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井下重力仪是一种高精度的、大体积的、测量整体密度的仪器。这是独特的,能够不需校正,而且是在定点位置直接测出整体密度。测量大范围岩石体积可以保证测量相对地不受泥饼、侵入带、井壁冲坏或套管的影响。大体积的密度测量的应用,对于有些地层评价问题确实增加了一个新的因次(深度)。在墨西哥湾地区的井中,用井下重力测井测得的整体密度指出,它和用伽(亻马)—伽(亻马)测井所测得的密度有很大的差异。而且,它们在超压的泥岩地层中并未表现出密度的变化。这些成果提出,虽然用小范围体积的仪器测得的物理参数能够很精确,但是它们也许不能作为地层真正特性的代表。在碳酸岩—泥岩地层剖面上,由井下重力测井得来的密度仅有一个例外,其余都与伽(亻马)—伽(亻马)测井测得的密度非常符合。这个0.18克/~3厘米的密度差归因于或者是对井眼的侧向孔隙度、岩性的侧向变化、或者是一个断层。井下重力仪的高精度和测量大范围岩石体积的能力,使它在测量低孔隙度方面,例如:利比亚的碎屑石英岩的低孔隙度、以及测量在套管后面的流体密度,例如在得克萨斯的气体,都特别有用。
Downhole gravity meter is a kind of high precision, large volume, measuring the overall density of the instrument. It is unique and can be calibrated without any correction, and it measures the overall density directly at a fixed point. Measuring a wide range of rock volumes ensures that the measurements are relatively unaffected by mud cake, intruding tape, wall damage, or casing. The application of large-volume density measurements does add a new dimension (depth) to some of the formation evaluation issues. In wells in the Gulf of Mexico, the overall density measured with downhole gravimetric logging indicates that it is significantly different from the density measured with gamma (gemma) -gamma logging. Moreover, they show no change in density in overpressured mudstone formations. These results suggest that although the physical parameters measured with small-scale instruments can be very accurate, they may not be representative of the true nature of the formation. On the carbonate-mudstone profile, the density derived from downhole gravity logging is one exception, and the rest closely matches the density measured by gamma geomagnetic logging. This density difference of 0.18 g / ~ 3 cm is attributed either to lateral wellbore porosity, lateral changes in lithology, or a fault. The high accuracy of downhole gravimeters and the ability to measure a wide range of rock volumes make it useful in measuring low porosity such as the low porosity of clastic quartz rocks in Libya and the measurement of fluid density behind casings such as in Texas The gas, are particularly useful.