Nanopores to Megafractures: Current Challenges and Methods for Shale Gas Reservoir andHydraulic FractureCharacterization
从纳米孔隙到宏观压裂：目前页岩气储层与水力压裂的挑战与方法
Journal of Natural Gas Science and Engineering, In Press, Accepted Manuscript, Available online 2 February 2016
Abstract:The past two decades have seen a tremendous focus by energy companies on the development of shale gas resources in North America, resulting in an over-supply of natural gas to the North American market in recent years. This shale gas “revolution” was made possible primarily through the application of drilling and completion technologies, particularly horizontal wells completed in multiple hydraulic fracturing stages (multi-fractured horizontal wells, or MFHWs). While these technologies have proven successful in commercializing the resource, imperfect understanding of basic shale gas reservoir properties, and methods used to characterize them, has perhaps led to inefficiencies in shale gas resource development and recovery that can be improved over time with further research.
The purpose of the current work is to 1) provide an overview of shale gas storage and transport mechanisms 2) summarize the challenges associated with evaluating key reservoir and hydraulic fracture properties and 3) discuss recent advances by the authors in the area of shale gas reservoir and hydraulic fracture characterization. For the latter topic, advances in multi-scale characterization techniques, from reservoir sample evaluation to production data analysis will be addressed, however an emphasis will be placed specifically on methods to evaluate reservoir and rock properties along the length of the horizontal well to enable selection of hydraulic fracture stage placement and improved well forecasting. Rock cuttings retrieved during drilling are typically the only reservoir samples obtained from horizontal wells, and therefore methods for quantitative assessment of pore structure, gas content, gas-in-place, permeability, fluid-rock interaction, and rock mechanical property assessment will be discussed. In particular, the following recent innovations by the authors are highlighted: 1) use of the Simplified Local Density (SLD) model to account for fluid property alteration from pore confinement, and to predict high pressure gas adsorption from low-pressure adsorption data collected for small amounts of cuttings samples 2) extraction of permeability/diffusivity from low-pressure adsorption rate data, also collected for small amounts of cuttings samples 3) use of a variable pressure, environmental SEM to assess fluid distribution and micro-wettability to support pore-scale modeling studies 4) estimation of unpropped hydraulic fracture permeability through generation of fractures in coreplugs under stress 5) use of microhardness tests to evaluate fine scale changes in “mechanical stratigraphy” and 6) use of sonic core-holders to provide measurements of dynamic rock mechanical properties for shale samples subject to in-situ stress. Modification of diagnostic fracture injection tests (DFITs), a common well-testing technique performed on shales to derive reservoir property and stress information but performed usually only at the toe of horizontal wells, to enable tests to be performed at multiple points along a horizontal well, will be proposed. Finally, advances in production analysis methods to account for effects such as pore confinement, relative permeability, and stress-dependent permeability will be reviewed, as will techniques for extracting hydraulic fracture properties through analysis of flowback data.
It is hoped that this summary will provide geoscientists and engineers with a comprehensive overview of shale gas reservoir and hydraulic fracture evaluation challenges and potential solutions, with a view to enabling more efficient shale gas extraction.
Induced mobility of inorganic and organic solutes from black shales usingwater extraction: Implications for shale gas exploitation
采用水提法的黑页岩无机与有机溶质诱导迁移性及其在页岩气开采中的应用
Applied Geochemistry, Volume 63, December 2015, Pages 158-168
Abstract:The study reported here evaluates the degree to which metals, salt anions and organic compounds are released from shales by exposure to water, either in its pure form or mixed with additives commonly employed during shale gas exploitation. The experimental conditions used here were not intended to simulate the exploitation process itself, but nevertheless provided important insights into the effects additives have on solute partition behaviour under oxic to sub-oxic redox conditions.
In order to investigate the mobility of major (e.g. Ca, Fe) and trace (e.g. As, Cd, Co, Mo, Pb, U) elements and selected organic compounds, we performed leaching tests with black shale samples from Bornholm, Denmark and Lower Saxony, Germany. Short-term experiments (24 h) were carried out at ambient pressure and temperatures of 100 °C using five different lab-made stimulation fluids. Two long-term experiments under elevated pressure and temperature conditions at 100 °C/100 bar were performed lasting 6 and 2 months, respectively, using a stimulation fluid containing commercially–available biocide, surfactant, friction reducer and clay stabilizer.
Our results show that the amount of dissolved constituents at the end of the experiment is independent of the pH of the stimulation fluid but highly dependent on the composition of the black shale and the buffering capacity of specific components, namely pyrite and carbonates. Shales containing carbonates buffer the solution at pH 7–8. Sulphide minerals (e.g. pyrite) become oxidized and generate sulphuric acid leading to a pH of 2–3. This low pH is responsible for the overall much larger amount of cations dissolved from shales containing pyrite but little to no carbonate. The amount of elements released into the fluid is also dependent on the residence time, since as much as half of the measured 23 elements show highest concentrations within four days. Afterwards, the concentration of most of the elemental species decreased pointing to secondary precipitations. Generally, in our experiments less than 15% of each analysed element contained in the black shale was mobilised into the fluid.
Afully coupled multiscaleshale deformation-gas transport model for the evaluation of shale gas extraction
全耦合多尺度页岩变形-气运移模型在页岩气开采评估中的应用
Fuel, In Press, Uncorrected Proof, Available online 22 March 2016
Abstract:Horizontal drilling and hydraulic fracturing are two enabling technologies to create a shale gas reservoir. For the created reservoir scale, we define shale blocks between hydraulic fractures as matrixes. In the matrix scale, flow processes are defined in the components of inorganic minerals and kerogens, respectively. Under this framework, a set of partial differential equations are derived to define various flow and deformation processes: (1) mechanical equilibrium equation that defines the shale deformation; (2) gas flow in the kerogen system of matrix; (3) gas flow in the inorganic system of matrix; and (4) gas flow in the hydraulic fracture system. For each of gas flow systems, a permeability or diffusivity model is derived to define its evolution. All systems are fully coupled through these permeability models and mass exchanges between different systems. The fully couple PDE system was solved by using COMSOL, a popular PDE solver. The model was verified against gas production data from the Marcellus Shale and the Barnett Shale, respectively. The verified model was applied to investigate the impact of adsorption parameters, flow regimes (Knudsen number), initial permeability of the inorganic matrix, and the effective stress variations on the gas production. Model results show that the Langmuir parameters affect both the cumulative gas production and the gas extraction processes; that the impact of flow regimes is closely related to the initial permeability of the inorganic matrix; and that the impact of effective stress variations on the permeability of hydraulic fractures is more significant than that on the matrix system.
Microscopic fracture characterization of gas shale via scratch testing
基于划痕测试的页岩气微观压裂特征
Mechanics Research Communications, In Press, Corrected Proof, Available online 4 January 2016
Abstract:We investigate the fracture properties of organic-rich shale at the microscopic scale by coupling advanced imaging techniques, fracture mechanics and micro-scale mechanical testing methods. We study three shale systems: Toarcian (Paris Basin, France), and Lower and Upper Woodford shale (Oklahoma, US). A material preparation procedure is designed so as to visualize the microstructure. Optical microscopy and scanning electron microscopy reveal a porous granular fabric with the grain size ranging from 30 to 100 μm as well as micron-sized air voids. Microscopic scratch tests are carried out, during which a stylus is pushed across the surface of a material under a prescribed monotonically increasing vertical load. We develop a fracture mechanics approach that takes into account the heterogeneity and anisotropy of gas shale. The microscopic scratch toughness predicted by the scratch fracture model is 2.35–2.98 MPa image, which is two times higher than the macroscopic fracture toughness. A microscopic examination of the fracture surface reveals toughening mechanisms such as particle pull out, crack front roughening and crack bridging. The methodology presented is new and will pave the way toward a mechanistic physics-based understanding of the fracture behavior of gas shale at multiple length scales. In turn, this will accelerate the design of optimum and efficient schemes to extract natural gas from unconventional shale.
Selenium enrichment in Carboniferous Shales, Britain and Ireland: Problem or opportunity for shale gas extraction?
英国与爱尔兰石炭纪页岩中的硒富集：页岩气开采困难与机遇？
Applied Geochemistry, Volume 66, March 2016, Pages 82-87
Abstract:The Carboniferous Bowland Shale in England, and its correlatives in Ireland, contain anomalously high concentrations of trace elements, including selenium (Se), molybdenum (Mo) and arsenic (As). High levels of these elements reflect high sulphur contents as these elements occur as trace constituents of pyrite. Anomalous Se in particular may have a volcanic provenance, from contemporary volcanic activity and/or drainage from Ordovician volcanogenic sulphide deposits. Following concern over the release of Se and As into groundwater during shale gas extraction in the US, the potential fate of Se and As during any future shale gas extraction from the Bowland Shale merits attention. It is at least an environmental issue that must be managed, but at best it could be an opportunity for extraction of Se in an environmentally sensitive manner.
