| Unconventional oil and gas reservoirs,such as shale oil/gas and tight gas,have become the focuses in the global oil and gas resources exploration and development.China has abundant shale oil/gas and other unconventional oil/gas resources,strengthening efforts to shale oil/gas exploration and development is of great significance to for promoting China's energy security and energy structure optimization.Hydraulic fracturing technology is the most effective way to improve the overall reservoir permeability and achieve exploration and development of unconventional oil/gas reservoirs.Fracture monitoring is the key to evaluating the effect of hydraulic fracturing,optimizing the fracturing plan and improving the efficiency of shale oil and gas development.The existing sonic logging,temperature logging and micro-seismic fracture monitoring technologies can determine the fracture location and height parameters,and radioactive trace logging can determine the location,height and width parameters of fractures.However,due to safety issues such as transportation and storage,the popularization and application of radioactive tracers are strictly limited.In this study,a non-radioactive gadolinium tracer imaging technique is proposed,which can not only evaluate the position,geometry and azimuth parameters of near-wellbore fractures,but also has the advantages of accurate identification of fracture parameters,no radioactive pollution,safety and environmental protection,and repeatable measurement.The purpose is to provide a new fracture monitoring method for the evaluation of hydraulic fracturing fractures,increase unconventional oil/gas production,and provide guarantee of China's energy security.Through relevant literature research,the development status of hydraulic fracturing monitoring technology at home and abroad are understood.Meanwhile,we have a clear understanding of the geological characteristics of shale and other unconventional geological formations,and the spatial distribution of fractures after hydraulic fracturing.Besides,the existing hydraulic fracturing monitoring methods are summarized,and capabilities and limitations of these methods are compared.Based on the neutron-gamma process,the mechanism of controlled crack neutron azimuth tracer imaging method is derived.After that,the neutron and gamma field distribution characteristics of the fractured formation and the unfractured formation are simulated and compared,and the conclusion is that elements with high thermal neutron capture cross-section in the fracture can significantly increase the capture gamma rays of the fracture.To achieve the fractures imaging by gadolinium tracer,a measurement system consisting of a D-T neutron source,four NaI gamma detectors and one He3 thermal neutron detector was designed,and its structural parameters were optimized.The spacing of NaI detector is 36 cm,and He3 is 45cm;The length of NaI detector is 10-12 cm,and the thickness of longitudinal gamma shielding is 5cm,and the thickness of neutron shielding is 1cm or 0cm.By analyzing the radioactive count statistics of the proppant and response sensitivity to fractures,it was concluded that the upper and lower limits of gadolinium content in the proppant decrease exponentially with the increasing fracture width.Besides,the imaging instrument has a 20 cm detection depth for gamma rays,and a 36 cm detection depth for thermal neutron.Finally,using the designed imaging instrument and marker proppant,an imaging forward simulation for cracks with different widths,heights,orientations,and inclinations was performed.And the response regular of forward simulation was obtained and the fracture parameters were calculated using the imaging map. |
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