The radial distribution of natural fracture density around the wellbore during drilled cuttings/produced water reinjection and well construction

Abstract

Drilled cuttings or produced water reinjection, drilling operations, and even wellbore stimulation are operations that can exploit the knowledge of natural fractures around wellbores. However, there is currently no robust model that quantifies Natural Fracture Density (NFD) around wellbores, to manage the highlighted operations. This is a problem when one considers environmental pollution caused by drilling waste leakage through natural fractures. During wellbore construction, natural fractures may enhance lost circulation, which causes well-control problems. Existing predictive models cannot quantify the NFD (which is in units of count per meter) around the wellbore because of the limitations in handling the total strain. Hence, it becomes difficult to understand how geomechanical parameters, wellbore geometry, or petrophysical variables affect the NFD. Thus, this study was carried out to eliminate these gaps. The strain energy of poroelastic rocks, fracture-area energy, and geomechanical analysis were combined to quantify the NFD. The total strain was decomposed into deviatory strain and volumetric strain, using the principal stresses. Then, based on geomechanical analysis, the tangential, axial, and radial stresses were incorporated, using Kirsch's wellbore stresses. The model was applied to a wellbore that is near the Atlantic Ocean, on the passive continental margin of the Niger Delta Basin, and comprising paralic sandstones of the Miocene. The results showed that the NFD decreased from the walls of the wellbore exponentially. Increasing the wellbore inclination reduced the NFD, but the increase in porosity caused the NFD to increase. The increase in Poisson's Ratio (PR) showed a different relationship with the NFD. The NFD reduced slightly as PR was increased from 0 to 0.2, but increased significantly as PR was increased from 0.2 to 0.5. In addition, the NFD oscillated sinusoidally with the azimuth, at some frequency. The results were validated by different literature sources and field observations.