A quantitative calculation method for fracture density using the neutron self-shielding modification and neutron-induced gamma logging

Abstract

As the critical parameter of hydraulic fracture evaluation, fracture density is significant for fracturing model optimization and production prediction. To compensate for the low response sensitivity of fracture density and the saturation of fracture evaluation in nonradioactive technology (NRT), we have developed a method based on modifying the neutron self-shielding effect to quantitatively calculate fracture density. The gamma counts of elements with content that did not change before and after fracturing are used to calculate the self-shielding correction factor. We establish the mathematical relationship between the corrected gamma counts of boron and fracture density to achieve a quantitative calculation of fracture density. Compared to NRT, the new method uses measured energy spectral information to eliminate the effect of neutron self-shielding on fracture density and improve calculation accuracy. Meanwhile, the scope or range of accurate fracture density estimation extends from the original 4% to more than 10%. Moreover, we further examine fracture response under different formations and borehole conditions using the Monte Carlo N-particle transport code. The salinity of formation water and borehole water has the most significant impact on fracture density calculation, shale content and porosity, lithology, and fluid type of formation; in contrast, borehole parameters are the least significant. Finally, in this paper, we illustrate the feasibility of the method through a continuous-depth numerical calculation.