Fracture detection with azimuthal seismic data, rock physics, and geomechanics
Keywords: seismic anisotropy
rock physics
azimuthal AVA modelling

Significant progress has been made for fracture detection in unconventional plays using advanced geophysical techniques such as the use of P-wave AVO algorithms in multi-azimuth data. In the past decade, it has been recognised that major amplitude distortions in anisotropic media occur, for wide-azimuth data, and that these might potentially be related to fracture distribution in the layered media. However, inversion of the azimuthally varying P-wave AVO gradient for the crack density is non-unique without additional information.

Aims
This research work aims to better understand the seismic anisotropy of shale rocks to improve future exploration by using fracture modelling and azimuthal AVO/AVA for fracture detection, and rock physics to develop a thorough understanding of the reservoir and quantify fracture anisotropy quantitatively and qualitatively. This is a potentially important unconventional oil and gas reservoir of the Late Jurassic age. Specifically, the project aims to improve future oil exploration by using AVAz variations for fracture detection and fracture models in effective media.

Scope
The scope of this research is to model fractures of the effective medium to see the impact on seismic amplitudes distortions in different azimuthal directions and determine the type of AVO present in the models to compare them with real data in shale-type rocks. Implementation is presented of a new methodology to estimate anisotropic brittleness, the azimuthal TOC and stress field and its comparison with hydraulic treatment pressure data, calculated from inverted volumes with good quality azimuthal seismic data. Rock physics and fluid replacement in fracture models in shale rocks approaches will be tested for (brine/oil/gas) to better understand the fluid substitution in azimuthal synthetic models and the AVAz curves.

Motivation
The motivation for this work is to develop a better understanding of the anisotropy within the Pimienta Formation, which will help understand the shale-type rock fabric through fracture modelling, azimuthal data for better reservoir characterization in unconventional reservoirs.

Methods
Based on the effective media theory of fracture models addressed by other researchers, codes were developed and tested to obtain an azimuthal seismic image depending on the type of fracture arrangement (e.g., Hudson, and Linear slip). Within the methodology, in the substitution of fluids, the Linear slip model was tested to verify the synthetic azimuthal response with three cases (Brine/Oil/dry). Azimuthal seismic information provided by PEMEX was used to perform AVO Azimuthal analysis within the Pimienta Formation in order to identify possible fracture swarms and be able to determine the direction of the fast velocity (V_fast) that is related to the direction of fractures and possible current stress. Then rock physics modelling of elastic moduli in an anisotropic way was estimated by developing codes to obtain brittleness, these elastic moduli (e.g., Young´s Modulus and Poisson's Ratio) were used to estimate the stresses in a sub-area in the study area using inverted volumes. Differential Effective Medium (DEM) and Effective Field Method (EFM) have been tested in the study area to estimate the elastic moduli.

Results
The Pimienta Formation in rock physics template analysis fit much better using friable-sand model instead of friable shale model, different values in anisotropy within Pimienta Formation were found according to the significant amplitude distortions seen in each fracture model which may explain the intrinsic anisotropy of the Pimienta Formation. The azimuthal AVO demonstrated the possible existence of fractures within the Pimienta Formation in the study area according to the differences in the fast velocity (V_fast) and slow velocity (V_slow) and the Thomsen’s parameters, the fluid-filled fracture models through fluid substitution analysis in shale rocks demonstrated amplitude changes depending on the azimuth direction. The elastic properties (e.g., Young´s Modulus and Poisson´s Ratio) were estimated in VTI/HTI modes using the inverted data and finally the stress field using hydraulic treatment pressure information.

Conclusions
In this research work, two main contributions can be addressed that may help other geoscientists for a better understanding of the anisotropy in shale rock type and can be widely applied for reservoir characterization on unconventional resources in other projects around the world and maybe applied naturally fractured carbonates.
1. A new equation for estimation of the TOC in different azimuths named “upscaled azimuthal TOC “using inverted data from azimuthal seismic data contributed to the novel implementation of the calculation of this attribute which had been experimentally tested on core analysis in different azimuths.
2. A novel approach for the estimation of anisotropic brittleness has contributed to the field of anisotropy on shale rock types by using vertical transverse isotropy (VTI) and horizontal transverse isotropy (HTI) using azimuthal sectors of inverted seismic information.
3. For this research project, applications were developed in the field of geophysics that addressed fractured models (e.g., Hudson, and Linear slip), anisotropic rock physics for the calculation of elastic properties in two main directions 0° (VTI) and 90° (HTI) for the calculation of anisotropic brittleness compared with other researchers in this field. Such applications can help the geoscientific community to model fractures in effective media in a versatile and easy to use way since most software does not handle this type of modelling.
4. These new contributions in geophysics applied to the oil industry were focused on the study of the Pimienta Formation; however, these contributions can be widely used in the development, understanding and characterization of unconventional and carbonate reservoirs around the world.
Finally, these contributions and all the analysis carried out in this research work will contribute to more efficient stages design for hydraulic treatment in horizontal wells in unconventional reservoirs, in order to reduce operating costs and the number of wells with the objective of obtaining the highest possible commercial production of hydrocarbon without ignoring the environmental impact and concerns.