Drag Wave Reservoir Detection via Monofrequency Signal
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Solution Overview
Problem
Current seismic subsurface imaging methods fail to accurately identify and map the extent and location of fluid-saturated subsurface reservoir formations, relying on non-unique and ambiguous results that do not effectively differentiate reservoir rocks from non-reservoir rocks based on porosity, permeability, and fluid content.
Innovation Solution
A method involving the transmission of a discrete monofrequency signal during seismic data acquisition, generating a Drag Wave that propagates at a slower velocity through permeable and fluid-saturated reservoirs, allowing for the detection of a unique lower frequency signature indicative of reservoir presence, and using a conversion factor to create a new swept frequency signal for cross-correlation with conventional seismic data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional swept frequency signals are used for seismic subsurface imaging, then the method is universally practiced and easy to implement, but the results are non-unique and ambiguous, failing to accurately identify reservoir formations
Solution Approach 1:
The patent segments the seismic signal transmission into two distinct parts: conventional swept frequency signals for general subsurface imaging and a separate monofrequency signal specifically tuned to generate the Drag Wave effect. This segmentation allows each signal type to serve its specialized function, with the monofrequency signal providing the unique reservoir identification capability through Drag Wave generation while the swept signal maintains broad applicability
Solution Approach 2:
The patent introduces the Drag Wave effect as an intermediary mechanism that mediates between the transmitted monofrequency signal and the recorded seismic data. The Drag Wave, generated when the monofrequency signal propagates through fluid-saturated porous formations, serves as a distinctive intermediary signature that directly indicates reservoir presence, bridging the gap between signal transmission and accurate reservoir identification
2Reliability
If monofrequency signal is transmitted to generate Drag Wave for direct reservoir signature, then reliable reservoir identification is achieved, but additional transmission steps are required at every nth station
Solution Approach 1:
The patent applies partial action by transmitting the monofrequency signal not at every station but only at selected nth stations throughout the survey area. This partial transmission strategy is sufficient to generate the Drag Wave effect for reservoir identification while significantly reducing the operational complexity compared to transmitting at every station, balancing reliability with ease of operation
Solution Approach 2:
The patent designs the seismic acquisition system to perform multiple functions: the conventional swept frequency signals continue to provide general subsurface imaging and structural information, while the added monofrequency signals at nth stations provide specialized reservoir identification through Drag Wave generation. This multi-functionality allows a single survey to achieve both broad geological characterization and specific reservoir detection
3Loss of information
If conventional swept frequency signals are used, then general subsurface imaging is obtained, but the method cannot differentiate reservoir rocks from non-reservoir rocks based on porosity, permeability, and fluid content
Solution Approach 1:
The patent applies local quality by using a specifically tuned monofrequency signal rather than a broad spectrum swept signal for reservoir detection. The monofrequency is selected to optimally generate the Drag Wave effect in fluid-saturated porous formations, concentrating the informational content about reservoir properties (porosity, permeability, fluid content) into a targeted frequency transmission rather than dispersing it across a broad frequency range
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables reliable identification of subsurface reservoirs by generating a unique lower frequency signal that is not present in non-reservoir rocks, reducing the need for unnecessary drilling and improving the success ratio of hydrocarbon exploration by providing a direct and accurate mapping of reservoir extent and location.
Implementation Method 1
Because of the difference in their propagating velocity, a Doppler shift in the primary frequency of the monofrequency takes place. A lower frequency is generated within the reservoir, which may be three to four times lower than the primary frequency of the originally transmitted monofrequency.
Implementation Method 2
The Drag Wave travels at a slower velocity, which is slower than the velocity of the compressional wave in the pore fluid itself. These two waves, the compressional wave and Drag Wave, both propagate through the reservoir formation simultaneously.
Data Source
AI summary
A monofrequency signal is used to record signature properties of subsurface reservoir formations. While recording conventional Vibroseis data after certain prescribed distances, the monofrequency signal is transmitted to evaluate the presence of reservoir rocks underneath that source location. When a compressional wave travels through a permeable and fluid-saturated reservoir formation, the Drag Wave travels through reservoir fluid interconnections at a slower velocity than the compressional wave in the rock matrix. Due to the Doppler Effect, a unique lower frequency is generated. This lower frequency becomes an indicator of the presence of reservoir formations. Its character depends on the tortuosity of pore interconnections, presence of pore fluids, and permeability. A transfer function is calculated to convert the swept frequency signal used for conventional seismic recording. This converted swept frequency signal is cross-correlated with the normally recorded signal. Only the presence of the reservoir formation is highlighted; non-reservoir formations are not displayed.