Acoustic Logging in Low Velocity Formations

The invention relates to a method and apparatus for logging the acoustic wave velocity in an earth formation surrounding a borehole containing liquid where the acoustic wave velocity in the formation is less than the acoustic wave velocity in the borehole liquid.

Acoustic well logging is a generally accepted method for obtaining information about subterranean earth formations surrounding wells or boreholes. Acoustic well logging can be used in the determination of formation lithology, density, porosity, the conversion of seismic time sections to depth sections, and the detection of fractures.

A conventional acoustic logging system includes a logging sonde suspended in the borehole liquid, a source attached to the sonde for generating compressional waves ("P-waves"), and two or more receivers attached to the sonde and spaced apart from the P-wave source for detecting P-waves in the borehole liquid. A P-wave in the borehole liquid generated by the source is refracted in the earth formation surrounding the borehole. The refracted wave propagates through a portion of the formation, is refracted back into the borehole liquid, and detected by two or more receivers spaced vertically apart from each other and from the P-wave source. The first arrival will be the P-wave which refracts along the borehole wall. The ratio of the separation between the two receivers to the time difference between the detections of the refracted P-wave by the two receivers yields the P-wave velocity in the formation. From this information, many characteristics of the formation can be determined.

Conventional acoustic logging is dependent on the respective velocities of the P-waves in the borehole liquid ("liquid velocity") and in the surrounding earth formation ("formation velocity"). Refraction necessary to facilitate acoustic well logging will not occur if the liquid velocity exceeds the formation velocity. The wave will not appropriately refract in the formation so that signals can be detected. Formation velocity is often less than liquid velocity in the top levels of a formation. For this reason, conventional acoustic logging cannot be performed in many formations in the first few hundred feet below the surface.

While acoustic well logging is a widespread method of collecting data, no conventional methods permit acoustic logging in formations where liquid velocity exceeds formation velocity ("low velocity formation"). All conventional acoustic logging methods require a formation velocity that is higher than the liquid velocity. The present invention is a method and apparatus for solving this problem.

According to this invention, gas bubbles are introduced into a zone of liquid around a logging device positioned in a borehole contemporaneously with the generation of compressional acoustic waves by an acoustic source in the logging device. The presence of bubbles in the zone of liquid reduces the compressional wave velocity therein so that it is less than the compressional wave velocity in the earth formation surrounding the borehole. Refraction can then occur in a manner facilitating acoustic logging of the formation.

A preferred apparatus for practicing the invention includes a gas injector ring which has perforations spaced substantially equidistantly on its outer edge. Compressed gas is delivered to the gas injector ring by a tube connected to a compressed gas source. Gas bubbles are then introduced through the perforations in the gas injector ring into the zone of liquid in which compressional waves are generated.

FIG. 1 is a front elevational view of a logging tool in a borehole having attached to it an acoustic source, two receivers, and a gas injector ring connected to gas compression means.

FIG. 2 is a schematic diagram of the path taken by a P-wave which refracts upon entering a formation surrounding a borehole where the velocity of the P-wave in the formation is greater than its velocity in the borehole liquid.

FIG. 3 is a top elevational view of a gas injector ring.

FIG. 4 is a front elevational view of a logging tool in a borehole having attached to it an acoustic source, two receivers, and a single injector hole located at the end of a tube connected to gas compression means.

The method and apparatus of the invention facilitates logging of the acoustic velocity of P-waves in an earth formation when the compressional wave velocity in the formation is less than the compressional wave velocity in the borehole liquid in which the compressional wave is generated.

The preferred embodiment of the invention may be more easily understood with reference to FIG. 1. A conventional logging sonde 10, with attached acoustic source 12 and two receivers 13 and 14 is placed in borehole 16. Receivers 13 and 14 may be of identical construction and hence interchangeable. One end of tube 18 is attached at location 20 to compressed gas source 21 located on surface 22 above borehole 16. The other end of tube 18 is attached to gas injector ring 24 located below sonde 10. Gas injector ring 24 includes at least one hollow spoke such as spoke 26, and hollow outer edge 28. At least one perforation such as perforation 30 on outer edge 28 permits gas to escape from the interior of the ring into borehole liquid 34 in zone 36 of borehole 16 adjacent to sonde 10. If two or more perforations are used, they should be spaced substantially equidistant from each other. Although six spokes and six perforations are illustrated, more or less than six spokes or perforations may be used.

In order for acoustic logging to succeed, P-waves must refract upon entering formation 32. This will occur if the formation velocity exceeds the velocity in liquid 34 in zone 36. To initiate logging, P-waves produced by source 12 travel through liquid 34 in zone 36 of borehole 16. The P-waves then enter formation 32 and refract so as to reenter borehole liquid 34 and arrive at receivers 13 and 14 if the formation velocity is greater than the liquid velocity. FIG. 2 is a schematic diagram of the path taken by the portion 40 of an acoustic wave which is traveling along critical angle 41. This portion 40 travels along the wall of formation 32 and is detected by receivers 14 and 13 respectively before any other wave. The difference in travel time between source 12 and receiver 13 and source 12 and receiver 14 permits determination of the formation velocity, which in turn is used to determine porosity and other properties of the formation. Critical angle 41 is determined by the known formula .theta.c= sin.sup.-1 {V.sub.l /V.sub.f }where V.sub.l is the compressional wave velocity in the borehole liquid, and V.sub.f is the compressional wave velocity in the formation. If the formation velocity is less than the the liquid velocity, this equation has no solution, and the refractions sought to be detected by receivers 13 and 14 do not occur.

Low formation velocity near the surface of the earth is generally caused by a low degree of consolidation. As the depth increases, the formation is compacted by pressure from sections of the formation above, increasing the consolidation. The actual depth at which the formation velocity exceeds liquid velocity depends primarily on the composition of the formation. Further, pressure inversion pockets may exist at depths where formation velocity would be expected to be greater than liquid velocity.

The invention permits logging in situations such as these where logging previously was not possible. Tube 18 carries gas from compressed gas source 21 to injector ring 24. FIG. 3 is a top elevational view of the preferred embodiment, which shows more clearly injector ring 24. After entering gas injector ring 24, gas is routed through spokes such as spoke 26 to outer ring 28. Gas escapes into borehole liquid 34 through perforations such as perforation 30, producing a substantially uniform distribution of bubbles which rise into zone 36. The gas bubbles lower the acoustic velocity of P-waves in bubble filled zone 36 of borehole liquid 34 so that it is less than the velocity in formation 32. The velocity of waves through a liquid is inversely proportional to the compressibility of the liquid. Since gas bubbles are extremely compressible in comparison to liquids, the presence of gas bubbles in a liquid will reduce the acoustic velocity therein. For example, E. Silberman, in "Sound Velocity and Attenuation in Bubbly Mixtures Measured in Standing Wave Tubes," Journal of the Acoustic Society of America, 29, pp. 925-933 (1957) ("Silberman"), reported that a 0.3% mixture by volume of air bubbles in water at a pressure of 109 kiloPascals and a temperature of 26.7.degree. C. reduced the liquid velocity from 1490 m/sec to 328 m/sec. This reduction would allow acoustic logging to be conducted in many loose sands and clays where velocities between 350 m/sec and 1050 m/sec are commonly found. If the formation velocity is greater than the reduced liquid velocity, the P-waves propagating from source 12 through zone 36 can refract upon entering formation 32, so as to permit acoustic logging.

Acoustic velocity is also inversely proportional to the density of the liquid, but the effect on liquid velocity due to the density change of the liquid caused by injection of gas bubbles therein is overshadowed by the magnitude of change in liquid velocity due to the compressibility change. The density of the liquid decreases slightly by comparison. Most types of gas are suitable for use in practicing the invention since most gases have a high compressibility compared to water.

Bubble diameter must also be controlled to prevent excessive attenuation of the logging signal. According to Silberman, the surface of each bubble will resonate at a frequency determined by the following formula:

Where

Po is the pressure at the bubble zone

.rho. is the density of the water gas mixture

.mu. is known as the Polytropic factor

.UPSILON. is the adiabatic constant, and

D is the diameter of the bubble.

The average resonant frequency of the bubbles must be greater than the logging frequency. If the average resonant frequency is near the frequency of the P-waves, the energy from the P-waves is used by bubbles in their resonance, causing attenuation of the logging signal. This attenuation may be so great that the signal will not be detected by the receivers, and logging fails.

Logging typically is performed at frequencies of 10-15 kHz to ensure refractions resulting in detectable refracted signals. A bubble resonant frequency of approximately 20 kHz will allow a sufficient margin of error so that excessive attenuation will not occur. Changing the bubble diameter is the most effective method of controlling the bubble resonant frequency. Pressure will increase by approximately 10 kiloPascals (kPa) for every meter of depth in the liquid. The density of the liquid-gas mixture will be determined by the volume of bubbles distributed. If these values are known, a desired bubble diameter can be determined.

The following example shows how bubble diameter can be adapted to yield the desired bubble resonant frequency. Using the resonant frequency formula, the minimum resonant frequency can be found when .mu.=1/.UPSILON.. The equation is reduced to:

In this example the logging will be attempted at a depth of 17 meters. The pressure at this depth will be approximately 170 kPa. It will be assumed that the borehole liquid will have a density of approximately 1000 kg/m.sup.3. The density will change as gas bubbles are introduced into the borehole liquid, but the volume of gas bubbles used makes this change insignificant. Where these pressure and density conditions exist, a bubble diameter of 0.1 mm will have a resonant frequency of 20 kHz. Since the pressure and bubble diameter are inversely proportional to each other it is preferred that when logging a borehole according to the invention, the perforations be continuously adjusted so that the bubbles formed have an average diameter giving a preferred average resonant frequency at each depth logging is conducted.

Other constructions of the apparatus are envisioned. FIG. 4 shows a single injector hole 48 located at the tip of tube 46 through which bubbles may be introduced from compressed gas source 21 into borehole liquid 34. Bubbles may alternatively be injected from injector holes positioned at various points near sonde 10.

The above description is merely illustrative of the present invention. Various changes in shapes, sizes, materials or other details of method and construction may be within the scope of the appended claims without departing from the spirit of the invention.