Data processing: measuring – calibrating – or testing – Measurement system in a specific environment
Reexamination Certificate
2001-09-24
2003-07-08
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
C702S018000
Reexamination Certificate
active
06591193
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of seismic prospecting and, more particularly, to migration of seismic data. Specifically, the invention is a method and apparatus for using tube-wave conversion to acquire offset checkshot survey data for the subsurface region in the vicinity of a well. Accurate migration traveltimes for the subsurface region may be derived from the offset checkshot survey data.
BACKGROUND OF THE INVENTION
In the oil and gas industry, seismic prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. A seismic prospecting operation consists of three separate stages: data acquisition, data processing, and data interpretation. The success of a seismic prospecting operation is dependent on satisfactory completion of all three stages.
In the data acquisition stage, a seismic source is used to generate a physical impulse known as a “seismic signal” that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different elastic properties). The reflected signals (known as “seismic reflections”) are detected and recorded by an array of seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The seismic energy recorded by each seismic receiver is known as a “seismic data trace.”
During the data processing stage, the raw seismic data traces recorded in the data acquisition stage are refined and enhanced using a variety of procedures that depend on the nature of the geologic structure being investigated and on the characteristics of the raw data traces themselves. In general, the purpose of the data processing stage is to produce an image of the subsurface geologic structure from the recorded seismic data for use during the data interpretation stage. The image is developed using theoretical and empirical models of the manner in which the seismic signals are transmitted into the earth, attenuated by the subsurface strata, and reflected from the geologic structures. The quality of the final product of the data processing stage is heavily dependent on the accuracy of the procedures used to process the data.
The purpose of the data interpretation stage is to determine information about the subsurface geology of the earth from the processed seismic data. For example, data interpretation may be used to determine the general geologic structure of a subsurface region, or to locate potential hydrocarbon reservoirs, or to guide the development of an already discovered reservoir. Obviously, the data interpretation stage cannot be successful unless the processed seismic data provide an accurate representation of the subsurface geology.
Typically, some form of seismic migration (also known as “seismic imaging”) must be performed during the data processing stage in order to accurately position the subsurface seismic reflectors. The need for seismic migration arises because variable seismic velocities and dipping reflectors cause seismic reflections in unmigrated seismic images to appear at incorrect locations. Seismic migration is an inversion operation in which the seismic reflections are moved or “migrated” to their true subsurface positions.
There are many different seismic migration techniques. Some of these techniques are applied after common-midpoint (CMP) stacking of the seismic data traces. Such “poststack” migration can be done, for example, by integration along diffraction curves (known as “Kirchhoff” migration), by numerical finite difference or phase-shift downward-continuation of the wavefield, or by equivalent operations in frequency-wavenumber or other data domains.
Other seismic migration techniques are applied before stacking of the seismic data traces. In other words, these “prestack” migration techniques are applied to the individual nonzero-offset data traces and the resulting migrated data traces are then stacked to form the final image. Prestack migration typically produces better images than poststack migration. However, prestack migration is generally much more expensive than poststack migration. Accordingly, the use of prestack migration has typically been limited to situations where poststack migration does not provide an acceptable result, e.g., where the reflectors are steeply dipping.
Regardless of the type of migration being used, an accurate migration velocity model and/or accurate migration traveltimes are required. Incorrect migration velocities and/or traveltimes can lead to at least two undesirable consequences. First, the resulting image may be poorly focused, making data interpretation difficult. Second, the reflectors may be mispositioned, a serious drawback in hydrocarbon exploration where accurate mapping of the subsurface structure is highly important. The effects of poor focusing and improper positioning are particularly apparent when migrating steeply dipping reflectors or when migrating in areas having significant lateral velocity variations.
Conventional methods for generating migration velocity models and/or migration traveltimes typically analyze seismic raypaths, which are inclined less than about 45 degrees with respect to the vertical. Unfortunately, accurate migration of steeply dipping reflectors, such as salt flanks and faults, also requires accurate migration traveltimes for raypaths that are closer to horizontal. Accurate traveltimes for horizontal or nearly horizontal raypaths may also be required for imaging subsurface areas having significant lateral velocity variations.
U.S. Pat. Nos. 5,696,735 and 6,002,642, both issued to J. R. Krebs, disclose a method for migrating seismic data using offset checkshot survey measurements. This method is particularly advantageous for imaging steeply dipping reflectors located in the vicinity of a well. According to this method, offset checkshot survey data are gathered from the subsurface region adjacent to the subsurface feature to be imaged. As illustrated in
FIG. 1
, these data typically are gathered using surface sources
10
and borehole receivers
12
located at various depths in the well
14
. Typically, the borehole receivers
12
are attached to a standard electric wireline
38
. The sources and receivers are placed in a geometry which results in raypaths
16
that are geometrically similar to the raypaths in the seismic data to be used in imaging the subsurface feature in question. The offset checkshot survey measurements are used to determine direct arrival traveltimes from the surface sources
10
to the borehole receivers
12
. These traveltimes may be used to generate a reflector-weighted migration velocity model to allow accurate migration of the reflector dips of greatest interest. Alternatively, the traveltimes may be used directly in migration routines that accept traveltime inputs. The method disclosed by Krebs may be used in time, depth, or Kirchhoff migration, in either two or three dimensions, and in either prestack or poststack applications.
Unfortunately, current methods of collecting offset checkshot survey data are very expensive and require substantial rig time. Typically, many tens of downhole receiver stations and surface shotlines are needed in order to acquire sufficient offset checkshot survey traveltime measurements for accurate migration of the subsurface area surrounding a well. For example, a complete three-dimensional offset checkshot survey of the subsurface area surrounding a well typically requires a grid of 30 or more surface shotlines and 30 or more downhole receiver stations. Such a survey may require as much as 900 hours of rig time, or even more, to complete using current methods of data acquisition. For this reason, the offset checkshot survey technique has not been used as widely as it should be, and when it has been used, it has typically been limited to acquiring incomplete data from only a few surface shotlines and downhole receiver stations, thus compromising the accuracy of the subs
Chen Sen-Tsuen
Krebs Jerome R.
Barlow John
ExxonMobil Upstream Research Company
Katz Gary P.
Le Toan M
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