Fracture Width Characterization by Integrated Analysis of Electrical Image and Acoustic Stoneley Reflectivity
Authors:   Halliburton, Lopez Eglee , Tracadas Philip ,   Halliburton, Hinz David ,   Halliburton, Gaibor Janeth ,   Halliburton, Barrett Peter ,   Halliburton
Published: November 2024

Assessment of fluid conductivity of fractures in interbedded formation types is challenging for resistivity imagers due to non-unique electrical conductivities of fluid and fracture fill. Natural fractures can enhance formation permeability and improve fluid flow; thus, it is important to characterize fracture networks that intersect the borehole and their fluid conductivity for applications such as water disposal, hydrocarbon production, geothermal fluid exchange, or carbon dioxide storage. This case study applies a multisensor workflow to assess fracture fluid conductivity in a large marine sandstone that is used as a water disposal field. The Ford, West (Wolfcamp) Field in the Delaware Mountain Group, located in Culberson, Texas, consists of mostly sandstones/siltstones interbedded with limestones and organic-rich shales. In many carbonate rocks, fractures can be filled with low-resistivity clay-sized particles. As a result, the electrical borehole image logs may misinterpret such fractures as open fractures. The high-formation resistivity tends to cause borehole images to highlight any conductive materials (such as fine clays) that may be present within the fractures and vugs, which cause potential ambiguity with similar readings from open fractures filled with brine. Traditional methods of calculating fracture aperture, such as Luthi-Souhaite, give an estimate of the aperture but have always been seen as qualitative without a means of calibration. Borehole image logs can identify fracture features with reasonable confidence, but fracture conductivity can only be verified with a pressure test. In a well drilled with WBM in this field, a microresistivity borehole imager and a sonic tool were logged and used in combination with a formation testing tool and injection rates and pressures to evaluate fluid conductivity of fractures crossing the borehole and to monitor well performance. This paper presents the results based on a new workflow that uses acoustic low-frequency Stoneley waves to characterize natural fractures (their conductivity and borehole washout volume) that intersect the borehole and compares these with the fracture aperture results from the Luthi-Souhaite method. With sonic logging, the Stoneley wave mode is essentially a low-frequency pressure wave traveling along a well’s axis. When the Stoneley wave intersects a fluid conductive fracture, pressure is released into the fracture in proportion to the conductivity of the fracture. Comparison of the direct signal to the secondary Stoneley wave created by this effect gives an estimate of effective fracture width or fluid conductivity by using a wideband frequency inversion model by Kostek et al. (1998). By combining Stoneley fracture analysis with the Luthi-Souhaite fracture computation from high-resolution borehole images, the truly open natural fractures can be characterized and calibrated within a formation while ignoring false signals from washouts, bedding changes, and sealed (not fluid-conductive) fractures. This methodology can be applied in traditional production scenarios, carbon capture and storage (CCS), or geothermal projects. It provides a way to calibrate the Luthi-Souhaite methodology with wireline data rather than needing core data, saving time and money. It can be effectively applied to existing wells where no core data are available, bringing accuracy to dynamic modeling.