Fracture density evaluation has always been challenging for the petroleum industry, although it is a required characteristic for reliable reservoir characterization. Production can be directly controlled by fracture density, especially in tight reservoirs. Previous publications introduced the method of using high thermal neutron capture cross-section contrast agents like boron carbide (CB4) to enhance the sensitivity of neutron logs to the presence of fractures. However, all of the studies focus on locating the proppants. In this paper, we propose to (a) enhance the propagation of the contrast agents into the secondary (natural and induced) fracture network using an externally applied electric field and (b) characterize the fracture density in un-propped area using the enhanced neutron porosity logs.
We perform numerical simulations to validate the feasibility of the proposed technique. A physical model derived from electrophoretic velocity and material balance formulations is proposed and solved to simulate the spatial distribution of contrast agents. Furthermore, we simulate neutron porosity logs by solving the neutron diffusion equation which significantly reduces the computing cost (from hours to a few seconds) compared to conventional use of the MCNP (Monte Carlo N-Particle Code) technique.
The simulation results confirmed that an external electric field can significantly enhance the transport of charged contrast agents into the secondary fracture network. Sensitivity analysis revealed that increasing particle zeta potential can efficiently decrease the transport time. Furthermore, we applied the introduced technique on synthetic cases with variable secondary fracture density ranging from 1% to 8%. The relative change of simulated neutron porosity before and after applying the electric potential field was up to 50% in a formation with 8% fracture density after applying an electric field for 6 hours. The proposed technique can potentially enable application of neutron porosity logs in fracture characterization, including assessment of secondary fracture density, if combined with other well logs.