Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-10-06
2001-10-30
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06311132
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of hydrocarbon exploration and production, and more particularly relates to detection of drilling hazards, especially subterranean shallow water flow sands.
BACKGROUND OF THE INVENTION
The use of seismic techniques to obtain information about subterranean geophysical features is very well-known in the prior art. Such techniques are commonly employed in the exploration for and production of hydrocarbons, e.g., natural gas and oil. The advantages and desirability of accurate characterization of subterranean features are self-evident.
Conventional compressional wave seismic land or marine acquisition techniques involve the use of an appropriate source to generate compressional energy and a set of receivers spread out along or near the Earth's surface to detect any seismic signals due to compressional energy being reflected from subsurface geologic boundaries. These signals are recorded as a function of time and subsequent processing of these signals, i.e. seismic data, is designed to reconstruct an accurate image of the subsurface. In simplistic terms, this conventional process has a compressional wave travelling down into the earth, reflecting from a particular geologic layer (due to impedance contrast), and returning to the receiver as a compressional wave. Data from such a process is referred to herein as “PP” data since compressional waves (P) propagate down from the surface (the first “P”) and back up to the surface (the second “P”). In reality, many different types of waves are created in conventional acquisition schemes, and the use of receivers with their sensitive axes oriented vertically (approximately parallel to particle motion for compressional waves), as well as the subsequent processing of the recorded data are designed so that the desired type or types of waves (such as signals representing PP data) is enhanced relative to other types of waves whose signals are considered noise.
On the other hand, so-called shear wave data is conventionally acquired by using a source which introduces particle motion transverse to the direction of wave propagation and then detecting the seismic signal with receivers. Two different types of shear waves (denoted herein as “S”) may be acquired: Sh, where the particle motion is perpendicular to, or across, the line from the source to the receiver or geophone; and Sv, where the particle motion is along, or in, the plane defined by the source, reflector, and receiver or geophone. While the characteristics and interpretation of these two types of shear waves may be quite different, both types of acquisition are denoted herein as SS to emphasize the symmetry resulting from the fact that both the downgoing wave (the first “S”) and reflected wave (the second “S”) are shear waves. Shear seismic data may provide additional information about the properties of the subsurface geologic layers which may be valuable in the exploration for hydrocarbons. See, for example, R. H. Tatham et al.; “V P/V S-A Potential Hydrocarbon Indicator”,
Geophysics
41, pp. 837-849 (1976).
Those of ordinary skill in the art will be aware that shear waves of the Sv type may also be generated by conversion from a compressional wave transmitted through or reflected from an impedance interface. In this so-called “converted shear” situation, the particle motion of the converted wave is transverse to the direction of wave propagation but in-line with respect to the source-receiver direction. These waves may be seen in conventional PP seismic records but it has been shown that their observation can be enhanced by modifying conventional compressional wave acquisition geophone axes slightly (i.e. placing geophones with their detection axes horizontally in-line rather than vertically). Seismic signals which are predominantly shear-waves may then be detected and may also be recorded. These waves arise from the partitioning of the energy of the compressional wave as it is reflected from an elastic interface. Shear waves of this type are variously referred to as converted waves, or PS waves, and are well known among exploration seismologists. See, for example, Ricker, et al., “Composite Reflections”Geophysics 15, pp. 30-50 (1950); see also U.S. Pat. No. 2,354,548, issued Jul. 25, 1944 to Ricker. When properly interpreted, converted shear wave data has been shown to be capable of providing information about the properties of the subsurface similar to no that provided by SS data.
There are two characteristics of converted waves (PS) which distinguish them from either conventional PP or SS waves. First, the travel path is asymmetric; compressional energy travels downward with a compressional velocity VP (Z), and after reflection travels upward with a shear velocity VS(Z). VP(Z) and VS(Z), (where Z represents the depth) are both generally functions of depth, Z, and VS is normally much less than VP. Second, since the shear velocity is usually much smaller than the compressional velocity in the same material, the velocity distribution of a converted wave (i.e. the velocities experienced by the energy travelling down and back up) is much broader than If the wave had been a pure compressional (PP) or pure shear (SS) wave over its entire path.
As is known by those of ordinary skill in the art, so-called “processed” seismic data is derived from raw seismic data by applying such conventional processing techniques as static correction, amplitude recovery, band-limiting or frequency filtering, stacking, and migration. The processed seismic data may be of either the so-called reflection coefficient data type or the integrated trace data type.
Once processed seismic data has been derived, this data must be correlated with such physical characteristics as reservoir continuity, reservoir thickness, pore fill fluid type (oil, gas, water, etc.), lithologic variation, and pay thickness, to name but a few. This correlation is commonly accomplished using seismic data (two or three dimensional) in conjunction with other inputs, such as well logs. Other ways of making this correlation include, e.g., analysis of surface out-crops and statistical modeling exercises.
Those of ordinary skill in the art will be aware that seismic techniques are traditionally employed to detect and characterize geophysical structures or features deep underground, generally in the subterranean regions where hydrocarbon deposits are likely to be found. On the other hand, seismic techniques are not traditionally employed in the oil industry for the purposes of detecting relatively shallower subterranean geophysical structures.
One type of shallow geophysical feature of particular interest is known as shallow waterflow sand. Shallow, overpressured sands constitute a severe hazard to drilling and facilities development because they tend to flow when penetrated. This causes significant drilling and cementing problems. Shallow flows can lead to washouts resulting in casing wear, buckled casing, and well re-entry problems. In some cases, shallow waterflows can breach the seafloor, resulting in loss of both the individual well and other prospect development sites. Over the years, shallow waterflow occurrences have been reported in various oil and gas fields or prospects. With a few exceptions, waterflow incidents occur at water depths exceeding 1,700feet with an average occurrence in 2,830 feet of water. In recorded cases, waterflow problem sands typically occur from 950 to 2,000 feet but have been reported as deep as 3,500 feet below the sea floor. In any event, for the purposes of the present disclosure, the term “shallow” as applied to subterranean measurements shall refer generally to various depths of up to as much as 3,500 feet or so below the sea floor.
In the Gulf of Mexico, one example of a region prone to shallow waterflow sand problems, the shallow waterflow sands were deposited as continental slope/fan sequences during the Late Pleistocene era. Individual sand-bearing units display slumping zones or debris flows with a chaotic seismic character and,
Bryndzia L. Taras
Dwan Fa S.
Pelletier John H.
Rosenquist Mark L.
McElheny Jr. Donald E.
Shell Oil Company
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