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[en] Overgrowths of hydraulic fracture height occur in reservoirs without stress barriers. It may decrease the efficiency of hydraulic fracturing, pose harm to well production, and increase the possibility of groundwater pollution. Hence, fracture height containment methods are recommended to restrain the overgrowths of fracture height. To enhance the effect of artificial barrier technology, it is improved by replacing the sand or the ceramic used for bridging the fracture tips by gel particles. This innovative technology could be named “artificial gel-barrier technology”. Experiments show that the design of artificial gel-barrier technology is more convenient and controllable, since the dilatability and the gelling performance of the selected gels are mainly controlled by salinity. Experiments also show that the break-through pressure of the gel-barrier is much larger than that of the classical barrier. Numerical simulations reveal that the artificial gel-barrier technology is more helpful and efficient than classical barrier technology. This improved technology has been applied to fracturing operations in Tahe oilfield of China; 73% of these operations were more efficient and the performances of these wells were better than those of adjacent wells.
[en] In developing internal fracture systems in coal beds, the initiation mechanism differs greatly from that of conventional ones and initiations may be produced beyond the wellbore wall. This paper describes the features of the internal structure of coal beds and RFPA2D simulation is used to attest the possible occurrence of initiation beyond the wellbore wall in coal bed hydraulic fracturing. Using the theory of elasticity and fracture mechanics, we analyse the stress distribution in the vicinal coal rock. Then by taking into consideration the effects of the spatial relationship between coal bed cleats and the wellbore, we establish a model for calculating both tensile and shear initiation pressure that occur along cleats beyond the wellbore wall. The simulation in this paper indicates that for shear initiations that happen along coal cleats, the pressure required to initiate fracture for cleats beyond the wellbore wall is evidently lower than that on the wellbore wall, thus it is easier to initiate shear fractures for cleats beyond the wellbore wall. For tensile failure, the pressure required to initiate tensile fracture for cleats beyond the wellbore wall is obviously higher than that for cleats at the wellbore wall, thus it is easier to initiate tensile fractures for cleats at the wellbore wall. On the one hand, this paper has proved the possible occurrence of initiations beyond the wellbore wall and has changed the current assumption that hydraulic fractures can only occur at the wellbore wall. On the other hand, the established theoretical model provides a new approach to calculating the initiation pressure in hydraulic fracturing. (paper)
[en] The prediction of hydraulic fracture height growth is of extreme interest in various engineering practices. To fully understand the dependence of the fracturing height in complex underground geological structures, it is crucial to develop theoretical and numerical hydraulic fracturing models to identify the key variables and their interactions to hydraulic fracture height containment. In this work, we adopt a three-dimensional hydro-mechanical coupling cohesive zone finite element model supported by logging data from a practical engineering project. A novel response surface method with Box–Behnken design is introduced to evaluate the statistical significance of tested variables and to forecast the optimal variable combinations for hydraulic fracture height. The key controlling variables are sorted according to their statistical significance from the analysis of variance, and only the minimum lateral stress ratios in mudstone layer and conglomerate layer are highly statistically significant. The minimum lateral in situ stress contrast is the predominant influence of fracture height containment regarding to their significant negative correlation, while the Young’s modulus contrast is probably not an important parameter in terms of direct control of fracture height. Upon optimization, 41 optimized variable combinations were achieved and could be served as references in regulating this engineering practice or even similar events.
[en] More recent public discourse has taken place regarding the potential correlation between seismic activity and hydraulic fracturing in shale gas reservoirs. Public fears about the risk of seismicity stem mainly from past earthquakes induced by conventional deep injections because the two types of projects share similar mechanisms of rock failure and fault activation. Although previous earthquake risks associated with fluid injection were not serious, the situation would be far more problematic if hydraulic fracturing in a shale gas reservoir triggered a similar-sized earthquake due to potential environmental issues. In fact, almost all documented injection-induced earthquakes have been associated with long-duration and high-volume injection rather than short-term (hours) pressurization (e.g., hydraulic fracturing). In general, hydraulic fracturing operations mostly induce microseismic events through rock failure and activation of small fractures. Although shale reservoirs in tectonically active zones pose a high risk of inducing large-magnitude seismic activities, the internal geological conditions and external stimulation conditions are impossible to be satisfied simultaneously to trigger activation of an entire fault and to result in a destructive earthquake during hydraulic fracturing operations.
[en] The significance of natural fractures in unconventional shale hydrocarbon formations has opened new research frontiers in hydraulic fracturing. Among many of its unique contributions to gas production, the interaction between natural fractures and hydraulic fractures has long been experimentally and computationally investigated. Here, we experimentally investigated the evolution of the fracturing network with a self-developed ultrasonic testing system. Laboratory experiments are proposed to study the propagation of hydraulic fracture in naturally fractured model blocks that contain random micro-fractures. Our analysis suggests that the total fracture width obtained by ultrasonic pulse velocity (UPV) can reflect the dynamic evolution of the fracturing network. The nonlinear fracturing network evolution process is closely related to the variation of the total fracture width. It is suggested from the total fracture width that the maximum fracture network can be realized with greater natural fracture density, large injection rate, and smaller stress ratio. The study presents a better insight into the response of random naturally fractured shale formation under hydraulic fracturing treatment by analyzing the variation of UPV in real time.
[en] Low porosity and permeability make it extremely difficult to develop shale oil and gas reservoirs. The stimulated reservoir volume is believed to have potential to obtain industry production by multi-stage or simultaneous fracturing in horizontal wells. The formation mechanism of network hydraulic fractures in fractured shale reservoirs remains poorly understood. In this article, a true tri-axial hydraulic fracturing system associated acoustic emission monitor was deployed to simulate hydraulic fracturing on shale outcrops. Results showed that the properties of natural fractures (such as aperture, orientation), compared to the viscosity and displacement of the fracturing fluid, affect the propagation direction of hydraulic fractures more predominantly. Each natural fracture in a natural fracture network can independently affect the hydraulic fracture. Low displacement (below the diffusion ability of a reservoir) fracturing tends to connect pre-existing fractures, while high displacement (surpass the diffusion ability of a reservoir) tends to create new fractures. After the breakdown pressure, an increase in injection rate results in more acoustic emission energy and induces new fractures. These results suggest that step-displacement fracturing technology is a possible mechanism to obtain effective fracture networks. Such an understanding would help to avoid unproductive, or sometimes destructive, costly segments of the hydraulic fracturing treatment design. (paper)
[en] Reservoir development for unconventional resources such as tight gas reservoirs is in increasing demand due to the rapid decline of production in conventional reserves. Compared with conventional reservoirs, fluid flow in water-bearing tight gas reservoirs is subject to more nonlinear multiphase flow and gas slippage in nano/micro matrix pores because of the strong collisions between rock and gas molecules. Economic gas production from tight gas reservoirs depends on extensive application of water-based hydraulic fracturing of horizontal wells, associated with non-Darcy flow at a high flow rate, geomechanical stress sensitivity of un-propped natural fractures, complex flow geometry and multiscale heterogeneity. How to efficiently and accurately predict the production performance of a multistage fractured horizontal well (MFHW) is challenging. In this paper, a novel multicontinuum, multimechanism, two-phase simulator is established based on unstructured meshes and the control volume finite element method to analyze the production performance of MFHWs. The multiple interacting continua model and discrete fracture model are coupled to integrate the unstimulated fractured reservoir, induced fracture networks (stimulated reservoir volumes, SRVs) and irregular discrete hydraulic fractures. Several simulations and sensitivity analyses are performed with the developed simulator for determining the key factors affecting the production performance of MFHWs. Two widely applied fracturing models, classic hydraulic fracturing which generates long double-wing fractures and the volumetric fracturing aimed at creating large SRVs, are compared to identify which of them can make better use of tight gas reserves. (paper)
[en] This project involves a semi-integration of collected rock stress data at Olkiluoto. The data included in the study involves overcoring and hydraulic fracturing data reported in the 2006 Site Report, supplemented with the additional data gathered since then, measurements for the power plant construction, and measurements in ONKALO. Despite the extensive data, and although the data display a fair correspondence with respect to stress magnitudes between methods, a few discrepancies between methods exist. The discrepancies primarily concern the orientation of the horizontal stresses and the magnitude of maximum horizontal stress. The objective of the study is help answer the remaining discrepancies in the collected data using a semi-integration approach on the overcoring data, i.e. a simplified application of the Integrated Stress Determination Method. The semi-integration involves two steps: (i) a brief re-evaluation of data; and (ii) forced overcoring stress calculations based on constraints derived from the other stress measurement techniques. The overall intension was to force overcoring data, which is the method displaying the largest scatter in results at the Olkiluoto site, to be consistent with a number of constraints that were derived from other stress measurement techniques. This exercise would outline the most probable constraints and in the extension, help identifying a stress model for the site. Regrettably, the results of the semi-integration were not completely satisfactory. The most important factor of this outcome is believed to be the failure to reduce the initial scatter through a re-evaluation. The remaining scatter after re-evaluation is judged too large to represent solely in situ stress variation; hence, measurement related errors are likely still present in the re-evaluated data set. A more in-depth analysis may to some extent decrease the scatter in data, but perhaps more effectively, develop a tool that simultaneously can derive the stress field from all stress measurement techniques. The variability in results is visualized by the low consistency of overcoring strains with the various constraints, although they clearly may represent also improperly defined constraints but may also reflect a reduced data quality or limitations of the acceptance criteria and algorithm. What can be concluded is that one principal stress is indeed vertical (or near vertical) and closely resembles the theoretical weight of the overburden. The results of constraining also horizontal stress orientations and magnitudes are though much less clear. Most likely, σH is directed NW-SW, i.e. a quite imprecise result. The report also involved analysis of observed core discing. The results of this study show that dominating disc type at the Olkiluoto site is the incomplete disc (52 % of all observations) and the dominating host rock pegmatitic granite (PGR; 49 % of all observations). Discing events with other disc shapes and in other rock types are significantly less frequent. The discing events could not be correlated with existing and larger deformation zones, although discing events may possibly be correlated to zone OL-BFZ002. Note that the analysis has only been made with respect to the major deformation zones, which does not preclude that discing events may be correlated to smaller scale zones
[en] Hydraulic fracturing tests designed to characterize the state of in situ stress were conducted in four deep boreholes. These tests are part of a site investigation program to assess the feasibility of constructing a nuclear waste repository in a basalt formation underneath the Hanford Site in southeastern Washington State. The results of these tests are examined in light of stress indicator data, which include core disking, borehole wall spalling, and fault plane solution data obtained at the site
[en] Hydraulic rock stress measurements were performed in the surface drillhole OL-KR40 (25 tests) and in the underground drillhole ONK-PP125 (10 tests) at the Olkiluoto site, Finland. The measurements were carried out in two separate campaigns with a total of 6 sleeve fracturing tests (SF), 17 hydraulic fracturing tests (HF), and 12 hydraulic tests on pre-existing fractures (HTPF). This report presents scope, objectives and performance of stress measurements in the two drillholes. Further, a description is given of the employed test equipment, quality assurance, testing methodology, and results of stress determination at the Olkiluoto site. The results of the stress measurement campaign in drillholes OL-KR40 and ONKPP125 were initially not optimal. The primary reason for difficulty in drillhole OLKR40 relates to the drillhole inclination, which is not optimal for hydraulic stress measurements. Hydraulic fracturing theory does not apply and the testing was therefore based primarily on HTPF approach. This technique requires a variety of fracture orientations in order to solve all components with precision. However, as a result of the drillhole inclination, the drillhole wall failed during pressurization and this physical constraint significantly limited the amount of successful HTPF tests. Because the induced fractures have similar orientation, the resolution of maximum horizontal stress magnitude and its orientation was not optimal based on conventional stress calculation. For drillhole ONK-PP125, few tests were undertaken and stress calculation results were not conclusive. For this borehole, despite the application of sleeve and hydraulic fracturing, axial fractures were not produced. As a result of these difficulties, a decision was made to employ en echelon theory. This theory investigates if the observed induced fractures can be explained by tensile or shear failure; thus it is a tool that may provide additional information that may help constrain the stress field. The application was successful and in drillhole OL-KR40, the stress field could be determined by joint inversion of hydraulic data and en echelon shear fractures. For drillhole ONK-PP125, the situation is more complex; shear failure and the very same stress field as in drillhole OL-KR40 could explain the collected data, but so could tensile failure given significantly inclined principal stress directions. However, both solutions outline that maximum principal stress is directed about E-W. The final result of the analysis of hydraulic rock stress data and en echelon fracturing of the drillhole wall suggest that the stress field at repository depth (400 m vertical depths) is: σH = 26.6 + 0.0225 (z-400) MPa σh = 16.6 + 0.0225 (z-400) MPa σv = 10.6 + 0.0265 (z-400) MPa (assumed) λ= 80 deg N (orig.)