Hydraulic fracturing plays an important role in improving productivity of unconventional reservoir. Gaining insight about the hydraulic fracture system, both intersecting and around the borehole, is crucial and can often help determine the economic success of a well and reservoir. At present, isotope tracer logging technology is one of the effective means to evaluate fracturing. The traditional radioactive (R/A) tracer technology can evaluate the fracture height and position, but it involves environmental, safety and regulatory issues. In addition, the non-radioactive tracer technology can locate proppants placement, but geometrical form or orientation of the fracture cannot be obtained.
A new non-radioactive tracer gamma ray image measurement system is proposed to acquire fracture geometry and orientation. For this measurement system, multi-detector array structure is adopted and it contains a D-T neutron source, four gamma detectors arranged in a ring with 90 degree between two of them and a He-3 detector whose spacing is further than gamma detectors spacing. In addition, a new kind of proppant which contains Gd2O3 with extremely high thermal neutron capture cross section is necessary. This new measurement system can realize neutron measurement of borehole and image of fractures in different orientation by gamma spectrometry.
When neutrons from D-T neutron source enter into the formation and undergo a set of reactions, and then thermal neutron is captured by the new proppant with Gd2O3 to emit the characteristic gamma rays. The yield of gadolinium is calculated from azimuthal gamma spectra based on differential elemental standard spectra. When the yield of gadolinium is converted to a fracture imaging around borehole, the concentration of current lines in the anomalies creates area distinctions with amplitude proportional to the contrast. The fracture filled with proppants incorporated with Gd2O3 shows a sharp contrast to the typically low-characteristic gamma ray counts for the surrounding matrix. The geometrical form or orientation of fractures are clearly shown in an image. On this basis, a new rapid identification algorithm based on digital image processing is adopted, which the fracture position, orientation and other parameters such as fracture width can be quickly calculated from borehole imaging. After formation fracturing, some proppants residue are in the well. It has a great influence on a fracture imaging around borehole to acquire information of fracture position, orientation and other parameters. Then thermal neutron count variation is recorded to reflect the change in the borehole.