Abstract
The finite difference (FD) method of monopole source is used to simulate the response of full-wave acoustic-logging in cave formations. The effect of the cave in the formation of borehole full-waves was studied. The results show that the radius of cave is not only linearly related to the first arrival of the compressional wave (P-wave), but also to the energy of the shear wave (S-wave). The converted S (S–S wave) and P-waves (S–P wave) are formed when the S-wave encounters the cave. If the source distance is small, the S–S and S–P waves are not separated, and the attenuation of the S-wave is not large, due to superposition of the converted waves. The S–P wave has been separated from the S-wave when the source distance is large, so the attenuation of the S-wave increases. The amplitude of the P and S–waves changes most when the distance of the cave to the borehole wall reaches a certain value; this value is related to the excitation frequency. The amplitude of the Stoneley wave (ST wave) varies directly with the radius of cave. If the radius of the cave is large, the energy of ST wave is weak. The scattered wave is determined by the radius and position of the cave. The investigation depth of a monopole source is limited. When the distance of the cave to the borehole wall exceeds the maximum investigation depth, the borehole acoustic wave is little affected by the cave. In actual logging, the development of the cave can be evaluated by using the first arrival of the P-wave and the energy of the S and ST waves.
Highlights
Caves are widely distributed in carbonate and igneous reservoirs, causing heterogeneity and making it difficult to evaluate the reservoirs by conventional logging
Based on the observation of cores, the reservoir spaces of igneous and carbonate formations can be divided into primary pores and secondary pores, according to their genesis [1]
It is very important to grasp the effect of caves on borehole acoustic wave propagation, which plays an important role in the detection and evaluation of caves
Summary
Caves are widely distributed in carbonate and igneous reservoirs, causing heterogeneity and making it difficult to evaluate the reservoirs by conventional logging. Based on the observation of cores, the reservoir spaces of igneous and carbonate formations can be divided into primary pores and secondary pores, according to their genesis [1]. These can be divided into matrix-dissolution pores, intragranular-dissolution pores and apricot-kernel dissolution pores in igneous formations [2]. Carbonate formations have porous caves, intergranular caves and fractured caves. The caves in the formation can be defined as voids, washouts or large pores of secondary porosity in a more general. It is very important to grasp the effect of caves on borehole acoustic wave propagation, which plays an important role in the detection and evaluation of caves. The effect of fractured and gas-bearing reservoirs on borehole acoustics has been investigated [6,7,8]
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