Jon Downton

Quantitative estimate of fracture density variations in the Nordegg with azimuthal AVO and curvature: A case study

We investigated the accuracy of surface seismic attributes in predicting fracture density variations within the Nordegg Formation in west central Alberta. We know from core, drill samples, well-log, and drilling data that the Nordegg zone is fractured to some degree. These fractures are of interest because the reservoir has very low permeability, and therefore natural fractures may materially affect well performance. 3D surface seismic techniques such as amplitude variation with azimuth or azimuthal AVO (AVAz), variation of velocity with azimuth (VVAz), curvature, and coherence techniques are all tools that have been used to predict fractures in a qualitative fashion. In this study, we wanted to understand how well these attributes predicted the reservoir quality in a quantitative fashion. Previous quantitative studies have used image log orientation data or estimated ultimate recoveries (EUR) in vertical wells as validation data. The conclusiveness of these studies has been subject to several problems: f...

Azimuthai simultaneous elastic inversion for fracture detection

Azimuthal AVO has proved to be useful to predict fractures. However, there are a number of limitations with the technique including the fact that only band limited fractional elastic parameters are estimated. Further, the method is derived for the case of an isotropic over HTI anisotropic half-space. It is not theoretically valid for the case of two anisotropic half-spaces. In order to overcome these limitations this paper develops and demonstrates a new (patent pending) azimuthal simultaneous elastic inversion. Azimuthand angle-limited seismic traces are modeled using the anisotropic Zoeppritz equation to generate equivalent reflectivity volumes which are then convolved with appropriate user defined wavelets, generating 3D volumes of the fracture parameters. HTI anisotropy, the lowest symmetry which describes fractures, is used in this implementation. Alternative parameterizations using rock physics models are examined to reduce and constrain the parameterization. The algorithm is demonstrated on both synthetic and real seismic data with good results.

The effect of interpolation on imaging and AVO: A Viking case study

The use of prestack interpolation prior to prestack migration to improve AVO analysis on image gathers is demonstrated on an exploration play. The interpolation achieves this improvement by reducing migration artifacts. AVO analysis attempts to estimate fundamental information from surface seismic data and likely will be used more frequently if the estimates can be more accurately produced. Land 3D seismic typically has poor and irregular sampling. This poor sampling creates migration noise, which is a material cause of inaccurate AVO estimates. Prestack 5D interpolation is applied prior to prestack migration and AVO analysis on the imaged gathers to address this noise problem. The interpolation algorithm includes offset and azimuth dimensions that preserve AVO information. This method is evaluated bycomparing the results to those of alternate approaches, such as superbinning, that suppress this kind of noise in AVO analysis. The evaluation is determined by comparing our ability to predict the reservoir q...

Exploration of uncertainty in bidirectional vector fields

While their importance is increasingly recognized, there remain many challenges in the development of uncertainty visualizations. We introduce two uncertainty visualizations for 2D bidirectional vector fields: one based on a static glyph and the other based on animated flow. These visualizations were designed for the task of understanding and interpreting anisotropic rock property models in the domain of seismic data processing. Aspects of the implementations are discussed relating to design, interaction, and tasks.

Causal fracture prediction: Curvature, stress, and geomechanics

We propose scaling volume curvature measurements with material property estimates to produce a superior prediction of natural fractures. Curvature is one of many, indirect, fracture-inferring attributes. It does not detect fractures, but is causally related to them through the assumption that increasing curvature relates to increasing strain. There are many other variables that are causally related to fractures. We propose that it would be advantageous to create combinations of these causal variables with curvature. Some of the most well known and important causes of variations in natural fracture density are material properties relating to brittleness. Material properties are critical geologically at all scales, from large-scale regional studies to prospect-level inquiries because the properties may vary significantly within individual formations and between formations. These vertical and lateral changes in material properties may be important and should be considered in fracture estimation, along with c...

The Effect of Interpolation On Imaging And Azimuthal AVO: A Nordegg Case Study

To address the issue of inadequate sampling, typical of land seismic data, an azimuthal AVO (AVAZ) processing flow should include interpolation and prestack migration prior to the AVAZ inversion. It is well established that seismic data should be prestack migrated before AVO, but the irregular sampling inherent in land data can introduce migration artifacts which distort the estimates of the AVO inversion. With the introduction of the extra dimension of azimuth this is even more a concern for AVAZ analysis. By performing 5D minimum weighted norm interpolation before PSTM, the wavefield is better sampled leading to better migration and AVAZ results. This has been demonstrated on synthetic examples. This paper demonstrates the benefits of this on a real dataset by comparing AVAZ processing sequences with and without interpolation and correlating the predictions quantitatively to well control. The interpolation/PSTM flow proceeding AVAZ inversion produces better correlations to the well control than only PSTM.

Introduction to special section: Fractures

Understanding the spatial distribution of natural fractures (orientation and magnitude) is important for drilling and producing from hydrocarbon reservoirs. In unconventional reservoirs (e.g., shale gas, shale oil, tight gas), natural fractures are the only pathways for fluid flow. If there are not

5D Interpolation, PSTM And AVO Inversion

To address the issue of inadequate sampling, typical of land seismic data, an AVO processing flow should include interpolation and prestack migration prior to the AVO inversion. It is well established that seismic data should be prestack migrated before AVO, yet the irregular sampling inherent in land data can introduce migration artifacts which distort the estimates of the AVO inversion. By performing 5D minimum weighted norm interpolation prior to the PSTM, the wavefield is better sampled leading to better migration and AVO results. By working in five dimensions, the algorithm can interpolate through gaps that are problematic for lower dimensional interpolators. The 5D interpolation is amplitude preserving and appears to improve the signal-to-noise ratio with minimal evidence of smearing. In order to support these assertions, a series of parallel processing test flows were performed and compared on a 3D seismic survey from Alberta, Canada with extensive well control. For each of these flows, Ostrander gathers at key wells, AVO attributes, and their ties to 29 wells were examined. The interpolation PSTM flow prior to AVO inversion produced the best correlation to the well control.