Method for acquiring data of azimuthal acoustic logging while drilling

Abstract

In a method for acquiring data of azimuthal acoustic LWD, when an acoustic LWD instrument rotates with a drilling tool at a certain depth, data is acquired by adopting an azimuthal equal-interval mode: a well circumference is divided into m sectors by azimuthal intervals Δθ, when a toolface angle of the acoustic LWD instrument is located in the k.sup.th sector, an acoustic transmitting source is controlled to transmit an acoustic signal, and an acoustic receiver measures the acoustic signal, digitizes it and then stores it as data in the k.sup.th sector; and the data is acquired for each sector in turn, and after the data is acquired in each sector for N times, the data acquisition at the current depth is completed. Meanwhile, as the drilling tool rotates and drills, the instrument acquires acoustic data at different depths and processes it to achieve azimuthal acoustic imaging.

Claims

1. A method for acquiring data of azimuthal acoustic Logging While Drilling (LWD), comprising: rotating a drilling tool in a well in a subterranean formation, wherein the drilling tool comprises an acoustic instrument having an acoustic transmitter and an acoustic receiver installed thereon; performing an azimuthal equal-interval acquisition mode for acoustic logging using the acoustic instrument at a first depth in the well, wherein the step of performing an azimuthal equal-interval acquisition mode for acoustic logging comprises the sub-steps of: a. dividing a circumference of the well into m sectors, each sector has an azimuthal angle of 360°/m, wherein m is an integer larger than one; b. measuring a toolface angle θ of the acoustic instrument; c. determining the acoustic instrument is in a k.sup.th sector when the toolface angle θ is larger than (k−1).Math.360°/m and smaller than k.Math.360°/m, wherein k is in a range from 1 to m; d. transmitting an acoustic signal from the acoustic transmitter into the subterranean formation when the acoustic instrument is in the k.sup.th sector; e. receiving the acoustic signal by the acoustic receiver from the subterranean formation; f. digitizing and storing the received acoustic signal as data in the k.sup.th sector, wherein data is waveform data; g. repeating sub-steps d, e, and f so that the k.sup.th sector receives N sets of data, wherein N is a positive integer; h. superimposing the N sets of data to obtain an superimposed data volume for the k.sup.th sector; and i. performing sub-steps c-h for a (k.sup.th+1) sector until (k.sup.th+1) equals m.

2. The method for acquiring data of azimuthal acoustic LWD according to claim 1, comprising performing acoustic logging using the acoustic instrument at a second depth in the well.

3. The method for acquiring data of azimuthal acoustic LWD according to claim 1, wherein the acoustic receiver comprises n sets of receiving transducer arrays, and the superimposed data volume for the k.sup.th sector is A(k, p), p=1, 2, . . . n, wherein p represents a serial number of the n sets of receiving transducer arrays and n is a positive integer.

4. The method for acquiring data of azimuthal acoustic LWD according to claim 3, further comprising processing the superimposed data volume in each sector by a slowness-time correlation (STC) method to obtain a compressional wave velocity and a shear wave velocity for each of the sectors at the first depth.

5. The method for acquiring data of azimuthal acoustic LWD according to claim 1, wherein the toolface angle is a gravity toolface angle measured by gravity accelerometers or a magnetic toolface angle measured by a magnetometer.

6. The method for acquiring data of azimuthal acoustic LWD according to claim 1, wherein m=8 or 16.

7. The method for acquiring data of azimuthal acoustic LWD according to claim 1, wherein the acoustic transmitter uses unipole transmission or dipole transmission.

8. The method for acquiring data of azimuthal acoustic LWD according to claim 7, wherein a transmission frequency of the unipole transmission is 10 kHz to 15 kHz.

9. The method for acquiring data of azimuthal acoustic LWD according to claim 7, wherein a transmission frequency of the dipole transmission is 2 kHz to 5 kHz.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram of an azimuthal equal-interval acquisition mode of azimuthal acoustic LWD; and

(2) FIG. 2 is a work flow diagram of an azimuthal equal-interval acquisition mode of azimuthal acoustic LWD.

DETAILED DESCRIPTION

(3) In order to make objectives, technical solutions and advantages of the present invention be clearer, the present invention will be further described in detail below in conjunction with accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

(4) Rather, the present invention encompasses any alternatives, modifications, and equivalent methods and solutions of the present invention as defined by appended claims and made within the spirit and scope of the present invention. Further, in order to provide the public with a better understanding of the present invention, some specific details are described in detail in the following detailed description of the present invention. The present invention may be fully understood by those skilled in the art without a description of these details.

Embodiment 1

(5) FIG. 1 is a schematic diagram of an azimuthal equal-interval acquisition mode of azimuthal acoustic LWD. FIG. 1 shows a cross section of positions of transmitting transducers. An instrument is in the middle of a wellbore. In a drilling process, the transmitting transducers rotate around the axis of the instrument. FIG. 1 shows a dipole transmitting source, including two transmitting transducers, or shows a unipole transmitting source, which only uses one transmit transducer. A round hole in the middle of the instrument provides a mud passageway, an angle θ is an included angle between the axis of the transmitting transducers and a position of a toolface angle reference value θ.sub.0 of the instrument.

(6) It is assumed that an azimuthal interval is Δθ, Δθ=360/m, and m is the number of measured sectors. By way of an example that the number of imaging sectors is 16, then Δθ=22.5 degrees. The toolface angle reference value θ.sub.0 of the instrument is equal to 0 degree, that is, for a gravity toolface angle, the axis of the transmitting transducers points to a high-side. When a magnetic toolface angle is used, the axis of the transmitting transducers points to a geomagnetic north direction.

(7) θ.sub.k represents an angular boundary of adjacent sectors, θ.sub.k=kΔθ, k=1 . . . m, and the k.sup.th sector is located between θ.sub.k-1 and θ.sub.k.

(8) When an azimuthal acoustic LWD apparatus acquires data at each depth point, firstly, according to an azimuthal equal-interval acquisition mode, starting from the first sector, data acquisitions of m sectors for the first time are sequentially completed, and then according to a requirement for the number of times of superposition, data acquisitions of m sectors for multiple times are completed. Coverage times of the data in each sector are the same, and are equal to the number of times of superposition. Next, a work flow of the azimuthal equal-interval acquisition mode of azimuthal acoustic LWD will be introduced specifically by steps. A method for acquiring data of azimuthal acoustic LWD acquires data by adopting the azimuthal equal-interval acquisition mode. The work flow of the azimuthal equal-interval acquisition mode of azimuthal acoustic LWD is as follows:

(9) (1) initializing a serial number of a to-be-measured sector, and measuring a first sector:

(10) when measurement is performed at each depth point, m sectors are, and firstly starting from the first sector, the serial number of the sector is set to k=1;

(11) (2) measuring a current toolface angle θ of the instrument, and determining whether the toolface angle θ of the instrument is within the to-be-measured sector or not:

(12) the gravity toolface angle or magnetic toolface angle is measured by utilizing accelerometers or magnetometers, when the toolface angle has not yet fallen into the to-be-measured sector, and re-measured until the toolface angle is in the to-be-measured sector, wherein

(13) the gravity toolface angle is measured by adopting two orthogonal accelerometers, which are installed radially along the drilling tool, and a sensitive axis of one of the accelerometers is oriented along the axis of the transmitting transducers. The magnetic toolface angle is measured by adopting two orthogonal magnetometers, installation directions of which are the same as those of the accelerometers, and installation positions of which need to avoid a magnetic interference area;

(14) (3) controlling an acoustic transmitting source to transmit directional radiation acoustic signals to a formation:

(15) the acoustic transmitting source is controlled to transmit acoustic energy pulses to the formation around a wellbore at an optimal frequency, when unipole transmission is adopted, a transmission frequency is selected to be 10 kHz to 15 kHz, and when dipole transmission is adopted, the optimum frequency is selected to be 2 kHz to 5 kHz;

(16) (4) controlling an acoustic receiver to synchronously acquire the data:

(17) the acoustic receiver includes n sets of receiving transducer arrays, and the receiving arrays synchronously acquire the data, and waveform data acquired for multiple times in each sector is superimposed to obtain a measured data volume A(k, p), p=1, 2, . . . n,

(18) k represents a serial number of the sector, and p represents a serial number of the sets of receiving transducer arrays,

(19) an acoustic wave field generated by the acoustic energy pulses is received by the receiving arrays in a process of downward propagating along the wall of a well and the formation around the well, and the receiving arrays sample received full-wave train signals to digitize waveforms. In order to save a storage space, for each sector, the data measured by the receiving transducers for multiple times is superimposed,

(20) (5) ending a current measurement and generating a sector step k=k+1:

(21) signal measurement and data superposition of the sector are completed, the serial number of the sector is incremented, and the next sector measurement is prepared;

(22) (6) determining whether all azimuthal measurements are completed or not k>m:

(23) when the serial number of the next sector is less than or equal to the number m of sectors, it indicates that all the sector measurements have not been completed yet, and the flow proceeds to the step (2); and when the serial number of the next sector is greater than the number m of sectors, it indicates that all the sector measurements have been completed, the flow proceeds to a step (7);

(24) (7) determining whether superimposing N measurements are completed or not:

(25) in order to improve the quality of the data, usually, data in each sector needs to be acquired for multiple times and the acquired data is superimposed, and when the required N times of superposition are completed, the data acquisition at this depth point is completed,

(26) when the required number of times of superposition are not completed, the process proceeds to the step (1), and the next round of data acquisition of all the sectors is performed,

(27) further, a step (8) may be performed for the data acquired; and

(28) (8) processing the data volume A(k, p) by utilizing a slowness-time correlation (STC) method to obtain compressional wave velocities V.sub.c(k) and shear wave velocities V.sub.s(k) of different sectors at the current measurement point,

(29) after the acquisition is completed, waveform data recorded by each sector is processed usually by adopting the slowness-time correlation (STC) method, a time difference of each component wave is calculated by providing a time window in a set of full-wave trains, finding compressional waves, shear waves and Stoneley waves by moving the time window at a certain slowness (time difference), and calculating a series of relevant coefficients, further the compressional wave velocity and the shear wave velocity of each azimuthal sector at a given depth are obtained, waveform data of an original acoustic wave is stored in a memory of a downhole instrument, and processing results are transmitted to the ground through a mud pulse telemetry technology in real time, and so far, an azimuthal equal-interval acquisition process of the current depth point is completed.

(30) Further, the method for acquiring data may further include a step (9):

(31) (9) performing acquisition at the next depth point:

(32) as the drilling process continues, the above-mentioned acoustic signal measurement and data processing processes are repeated, thereby obtaining compressional wave and shear wave velocities of multiple sectors at different depths, and realizing azimuthal LWD imaging. A schematic diagram of the work flow of the steps (1) to (9) is shown in FIG. 2.