The Numerical Calculation Of Acoustic Logging And Its Response Characteristics

Acoustic logging is an important member of geophysical exploration methods.Geological reservoirs using acoustic logging detection state is indispensable in today’senergy development research component. Cracked reservoir geological body as animportant medium, has been the focus of one of the objects geophysical research.However, the cracked medium has complex geological structure, a wide range ofinclusions, cracks distribution and arrangement of the relationship varied, which haveincreased the difficulty researchers analyze cracked reservoir. Especially for acousticlogging method, because distribution of the well field, it is necessary to numericalsimulation of the cracked reservoir at the well site.During surveys, water layers may interfere with the detection of oil layers. Inorder to distinguish between oil and water layers, research on the properties of welldiameters and oil and water layers and their relation to acoustic logging rules sessential. On the basis of Hudson’s crack theory, we simulated oil and water layerswith different well diameters or crack parameters (crack angle and crack numberdensity and crack aspect ratio). We found that wave velocities show the same trend foroil and water layer, and the velocities increase as the crack angle increases from20°to90°. P-wave velocity increases linearly in the water layer, while it undergoes aladder type in the oil layer. P-wave velocity increases at higher rate in an oil layer in acracked medium with high crack angles than in one with low crack angles; P-wavevelocity is insensitive to crack parameters when the crack angle in the oil layer issmall; however, the velocity is sensitive to the crack parameters when the crack anglein the oil layer is large. Although the trend of S-wave velocity is similar to that ofP-wave velocity, trends of the former in oil and water layers are more consistent thanthose of the latter.P-wave and S-wave velocities reduce with increase in the crack number densityfrom0.00to0.40. In the water layer, P-wave velocity decreases drastically as thecrack number density increases from0.00to0.05; the variation in velocity reaches large part of the total attenuation. In the oil layer, P-wave velocity decreasesdrastically as the crack number density increases from0.00to0.09. Overall, theattenuation ratio of P-wave velocity for the oil layer is less than that for the waterlayer. We studied the inflection points of velocity attenuation and found that theinflection point appears later in the oil layer. S-waves exhibit different behaviours inoil and water layers; when the number density of cracks increases from0.07to0.13,S-wave velocity decreases linearly in the water layer, but in the oil layer, the trendshows an inflection. Further, the velocity decreases at a higher rate in the water layer.P-and S-waves undergo similar changes as the crack aspect ratio increases from0.0to1.0. Their velocities are greatly attenuated when the aspect ratio range is withinthe range0.0–0.03, and the attenuation is greater in the oil layer than in the waterlayer. Stoneley wave velocity is not influenced by the crack aspect ratio. In thefull-waveform logs for the oil and water layers, wave amplitudes in the water layerare considerably greater than those in the oil layer; this difference between theamplitudes in the oil and water layers is most pronounced in the case of Stoneleywaves. The velocities of P-, S-, Stoneley, and mud waves do not show great changes.P-wave amplitude is slightly smaller in the oil layer than in the water layer. Theamplitudes of S-and pseudo-Rayleigh waves are higher in the oil layer than in thewater layer.The two-dimensional spectra for the water and oil layers demonstrate that on thewhole, the two-dimensional spectrum of oil layer’s colour is darker than the colour ofthe water layer, and the spectrum values are higher in the case of the oil layer, thedifference implying that the contribution of these modes and the maximum excitationintensity values are greater in the oil layer than in the water layer. Therefore, in actuallogging exploration we can compare the spectrum of the water layer with that of theunknown formation, to determine if the formation is likely to contain oil. Waveamplitudes show the same trend for compressional wave, and the amplitudes decreaseas the well radius increases form0.04m to0.16m. Compressional wave amplitudesdecrease at lower rate in oil layer with a small well radius range (0.04m to0.06m)than in water layer with the same well radius range; the variation ratio of the amplitude only reaches25.5%of the total decrease in the former case, while it reaches79.8%in the latter case. In other words, compressional wave amplitudes areinsensitive to well diameter when the well diameter is small and in the oil layer;however, the amplitudes are sensitive to the small well diameter when the waves arein the water layer. We studied the inflection points of amplitude attenuation and foundthat the inflection point appears later in the oil layer. The differences of compressionalwave amplitudes between each crack angle are greater in the case of the oil layer thanin the case of the water layer.Wave amplitudes in the water layer are considerably greater than those in the oillayer. The difference between the amplitudes for large well radius and small wellradius is most pronounced in the case of Stoneley waves. The difference of Stoneleywave amplitude between the crack parameters is greater in the case of the water layerthan in the case of the oil layer. The wave energy sensitivity in the cracked mediumwith high crack angle or low number density of cracks, compressional and Stoneleywave energy is sensitive to well diameters; further, the energy is more sensitive towell diameters when the waves propagate through the water layer than when theypropagate through the oil layer. We studied the inflection points of Stoneley waveenergy attenuation sensitivity and found that the higher the crack number density is,the later the inflection point appeared in the water layer. We conclude that thepropagation of an elastic wave is affected by medium composition and well diameter,and the influence has certain regularity. These results can guide further reservoirlogging field exploration work.