TECHNICAL FIELD

The present disclosure generally relates to generating seismic velocity models.

BACKGROUND

In geology, sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments. Generally, geologists distinguish facies by aspects of the rock or sediment being studied. Seismic facies are groups of seismic reflections whose parameters (such as amplitude, continuity, reflection geometry, and frequency) differ from those of adjacent groups. Seismic facies analysis, a subdivision of seismic stratigraphy, plays an important role in hydrocarbon exploration and is one key step in the interpretation of seismic data for reservoir characterization. The seismic facies in a given geological area can provide useful information, particularly about the types of sedimentary deposits and the anticipated lithology.

In reflection seismology, geologists and geophysicists perform seismic surveys to map and interpret sedimentary facies and other geologic features for applications, for example, identification of potential petroleum reservoirs. Seismic surveys are conducted by using a controlled seismic source (for example, Vibroseis™ or dynamite) to create a seismic wave. The seismic source is typically located at ground surface. The seismic wave travels into the ground, is reflected by subsurface formations, and returns to the surface where it is recorded by sensors called geophones. The geologists and geophysicists analyze the time it takes for the seismic waves to reflect off subsurface formations and return to the surface to map sedimentary facies and other geologic features. This analysis can also incorporate data from sources, for example, borehole logging, gravity surveys, and magnetic surveys.

One approach to this analysis is based on tracing and correlating along continuous reflectors throughout the dataset produced by the seismic survey to produce structural maps that reflect the spatial variation in depth of certain facies. These maps can be used to identify impermeable layers and faults that can trap hydrocarbons such as oil and gas. In some instances, tracing can involve the process of migration (for example, imaging), which involves the repositioning of seismic data so that a more accurate picture of subsurface reflectors is given. Performing migration calculations can include determining/approximating the seismic velocities in the subsurface at a multitude of points.

SUMMARY

This specification describes systems and methods in which a dynamic geomechanical forward model is used to enhance seismic velocity models when compared to traditional techniques for seismic velocity modeling. Raw seismic data is used to generate an initial tomographic velocity model. The geomechanical model is used to simulate the tectonic evolution of geological structures. Results from the geomechanical model simulations are used to gain a better understanding of the impact that the mechanical evolution of sediments and salt have on the spatial variability of sediment densities and pore-pressures. These results are used to generate a geomechanical velocity model. A second tomographic velocity model is generated using the initial tomographic velocity model and the geomechanical velocity model.

In at least one aspect of the present disclosure, a method is provided. The method includes generating a first tomographic velocity model based on received raw seismic data. The method includes generating one or more initial seismic images based at least partially on the first tomographic velocity model. The method includes generating a geomechanical model based at least partially on received geomechanical data and the one or more initial seismic images. The method includes generating a plurality of geomechanical outputs by performing one or more geomechanical simulations using the geomechanical model. The method includes generating a geomechanical velocity model based at least partially on the geomechanical outputs. The method includes generating a second tomographic velocity model based at least partially on the first tomographic velocity model and the geomechanical velocity model.

The method can further include generating one or more updated seismic images based at least partially on the second tomographic velocity model. Generating one or more initial seismic images can include performing reverse time migration. Generating one or more initial seismic images can include performing Kirchoff migration. Generating a second tomographic velocity model can include warping the geomechanical velocity model to the first tomographic velocity model.

Generating a first tomographic velocity model can include determining one or more interval velocities. The received geomechanical data can include evolutionary tectonic data.

The geomechanical model can be a dynamic forward geomechanical model. The geomechanical outputs can include at least one of: an effective mean stress value, an effective stress value, a density value, or a bulk modulus value.

In at least one other aspect of the present disclosure, a system is provided. The system includes a raw seismic data module configured to obtain raw seismic data. The system includes tomographic velocity modeling module configured to receive the raw seismic data and generate a first tomographic velocity model based on received raw seismic data. The system includes a seismic imaging module configured to generate one or more initial seismic images based at least partially on the first tomographic velocity model. The system includes a geomechanical modeling module. The geomechanical modeling module is configured to generate a geomechanical model based at least partially on received geomechanical data and the one or more initial seismic images, and generate a plurality of geomechanical outputs by performing one or more geomechanical simulations using the geomechanical model. The system includes a geomechanical velocity modeling module configured to generate a geomechanical velocity model based at least partially on the geomechanical outputs. The tomographic velocity modeling module is further configured to generate a second tomographic velocity model based at least partially on the first tomographic velocity model and the geomechanical velocity model.

The seismic imaging module can be further configured to generate one or more updated seismic images based at least partially on the second tomographic velocity model. The seismic imaging model can be configured to perform reverse time migration to generate the one or more initial seismic images comprises performing reverse time migration. The seismic imaging model can be configured to perform Kirchhoff migration to generate the one or more initial seismic images. The tomographic velocity modeling module can be further configured to determine one or more interval velocities to generate the first tomographic velocity model. The velocities are typically in units of meters/second (m/s).

The geomechanical model can be a dynamic forward geomechanical model. The geomechanical outputs can be at least one of: an effective mean stress value, an effective stress value, a density value, or a bulk modulus value.

The tomographic velocity modeling module can be further configured to warp the geomechanical velocity model to the first tomographic velocity model to generate the second tomographic velocity model.

In at least one aspect of the present disclosure, one or more non-transitory computer readable media are provided. The one or more non-transitory computer readable media are store instructions configured, when executed by at least one processing device, the at least one processing device to perform operations that include generating a first tomographic velocity model based on received raw seismic data. The operations include generating one or more initial seismic images based at least partially on the first tomographic velocity model. The operations include generating a geomechanical model based at least partially on received geomechanical data and the one or more initial seismic images. The operations include generating a plurality of geomechanical outputs by performing one or more geomechanical simulations using the geomechanical model. The operations include generating a geomechanical velocity model based at least partially on the geomechanical outputs. The operations include generating a second tomographic velocity model based at least partially on the first tomographic velocity model and the geomechanical velocity model.

The operations can further include generating one or more updated seismic images based at least partially on the second tomographic velocity model. Generating one or more initial seismic images can include performing reverse time migration. Generating one or more initial seismic images can include performing Kirchoff migration. Generating a second tomographic velocity model can include warping the geomechanical velocity model to the first tomographic velocity model.

Generating a first tomographic velocity model can include determining one or more interval velocities. The received geomechanical data can include evolutionary tectonic data.

The geomechanical model can be a dynamic forward geomechanical model. The geomechanical outputs can include at least one of: an effective mean stress value, an effective stress value, a density value, or a bulk modulus value.

Embodiments of these systems and methods can include one or more of the following advantages. The present disclosure provides means for generating velocity models for use in subsurface imaging. The described systems and methods for producing velocity models support the mapping of lateral variations in seismic velocities as a function of lateral positions with respect to key geological structures such as salt diapers, salt walls or canopies, as well as a function of depth. In contrast, conventional methods of generating velocity models predominantly employ the usage of depth as a key parameter when calibrating seismic velocity across a region of interest. This approach ignores lateral variations in seismic velocities arising from anomalous compaction and pore-pressure variations in the sediments near salt bodies.

The systems and methods described in this specification use dynamic geomechanical forward model to simulate the tectonic evolution of geological structures near or beneath salt structures proximate to sedimentary layers. The results of the geomechanical model simulations are used to constrain recorded seismic velocities based on the impact of the mechanical evolution of sediments and salt on the spatial variability of the sediment densities and pore-pressures. Using a dynamic geomechanical model, as opposed to a static geomechanical model, allows these systems and methods to account for anomalous dilation or compaction of an area of interest, both of which are related to geological history. The incorporation of geological history using the dynamic geomechanical model can be especially important when sediment deposition and salt deformation are coeval (for example, having the same age or date of origin), in which case enhanced compaction or dilation near salt bodies probably results in anomalous seismic velocities. This approach allows for more efficient well planning, especially near salt bodies. Consequently, relative to traditional techniques for velocity modeling for subsurface imaging, these systems and methods can reduce errors when estimating the location of interpreted reservoirs and can provide greater accuracy when imaging subsurface layers in a region of interest.

The details of one or more embodiments of these systems and methods are set forth in the accompanying drawings and the following description. Other features, objects, and advantages of these systems and methods will be apparent from the description and drawings, and from the claims.

