Charlotte Botter

Combination of the discontinuous Galerkin method with finite differences for simulation of seismic wave propagation

We present an algorithm for the numerical simulation of seismic wave propagation in models with a complex near surface part and free surface topography. The approach is based on the combination of finite differences with the discontinuous Galerkin method. The discontinuous Galerkin method can be used on polyhedral meshes; thus, it is easy to handle the complex surfaces in the models. However, this approach is computationally intense in comparison with finite differences. Finite differences are computationally efficient, but in general, they require rectangular grids, leading to the stair-step approximation of the interfaces, which causes strong diffraction of the wavefield. In this research we present a hybrid algorithm where the discontinuous Galerkin method is used in a relatively small upper part of the model and finite differences are applied to the main part of the model.

The impact of faults and fluid flow on seismic images of a relay ramp over production time

Relay ramps can act as conduits for fluid flow in producing hydrocarbon reservoirs, but the two bounding faults are often at the limit of seismic resolution. To study the impact of relay ramps and their fluid composition on seismic data, we present an integrated workflow combining flow simulation in a geomodel of an outcrop relay ramp, forward seismic modelling and seismic-attribute-based volume extraction. In the chosen outcrop of the Arches National Park (Utah, USA), the petrophysical properties are conditioned by deformation bands present in the sandstone, and are used to run a simple water injector–oil producer fluid-flow simulation. Pre-stack depth-migration seismic images are obtained at t = 0, t1 = 10 and t2 = 20 years of the flow simulation. The seismic image porosity changes at t = 0 when the model is oil-saturated, whereas the water–oil contacts have stronger amplitude contrasts at later stages. With an adapted attribute-based workflow, we are able to extract geobodies corresponding to the faults and the relay ramp from the three seismic cubes. By varying workflow parameters, we also show reservoir and acquisition conditions that can affect the resolution of the relay ramp on the seismic image either positively or negatively.

The Effect of Fluid Flow in Relay Ramps on Seismic Images

Using an integrated workflow from an outcrop model in the Arches National Park, Utah, fluid flow simulation to seismic modelling, we intend to study the impact of a relay ramp system and its fluid composition on seismic images. Faulting in porous sandstone is associated to deformation bands that decrease the porosity and permeability locally. Based on those modified petrophysical properties, we run a fluid flow simulation and a ray-based pre-stack depth migration (PSDM) simulation to evaluate the impact of parameters such as illumination or wave frequency. We study the relay ramp at two stages of the fluid flow simulation: at the beginning and at the end. Only the changes in porosity around the faults are visible at the beginning, while fluid contacts affect also the model at the end. Resultant seismic images are able to show reflection and diffraction for the two faults when constant fluid saturation at the beginning. However, the thin layer of water at the top of model has a much stronger impact at the end and the faults can hardly be interpreted. Our methodology provides ways to better understand how faulting impact seismic, and therefore to tune acquisition and processing parameters for fault characterization.

From Geomechanical Modelling to Seismic Imaging of 3D Faults

Faults are 3D narrow zones of highly and heterogenously strained rocks, with petrophysical properties differing from the host rock, and are primary controls on fluid flow in reservoirs. We present a synthetic workflow to assess the potential of seismic data for imaging fault structure and internal properties. The workflow is based on a discrete element model (DEM) of faulting, simple relations to modify the seismic properties based on volumetric strain, and a ray-based modelling (pre-stack depth migration or PSDM simulator). Parameters such as wave frequency and their impact on the resulting seismic image are evaluated with the PSDM simulator. We illustrate the application of the workflow to a large displacement 3D normal fault in an interlayered sandstone-shale sequence in two mdoels, one with constant fault slip and the second with linearly variable fault slip along strike. DEM produces realistic fault geometries and strain fields. Seismic cubes at a high wave frequencies show the complexity of the faults, with reflectors offset and laterally affected. As the wave frequency decreases the fault traces become simplier. Seismic extracted fault geobodies make a direct link between the seismic, the DEM and the internal properties of the faults.

The Impact of Faults and Fluid Flow on Seismic Images of a Relay Ramp over Production Time

Relay ramps act often as conduits for fluid flow in producing hydrocarbon reservoirs. The two bounding faults can however be at the limit of seismic resolution and not correctly interpreted. We present an integrated workflow combining flow simulation in a geomodel of an outcrop relay ramp, forward seismic modelling and seismic attribute-based volume extraction. In the chosen outcrop of the Arches National Park (Utah, USA), the petrophysical properties are conditioned by deformation bands present in the sandstone, and are used to run a simple water injector–oil producer fluid-flow simulation. Pre-stack depth-migration seismic images are obtained at several stages of the flow simulation, showing fluid contact evolution. The seismic image porosity changes when the model is oil-saturated, whereas the water–oil contacts have stronger amplitude contrasts at later stages. With an adapted attribute-based workflow, we extract geobodies corresponding to the faults, the relay ramp and the fluid bodies from the three seismic cubes.

Seismic Characterization of Fault Facies Models

Faults play can enhance or restrict fluid flow in reservoirs. Interpretation of seismic data is a key method for studying subsurface features, but the internal structure and properties of faults are often at the limit of seismic resolution. We present an integrated workflow to investigate the seismic response of a sandstone reservoir-scale fault zone model populated with fault facies based on deformation band distributions. We generate synthetic seismic cubes of the fault facies model for several wave frequencies and under realistic conditions of reservoir burial and seismic acquisition. The fault zone definition and the amount of details are highly dependent on wave frequency. We can use seismic attributes, such as tensor and envelope, to characterize the fault volume and its internal structure. Based on these attributes, we can subdivide the fault zone into several seismic facies from core to damage zone. Statistical analyses show a correlation between the seismic attributes and the fault internal structure, although seismic facies, due to their coarser resolution, cannot be matched to individual fault facies.

Mechanical Modelling and Seismic Imaging of Fault Zones

Faults are 3D zones of deformed rock that play a major role in controlling fluid flow in reservoirs. Fault zones are difficult to characterize: outcrops give limited view and seismic is limited by resolution and image quality. We propose an integrated approach to study fault zone evolution and its impact on seismic. We model fault zones using the discrete-element method (DEM). The finite strain of these models is used to condition seismic properties. Finally, seismic imaging of the DEM analogues is performed. An example is presented for a normal fault zone (60 m of fault displacement) in a 2D shale-sandstone layered model of size 1 x 0.5 km at 1 km depth. The fault zone has a complex distribution of shear and volumetric strain. Density, seismic velocities and reflectivities are conditioned by the volumetric strain of the DEM. Seismic imaging shows a response from the fault zone. Enhancing this part of the image is a challenge in acquisition and processing. Our approach can be extended to 3D. Future research will involve denser particles assemblages (smaller particles), and new techniques to pick much of the energy from the fault zone.