Fracture Aperture Computation
Fracture aperture plays a crucial role in oil production and hydrogeology. It predominantly controls fluid transport in fractured rock masses. When oil production declines, the secondary production stage begins in the form of water flooding, leading to an increase in water saturation in the fracture network. Therefore, reliable and accurate methods for determining fracture aperture and the permeability of fractured rocks are essential.
The computation of fracture aperture from image data is not an exact science. The methodology adopted by HRP involves using the conductivity and estimating the inflexion points from distributions as seen in the second stage of the figure. The methodology relies on there being a consistent difference between fracture conductive value and the surrounding rock, thus creating an anomaly or feature. Luthi and others have done considerable work in the field of fracture aperture computation such as the following references: Hornby, B. E., Luthi, S. M., & Plumb, R. A. (1992, January 1). Comparison Of Fracture Apertures Computed From Electrical Borehole Scans And Reflected Stoneley Waves: An Integrated Interpretation. Society of Petrophysicists and Well-Log Analysts and S. M. Luthi and P. Souhaité (1990). ”Fracture apertures from electrical borehole scans.” GEOPHYSICS, 55(7), 821-833.
Ghosh, K, Harvey, N and Gales, R, Borehole image based aperture characterization to identify primary versus secondary fracture opening, AAPG Annual Convention, AAPG Annual Convention and Exhibition, Houston, Texas, 2017
The fracture width is actually calculated using the digitized trace from the fracture pick and the inflexion points are identified. The width is calculated as being approximated by the width of the anomaly. This anomaly is measured at right angles or orthogonally across the image.
The circumferential length uses a similar methodology to locate the fracture and when the conductive difference is above the threshold, the fracture is said to exist and the detection length is added to the circumferential length. The thresholds used should remain consistent when performing a multi-well study. This is shown in the center part of the figure above.
A profile of conductivity is illustrated in an example above.
All fractures were picked as planes through the wellbore. Cumulative fracture length is also computed and the relationship to aperture is important (see Subinear scaling of fracture aperture versus length, Olson, J, 2003). If the fracture is convex or concave and continuous around the wellbore the actual length measured will be greater than the length of a planar feature. An example of data from a well in Permian basin, Texas is presented to the right. Below the figure showing aperture versus length as a percentage is a reproduction of Figure 5 from the same paper.
In the case of the imaging tools, because of gaps between the pads, a cumulative length greater than 80% indicates that the fracture most likely is continuous around the borehole.
By using the approach and comparison of length to width HRP are able to provide information that can be extended beyond the well bore and provide insights into fracture length. Contact HRP here for more information
