Phase Velocity Processing For Acoustic Logging-While-Drilling Full Waveform Data
This paper presents the principle and demonstrates the advantages of using a Phase Velocity Processing technique (PVP) applied to the full wave acoustic data recorded by a logging-while-drilling tool. Phase Velocity Processing is particularly well suited for handling the characteristics of LWD data that may degrade the results of Semblance method. Among them are: large tool-mode content, high signal attenuation across the receiver array, and strong road noise. Another advantage of applying PVP is its high vertical resolution: due to the common receiver mode of operation vertical resolution of the calculated slowness is equal to the offset between adjacent receivers. By comparison, the Semblance method delivers results averaged across the entire receiver array span. The PVP algorithm employs series of Fast Fourier Transform procedures to (a) filter the data using Heisenberg’s method, (b) define time domain distribution of the phase arrivals using a Hilbert transform, and finally (c) perform the Phase Velocity analysis. Large number of phase arrival points provides efficient mechanism to calculate standard deviation of the results, quality indicators are added to show how well the algorithm was able to calculate both compressional and shear slownesses. Processing package includes filters operating in the frequency, time, slowness or space domains to minimize competing acoustic modes, especially the tool arrivals that are frequently present in LWD waveform data. Examples demonstrate improvement of the slowness calculation performed with the PVP over the results delivered by the Semblance method. The results obtained in attenuating formations, across the bed boundaries, and in thin beds are presented. The ability of the new filtering technique to identify and suppress the tool mode arrivals is also demonstrated. INTRODUCTION The acoustic logging-while-drilling tools that were introduced six years ago (Minear et. al., 1995, Heysse et. al., 1996, Minear et. al., 1996) are by necessity full waveform recording devices. The drilling noise and the requirement for rigidity in an LWD environment prevented the use of a simple threshold technique as employed in the older wireline acoustic tools. The semblance method was the most commonly utilized processing technique at the time. It was implemented in the form of embedded firmware for downhole processing and as the surface system application for memory waveform data analysis. Semblance processing has a number of drawbacks, such as its requirement for constant amplitude and velocity across the receiver array, the smoothing of the waveforms across the receiver span, the ability to mask defective data collected by an under performing tool, and not in the least its numerical complexity and therefore long execution time. We introduce here the Phase Velocity Processing algorithm implemented to process full waveform data recorded with a loggingwhile-drilling acoustic tool. ACOUSTIC LWD WAVEFORM AND LWD TOOL ARCHITECTURE The very nature of acoustic measurements performed under drilling conditions has necessitated the recording of full waveform data. This has complicated not only the construction of the acoustic LWD tools but also the downhole processing, as realtime sonic slowness calculations are required. A constant presence in LWD waveforms is tool mode, defined as the signal traveling from the transmitters to the receivers, either directly in the tool body or deflected by the borehole wall. The isolator sections in acoustic LWD tools are not perfect due to the fact that the LWD devices, contrary to wireline units, need to be absolutely rigid: the collar strength cannot be compromised by the introduction of the electronics and isolators. Tool mode signal is
Initial Results from an Acoustic Logging-While-Drilling Tool
This paper presents the initial results from field tests of an acoustic logging-while-drilling tool. Over 60,000 ft of well have been logged with 6.75-in.- and 8-in.-diameter versions of the tool. Various types of formations were encountered with compressional slowness values ranging from 55 {mu}sec/ft to 170 {mu}sec/ft. The tool is configured as a borehole-compensated device; transmitters are located on opposite sides of an array of four receivers. An ultrasonic standoff transducer provides a tool-to-borehole wall distance measurement for data quality evaluation and processing. Slowness values are computed downhole using real-time waveform processing. Full-waveform data are stored for retrieval and re-processing at the surface. Interactive processing software allows an engineer to reprocess log intervals of poorer log quality and improve the slowness log computed downhole. Downhole-processed and post-drilling-processed slowness logs are compared, and generally show close agreement. These logs also agree well with wireline slowness logs. Rugose boreholes and attenuating formations can reduce waveform quality. A new rapid-fire technique acquires multiple estimates of slowness as the drillstring rotates through only a fraction of a revolution, and improves slowness quality in rugose boreholes. Shear-wave energy is clearly apparent in most waveform data. Waveforms can be processed to yield both shear and compressionalmore » slowness values. These can then be used to estimate porosity, compute elastic moduli of the rock (for example, Poisson`s ratio, Young`s modulus, bulk compressibility, and shear modulus), and to compute synthetic seismograms for correlation with seismic profiles. Some of these applications of compressional and shear slowness data are discussed with examples.« less
Analysis of the stress wave effect during coal breakage by a high-pressure abrasive air jet
In view of the defects of borehole collapse, inhibition of gas desorption and migration of gas existing in hydraulic fracturing and other hydraulic permeability–increasing measures for soft coal seams with low-permeability technology is proposed for coal breakage by a high-pressure abrasive gas jet for relieving pressure and increasing permeability. The comparative analysis of gas jet flow field structure between convergent nozzle and Laval nozzle has been given by numerical simulation. For Laval nozzle, the expansion wave and compression wave alternate and move forward steadily in gas jet and vanish when potential core length reaches maximum. So, the Laval nozzle can form more stable flow filed structure of gas jet and avoid shock wave in gas jet. Furthermore, a high-speed camera is adopted to analyze the jet structure and verify the conclusion of numerical simulation. Based on thermodynamic theory, this article calculates and analyzes the critical local sound velocity and pressure generated from the stress wave during the process of coal breakage by the gas jet. Furthermore, experimental coal breakage by a high-pressure abrasive gas jet is carried out. The high-pressure abrasive gas jet impacts the coal body as a quasi-static load and a dynamic load and forms corrosion pits on the surface of the coal body. Penetrating cracks are formed within the coal in the pattern of the loaded stress wave which leads to coal breakage. The effects of porosity and permeability on the propagation of the stress wave in coal are analyzed by establishing the dispersion equation for the spread of the stress wave in coal. The results show that porosity has a significant effect on wave velocity and that the attenuation of the stress wave is intensified with an increase in porosity. Moreover, the stress wave attenuation is more obvious at high frequency. The effect of permeability on the wave velocity is not significant at low frequencies. In contrast, at high frequency and relatively low permeability, the wave velocity increases with the permeability, and the attenuation of the wave velocity initially increases and then decreases. When the permeability is greater than 10−11 m2, the wave velocity is not affected by the permeability. However, the stress wave is not attenuated.