, capillary pressure), reservoir-rock and -fluid properties, fracture-fluid properties, proppant placement, fracture-treatment parameters, and subsequent flowback and field operations. Changing SRV and fracture conductivity as production progresses further adds to the complexity of this problem. Numerical simulation is the best and most-practical approach to investigate such a complicated blend of mechanisms, parameters, their interactions, and subsequent effect on fracture-fluid cleanup and well deliverability. In this paper, a 3D, two-phase, dual-porosity model was used to investigate the effect of multiphase flow, proppant crushing, proppant diagenesis, shut-in time, reservoir-rock compaction, gas slippage, and gas desorption on fracture-fluid cleanup and well performance in Marcellus Shale. The research findings have shed light on the factors that substantially constrain efficient fracture-fluid cleanup in gas shales, and we have provided guidelines for improved fracture-treatment designs and water management.
Conventional hydrocarbon reservoirs, from an engineering and economic standpoint, are the easiest and most cost-efficient deposits to develop and produce. However, as economic deposits of conventional oil/gas become scarce, hydrocarbon recovered from tight sands and shale deposits will likely fill the void created by diminished conventional oil and gas sources. The purpose of this paper is to review the numerical methods available for simulating multiphase flow in highly fractured reservoirs and present a concise method to implement a fully implicit, two-phase numerical model for simulating multiphase flow, and predicting fluid recovery in highly fractured tight gas and shale gas reservoirs. The paper covers the five primary numerical modeling categories. It addresses the physical and theoretical concepts that support the development of numerical reservoir models and sequentially presents the stages of model development starting with mass balance fundamentals, Darcy’s law and the continuity equations. The paper shows how to develop and reduce the fluid transport equations. It also addresses equation discretization and linearization, model validation and typical model outputs. More advanced topics such as compositional models, reactive transport models, and artificial neural network models are also briefly discussed. The paper concludes with a discussion of field-scale model implementation challenges and constraints. The paper focuses on concisely and clearly presenting fundamental methods available to the novice petroleum engineer with the goal of improving their understanding of the inner workings of commercially available black box reservoir simulators. The paper assumes the reader has a working understanding of flow a porous media, Darcy’s law, and reservoir rock and fluid properties such as porosity, permeability, saturation, formation volume factor, viscosity, and capillary pressure. The paper does not explain these physical concepts neither are the laboratory tests needed to quantify these physical phenomena addressed. However, the paper briefly addresses these concepts in the context of sampling, uncertainty, upscaling, field-scale distribution, and the impact they have on field-scale numerical models.
It is anticipated that increasing pressure for cleaner burning fuels and lower carbon dioxide (CO 2 ) emissions will cause a shift in global energy demand from oil to natural gas. In the near future, natural gas is expected to replace crude oil as the fuel of choice for energy production and transportation. In Trinidad and Tobago, natural-gas production has already surpassed crude-oil production. Natural gas accounts for 80% of the country's energy export, but the reserves-to-production ratio is only 7 years (year 2022). Consequently, the Ministry of Energy has taken steps to supplement the natural-gas resource base by supporting initiatives that can potentially bolster the nation's proven gas reserves. Such initiatives include invitations to tender on deepwater blocks offshore Trinidad and Tobago's gas-rich east coast.Even though initiatives are under way to boost conventional natural-gas reserves, effort was not placed on identifying and/or characterizing unconventional gas resources such as natural-gas hydrates. Furthermore, the potential hazards of submarine gas hydrates on deepwater exploration and production (E&P) activities on Trinidad and Tobago's east coast were not assessed. The results presented in this manuscript provide oil-and-gas operators with a means of proactively managing the risk associated with natural-gas hydrates. More importantly, this study acts as a necessary precursor to future studies in characterizing and, later, harnessing the energy potential of Trinidad-and-Tobago's natural-gas-hydrate deposits.
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