Characterizing Hydraulic Fractures
Knowing the final position and orientation of hydraulic fractures can be important in assuring that maximum efficiency is achieved for an environmental remediation system. Unfortunately, the only methods available for absolutely determining the form of a fracture are excavation and/or soil coring. Both of these methods are invasive and could negate the original purpose of fracture emplacement. Therefore, remote methods have been developed to allow estimation of fracture form without disrupting the fracture itself. These methods involve interpretation of injection pressure logs and/or monitoring deformation of the land surface during the fracturing event.
Pressure Logs - A relatively high borehole pressure is required to “break” the formation. The break represents fracture initiation, which is followed by fracture propagation. The pressure required to propagate a fracture through the formation is typically significantly less than breaking pressure, however, some appreciable injection pressure is still required. Pressure measurements during fracture propagation provide real-time information about fracture orientation. This is because horizontal and vertical fractures generate distinctly different signatures in pressure logs. Pressure log interpretation provides fairly low resolution with respect to determinations of fracture orientation, and provides no indication of spatial distribution. However, if high resolution is not required, it avoids the expense of more involved analysis.
Surface Deformation - The act of injecting material into the subsurface causes deformation of the surrounding media. This is especially true during processes like hydraulic fracturing, which opens and fills new space within the soil or rock. The accommodation of these changes extends in all three dimensions. Accommodation in the vertical dimension is represented by slight variations in elevation and angle of the ground surface relative to gravity. Measurement of these shifts provides data from which the form of the underlying fracture can be inferred. We employ two different methodologies for characterizing surface deformation in the field: Uplift measurement by theodolite and angle measurement with tilt meters.
Uplift - Formation is displaced a distance equal to the fracture aperture as a fracture propagates. Much of this displacement is distributed in all 3-dimensions and accommodated by compression of the formation material. None the less, some upward displacement of the ground surface also occurs. Upward displacement of the ground surface (also known as heave or uplift) can be measured using surveying equipment. The observation station is placed well outside the expected radius of the fracture and remains stationary.
Typical array used to measure and characterize deformation at the ground surface. Fracture thickness and surface uplift are exaggerated for illustrative purposes.
Angle - The ground surface is uplifted during fracture propagation, however, the uplift does not occur uniformly. This results in some angular deformation of the ground surface, which can be measured by tiltmeters. These induced changes in surface angle tend to be extremely small. FRx uses AGI Series 700 and Series 800 tiltmeters that are capable of resolutions up to 1 microradian (equivalent to a slope of 1 millimeter per kilometer) or better. This resolution allows collection of meaningful data that, along with uplift data, can be inverted by a material deformation model to provide approximations of fracture form.
Example of surface expression results as measured by tiltmeters. Arrows point down-dip with lengths that represent tilt magnitude.