To improve the vertical resolution and statistical precision of C/O measurement, a novel logging tool structure is proposed. Inside the eccentric tool, both near and far γ-ray detectors are set close to the inner wall of casing, and besides the shield between the neutron source and near detector, another y-ray shield between the detectors and borehole fluids is also employed, which makes both detectors mainly collect formation information. By using Monte-Carlo method, inelastic spectra in different spacings of an 89 mm OD C/O tool, which is positioned against a 125 mm ID casing with an eccentric device, are simulated under 4 kinds of conditions, such as water or oil in the borehole, and water or oil saturated sandstone formation with 35% porosity. The relationship between C/O ratio and spacing in the aforementioned conditions is calculated, which shows that the borehole effect can be reduced and more formation information can be obtained by using the eccentric C/O logging tools. Besides, the response features of C/O logging tools with various Ods are simulated and the calculated results are compared. It is recognized that the tool with larger outer diameter is less influenced by borehole fluids and has higher formation distinguishing ability. A new toll design scheme with parameters of 89 mm OD, 5 cm distance between the source and shield, more than 30 cm near spacing and the shortest far spacing is suggested.%为提高碳氧比测井的纵向分辨率和统计精度,提出一种新型仪器结构,将测井仪推靠至套管内壁,在探测器与井液间设置伽马射线屏蔽体,让近、远探测器均记录主要来自地层的伽马射线.针对89 mm外径碳氧比测井仪推靠至125 mm内径套管内壁且套管内分别充满油或水、地层分别为35％孔隙度的饱和油砂岩或水砂岩等4种情况,采用蒙特卡罗方法模拟不同源距处面向地层与面向井眼的非弹伽马能谱,获得了地层或井液变化后碳氧比差值随源距变化关系.推靠式测井可显著减小井眼效应,对地层变化更灵敏.针对不同外径的碳氧比测井仪,模拟了套管内为水、地层分别为35％孔隙度的饱和油砂岩与水砂岩时的测井响应,得到地层改变后碳氧比差值随源距变化关系.外径越大且源屏距越小,则仪器受井液影响越小、识别地层能力越强.推荐89 mm外径、5 cm源屏距、短源距大于30 cm、远探测器尽量靠近近探测器的推靠式仪器设计方案.
展开▼
