Oil & gas operations often recover water from the subsurface, mostly this material is a nuisance. Produced water usually is highly saline waste that needs to be disposed of. The most common means of getting rid of this material is by reinjection back into the ground, although rarely is this reintroduction into the same formation from which it was recovered. Produced water injection, when conducted at volume rates that are too high, can result in induced seismicity. Therefore, it is important to have a clear understanding of the porosity and permeability characteristics of the rock mass receiving the wastewater injection. This is where borehole images come in.
In the Midland Basin, shallow disposal zones (Delaware Mountain Group) are becoming overpressured due to the large volume of water being injected from the lower producing horizons. These overpressured zones are causing drilling problems and expensive remediation. An alternative to these shallow injection zones is the Lower Paleozoic section from the Mississippian to the Ordovician. A particularly useful stratigraphic unit is the Upper Cambrian – Lower Ordovician Ellenburger formation characterized by high-capacity disposal potential.
Ellenburger porosity is associated with karstic solution-collapse brecciation, enhanced natural fractures, and vuggy matrix porosity from dolomitization. Because of the unique porosity associated with karst, fractures, and vugs, conventional porosity logging tools provide an incomplete picture of pore space in the Ellenburger. Borehole image logs are able to identify unique porosity zones to improve injection efficiency for water disposal wells. Knowing where the enhanced permeability zones are situated along the wellbore is useful for improving the effectiveness of acid stimulation by targetting only the best potential injection intervals. Borehole image logs identify the orientation of the maximum horizontal stress and the spatial geometry of the natural fracture system. Combining image log fracture identification with a geomechanical study identifies the fractures that are critically-stressed; fractures that are most likely to be reactivated by increased pressure due to fluid injection.
Mapping these enhanced flow zones with image logs and seismic can improve injection capacity, reduce well interference and reduce flow instability in the field area.
Image log data contributes directly to predicting reservoir behavior during injection. Borehole images provide a means of understanding rock properties. They establish the spatial geometry of bedding, fractures, and faults, they determine the orientation of the present-day stress and, they permit identification of karst breccia and vuggy porosity. Combining image attributes with geomechanics and seismic data enhances our understanding of the spatial variation of transient fluid flow and pressure profiles within the injection zone. Establishing the spatial geometry of critically-stressed faults and fractures helps determine pressure profiles and optimum well spacing to minimize the potential for triggering induced seismicity.
