Processing sonic waveform measurements

Details Processing sonic waveform measurements Processing sonic waveform measurements

2002-01-18

2003-11-25

Barlow, John (Department: 2863)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

C703S010000

Reexamination Certificate

active

06654688

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods for processing sonic waveform measurements, particularly sonic waveform measurements made for the purpose of characterising properties of underground formations. The invention in particular relates to methods for determining the best value for a parameter that has been determined in a number of different ways.
RELATED ART
It has been know for some time that it is possible to determine properties of underground formations using measurements of acoustic/sonic waves that have passed through the formations. The basic technique comprises placing a tool comprising a spaced sonic source and receiver in the borehole and using the source to generate sonic waves which pass through the formation around the borehole and are detected at the receiver. Sonic waves can travel through rock formations in essentially two forms: body waves and surface waves. There are two types of body waves that travel in rock: compressional and shear. Compressional waves, or P-waves, are waves of compression and expansion and are created when rock formation is sharply compressed. With compressional waves, small particle vibrations occur in the ,same direction the wave is travelling. Shear waves, or S-waves, are waves of shearing action as would occur when a body is struck from the side. In this case, rock particle motion is perpendicular to the direction of wave propagation. The surface waves are found in a borehole environment as complicated borehole guided waves which come from reflections of the source waves reverberating in the borehole. The most common form or borehole-guided, surface wave is the Stoneley wave.
FIG. 1
shows a series of sonic waveforms such as would be recorded in a borehole from a monopole (omnidirectional) source with the first arrivals of the compressional (P), shear (S) and Stoneley (St) waves at the receiver marked. In situations where dipole (directional) sources and receivers are used, an additional shear/flexural wave propagates along the borehole and is caused by the flexing action of the borehole in response to the dipole signal from the source The flexural wave typically travels at about the same speed as the shear wave, slower than the compressional wave. (It is to be noted that sonic waves will also travel through the fluid in the borehole and along the tool itself. With no interaction with the formation, these waves carry no useful information and run on wireline or coiled tubing or the like, or alternatively can be a logging while drilling tool located in a drill string being used to drill the borehole.
The speeds at which these waves travel through the rock are controlled by rock mechanical properties such as density and elastic dynamic constants, and other formation properties such as amount and type of fluid present in the rock, the makeup of the rock grains and the degree of intergrain cementation. Thus by measuring the speed of sonic wave propagation in a borehole, it is possible to characterise the surrounding formations by parameters relating these properties. The speed or velocity of a sonic wave is often expressed in terms of 1/velocity and is called “slowness”. Since the tools used to make sonic measurements in boreholes are of fixed length, the difference in time (&Dgr;T) taken for a sonic wave to travel between two points on the tool is directly related to the speed/slowness of the wave in the formation.
An example of a tool for use in a borehole for sonic measurements is the DSI tool of Schlumberger which is shown schematically in FIG.
2
. The DSI tool comprises a transmitter section
10
having a pair of (upper and lower) dipole sources
12
arranged orthogonally in the radial plane and a monopole source
14
. A sonic isolation joint
16
connects the transmitter section
10
to a receiver section
18
which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge
20
is connected at the top of the receiver section
18
and allows communication between the tool and a control unit
22
located at the surface via an electric cable
24
. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire (digitised) waveforms. The modes are: upper and lower dipole modes (UDP, LDP)—waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole mode—waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley mode—monopole waveforms from low frequency firing of the monopole source; P and S mode (P&S)—monopole waveforms from high frequency firing of the monpole transmitter; and first motion mode—monopole threshold crossing data from high frequency firing of the monopole source.
One way to determine compressional, shear and Stoneley slownesses from these measurements is to use slowness-time-coherence (STC) processing. STC processing is a full waveform analysis technique which aims to find all propagating waves in the composite waveform. The processing adopts a semblance algorithm to detect arrivals that are coherent across the array of receivers and estimates their slowness. The basic algorithm advances a fixed-length time window across the waveforms in small, overlapping steps through a range of potential arrival times. For each time position, the window position is moved out linearly in time, across the array of receiver waveforms, beginning with a moveout corresponding to the fastest wave expected and stepping to the slowest wave expected. For each moveout, a coherence function is computed to measure the similarity of the waves within the window. When the window time and the moveout correspond to the arrival time and slowness of a particular component, the waveforms within the window are almost identical, yielding a high value of coherence. In this way, the set of waveforms from the array is examined over a range of possible arrival times and slownesses for wave components. STC processing produces coherence (semblance) contour plots in the slowness/arrival time plane. Regions of large coherence correspond to particular arrivals in the waveforms. The slowness and arrival time at each coherence peak are compared with the propagation characteristics expected of the arrivals being sought and the ones that best agree with these characteristics are retained. Classifying the arrivals in this manner produces a continuous log of slowness versus depth. For dispersive waves, the STC processing is modified to take into account the effect of frequency. As the output of STC processing is a coherence plot, the coherence of each arrival can be used as a quality indicator, higher values implying greater measurement repeatability. When processing dipole waveforms, one of the coherence peak will correspond to the flexural mode but with a slowness that is always greater (slower) than the true shear slowness. A precomputed correction is used to remove this bias.
To compensate for variations in measurements due to the borehole rather than due to the formation a series of measurements are made across an interval in which the formation properties are expected to vary little, if at all. In its simplest form, the interval corresponds to the extent of the receiver array, and the waveforms at each receiver station measured for a given firing of a source (“receiver array” or “receiver mode” or “Rec.”). In simple STC processing, all receiver stations are considered. In multishot STC processing (MSTC), sub-arrays of receiver stations within the receiver array are considered, for example a sub-array of five receiver stations in a receiver array of eight receiver stations (other numbers or receiver stations in the sub-array can be used depending on requirements). In this case, the same formation interval corresponding to the extent of a five receiver station sub-array ca

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