How high can fracture porosity become in the ultra-deep subsurface?

Abstract

Knowledge of how high can fracture porosity become in the ultra-deep burial conditions is important but remains problematic. Fracture aperture and porosity are measured using X-ray computed tomography (CT) at atmospheric pressure and then calculated by image logs. Special attention is paid to how high fracture porosity can become in ultra-deep (>6000 m) settings, and which situations will result in high fracture porosities. In situ stress magnitudes, which can be calculated using well logs, control fracture performances, and dissolution along fracture improve fracture porosity at ultra-deep burial depths. Low horizontal stress difference (Δσ < 25 MPa), very high fracture density will result in a high fracture porosity. Fracture porosity can keep as high as 2.0% in relatively low in situ stress conditions even at ultra-deep burial depths. In intense in situ stress conditions (Δσ > 45 MPa), a high degree of dissolution along the fracture dramatically increases fracture porosity. Dissolution will result in the vuggy fracture planes and improve fracture porosity up to 2.0%. The results provide insights into the detection, characterization, and modeling of subsurface fractures.