Fig 2 - available via license: CC BY-NC-ND
Content may be subject to copyright.
Source publication
In this paper, a quantitative seismic prediction technique of multi-parameter shale brittleness index suitable for the environment with complex structural stress was developed in order to confirm highly brittle intervals and favorable fracturing zones in the shale reservoirs in the Jiaoshiba Block, Fuling shale gas field, Sichuan Basin. Firstly, th...
Context in source publication
Context 1
... Jiaoshiba Block is located both in the separated blocktype fold in the eastern Sichuan Basin, and in the Wanzhou complex syncline to the west of Qiyueshan fault at the boundary of the basin [16,17]. Generally, it presents a diamond faulted anticline trending in NeE (Fig. 2). As a part of the Sichuan Basin, the Jiaoshiba structure has experienced the evolution of cratonic depression in the Early Paleozoic, cratonic rifting in the Late Paleozoic, and foreland depression in the Mesozoic. In this process, it was reformed by multi-stage multi-directional intense tectonic movements with the dominance of nappe ...
Similar publications
The main goal of the study was to enhance and improve information about the Ordovician and Silurian gas-saturated shale formations. Author focused on: firstly, identification of the shale gas formations, especially the sweet spots horizons, secondly, classification and thirdly, the accurate characterization of divisional intervals. Data set compris...
Citations
... To more comprehensively and quantitatively evaluate the frackability of the various lithofacies, mechanical and mineral brittleness indices were used for characterization [66,84]. The mechanical and mineral brittleness indices were calculated based on Young's modulus, Poisson's ratio, and the contents of different minerals [85][86][87]. The mechanical brittleness index was obtained via Young's modulus and Poisson's ratio, and the mineral brittleness index was obtained based on the contents of different minerals ( Table 1). ...
Lacustrine shale is characterized by rapid lithofacies transformation and compositional heterogeneity, which present challenges in shale oil sweet spot evaluation and distribution prediction and should be systematically studied. Field emission-scanning electron microscopy (FE-SEM), low-pressure adsorption isotherm analysis, mercury intrusion porosimetry (MIP), and triaxial compression testing were employed to comprehensively analyze the oil-bearing capacity, reservoir properties, fluidity, and frackability of different lithofacies. Via analyses of mineral composition, total organic carbon (TOC) content, and sedimentary structure, seven lithofacies were identified: organic-rich calcareous shale (L1), organic-rich laminated calcareous mudstone (L2), organic-rich laminated carbonate-bearing mudstone (L3), intermediate-organic laminated calcareous mudstone (L4), organic-poor laminated calcareous mudstone (L5), organic-poor thin-bedded calcareous mudstone (L6), and organic-rich laminated silty mudstone (L7). Considered together, the oil-bearing capacity, reservoir properties, fluidity, and frackability suggested that the L1 and L7 lithofacies were high-quality sweet spots, with satisfactory oil-bearing capacity (TOC>3.5%; S1>10 mgHC/grock), well-developed pores and microfractures, notable fluidity (as indicated by a high oil saturation index value), and suitable brittleness. The sweet spot distribution was predicted according to multiresolution graph-based clustering analysis of well logs. The results indicate that comprehensive research of the key factors for shale oil and lithofacies prediction can promote sweet spot prediction and enhance shale oil exploration.
... The role of modulus of elasticity (E) and Poisson ratio (υ) in determining the brittleness index of rock has been investigated, indicating that the increase in E and υ leads to an increase and a decrease in the rock brittleness, respectively (Rickman et alii, 2008). Li & Li (2018) used a quantitative seismic prediction method to determine BI on shale, using modulus of elasticity, Poisson ratio, and mineralogical compositions. Samaei et alii (2018) also used uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), specific gravity, and rock material to calculate brittleness. ...
... The main reason for the decrease in brittleness in the saturated state is the presence of clay minerals, in this formation particularly parameters such as BI, different properties of rock mass are effective and simultaneous study of the effect of these properties in determining different geotechnical parameters can provide more realistic results. One way to achieve this goal is to use multivariate regressions (Kaunda & Asbury, 2016;Lashkaripour et alii, 2018;Yagiz et alii, 2018;Li & Li, 2018;Karami et alii, 2021;Moradi et alii, 2021 ...
... Similarly in isotopic or marginally anisotropic rocks, UCS and Vp or Vs are inversely correlated to porosity. A multivariate relationship using mechanical properties was proposed to predict BI in shale rock, which provides valid results (Li & Li, 2018). ...
... After the fracturing of a shale reservoir, the initiation and extension of fractures are the key problems during fracturing, and the brittleness index is an important parameter affecting the efficiency of fracturing. In the aspect of brittleness evaluation of shale, a large number of researchers have obtained a great deal of understandings from experiment evaluation, prediction through well logging, and its engineering application [4][5][6][7][8][9][10][11]. The results of these studies played an important role in the evaluation of the fracability in shale gas reservoirs. ...
The rock physics experiments and fracture toughness tests of shales from the Lower Silurian Longmaxi Formation in the Sichuan Basin in China were carried out. Based on this, the calculation model of the fracture toughness was constructed, thus, the single well evaluation of the fracture toughness in shale formation would be obtained based on the well logging data, which can be used to summarize the spatial distribution characteristics of the fracture toughness in the shale formation. However, it is difficult to obtain transverse distribution characteristics of fracture toughness in shale formation based solely on the well logging data. Therefore, in order to investigate the spatial distribution of the fracture toughness, jointing well logging and seismic method could be adopted to quantitatively predict the fracture toughness in shale formation. The results show that fracture toughness of shales is sensitive to acoustic interval transit time and wave impedance. The prediction model of the fracture toughness of shales was constructed, which had a good prediction effect. The fracture toughness values of shales from the Upper Silurian Wufeng-Longmaxi Formation were larger, whereas those of shales from the Lower Silurian Wufeng-Longmaxi Formation were lower. The fracture toughness is mainly distributed in strips along the vertical direction while the distribution area is continuous in the lateral direction, indicating that it has obvious stratification characteristics.
In situ stress distribution and mechanical properties both control the engineering performance of shale, coal, and tight sandstone gas/oil reservoirs. A clear understanding of the above parameters is of significance for engineering design and gas/oil recoveries. The detailed utilization of well logging data combined with basic laboratory tests can provide insight to both mechanical property and in situ stress distributions. The comprehensive combination of in situ stress and mechanical properties is presented here to clarify the understanding of unconventional reservoirs. The acquisition of static and dynamical parameters, brittleness of shale and present in situ stress magnitude is presented with a case study from southern China. The Lower Silurian Longmaxi formation is one of the most promising shale gas reservoirs in China. In this chapter, Longmaxi formation shale samples from the Jiaoshiba area, Sichuan Basin, China, were selected for laboratory experiments, and a series of testing methods such as scanning electron microscope, uniaxial compression, and triaxial compression were carried out. The analysis presented combines logging data with the geomechanics and petrophysical properties of the Longmaxi formation shale. The results reveal the mechanical properties and in-situ stress characteristics of this shale reservoir, and provide the necessary information for field hydraulic fracturing and wellbore stability applications in the Longmaxi shale gas development process.
Differentiating brittle zones from ductile zones in low permeability shale formations is imperative for efficient hydraulic fracturing stimulation. The brittleness index (BI) is used to describe the rock resistance to hydraulic fracture initiation and propagation and measures the ease at which complex fracture networks can be created. In this study, we constructed brittleness templates through the correlation of fundamental rock properties and geomechanical characterization. We then employed the templates to distinguish the brittle, ductile, and brittle-ductile transition zones in the Longmaxi shale gas reservoir, Sichuan Basin of southern China. The approach works in two steps. First, we suggest a new expression for the mineralogical BI by their respective weights based on the analysis of correlation coefficients between mechanical testing and XRD results. Second, we correlate TOC, porosity, pore fluid, natural fractures, and improved BI model with multiple elastic properties to define the brittle, ductile, and transitional zones in the Longmaxi shale gas reservoir of China. Compared with the traditional mineralogy-based BI definition, the improved BI model differentiates the brittle and ductile zones and provides a better sense of the most suitable fracturing regions. Our results show that the brittleness templates, which combine fundamental rock properties, improved BI model, and geomechanical characterization led to identifying favorable zones for hydraulic fracturing and enhanced shale characterization. The proposed brittleness templates’ effectiveness was verified using data from horizontal wells, offset wells, shale gas wells from different origins, laboratory core testing, and seismic inversion of BI across the studied wells.
The lamellation fractures in marine shales are important to the accumulation and preservation of shale gas, whose characteristics and controlling factors are still unclear. This paper focus on the characteristics descriptions and controlling factor analyses based on core observations, thin sections, and scanning electron microscope (SEM) experiments. The marine shales of Paleozoic Wufeng Formation and Longmaxi Formation in the Fuling Area of Eastern Sichuan Basin are taken as an instance. The lamellation fractures are occurring along the direction of the lamellations, partially of them curved, bifurcated, and converged. The apertures of fractures typically range from 1 to 500 μm. Only a few (<1%) lamellation fractures are filled with quartz, calcite, pyrite, and bitumen. After a series of analyses, developments of lamellation fractures are controlled by total organic carbon (TOC), brittle minerals, laminae, and pyrite. (1) The organic matters control the formation and evolution of lamellation fractures by generating gas and resulting in overpressure. An increase in the TOC content of 1% can increase the lamellation fracture density by 0.26 cm⁻¹. (2) Brittle minerals, especially authigenic quartz, is also an important controlling factor to the development of lamellation fractures. When BI increases by 10%, the lamellation fracture density increases by 19%. (3) Besides, siliceous and carbonate laminae in the shale are more suitable for lamellation fractures to develop. When the laminae density is smaller than 4 cm⁻¹, it is easier to develop lamellation fractures with higher laminae density. However, too densely-distributed laminae will suppress the formation of lamellation fractures. (4) Higher pyrite content is good for the development of lamellation fractures. Lamellation fracture density will increase by 9%, with pyrite content increased by 1%. For the shale at the bottom of the Wufeng Formation - Longmaxi Formation, lamellation fractures develop extensively due to the high TOC and pyrite content, great brittleness, and moderate laminae density. The density of lamellation fractures decreases from the bottom to the top.
The brittleness of shale gas reservoir determines the effect of fracturing reconstruction, which is of great significance for improving shale productivity. The study of shale brittleness evaluation can provide basic data for fracturing design. The mineral composition characteristics of shale gas reservoir in the early Cambrian Qiongzhusi formation of Huize area, Eastern Yunnan Province are reported in this paper, and the brittleness is evaluated according to the proportion of brittle minerals in total minerals. The results show that the mineral composition of the reservoir is complex. After comparing the brittleness index calculated by different formulas, it is considered that the minerals with high young’s modulus and low poisson’s ratio should be considered as brittle minerals, which is conducive to the universality of the brittleness index formula and the objectivity of reservoir brittleness evaluation. Shale gas reservoir of Qiongzhusi formation is obviously divided into two sections: the layers of 1 to 7 with a large brittleness index (77.60% on average) on the lower part are beneficial to reservoir fracturing reconstruction, while the layers of 8 to 14 with a small brittleness index (37.78% on average) on the upper part are not conducive to reservoir fracturing reconstruction. This study is conducive to guiding further exploration and development of shale gas in the research area.
