Hydraulic fracturing of unconventional reservoirs in the Powder River Basin in Wyoming and Montana is a growing source of oil and gas production. However, shale and tight-oil reservoirs in the region have high rates of decline in production compared to conventional oil and gas extraction, severely limiting well life. The full reasons for these high decline rates are unclear and have been attributed to a number of causes, including porosity decrease from fines migration. Recent field and experimental studies have shown that water-rock interaction with hydraulic fracturing fluid can cause mineral precipitation in the reservoir subsurface. Experimental studies into water-rock interaction also suggest that reservoirs are sensitive to changes in mineral surface area and to oil adhering to the mineral grains. This study tests the potential effect on water-rock interaction of removing residual oil from unconventional reservoir rock at reservoir conditions as found in the Powder River Basin in Wyoming. Rock samples from the Parkman Sandstone in the Powder River Basin, Wyoming were combined with synthesized formation water at in-situ reservoir conditions and reacted for ~35 days to approach steady-state. A simulated hydraulic fracturing fluid was then injected and reactions proceeded for another ~35 days. Fluid samples were collected throughout the experiment. One experiments uses rocks chemically processed to remove residual oil (low-residual oil, or LRO) and one uses rocks that retain residual oil (high-residual oil, or HRO). All experiments use 0.5–1 cm rock cubes to emulate the interface between fractures and the rock matrix. Analyzed chemistry results from aqueous samples collected during the experiments indicate water-rock interaction with both carbonates and clay minerals. Observation of rock recovered from the experiments shows changes to mineralogy visible in microscope or SEM. Fluid results suggest that unconventional reservoir rock with less residual oil at the mineral face is more prone to carbonate dissolution than reservoir rock with residual oil at the fracture face. Little evidence of precipitation or dissolution was observed on the recovered rock after experiments; however, water-rock interaction at the timescales of these experiments is not likely to cause significant changes to in-situ reservoir porosity or permeability. The water-silicate interaction trend suggests that the fluid chemistry may favor smectite or other clay precipitation at timescales beyond those represented in the experiments.

A comprehensive overview of the key geologic, geomechanical and engineering principles that govern the development of unconventional oil and gas reservoirs. Covering hydrocarbon-bearing formations, horizontal drilling, reservoir seismology and environmental impacts, this is an invaluable resource for geologists, geophysicists and reservoir engineers.

Unconventional petroleum reservoirs have become important resources for energy production. Flowback fluid produced from hydraulically fractured reservoirs is typically analyzed after hydraulic fracturing fluid is injected into the reservoir and the well has been shut-in for weeks. However, geochemical reactions between reservoir rock and injected fluid are known to occur on the order of a few days, a timeframe less than the typical shut-in period of a hydraulically fractured reservoir. Two laboratory experiments were performed to analyze the potential for geochemical reactions between reservoir rock and injected fracturing fluid within this timescale. Core from the Niobrara Formation (chalk and marl), a productive unconventional reservoir in the Denver-Julesburg Basin, Colorado, USA, and alkaline hydraulic fracturing fluid (pH=10.7) were reacted at reservoir conditions 113 °C (235 °F), 27.5 MPa (3988 psi)) for ~35 days. Temporal evolution of aqueous geochemistry and thermodynamic analysis of both experiments indicates 1) rapid pH neutralization by carbonate mineral reactions; 2) non-stoichiometric dissolution of Mg-calcite and formation of secondary calcite; 3) aluminosilicate mineral dissolution in the first 100 hours; and 4) secondary clay mineralization after 100 hours. Dissolution of barite is also indicated for both experiments, however, termination of the marl experiment produced barite scaling. Secondary precipitation of carbonate and silicate minerals is inferred in fluid chemistry but not observed using standard scanning microscopy and x-ray diffraction. The absence of secondary mineralization indicates limited reaction between alkaline hydraulic fracturing fluid and Niobrara Formation chalk and marl and thus little impact of fluid-rock interactions to extraction of fluids from unconventional reservoirs.

The advancements of hydraulic fracturing techniques ensure the improved fracture surface areas that are open to fluids flow. The induced microcracks accelerate the fluid communications between fractures and the fracture adjacent rock matrix at fracture surface. In brittle rocks, the generated fracture network puzzles engineers since the induced hydraulic fractures and activated pre-existing fractures challenge the stimulated reservoir volume (SRV) characterizations. Furthermore, the necessary engineered justifications of each stage due to lateral heterogeneity of the reservoir and the stress shadow effect (in-situ stress increase along the wellbore) even introduce another level of complexity of the effective fracture drainage complexity. Simultaneous fracture growth becomes difficult, resulting in variations of fracture half lengths, within a stage, and among stages. The failure planes of the rock, from mode I, mode II and the combination of them, are not smooth and parallel; instead, they are usually associated with certain surface roughness and non-planar morphology, which in turn inhibit the ideal Poiseuille flow in the fracture. As a result, the fundamental studies of non-planar and rough complex fracture paths to the proppant transport are essentially inevitable.

Scientific understanding of fluid flow in rock fracturesâ€"a process underlying contemporary earth science problems from the search for petroleum to the controversy over nuclear waste storageâ€"has grown significantly in the past 20 years. This volume presents a comprehensive report on the state of the field, with an interdisciplinary viewpoint, case studies of fracture sites, illustrations, conclusions, and research recommendations. The book addresses these questions: How can fractures that are significant hydraulic conductors be identified, located, and characterized? How do flow and transport occur in fracture systems? How can changes in fracture systems be predicted and controlled? Among other topics, the committee provides a geomechanical understanding of fracture formation, reviews methods for detecting subsurface fractures, and looks at the use of hydraulic and tracer tests to investigate fluid flow. The volume examines the state of conceptual and mathematical modeling, and it provides a useful framework for understanding the complexity of fracture changes that occur during fluid pumping and other engineering practices. With a practical and multidisciplinary outlook, this volume will be welcomed by geologists, petroleum geologists, geoengineers, geophysicists, hydrologists, researchers, educators and students in these fields, and public officials involved in geological projects.

Unconventional reservoirs are usually complex and highly heterogeneous, such as shale, coal, and tight sandstone reservoirs. The strong physical and chemical interactions between fluids and pore surfaces lead to the inapplicability of conventional approaches for characterizing fluid flow in these low-porosity and ultralow-permeability reservoir systems. Therefore, new theories and techniques are urgently needed to characterize petrophysical properties, fluid transport, and their relationships at multiple scales for improving production efficiency from unconventional reservoirs. This book presents fundamental innovations gathered from 21 recent works on novel applications of new techniques and theories in unconventional reservoirs, covering the fields of petrophysical characterization, hydraulic fracturing, fluid transport physics, enhanced oil recovery, and geothermal energy. Clearly, the research covered in this book is helpful to understand and master the latest techniques and theories for unconventional reservoirs, which have important practical significance for the economic and effective development of unconventional oil and gas resources.