Recent studies of continuous acoustic velocity logs indicate that these logs may provide important assistance in differentiating gas, oil, and water saturations in reservoir rocks. In general, velocities are appreciably lower in sands carrying oil or gas than in water‐saturated sands of otherwise similar character. Specific examples from field logs illustrate this application. Laboratory measurements have been made of acoustic velocity of synthetic and natural rocks. Published studies, both empirical and theoretical, of other workers concerned with the transmission of sound in porous media have been considered. All of these at least qualitatively confirm the conclusions drawn from field data.
Shale masses are here defined as large bodies of shale at least several hundred feet in thickness. These may be formed either as diapiric masses or as depositional masses. The shale masses are like salt masses and the two are many times combined to form domal masses; they both may form the updip seal for stratigraphic accumulation of oil. The shale masses exhibit the following properties by comparison to the normal section: (1) low velocities—in the range of 6,500 to 8,500 ft/sec with very little increase of velocity with depth, (2) low densities—estimated to be in the range [Formula: see text] to [Formula: see text], (3) low resistivities—approximately 0.5 ohm‐m, and (4) high fluid pressures—about 0.9 overburden pressure. These properties all seem to be caused by the high porosity and low permeability of these large shale masses. Maps and cross sections of an example area block 113, Ship Shoal Area are shown. The low shale velocities were measured by acoustic logs and verified by refraction shooting. The low densities were deduced from gravity maps. The low resistivities are shown on electric logs, and high pressure is evidenced by the drilling difficulties with heaving shales. These physical properties allow the outlining of the shale mass by one or more of the following ways: the gravity method is used to outline the low density material, the seismic reflection method is used to outline the lack of reflection contrast and in some cases map the velocity configuration, the seismic refraction method is used to indicate the velocity of the anomalous mass, thereby differentiating between shale and salt.
Difficulties occur in obtaining accurate two‐receiver velocity logs in formations sensitive either to damage by exposure to drilling mud or to mechanical stress relief. Some shales are so altered by the drilling operation that their elastic properties are modified. Vertical velocity measured immediately adjacent the boreface is lower than if it were measured at a greater radial distance from the bore. These damaged shales require relatively deep penetration by the acoustic signal; consequently, the transmitter‐to‐first‐receiver spacing in a two‐receiver velocity logging system should be long enough to refract the sound waves through virgin formation. Experiments in one predominantly shaly section show a difference of almost 10 percent between times measured using transmitter‐to‐first‐receiver spacing of 4.3 ft compared to 8.8 ft. A limited amount of field data suggest that sodium montmorillonite is the clay type most sensitive to hydration and swelling. Studies of areal prevalence of the shale damage problem are incomplete.
Paragraph II: Kaarsberg states that mechanical stress relief should not be ruled out as an explanation of low velocity adjacent the borehole. If the volume of the material stressed increases, the elastic moduli decrease. Therefore, velocity immediately adjacent the bore decreases. This is indeed a powerful point and properly should have been discussed more fully.
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