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Abstract A numerical study of hydraulic fracture growth from a circular borehole under plane strain conditions is presented. The coupling of fluid flow and rock deformation plays a key role in the fracture reorientation process and in determining the curved fracture path formed. The fracture path is given as a function of both the nondimensional parameter β, which has been previously described in the literature and applies to fracture growth that is dominated by rock fracture toughness, and the recently derived nondimensional parameter χF, which applies to fracture growth that is dominated by fluid viscous dissipation. The results show that the values of β and χF determine the fracture trajectory as the fracture grows from the wellbore and eventually reorients parallel to the maximum far-field stress direction. Thus, for viscous dominated conditions that are typical of field application of hydraulic fracturing for stimulation, χF is a measure of the development of nearwellbore fracture tortuosity. A larger value of χF implies a stronger curvature and fracture tortuosity. The value of χF is reduced by increasing the viscosity and injection rate of the fluid.
- North America > Canada > Alberta (0.46)
- North America > United States > Texas (0.28)
ABSTRACT: Unconventional reservoirs depend on drilling horizontal wells and hydraulically fracture them to commercially produce hydrocarbons. The orientation of the laterals with respect to the in situ minimum horizontal stress is a key parameter, because it controls the direction of fracture propagation, the pressure needed to initiate the fracture and the stimulated rock volume (SRV), and therefore the hydrocarbon production of the well. In addition, in a full-field scale, wellbore azimuth affects the number of wells that fit in a specific area. In the Neuquén Basin, a dominant E-W orientation is expected for the maximum horizontal stress, due to the position of the basin relative to the subduction zone. For this reason, the optimum well orientation in most of the assets is N-S. Nevertheless, a comprehensive analysis of breakout orientation in image logs from wells located across the basin reveals deviations from the expected direction of maximum horizontal stress in different areas of the basin. This implies that drilling wells in a N-S direction may not always be the best option from a stimulation point of view. To analyze the effect of wellbore azimuth on hydraulic fracture growth and initiation pressure, several FEM simulations were run using different wellbore orientations. As expected, results show that the best orientation is parallel to the minimum horizontal stress, because in this case hydraulic fractures grow perpendicular to the wellbore. When the well trajectory deviates from this preferred orientation, the effective length of the lateral diminishes, which means that the same SRV could be reached with a shorter well at the optimum orientation. Finally, a computational code for a perforated well in an arbitrary direction was implemented to calculate the breakdown pressure for every perforation along the well. Given the same stress regime, the results show that the initiation pressure varies in the circumference of the wellbore, and although the minimum value is almost the same for every wellbore azimuth the maximum is different. This means that in some cases the pressure needed to initiate a fracture is different for each perforation, which can lead to a lower cluster efficiency. In the worst-case scenario, when the well is misaligned with the minimum horizontal stress cluster efficiency decreases dramatically and about 70% of the perforations become inactive.
- South America > Argentina > Neuquén Province > Neuquén (0.68)
- North America > United States > Texas (0.47)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Compressional Tectonics (0.54)
- South America > Argentina > Patagonia > Neuquén > Neuquen Basin > Vaca Muerta Shale Formation (0.99)
- North America > United States > West Virginia > Appalachian Basin > Marcellus Shale Formation (0.99)
- North America > United States > Virginia > Appalachian Basin > Marcellus Shale Formation (0.99)
- (4 more...)
Summary Field measurements of fluid pressure inside hydraulic fractures have shown rapid pressure declines along the fracture length. The consequence of this pressure profile is rapidly tapering fracture width. This means that a disproportionate volume of fluid and proppant injected inside hydraulic fractures remains near the wellbore, thus creating excessive near-wellbore and substantially less far-field fracture conductivity. This explains why history matching of oil well production figures yields much lower effective fracture lengths than when the same exercise is performed for gas wells, as oil wells require higher fracture flow capacity because of their higher permeability. The rapid tapering of the fracture width also restricts the movement of the proppant inside the fracture, causing its accumulation near the wellbore. As the treatment progresses, and if sufficient proppant volume has been injected inside the fracture, the near-wellbore segment of the fracture can begin to fill with proppant, thus reducing the open width available for further movement of the fluid. Essentially, accumulation of proppant near the wellbore reduces the fracture width available for fluid flow, which then results in higher frictional pressure losses inside the fracture, further skewing the pressure distribution and eventually leading to screenout. Introduction Over the last two decades, the industry has been gradually recognizing the complexity of the theory and practice of hydraulic fracturing. Simple theories of tensile fracturing (Haimson and Fairhurst 1967) evolved successively into concepts of near-wellbore tortuosity (Aud et al. 1994), of the presence of branches and shear fractures (Weijers et al. 2000), and of off-balance growth (Daneshy 2003). The hydraulic fracture is now viewed as extending under a mixture of tensile and shear forces and containing numerous branches, with the extension occurring randomly around the fracture tip and highly influenced by local inhomogeneities and planes of weakness. Randomly distributed proppant packs forming behind narrow shear fractures are now offered as alternatives to simple viscosity-dominated proppant transport models (Daneshy 2005). The natural consequence of these developments is the recognition that our simple models of pressure distribution inside the fracture also need revision and re-evaluation. This paper extends the new fracturing concepts into analysis of fluid pressure variations inside the fracture and reviews their impact on fracture shape, fracture conductivity, analysis of post-frac treatment pressure analysis, and the causes of screenout, as well as the other major application of hydraulic fracturing, namely in-situ stress measurement. Fluid pressure variations observed during a fracturing treatment have always been very complex and unpredictable. Most hydraulic fracturing theories assume the fluid pressure to be relatively constant inside the fracture, except for the small region near its tip. Under this condition, tensile fracturing models coupled with energy balance equations predict that fluid pressures tens or a few hundred psi in excess of the least in-situ principal stress will be sufficient to extend a hydraulic fracture. These same models predict the extension pressure to be relatively flat or slightly decreasing. Yet actual hydraulic fracturing pressures sometimes vary by several thousand psi. One explanation offered for such pressure variations has been fracture containment (Nolte and Smith 1979). The hypothesis offered is that fluid pressure in a vertically contained fracture would increase during the treatment and that constant or decreasing pressure means the fracture is experiencing excessive vertical growth. However, in spite of its simplicity and appeal, this theory cannot explain how the enormous energy contained in the pressurized fluid is consumed inside the fracture, nor how the formation can resist the very large stresses created by these pressure increases.
ABSTRACT: The injection of large quantities of treating fluid and proppants during fracture stimulation of low permeability formations causes local insitu stress changes, sometimes referred to as stress shadowing or stress interference. Recent procedures for improving production have led to closer spacing of clusters, from approximately 80 feet spacing a few years ago to about 20 feet spacing now, or less in some cases. That is, there are now about four times the number of potential fracture initiation locations (clusters) per foot of lateral well, while the sand pumped per foot of lateral continues at about 1700-1800 pounds per foot. With four times the number of clusters and the same mass of sand pumped, the stress disturbance of one cluster to another cluster has changed. This paper addresses the near wellbore stress interference effects for close cluster spacing. Numerical simulations are presented using a robust linear-elastic 3-D hydraulic fracturing computer code that calculates fracture ‘bending’ and fracture width change due to stress interference. The stress interference that changes the fracture width is the most significant because the resistance to fracture fluid flow at high velocities in these narrow channels is the primary driver of stress shadowing fracture geometry changes.
Subcritical Propagation of Hydraulic Fractures by Step Loading
Wang, Pu (College of Petroleum Engineering, China University of Petroleum) | Zhang, Guangqing (College of Petroleum Engineering, China University of Petroleum) | Wu, Yingqiang (Oil and Gas Technology Research Institute PetroChina Changqing Oilfield) | Zhou, Dawei (College of Petroleum Engineering, China University of Petroleum) | Nie, Yuanxun (College of Petroleum Engineering, China University of Petroleum)
ABSTRACT Hot Dry Rock geothermal exploitation is an important energy exploitation mode, and the subcritical propagation of hydraulic fracturing will significantly reduce breakdown pressure and improve recovery efficiency. Previously, double-twist test was used to measure subcritical crack extension for predicting the material life, which is not suitable to the hydraulic fracture extension. In this paper, a new step loading experiment method is proposed, and the subcritical fracture propagation behavior caused by fluid-solid coupling was studied by changing the step loading time. In the experiment, the true tri-axial hydraulic fracturing equipment was adopted, and guanidine gel solution was 1% in concentration for the fracturing fluid, and 70% of the fracture pressure was taken as the initial step pressure. During the experiment, acoustic emission was used to detect damage accumulation and fracture propagation. The effects of stress difference and pressurization time on subcritical crack propagation were analyzed. The research shows that (1) The step loading method can effectively reduce the breakdown pressure in both horizontal and vertical wells. When the step loading time interval is 10 min, the breakdown pressure under horizontal well conditions is reduced from 42.3 MPa to 41.1 MPa with a reduction of 2.5%. Under vertical well conditions, the breakdown pressure is reduced from 21.1MPa to 20MPa with a reduction is 4%. (2) Branching fractures are extensively induced after step loading according the fracture morphology. (3) RA and AF calculated from acoustic emission data show that shear cracks prevail in the step loading. It is vital to study the subcritical crack propagation, especially in enhanced geothermal system, which is of great significance to improve the economic and effective development of geothermal mining. 1. INTRODUCTION In recent years, the fracture mechanics is used to study the propagation of rock cracks. Experiments have shown that the crack will initiate when the stress intensity factor at the crack tip is higher than the fracture toughness. When the stress intensity factor at the crack tip is much lower than the fracture toughness, the crack can still propagate with a very slow speed, resulting from fatigue or stress corrosion. This stable, quasi-static crack propagation is called subcritical extension. This fact has been mentioned in the past (Smith, 2015).
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.48)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource (0.69)
- Asia > China > Shanxi > Ordos Basin > Changqing Field (0.99)
- Asia > China > Shaanxi > Ordos Basin > Changqing Field (0.99)
- Asia > China > Ningxia > Ordos Basin > Changqing Field (0.99)
- (2 more...)