Summary
Unconventional reservoirs such as gas shales and tight gas sands require technology-based solutions for optimum development because of the undeveloped matrix pores and poor permeability. Hydraulic fracturing is one of the most critical technologies. The quantitative characterization of hydraulic fractures is of great significance to the stimulation evaluation of the reservoir, but there is still a lack of fine, effective and systematic evaluation methods. 3D optical scanning technology is widely used in the quantitative characterization of rock fracture morphology for its advantages of high speed, convenience, high precision, and nondestructive testing. In this study, after the indoor hydraulic-fracturing simulation experiments, 3D optical scanning was used to visualize the fracture network. On this basis, two aspects of quantitative evaluation methods for stimulation effectiveness were established, including: (1) evaluating the local conductivity (permeability) of different fractures by cutting hydraulic-fracturing samples. Then combining local conductivity of different fractures with the overall stimulated reservoir area, which could be more reasonable to evaluate the stimulation scope of the reservoir; (2) calculating the fractal dimension (FD) of the 3D spatial structure based on the point-cloud processing, which could directly reflect the complexity of the fracture network. Finally, a new evaluation index for stimulation (Es) was established to comprehensively assess the stimulation effectiveness of the reservoir, which was applied and verified through the indoor fracturing simulation experiments of tight sandstone from the Ordos Basin, China.
Marine
and lacustrine shales are two major important types of organic-rich
shales that are usually deposited in the stable ocean basin and deep
lake/swamp settings, respectively. In the past decade, commercial
development of marine shale gas resources has been achieved. The underdeveloped
lacustrine shale also showed great oil and gas potential. Is it feasible
to simply copy the development mode of marine shale gas to lacustrine
shale? This is what this paper would attempt to answer from the viewpoint
of rock mechanics. Using borehole cores from two typical marine and
lacustrine shale gas wells, a series of experiments, including X-ray
diffraction (XRD), triaxial compression, fracture surface three-dimensional
(3D) scanning, and scanning electron microscopy (SEM), were implemented
to comprehensively reveal the mechanical difference between marine
and lacustrine shales. Postfailure fracture complexity and surface
roughness were quantified by fractal dimension and morphology reconstruction,
respectively. A new index BInew, considering the influences
of the complete stress–strain curve and fracture characteristics,
was proposed to evaluate the brittleness of these two types of shales.
Results showed that the mechanical properties of marine and lacustrine
shales were quite different and should be exploited by following their
own characteristics. Mechanical parameters of lacustrine shale, such
as compressive strength, Young’s modulus, cohesive strength,
and internal friction angle, were much lower than those of marine
shale. Different from the typical shear plus bedding splitting failure
mode of marine shale, lacustrine shale usually formed only one smooth
shear fracture. Based on the newly proposed index, the brittleness
of lacustrine shale was significantly lower than that of marine shale.
Huge discrepancy in quartz and clay mineral contents and the consequent
petrophysical structure divergence were the reasons for the marked
mechanical difference between marine and lacustrine shales. Differentiation
strategies and effects of hydraulic fracturing for these two kinds
of formations were also discussed.
Competitive propagation of fractures initiated from multiple perforation clusters is universal in hydraulic fracturing of unconventional reservoirs, which largely influences stimulation. However, the propagation mechanism of multi-fractures has not been fully revealed for the lack of a targeted laboratory observation. In this study, a physical simulation experiment system was developed for investigating the initiation and propagation of multi-cluster hydraulic fractures. Different from the traditional hydro-fracking test system, the new one was equipped with a multi-channel shunting module and a strain monitoring system, which could guarantee the full fracture extension at each perforation clusters and measure the internal deformation of specimens, respectively. Several groups of true tri-axial fracturing tests were performed, considering the factors of in situ stress, cluster spacing, pumping rate, and bedding structures. The results showed that initiation of multi-cluster hydraulic fractures within one stage could be simultaneous or successive according to the difference of the breakdown pressure and fracturing fluid injection. For simultaneous initiation, the breakdown pressure of the subsequent fracture was lower than or equal to the value of the previous fracture. Multiple fractures tended to attract and merge. For successive initiation, the breakdown pressures of fractures were gradually increasing. The subsequent fracture tended to intersect with or deviated from the previous fracture. Multiple fractures interaction was aggravated by the decrease of horizontal stress difference, bedding number and cluster spacing, and weakened by the increase of pump rate. The propagation area of multiple fractures increased with the pump rate, decreased with the cluster spacing. The strain response characteristics corresponded with the initiation and propagation of fracture, which was conducive to understanding the process of the fracturing. The test results provide a basis for optimum design of hydraulic fracturing.
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