Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Reexamination Certificate
1998-09-14
2002-05-14
Brown, Glenn W. (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
C367S038000
Reexamination Certificate
active
06388947
ABSTRACT:
The present invention relates generally to crosswell seismic mapping or imaging and more particularly to a method for producing high quality three dimensional mapping or imaging using at least one crosswell data profile in combination with other downhole derived data which may include one or more other crosswell data profiles.
In the field of geophysics, the knowledge of the subsurface structure of the ground is useful, for example, in the extraction of mineral resources such as oil and natural gas. In the past, a number of different seismic imaging methods, including tomographic methods and reflectance imaging methods, have been implemented with the goal of rendering images which impart such knowledge of the subsurface geologic structure. With regard to an oil or gas reservoir, the availability of an accurate prior art seismic image, as interpreted by a geoscientist reasonably versed in its use, can provide the capability to extract additional production from a reservoir than would otherwise be possible. Thus, in view of the earth's limited resources and the expense encountered in recovery of such resources, prior art seismic imaging has become increasingly important.
One area of particular importance with regard to prior art seismic methods is that of crosswell seismic imaging. Conventional crosswell seismic imaging typically utilizes a pair of boreholes in proximity to the reservoir. In the first of these boreholes, a seismic source is deployed for emitting seismic energy into the region of interest, often as a swept frequency signal (chirp) which covers a predetermined frequency range. The source is selectively moved between a series of positions within the first borehole at predetermined times. The seismic energy passes through and around the subterranean region of interest to the second one of the pair of boreholes. A receiver array is typically deployed within the second borehole and, like the seismic source, the receiver array is moveable between a series of positions within the second borehole. It should be appreciated that the subterranean region of interest may comprise a zone which is at a known or estimated depth range below the surface. In this case, the receiver array and source positions are adjusted accordingly such that the positions are spaced across the zone of interest rather than extending all the way to the surface. By transmitting from each source position in the first borehole while receiving data at each and every receiver array position in the second borehole, a seismic crosswell dataset is generated. Crosswell seismic datasets typically offer advantages in resolution over surface seismic datasets which are more commonly practiced.
After having generated a seismic dataset, the task of using the dataset to produce a crosswell image or tomogram may be undertaken. In this regard, it should be appreciated that the crosswell seismic dataset comprises a large, complex set of information which is rich in detail relating to the geologic structure and material properties of the subterranean region of interest. Due to this complexity, certain known types of data such as, for example, direct arrival traveltimes are generally obtained from the profile and utilized iteratively in a mathematical model of the subterranean region to establish the geologic structure and material properties within some approximation. While a number of effective methods for generating models have been developed in the prior art, it should be appreciated that most conventional crosswell imaging is limited to producing a two dimensional tomogram in the plane defined between two generally vertically extending boreholes. The usefulness of even a two dimensional high resolution tomogram depicting a “slice” of the subterranean region between a pair of boreholes is not diminished by the teachings and discoveries disclosed herein. However, these discoveries in conjunction with other recent developments in the prior art are thought to provide sweeping improvements.
One recent development relates to the practical acquisition of individual crosswell datasets. In the past, data was collected between a pair of boreholes in the manner described above. The collection of even one crosswell dataset was prohibitively expensive, at least in part due to relatively inefficient methods. More recently, however, improvements in data acquisition technology have been made which have dramatically increased data collection efficiency between a pair of boreholes. Such improvements include, for example, stronger sources, shooting “on-the-fly” and the use of multilevel receiver strings. Based on improved collection efficiency alone, the amount of data which may be collected in a single day has increased by a factor of at least ten. Such a dramatic improvement makes crosswell seismic even more attractive and opens the potential for crosswell surveys over large areas of existing and prospective reservoirs. Of course, the advantages of crosswell seismic such as, for example, high resolution (on the order of well log resolution) remain attendant to its use. However, there remains a need for a corresponding improvement in the all important image generating process for dealing in an effective way with this vast increase in the amount of available data, particularly in the instance where a number of crosswell datasets are available for a single field or reservoir. This need as well as other needs for improvement will be discussed in further detail below.
A number of crosswell datasets may be available for a particular reservoir or region for two reasons. First, the datasets may be obtained in the conventional manner between pairs of boreholes across the region. Second, using more recent data acquisition technology, crosswell data surveys may now be conducted across a region penetrated by a plurality of boreholes by deploying a source in one of the boreholes and deploying receiver strings in each of the remaining boreholes so as to simultaneously record the output of the source to at once provide a plurality of datasets. In this regard, one of the more telling limitations of current crosswell technology resides in the fact that conventional crosswell technology provides only the capability to use each dataset apart from the other datasets to generate a two dimensional image. That is, the geoscientist is presented with a plurality of independent, two dimensional images, one for each dataset. In most practical cases, two dimensional planar images are an approximate depiction of the true complex, three dimensional earth structure. Unfortunately, a more detailed limitation has been observed in this regard to the extent that intersecting planar images produced between different pairs of boreholes in the same region commonly exhibit inconsistencies at intersection points of the two images so as to present a dilemma as to the actual structure. Presently, the only solution to this dilemma has been found in questionably effective techniques such as interpolation. Applicants are not aware of any effective prior art technique for producing a uniform image across a region in view of a plurality of independently generated crosswell seismic planar images, irrespective of the fact that the datasets resulting in the images may even have been obtained simultaneously.
Still another and practically significant limitation of conventional crosswell seismic imaging is found in situations where boreholes from which a crosswell dataset is generated exhibit significant deviation from the vertical direction. One inherent assumption of conventional crosswell imaging is that of producing an image of the plane defined between a pair of vertically extending boreholes. Unfortunately, this assumption is problematic in instances of significant borehole deviation. In the past, correction schemes have been introduced in an attempt to compensate for such deviation. However, there remains a need for a more effective solution. Similarly, conventional crosswell imaging assumes generally two dimensional earth structure with limited geological complexity. Wh
Bube Kenneth P.
Rector, III James W.
Washbourne John K.
Brown Glenn W.
Jolly Anthony
Pritzkau Michael
TomoSeis Inc.
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