The present invention relates to apparatus and methods for oil field acoustic logging while drilling.
In the oil and gas industry, underground formations are probed and characterized by well logging tools. Acoustical properties of a formation, such as compressional (P) and shear (S) wave speeds, are measured with sonic tools and have applications in seismic correlation, petrophysics, rock mechanics and other areas. Traditionally, these measurements are conducted after or in between drilling and are called wireline logging. In wireline logging, sonic monopole tools are used to measure P in all formations and S in fast formations; sonic dipole tools are used to measure S in both fast and slow formations (Kurkjian and Chang, 1986). Exxon reported experimental work on the concept of wireline quadrupole tools (Chen 1989, Chen and Eriksen 1991, Winbow etc. 1991). In the last decade there has been a trend towards measuring the rock properties while drilling is taking place, that is Logging While Drilling or LWD. Schlumberger""s ISONIC tool is an example of such LWD tools. The ISONIC is a monopole sonic tool. Other companies in the industry have commercialized combined monopole and dipole LWD tools. For all sonic tools, tool borne energy, which contains no information about the formation, is sought to be removed or attenuated. For wireline tools, the tool waves are reduced with designs such as slotted sleeves, isolation joints and flexible tool structures. For LWD tools, tool waves carried by the prominent and stiff tool body, which is essentially the drill collar with sensors and electronics mounted on it, is a more serious challenge; hence deriving formation P and S speeds from LWD data is not nearly as straightforward as in wireline.
A sonic tool has usually one or more sources and multiple receivers. There are multiple paths for the acoustic energy emitted by the source to reach the receivers. The part of acoustic energy propagating through the formation and mud provides useful information for characterizing the formation. The part of acoustic energy propagating through the tool body does not contain useful information and is sought to be removed.
A monopole logging tool, wireline or LWD, employs single or multiple monopole acoustic source(s) as well as receivers; they transmit and detect acoustic energy uniformly in all azimuthal directions in the plane perpendicular to the tool axis. It is well understood based on wave propagation theory that a monopole tool can excite and detect P and Stoneley waves in all formations, regardless of formation speed. In addition, a monopole tool is capable of generating and detecting S waves in fast formations where the formation shear speed is faster than the sound speed in the mud. In wireline monopole tools, the tool borne energy is delayed and suppressed by techniques such as slotted receiver housings and/or damping. In LWD monopole tools, dealing with the tool waves is a more difficult issue. Schlumgerger""s ISONIC tool achieves tool wave attenuation over a selected frequency band with a specially designed periodic array of grooves machined on the collar section between the transmitter and receivers. This technique is disclosed in U.S. Pat. No. 5,852,587 to Kostek et al., issued Dec. 22, 1998.
A dipole tool generates and receives the flexural mode in a borehole surrounded by a formation. The term xe2x80x9cDipolexe2x80x9d refers to the azimuthal profile cos xcex8 for the transmitter, receivers and the acoustic field associated with the flexural mode. The flexural mode propagation asymptotes to the formation shear speed at the low frequencies, and to the mud-formation interface wave speed at high frequencies. Thus S wave speed of the formation can be derived from the measured flexural mode (Kurkjian and Chang 1986). Wireline dipole tools use flexible (i.e. acoustically slow) or acoustically transparent housings for receivers to minimize or avoid tool effects on the measured borehole flexural mode, in other words, to approximate the fluid-filled (no tool) borehole condition. These tools are also designed with some form of acoustic isolator or attenuator between the source and receivers to reduce the excitation of tool waves.
Applying the wireline dipole shear technique to LWD encounters serious challenge of tool wave interference. LWD tools can not be made very flexible or slow as wireline tools because of tough drilling conditions in which LWD tools need to operate. The body of an LWD tool, essentially a drill collar, provides a propagation path of acoustic energy between the acoustic source and receivers. Both borehole flexural and tool flexural waves are present in similar frequency and slowness ranges and thus can not be easily separated. In addition, the presence of the tool alters the borehole mode from the no tool condition; the alteration is the most substantial under conditions where the borehole and tool flexural waves are close in speed (Hsu and Sinha 1998, Rao and Vandiver 1999). These two coexisting borehole and tool flexural modes can be predicted by modeling. However, to extract formation shear from these altered modes is much more difficult and less robust than extracting shear in the wireline situation where only the borehole flexural mode is present. One typical approach to alleviate the problem is to attenuate the tool borne energy with attenuators placed in the drill collar in between the transmitter and receivers (Birchak etc 1996, Molz and Leggett 2000). However, even if the tool borne energy is substantially attenuated and becomes weaker than the borehole flexural mode signal, the resultant dispersion of the borehole flexural mode will typically not asymptote to the formation shear speed. This can significantly reduce the accuracy of extracting formation shear from the data.
Borehole quadrupole mode has similar dispersion characteristics as that of the borehole dipole mode, with the quadrupole dispersion taking place at higher frequencies relative to the dipole mode. Quadrupole refers to the azimuthal profile cos2xcex8 of transmitter, receivers, and acoustic field associated with the quadrupole modes. Both dipole and quadrupole modes asymptote to the fluid-formation interface wave speed at high frequencies and to the formation shear speed at low frequencies. More specifically, the quadrupole mode actually crosses the shear speed.; however, there is practically no energy at frequencies below the crossing point. Exxon reported on a wireline quadrupole experimental tool for S wave measurement (Chen 1989, Chen and Eriksen 1991, Winbow etc. 1991). This tool is disclosed in U.S. Pat. No. 4,932,003 to Winbow et al., issued Jun. 5, 1990, and in U.S. Pat. No. 5,027,331 to Winbow et al., issued Jun. 25, 1991. In the Exxon experimental quadrupole tool, an acoustic isolator between source and receivers was mentioned, and no consideration of tool effect was reported.
The present invention provides, in a preferred embodiment, a tool for generating signals indicative of shear wave slowness of the formation surrounding a borehole containing a fluid and a drill string. The tool is adapted for use with a drill string having a plurality of drill collars jointed together. The tool comprises: a collar portion defining an axis and adapted for mounting in a drill string; one or more quadrupole sonic transmitters mounted to the collar portion at a first location; and a quadrupole sonic receiver array mounted to the collar portion at a second location, the second location spaced apart from the first location.
The transmitter is preferably embedded in an annular groove in said collar portion. The receiver array includes an axial array of transducers, each transducer embedded in an annular groove in said collar portion. The transmitter and receiver are adapted for mounting to a thick wall drill collar.
The tool also includes electronics, mounted within said collar portion, for converting signals indicative of shear wave slowness of the formation into digital data indicative of shear wave slowness of the formation; receiver electronics with means for storing said digital data; and means for downhole processing and transmitting formation shear slowness data to the surface by mud-pulse or other suitable drill-string telemetry.
The present invention also provides a quadrupole sonic transmitter, for use in a tool for generating signals indicative of shear wave slowness of the formation surrounding a borehole containing a fluid and a drill string, the tool adapted for use in a drill string having a plurality of drill collars jointed together, said tool having a collar portion defining an axis, said quadrupole sonic transmitter having an azimuthal array of four equally spaced transducer quadrants, each quadrant including at least one electro-acoustical transducer element. In a preferred embodiment, each quadrant includes a quarter-circle array of piezoelectric transducer elements, the elements of the four quadrants collectively arranged as a quadrupole piezoelectric ring transducer. Each quadrant has a predefined acoustic power output profile across the elements of each quarter-circle array, such that the elements of the four quadrants collectively produce an acoustic wave that approximates the quadrupole profile of cos2xcex8. The output of these elements can be shaped to approximate this quadrupole profile more closely by varying the physical size of these elements or varying the driving voltage of these elements following the cos2xcex8 azimuthal pattern. The transmitter is adapted for embedding within a protective encapsulating material in an annular groove in the collar portion of said tool.
In one alternative embodiment, each transducer quadrant includes a stack of piezoelectric elements.
In another alternative embodiment, each transducer quadrant includes at least one pressure balanced backing bender transducer.
In another alternative embodiment, each transducer quadrant includes at least one magnetostrictive transducer.
In another alternative embodiment, each transducer quadrant includes at least one electro-dynamic transducer.
The present invention also provides a quadrupole sonic receiver, for use in a tool for generating signals indicative of shear wave slowness of the formation surrounding a borehole containing a fluid and a drill string, the tool adapted for use in a drill string having a plurality of drill collars jointed together, said tool having a collar portion defining an axis, said quadrupole sonic receiver having an azimuthal array of four equally spaced transducer quadrants, each quadrant including at least one electro-acoustical transducer element. In a preferred embodiment, each quadrant includes a quarter-circle array of piezoelectric transducer elements, the elements of the four quadrants collectively arranged as a quadrupole piezoelectric ring transducer. Each element of each quadrant has a predefined sensitivity profile across the elements of each quarter-circle array, such that the elements of the four quadrants collectively produce an acoustic wave sensitivity that approximates the quadrupole profile of cos2xcex8. The receiver is adapted for embedding within a protective encapsulating material in an annular groove in the collar portion of said tool.
In alternative embodiments the transducer quadrant includes either a stack of piezoelectric elements, or at least one pressure balanced backing bender transducer, or at least one magnetostrictive transducer, or at least one electro-dynamic transducer.
Another alternative but not preferred embodiment is to have receiver transducers sensing the pressure only around a selected azimuthal angle, for example, aligned with one of the four quadrants of the transmitter. Such a receiver array has much less capability in rejecting other spurious modes.
A system is provided for determining shear wave slowness of the formation surrounding a borehole, the borehole containing a fluid, the system comprising: a tool for generating signals indicative of formation shear wave slowness, the tool having a collar portion defining an axis and adapted for mounting in a drill string, a quadrupole sonic transmitter mounted to the collar portion at a first location, and a quadrupole sonic receiver array mounted to the collar portion at a second location, the second location spaced apart from the first location; and computing means for calculating formation shear wave slowness from said signals. Dispersive wave processing includes means for dispersive slowness time coherence processing.
The present invention provides a method for determining shear wave slowness of the formation surrounding a borehole containing a drill string and a fluid, the method comprising: propagating quadrupole wave energy into the formation from a first location on the drill string; detecting quadrupole dispersive waveforms in a receiver array at a second location on the drill string at a distance from the first location to obtain digital data indicative of shear wave slowness of the formation; and calculating formation shear wave slowness from said digital data. Calculating formation shear wave slowness includes dispersive wave processing. Dispersive wave processing includes dispersive slowness time coherence processing. Calculating formation shear wave slowness includes: a) filtering the recorded signals to remove the collar quadrupole mode from the data, b) Fourier transforming at least a portion of said signals to obtain Fourier transformed signals; c) backpropagating said Fourier transformed signals utilizing a plurality of pre-computed, model-based quadrupole dispersion curves to obtain a plurality of sets of backpropagated signals; d) stacking said backpropagated signals of each set to provide stacked sets; and e) using said stacked sets to determine formation shear wave slowness.
In one embodiment, the method uses formation quadrupole dispersive waveforms having a range of frequencies whose maximum frequency is approximately equal to the cut-off frequency for collar quadrupole mode propagation, and whose minimum frequency is approximately equal to the frequency at which formation quadrupole mode is expected to propagate at a slowness approximately equal to the formation shear slowness.
The method uses a pre-selected range of frequencies, for example 4-8 kHz, depending on the tool size and, if available, the expected formation slowness. The method uses filtering for formation quadrupole mode data, wherein filtering includes frequency domain filtering, slowness domain filtering, or frequency-slowness domain (two dimensional) filtering.
The present invention provides a method for determining shear wave slowness in formation surrounding a borehole containing a drill string and a fluid, the method comprising: a) obtaining from a plurality of sonic detectors, quadrupole pressure data indicative of formation shear wave slowness; b) Fourier transforming at least a portion of said data to obtain Fourier transformed signals; c) backpropagating said Fourier transformed data utilizing a plurality of quadrupole dispersion curves to obtain propagated data representing a plurality of sets of backpropagated signals; d) stacking said backpropagated data to provide stacked sets data; and e) processing said stacked sets data to determine formation shear wave slowness.
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