Study of a Method to Evaluate Hydraulic Fracturing near Wellbore Using Thermal Neutron Detection Based on Gd Tracer
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摘要: 水力压裂是提高非常规油气开采的重要手段，支撑剂位置及裂缝参数是评价压裂效果的重要因素.提出了以Gd作为示踪剂探测热中子来评价井周裂缝方法，将热中子双组扩散理论与数值模拟方法相结合，定义热中子变化参数WTN来指示裂缝宽度，并分析了岩性、井眼尺寸、地层孔隙度、地层水矿化度及含油饱和度对WTN的影响.模拟结果表明：随着裂缝宽度的增加，WTN先呈指数增加后趋于平缓；而地层孔隙度越大、地层水矿化度越小，WTN越大；井眼尺寸和地层含油饱和度的变化对WTN影响较小.利用蒙特卡罗方法建立了地层孔隙度、矿化度石灰岩地层压裂前后的数值计算模型，模拟研究了不同深度地层的热中子分布，得到了近远探测器热中子计数FAR、NEAR和WTN的响应曲线，最终处理裂缝参数结果与设定模型相吻合，验证了利用Gd示踪热中子探测方法可以来评价井周裂缝.Abstract: The proppant placement and fracture geometric parameters evaluation in hydraulic fractures are essential to optimize the stimulation strategy in unconventional oil and gas production. A new method adopting Gd as a tracer to detect hydraulic fracture near wellbore is proposed in this paper. Based on the numerical simulation and thermal neutron diffusion theory, the induced fracture width evaluation parameter WTN is defined. Then, the effect of lithology, porosity, borehole diameter, salinity and oil saturation to determination of fracture width is analyzed. The results indicate that WTN exponentially increases before leveling off with the increases of induced fracture width. The larger the porosity, and the smaller the formation water salinity, the smaller the WTN is. The dimension of hole and oil saturation have little effect on WTN. Monte Carlo method is employed to build formation model before and after hydrulic fracturing and thermal neutron distribution in strata of different depths is simulated. Also, the response curves of thermal neutron count NEAR, FAR and WTN are obtained. It is found that the fracture parameter results are consistent with the setting model, showing that the thermal neutron detection method based on Gd tracing can be used to evaluate the fracture of near wellbore.
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Key words:
- hydraulic fracturing /
- fracture width /
- gadolinium tracer /
- thermal neutron /
- Monte Carlo N-Particle(MCNP) /
- geophysics
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图 3 不同裂缝宽度条件下探测器热中子及WTN的响应关系
Fig. 3. Response curves of thermal neutron and WTN under different fracture width
图 4 不同孔隙度条件下热中子计数率及WTN对裂缝宽度的响应
Fig. 4. The response curves of the thermal neutron count rate and the WTN to fracture width under different porosity
图 5 不同孔隙度的WTN校正后与裂缝宽度的关系
Fig. 5. The relationship between the corrected WTN of different porosity and the fracture width
图 6 不同矿化度条件下热中子及WTN对裂缝宽度的响应
Fig. 6. The response of thermal neutron and WTN to fracture width under different salinity
图 8 不同井眼尺寸条件下热中子计数率及WTN对裂缝宽度的响应曲线
Fig. 8. The response curves of thermal neutron count rate and WTN to fracture width under different borehole size
图 9 不同地层岩性条件下热中子计数率及WTN对裂缝宽度的响应曲线
Fig. 9. The response curves of thermal neutron count rate and WTN to fracture width under different Lithology
图 10 不同裂缝宽度条件下热中子计数率及WTN对含油饱和度的响应曲线
Fig. 10. The response curves of thermal neutron count rate and WTN to oil saturation under different fracture width
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