Shale compaction and sonic logs

Details Shale compaction and sonic logs Shale compaction and sonic logs

2000-04-18

2002-06-25

McElheny, Jr., Donald E. (Department: 2862)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

Reexamination Certificate

active

06411902

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method of predicting acoustic wave performance in sediments. In particular, the invention relates to a compacted shale model and applications of the model to generate theoretical sonic logs useful in seismic studies. The model predicts acoustic velocity with depth (sonic log) and allows procedures such as sonic log editing and quality control.
Background Art
Acoustic wave propagation in sedimentary rock sequences, the subject of sonic logs, is of fundamental interest in petroleum exploration. It provides a key linkage between geophysical data acquisition and interpretation, and the rock properties which are of basic interest to geologists. Compression (and more recently shear) wave data are commonly acquired during borehole logging operations. These data are subsequently used in and are often critical to interpretation of rock properties, reservoir analyses, seismic interpretation and basin analyses.
The linkage between rock properties measured in boreholes and the interpretation of similar properties from seismic data is provided by measurements recorded in-situ by logging tools, and by detailed laboratory measurements on rock samples recovered during drilling. The properties of greatest interest in this process are acoustic performance (of both compression and shear waves) and bulk density. The borehole environment and logging process often adversely affect acquisition of good quality acoustic log data over substantial intervals of section, resulting in poor ties of well data to seismic, and inferior quality acoustic velocity data.
The quality of the borehole log data is often affected by petrophysical properties (fractures, compaction, hydrocarbon content), borehole environmental factors (mud properties, borehole surface conditions) and acquisition parameters (logging speed, signal generation and detection techniques). The data so acquired are calibrated by comparing the integrated borehole signal with independently measured interval velocity data (check-shot data). Misfit between integrated log data and check-shot data primarily arises because noise is commonly incorporated in the log signal, and because of different acoustic frequencies employed in the two techniques (Ward, R. W. and Hewitt, M. R., 1977, Monofrequency borehole travel time survey: Geophysics, 42, 1137-1145). Goetz et al (1979, An investigation into discrepancies between sonic log and seismic check-shot velocities: Australian Petroleum Exploration Association. J.. 19, 2, 131-141) provided a complete discussion of error sources of the two processes.
Corrections are applied to the borehole data to force a fit to the check-shot data, and the emergent calibrated sonic log is then used as input to further studies, particularly seismic modelling. The tie of well data to seismic, and the interpretation of seismic character of sedimentary packages (seismic stratigraphy) is fundamental to interpretation of basic structural evolution, the history of deposition and present geometry.
Sonic signal degradation, particularly in near-surface and less compacted rocks, often leads to substantial editing being required before the integrated signal agrees acceptably with the check-shot data. Lack of a suitable technique (both in terms of physical modelling and operational efficiency) for systematic noise removal and editorial replacement of intervals of suspect data has hitherto resulted in linear interpolation being the most commonly used method of noise removal from the sonic log.
Under normal circumstances, in generally subsiding depositional basins, progressively increasing overburden load due to increasing depth of burial, results in sequence compaction, with porosity reduction, increased bulk density and improved acoustic propagation efficiency. Observation of sonic logs clearly shows a general increase in acoustic p-wave velocities of propagation with increasing depth of burial (Telford, W. M., Geldart, L. P., and Sheriff, R. E., 1990, Applied Geophysics, (Second Edition), Cambridge University Press). Exceptions exist, and are primarily lithology-dependent. Several recent papers have used various compaction models to quantify these changes, and to use the data for studies of basin evolution.
An exponential decay model for the density-depth function was proposed by Stegena, L. (1964, The structure of the earth's crust in Hungary, Acta Geologica, Budapest 8, 413-431), and Korvin, Gabor (1984, Shale compaction and statistical physics: Geophysical Journal of the Royal Astronomical Society, v. 78, p. 35-50) developed a mathematical proof of the exponential decay model for shale compaction. This model has not been widely used (Japsen, Peter, 1998, Regional velocity-depth anomalies, North Sea Chalk: a record of overpressure and Neogene uplift and erosion: AAPG Bulletin, v82, No 11, p. 2031-2074, Heasler, Henry P., and Kharitonova, Natalya A., 1996, Analysis of sonic well logs applied to erosion estimates in the Bighorn Basic, Wyoming: AAPG Bulletin, v. 80, No. 5. p. 630-646). Difficulties in the use of such a model arise from the often complex mix of lithologies and absence of observational data for most lithologies other than shale.
Japsen uses a segmented linear model for North Sea Chalk. Gassman, F., (1951, Ueber die elastizitat poroser medien: Natur. Ges. Zurich, Vierteljahrssch. V. 96, p. 1-23) introduced a physical model for compressional wave velocity in porous rocks, and this has been recently applied to quantify variations in sonic p-wave performance in sandstone reservoirs (Alberty, Mark, 1996, The influence of the borehole environment upon compressional sonic logs: The Log Analyst, v. 37, p. 30-44).
OBJECT OF THE INVENTION
It is an object of the invention to use data acquired in association with boreholes in an improved manner by means of mathematics based processing to generate synthetic sonic logs. It is a particular object of the invention to provide methods by which to overcome problems such as the defects in actual logs, which logs are often compromised by borehole engineering, environmental difficulties, and by operational considerations. More particularly, the invention may provide an improvement over the considerable post-acquisition editing and re-calibration of the sonic signal which has been required so as to yield acceptable agreement between integrated sonic and check-shot measured interval travel times.
Explanation of the Invention
A study of the existing methods of editorial calibration, and a consideration of the underlying physical and mathematical processes, has led to a re-examination of the physics and mathematics of compactive modelling, and to the development of methods of use as outlined hereinbelow.
Because sedimentary rock burial history determines density, applying the invention allows interpretation of rock velocity in terms of burial history (the depth z in equation 4 below).
Systematic response of p-wave propagation efficiency to progressively increasing compaction is approached by initial consideration of the response of a relatively pure lithology. A marine shale with low total organic carbon content, buried progressively but sufficiently slowly that a normal pore pressure gradient is maintained, is first considered.
Compaction algorithm.
An exponential decay model is used below (after Korvin, 1984), wherein the shale density &rgr; progressively changes with burial depth:
&rgr;(
z
)=&rgr;
∞
+(&rgr;
0
−&rgr;
∞
)
e
−kz
  (1)
where &rgr;(z) is the bulk density at depth z, k is a compaction constant,
∞
& &rgr;
0
are respectively the bulk density at infinite depth and at the mudline and e is the exponential constant.
Boundary considerations yield clearly defined limits to the above. At the mud-line, as clastic debris (fragments of pre-existing rock) first accumulates, the newly deposited material will have a bulk density (&rgr;
0
) similar to that of the water of deposition, that is, about 1.022 for seawater, and 1.000 for fresh water. Upon burial, initial consolidation is rapid,

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