In acoustic well logging, acoustic waves emitted from an acoustic source propagate through earth formations surrounding the well, and are received by an array of transducers spaced a distance away from the source. Measurements of acoustic wave arrival times and propagation velocities provide an important source of quantitative information for locating hydrocarbon bearing reservoirs and assessing their economic value.
In elastic earth formations, such as porous sandstone containing water or hydrocarbons in the pore space, the acoustic wave divides into at least two components traveling at different acoustic velocities. A compressional wave, also known as a primary or P-wave, is commonly the fastest acoustic wave and appears at the receiver before the other waves. This wave arises from compression of the formation, and is related to the compressibility properties of both the mineral content of a porous formation and the compressibility properties of fluid contained in the pores. A shear wave or S-wave typically follows the arrival of the P-wave. It arises from transverse shear motion of the formation, and is affected primarily by the shear stiffness of the mineral components of a porous formation. In formations with substantial anisotropy, the shear wave may be further subdivided into two components traveling at different velocities; a component polarized in one direction (called the x-shear wave), and another component polarized in a perpendicular direction (called the y-shear wave).
Measurement of wave velocities of formation materials surrounding the borehole provides important quantitative information about both mineral matrix and pore fluids. In particular, a combination of compressional and shear velocities allows the location, identification, and economic value assessment of hydrocarbons, including both oil and gas. Measurement of wave velocities between separate boreholes provide information about geologic structures between the boreholes, and provides useful information for identifying bypassed oil pockets and monitoring the effectiveness of oil field production. Furthermore, measurement of arrival times from reflected waves provides important information about the location and properties of geological structures, such as hydrocarbon-bearing zones, at a distance away from the borehole.
Additional undesirable acoustic waves called tube waves are commonly produced when an acoustic source is located in a borehole containing fluid such as drilling mud or water. The tube wave, sometimes also called a Stoneley wave, is a low frequency (less than 10 kHz) compressional wave which is propagated in the borehole fluid but whose velocity may be modified by the properties of the surrounding formation. At high frequencies (greater than 10 kHz), there are in general a multiplicity of complicated waves traveling through the borehole fluid at different velocities. These additional waves represent undesirable noise which obstructs the measurement of shear wave and compressional wave velocities.
Many tools have been developed for acoustically logging a wellbore to obtain wave velocity data. A typical acoustic well logging tool comprises a sonde containing an acoustic source and one or more spaced receivers together with processing means to identify a selected acoustic wave and measure its velocity. The sonde is raised and lowered in the wellbore on a wireline logging cable which provides operating power for transmitter and receivers and transmits information from the sonde to the surface. The transmitter is pulsed in a timed sequence as the sonde is raised in the wellbore to generate acoustic signals which pass into the formation and are passed back into the wellbore where they are detected by the receivers in the sonde. The detected signals are then transmitted through the cable to the surface where they are processed and recorded to produce the desired acoustic log of the wellbore.
The earliest acoustic logging tools to be commercially developed, sometimes called sonic tools, provided for measurements of compressional wave velocity in uncased wellbores located in hard rock formations. In this situation, since the compressional wave is the fastest of all wave modes, detection of a first arrival in the wave train at spaced apart receiver locations determined interval transit time (usually expressed in microseconds per foot), from which compressional wave velocity was readily obtained. Acoustic energy sources, or transmitters, used in sonic tools were originally made of magnetostrictive material, which is magnetic material that deforms in the presence of a magnetic field. Later tools used piezoelectric crystals that deformed in the presence of an electric field. Magnetostrictive or piezoelectric transmitters for sonic tools operated at the resonant frequency of the transmitter, which was typically between 10 and 30 kHz.
Sonic tools heretofore devised have been limited by a variety of factors. Perhaps most important is the restriction of the sonic log to measurements of compression wave velocity only, although it is recognized that shear wave velocity measurements contain vital information for characterizing formation properties. Furthermore, the relatively low power level of magnetostrictive or piezoelectric transmitters limited the spacing between transmitter and receiver and also restricted the sonic log to open-hole (uncased) service. Short spacing and high frequencies in prior devices caused the measurement of formation compressional waves to be representative of formation material to within only a few inches of the borehole wall.
More recently, full-wave acoustic logging tools have been developed which record acoustic pressures for an extended period of time after first arrival of a return signal at the tool receiver section. It was anticipated that analysis of the recorded wave would detect arrival of the shear wave and provide valuable data on shear wave velocity. However the presence of tube waves which travel through the mud around the tool and also tool vibrational waves which travel through the tool body have seriously obstructed the accurate determination of arrival time and propagation velocity of the shear wave.
In order to compensate for interference of undesirable waves new analysis methods were developed which estimated shear propagation velocity using acoustic pressures for an extended period of time after first arrival. However, these analysis methods did not provide an exact measurement of phase velocity, which is the desired true propagation velocity of the formation material.
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