Abstract The microseismicity associated with hydraulic fracturing in unconventional reservoir (i.e. shale gas play) has been investigated in the past several decades. Few experimental studies with respect to the focal mechanism and stress inversion was conducted, especially for Glutenite reservoir. In this study, the glutenite core was taken from the underground of 2600 m. Next, we performed scaled hydraulic fracturing tests on the cubic core (50×50×50mm) under geological principle stress condition in true tri-axial stress cell. Meanwhile, we monitored wellbore and pore pressure, and micro-seismic events during the fracture propagation from six faces of the cubic rock. Micro-seismic survey and events were interpreted to identify the induced fractures distribution in three dimension. Source mechanism and stress inversion were analyzed by moment tensor decomposition. The correlation of failure plane from microseismicity and tested sample implied that the microseismic events were accurately localized. The distribution of microseismic events from secondary and reopening tests indicated that the hydraulic fracturing induced microseismicity are mainly caused by significant tip effect (i.e. reactivate preexisting natural fractures). Based on source mechanism analysis, we found that the most of the failure are dominated by double-couple (DC). The correlation between original principle stress state and the one from STESI inversion indicated that the direction of principle stresses, especially for σ2 and σ3 inversed from reopening test, can be highly influenced by the hydraulic induced fracture or weak planes during secondary fracturing test.
Production performance of shale oil and gas wells is complicated due to the uncertainty of fracture geometry and heterogeneity of reservoir properties, et al. Especially, it is difficult to link the engineer parameters such as stage, cluster, proppant, fracture fluid to well production, which is significant for fracturing design and optimization. The purpose of this study is trying to find out this link and reveal the production performance and influence factors for shale oil and gas reservoirs. A large number of analytical modeling has done and the sensitivity of production influence factors has studied. Moreover, a field case of shale oil and gas basin in US is analyzed and compared with the theoretic study results. Sensitivity study showed that the production rate increases with fracture half-length, conductivity and horizontal well length increase, while declines with the fracture spacing increases. All of the engineering parameters have optimal values. Field case stated that the influence rules of engineering parameters on production are consistent with the theoretical modeling results, but it is more complicated. It is shown that production has positive correlation with horizontal well length, and has ascending trend with stage, cluster, fracture fluid, proppant increase although it is not obvious. However, the production contribution per increment of their values will decline when they increase. In most cases, the increase of engineering parameters could not lead to production decline, and will only add the operation cost and difficulty. So in order to design the engineering parameters, the optimal value should be determined based on economic indexes. The link between fracturing parameters and fracture characteristics is significant. Fracturing monitoring or fracture propagation modeling is required and necessary to understand well performance for shale oil and gas reservoirs.
Abstract Hydraulic fracturing is a coupled multi-physics and scale-dependent process requiring an extensive numerical-laboratory appraisal to assess feasibility in the field. Developing a robust model of hydraulic fracture propagation requires knowledge of the time evolution of the fracture’s geometrical attributes, e.g., width/aperture and length/radius. However, it is inherently challenging to directly measure even the simplest fracture attribute (i.e., radius) within a rock sample subjected to in-situ stresses in the laboratory, let alone in the field. In this study, an analytical model ( R d ) is developed based on Poiseuille’s law. Scaling laws and dimensional analysis are used to define propagation regimes; and non-linear hydro-mechanical coupling is accounted for the near-tip region. This model aims to predict the time evolution of radius for a homothetic penny-shaped hydraulic fracture when the fracture opening, and internal pressure gradient are known. Based on the available experimental data from literature, we quantify the growth of the fracture radius using linear elastic fracture growth model ( R E ); tip asymptotic solutions ( R V and R T ); semi-analytical solutions ( R S ); and the model R d . A comparison of the four analytical models with published experimental data reveal that (i) the asymptotic solutions are limited to linearly elastic and homogeneous materials, i.e., PMMA; (ii) the semi-analytical solutions ( R S ) is only suitable for late-time propagation (iii) the performance of the linear elastic model ( R E ) poorly matches the experimental data, especially for unstable propagation situations; (iv) the new R d model takes advantage of a robust reconstruction of the temporal radius growth of hydraulic fracture problems under realistic stress conditions, and including multiscale propagation regimes, cohesive effects, as well as stable and unstable propagation regimes of geomaterials.
Abstract Hydraulic fracturing is a coupled multi-physics and scale-dependent process requiring an extensive numerical-laboratory appraisal to assess feasibility in the field. Developing a robust model of hydraulic fracture propagation requires knowledge of the time evolution of the fracture’s geometrical attributes, e.g., width/aperture and length/radius. However, it is inherently challenging to directly measure even the simplest fracture attribute (i.e., radius) within a rock sample subjected to in-situ stresses in the laboratory, let alone in the field. In this study, an analytical model is developed to predict the time evolution of the radius for a penny-shaped hydraulic fracture. This model ( R d ) predicts the fracture opening and internal pressure gradient using Poiseuille’s law and assuming a self-similar propagation. Scaling laws and dimensional analysis are used to define propagation regimes; and non-linear hydro-mechanical coupling is accounted for in the near-tip region. We also quantify the growth of the fracture radius using linear elastic fracture growth model ( R E ); and tip asymptotic solutions ( R V and R T ). A comparison of the three analytical models with published experimental data reveal that (i) the asymptotic solutions are limited to linearly elastic and homogeneous materials, i.e., PMMA; (ii) the performance of the linear elastic model ( R E ) poorly matches the experimental data, especially for unstable propagation situations; (iii) the new R d model takes advantage of a robust reconstruction of the temporal radius growth of hydraulic fracture problems under realistic stress conditions, and including multiscale propagation regimes, cohesive effects, as well as stable and unstable propagation regimes of geomaterials.
Understanding the propagation of hydraulic fracture (HF) is essential for effectively stimulating the hydrocarbon production of unconventional reservoirs. Hydraulic fracturing may induce distinct failure modes within the formation, depending on the rheology of the solid and the in-situ stresses. A brittle-to-ductile transition of HF is thus anticipated with increasing depth, although only scarce data are available to support this hypothesis. Here we carry out laboratory hydraulic fracturing experiments in artificial geomaterials exhibiting a wide range of rheology: cubic samples 50x50x50 mm3 in size are subjected to true triaxial stresses with either a low (σv = 6.5 MPa, σH =3 MPa, and σh =1.5MPa), or a higher (15 MPa, 10 MPa, and 5MPa) confinement. The 3D strains induced by hydraulic fracturing are monitored and interpreted; X-ray Computed Tomography (CT) imaging is used to document the HF geometry; and viscoelastic modelling of the tested materials is also conducted to explain the distinct geometry of hydraulic fracture subjected to the stress state. Finally, a correlation between the normalized fracture area (AFN) and the brittleness index (BI) of tested samples is introduced. Our results reveal that: (i)The intermediate stress plays a profound role in hydraulic fracture propagation subjected to the normal faulting regimes (i.e., the transitional intermediate strain observed from brittle to ductile samples); (ii) The orientation angle of hydraulic fracture is highly inclined to the maximum horizontal σH (or vertical σv) stresses in brittle/semi-brittle samples; as BI decreases, the angle inclination is reduced for that of semi-ductile samples, finally reaches to zero (parallel to σH and σv) in ductile sample. (iii) The normalized fracturing area (AFN) decreases as the decrease of BI among different samples under either low or higher confinement. The results of viscoelastic modelling explain the distinct characteristics of hydraulic fracturing induced deformation among the tested samples subjected to true triaxial stress state. This study reveals the importance of understanding the underground brittle-to-ductile behaviour of hydraulic fracture prior to the field implementation.
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