Oil and gas operations often extract water from the subsurface; however, this material is mostly a nuisance. Produced water is typically a highly saline waste that requires proper disposal. The most common method for managing this waste is by reinjecting it back into the ground, although it is rarely reintroduced into the same formation from which it was extracted. When produced water injection occurs at excessive volume rates, it can lead to induced seismicity. Therefore, understanding the porosity and permeability characteristics of the rock mass receiving the wastewater is crucial. This is where borehole images become essential.
In the Midland Basin, shallow disposal zones, particularly the Delaware Mountain Group, are becoming overpressured due to the significant volume of water being injected from lower-producing horizons. These overpressured zones result in drilling challenges and costly remediation efforts. An alternative to using these shallow injection zones is the Lower Paleozoic section, which spans from the Mississippian to the Ordovician. The Upper Cambrian-Lower Ordovician Ellenburger Formation, known for its high-capacity disposal potential, is particularly valuable.
The porosity of the Ellenburger Formation is linked to karstic solution-collapse brecciation, enhanced natural fractures, and vuggy matrix porosity from dolomitization. Due to the unique porosity associated with karst features, fractures, and vugs, conventional porosity logging tools may not provide a complete understanding of the pore space within the Ellenburger. Borehole image logs can identify these unique porosity zones, thereby enhancing the injection efficiency for water disposal wells. Identifying the locations of enhanced permeability along the wellbore helps improve the effectiveness of acid stimulation by targeting the most suitable injection intervals. Additionally, borehole image logs can reveal the orientation of maximum horizontal stress and the spatial geometry of the natural fracture system.
By combining fracture identification from image logs with geomechanical studies, it is possible to pinpoint critically stressed fractures that are most likely to be reactivated by increased pressure from fluid injection. Mapping these enhanced flow zones using image logs and seismic data can improve injection capacity, reduce well interference, and minimize flow instability in the field area.
Image log data plays a crucial role in predicting reservoir behavior during injection. Borehole images help us understand rock properties by revealing the spatial geometry of bedding, fractures, and faults. They also provide insights into the orientation of current stress and facilitate the identification of karst breccia and vuggy porosity. By combining image attributes with geomechanical and seismic data, we can better understand how fluid flow and pressure profiles vary within the injection zone. Establishing the spatial geometry of critically stressed faults and fractures is essential for determining pressure profiles and optimal well spacing, which helps minimize the risk of triggering induced seismicity.
