Hydraulic fracturing is commonly used to create pathways to allow oil and natural gas to move through the rock to the well and to enable it to be pumped to the surface. However, hydraulic fracturing is well known to generate seismic events. The process of hydraulic fracturing generates a network of new induced faults and reactivates dilated natural fractures. Detecting the minimum resolvable fault displacement relies on several factors. The intensity of any induced event occurring along a fault can be established by implementing Kanamori and Anderson’s (1975) relationship between fault dimensions and magnitude, constrained by slip length.
Events related to hydraulic fracturing normally have a rupture length of not more than a few hundred metres and a slip in the range of millimetres. A number of sensitivity studies have been performed to investigate if the total fracture volume, aperture and porosity are critical to the fracture length. Other researchers have also focused on the effect that the in-situ stress ratio, cohesion, injection rate and internal friction angle have on the natural fractures. It has been reported that injection rate has an effect on fracture complexity, and an increase in stimulated fracture area has been reported for increasing injection rate.
In-situ stress ratio, cohesion and internal angle of friction were all found to affect the morphology of the fracture network and faults configured toward the maximum stress direction were found to improve the complexity of the fracture network. Keele University researchers, Dr Rachel Westwood, Samuel Toon, Professor Peter Styles and Professor Nigel Cassidy (currently Head of Civil Engineering at the University of Birmingham) have studied the effect of hydraulic fracturing near to a pre-existing fault. By altering the natural fracture networks intensity and operational parameters, including pumping time, flow rate and differential pressure, they assessed the effect these parameters had on the fracture network area, flow distance and the minimum lateral distance that hydraulic fracturing should occur from a pre-existing fault so as not to reactivate it, therefore, mitigating the effects of felt seismicity. Their research work is published in peer-reviewed journal, Fuel and also in Geomechanics and Geophysics for Geo-Energy and Geo-Resources.
The research team adopted a Monte Carlo numerical modelling approach to perform the sensitivity analyses. They obtained the lateral respect distance from computations of the Coulomb stress change of the rock formation surrounding the injection stage.
The authors observed that the flow rate had the smallest rate of change for 3700m2 per 0.01m3/s fracture area and flow distance of 8.3m per 0.01m3/s. The authors also found that the differential pressure had the most significant effect on the stimulated fracture area, when it was less than 2MPa at 31,029m2/MPa. The pumping time had the most significant impact on the flow distance, and the stress threshold value on lateral respect distance.
The findings of the study indicated that in order to reduce the lateral distance, a compromise was required between fracture area and flow distance. The findings also provide important guidance for operational practice, by establishing the potential area of the induced fracture network and the stress field generated under realistic hydraulic fracturing conditions, which is a critical aspect of risk assessment.
Rachel F. Westwood, Samuel M. Toon, Nigel J. Cassidy. A sensitivity analysis of the effect of pumping parameters on hydraulic fracture networks and local stresses during shale gas operations. Fuel, volume 203 (2017), pages 843–852.
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Rachel F. Westwood, Samuel M. Toon, Peter Styles, Nigel J. Cassidy. Horizontal respect distance for hydraulic fracturing in the vicinity of existing faults in deep geological reservoirs: a review and modelling study. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, volume 3(4) (2017), pages 379–391.Go To Geomechanics and Geophysics for Geo-Energy and Geo-Resources