Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-06-13
2004-01-13
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C367S041000
Reexamination Certificate
active
06678617
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to mapping the open natural fractures in the hydrocarbon reservoirs, using two surface-generated seismic signals by measuring their elastically nonlinear interaction caused due to their transmission through the open fractures.
2. Description of the Prior Art
In most of the carbonate and certain sandstone reservoirs, natural fractures are encountered that are open and control the directional permeability and the effective flow pathways for the hydrocarbons. Mapping these fractures and their orientation is the key to the economic recovery of hydrocarbons from these reservoirs. At present, natural fracture characterization is of increasing importance, since the industry is venturing into increasing their producible reserves from the existing fields that are showing production decline.
Natural fractures in the subsurface rocks are usually vertical and are mostly found in the formations that have gone through structural deformation or have experienced regional stresses. These fractures commonly terminate at lithologic discontinuities within the reservoir formations. These fractures can be closely or widely spaced and irregularly distributed. Quite often, swarms of fractures are encountered with unfractured intervals in between. Economic hydrocarbon production from the fractured reservoirs requires an optimal access of the wellbore to the open fractures. This makes it extremely important that an accurate map of the open fracture system should be available prior to any field infill and development program.
In many cases, fractures are difficult or impossible to map adequately by using currently available technologies. Physical measurements through cores and well logs are limited to the vicinity of the wells drilled in the reservoir. The density of sampling the reservoir rock using cores and well logs quite often is not sufficient to provide any useful information regarding the orientation and the location of the fractures. This is due to two main characteristics of the majority of the wells that are drilled:
1) Both the wells and the fractures are generally vertical and parallel to each other; and
2) The wellbore is smaller than the fracture spacing between the larger fractures.
Horizontal drilling—where the cost of drilling a well is high—has to be designed to take full advantage of the natural fractures that are open, by mapping their location and their orientation. Since a single horizontal well is limited in producing from a few layers of the reservoir, it is important to identify the part of the reservoir from which the production can be optimized, prior to drilling the well. This requires that the specific fractured beds should be identified prior to any drilling commitments.
This invention uses the elastically nonlinear interaction between two seismic waves as they propagate through the open fractures in the reservoir formation. Two compressional seismic signals are used. One is a higher frequency swept signal (‘carrier’ wave) transmitted from the surface, using a surface seismic vibratory source. This ‘carrier’ wave penetrates the reservoir and travels through the fractured rock and after being reflected from the formation directly below the fractures, is recorded by the receivers that are located on the surface. The second is a lower-frequency seismic signal (‘modulation’ wave) that is also transmitted from the surface using a similar source like a surface seismic vibrator. The nonlinear elastic interaction between the ‘carrier’ wave and the ‘modulation’ wave, as both the waves travel through the open fracture, is measured.
The nonlinear interaction between the two compressional waves, mentioned above, will be zero when the ‘modulation’ source and the ‘carrier’ source are located parallel to the fractures or directly above them. The interaction will also be zero when the sources and the receivers are located on the surface in such a manner that the recorded reflected signal does not intersect any subsurface open fractures.
By moving the surface sources to different locations on the surface and recording the reflected seismic signals from the subsurface formations by multiple receivers located on the surface, the measurements of the nonlinear interaction between the ‘carrier’ and ‘modulation’ waves can be used to determine the location and orientation of the fractures.
Since practically all the subsurface fractures are vertical, the fracture width of the open fractures is not modulated when the ‘modulation’ and the ‘carrier’ surface sources are directly above them. The ‘modulation’ is maximized when the ‘modulation’ source is at or near right angles to the fractures and at a distant offset, so that the ‘modulation’ seismic signal is arriving at the fracture at a wide angle.
In a three dimensional (3-D) seismic recording, the surface receivers are located over a large surface area, providing a good distribution of source/receiver offset distances. Multiple receiver layout configurations are used to provide an even distribution of source/receiver azimuthal angles for the reflection paths of the seismic signals. The art and knowledge to use different configurations of seismic sources and receivers for 3-D recording are well known in the seismic-imaging industry, and need not be described in detail in this invention. Once the main concept of this invention is understood, anyone familiar with 3-D seismic recording can use this invention to map the orientation and location of the open fractures by analyzing the data representing the nonlinear interaction between the two transmitted waves, recorded by receivers over large distribution of source/receiver azimuthal angles and offset distances.
SUMMARY OF THE INVENTION
Briefly, the present invention provides a new and accurate seismic method of mapping the orientation and location of the open natural fractures that are common in the hydrocarbon reservoirs. Two predetermined seismic signals are used. One is a higher frequency swept signal referred to in the description as a ‘carrier’ signal that is in a higher frequency range compared to the lower frequency signal, which is termed as a ‘modulation’ signal. The ‘carrier’ signal is transmitted using a vibratory surface seismic source located in a pattern designed for conventional 3-D seismic recording. The lower frequency source is also located on the surface and can be easily deployed in any geometric pattern that is considered necessary to map the location and orientation of the fractures. For this particular description both the seismic surface sources that generate the ‘modulation’ and the ‘carrier’ seismic signals are located at the same surface location. Since the lower frequencies are less attenuated as they travel through the earth, the level of the ‘modulation’ signal available at the subsurface fractures would be larger in amplitude compared to the higher frequency ‘carrier’ signal.
Experiments in rocks show a large nonlinear elastic wave response, far greater than that of gases, liquids, and most other solids, The large response is attributed to structural discontinuities in the rocks such as fractures (P. A. Johnson and K. R. McCall, Los Alamos National Laboratory, Los Alamos, N.Mex.). Two compressional waves, as they propagate through a fractured rock that acts as an elastically nonlinear medium, interact with each other. Due to this nonlinear interaction, the sum and difference frequencies of the two primary waves are created. These new frequencies constitute the ‘interaction’ wave that travels along with the primary waves. The amplitude of the summed frequencies or the ‘interaction’ wave is a function of the amplitudes of the two primary waves and the propagation distance through the nonlinear rock. The amplitude of the ‘interaction’ wave is proportional to the product of the primary wave amplitudes. Its amplitude grows with propagation distance due to nonlinearity, and decays with distance due to attenuation. Reference U.S. Pat. No. 6,175,536 (Khan), where the interaction of the two crosswell
Barlow John
Le Toan M
Nonlinear Seismic Imaging Inc.
Nonlinear Seismic Imaging, Inc.
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