Oil & Gas Technology – Brazil Edition

Printed with permission from Cavendish Group International Ltd. June 2013

Fractured Carbonates Require a New Level of Predictive Understanding
Kevin McKenna, Vice President, Technology Solutions, SIGMA³ Integrated Reservoir Solutions

Naturally fractured carbonate reservoirs continue to play an important growth role for oil and gas exploration and production globally. As oil companies search for opportunities to optimize their offshore exploration, drilling, and production efforts, operators are forced to address multiple challenges, such as locating the most productive and profitable areas, drilling the fewest wells and leveraging their trajectories while making the most of reservoir properties, including physical rock attributes like permeability, porosity, geopressure and natural fractures.

Natural fractures play a major role in effective well positioning and drilling planning programs, particularly in fractured carbonate reservoirs where heterogeneous characteristics -- including variability in well rates, rapid pressure declines, and poorly connected fractures that impede flow -- create specific challenges for geoscientists and engineers. A thorough understanding of fracture location, density and orientation is essential. Integrated, real-time technology provides an optimized pathway for building predictive natural fracture models based on the extensive use of geophysics.

A Unified Model for Predictive Reservoir Understanding
Combining best-in-class geoscience and engineering technology, SIGMA³ integrated services experts leverage decades of industry experience and our proprietary CRYSTAL™ software to generate seismically driven geologic models constrained by seismic and well data. Using our Real-Time Dynamic Earth Modeling (RTDM™) workflow, real-time completions data are visualized and interpreted in tandem with the geologic models, well geometries, and field-scale reservoir property predictions - leading to better drilling decisions, and better outcomes for clients to:

  • Understand actual fracture location, density, and orientation
  • Build integrated, predictive reservoir models including natural fractures
  • Drill the most prolific wells with optimal locations


A key strength of CRYSTAL™ software is its ability to generate robust 3D geocellular grids from interpreted single-z and multi-z horizons and faults, and represent hundreds of faults directly in the reservoir. Users can visualize, edit, and manipulate attributes and user-defined reservoir properties in time or depth, and upscale the reservoir models to any grid size while keeping the geologic features and plan optimal well trajectories.

Reveal New Details with Highest-Resolution Imaging and Inversion

CRYSTAL users can also combine pre- and poststack inversion and spectral methods to predict rock properties, fracture networks and hydrocarbon indicators directly from well and seismic data. Using ThinMAN™, the powerful spectral inversion technique, Subsurface GeoEngineering teams can perform "space-time adaptive" spectral inversion, which combines non-stationary wavelets with optimization algorithms to generate high-resolution images of their reservoirs (Figure 1).


Figure 1: High-resolution elastic inversion calculated in faulted framework. SIGMA³ workflows maximize the resolution and detectability of seismic data while preserving amplitude for quantitative interpretation.

 

Within the SIGMA³ sequential geologic modeling workflow, these techniques facilitate the identification and modeling of the many facies and textures that subtly impact hydrocarbon flow and production in carbonate reservoirs (Figure 2). Quantifiable relationships are established between seismic data, facies types, rock properties, and ultimately production. Independent QC steps at each stage of the modeling process assure the desired predictive outcome.


Figure 2: Fractured carbonate model built using SIGMA³ seismically driven reservoir modeling workflow shows subtle facies and texture differences important for fluid flow.

 

Built for optimization of naturally fractured reservoirs
A critical component of the CRYSTAL platform, the Continuous Natural Fracture Modeling (CNFM™) workflow is used to identify reservoir sweet spots influenced by natural fractures, and predict user-defined properties across the entire geomodel. CNFM leverages a structured, proprietary neural network to find and rank correlations that exist between large volumes of well data, production data, and seismic attributes. The neural network is first trained on a subset of available wells (e.g., using "fracture indicator" logs from FMI, etc.), and then validated by testing its predictions on a separate subset of wells (Figure 3).

 


Figure 3: Anisotropy map showing seismically derived dominant orientation and density of fractures within grid cells in a stratigraphic layer.

 

CRYSTAL's seismically driven reservoir modeling generates 3D models of Young's Modulus, Poisson's Ratio, brittleness, and other key properties that impact production. SIGMA³ has used CNFM predictions to enable Subsurface GeoEngineering teams across the globe to perform field-level planning and optimization and achieve accurate estimates of reservoir properties.

The SIGMA³ sequential geological modeling workflow optimizes the integration of reservoir geophysics with reservoir modeling. For fractured carbonate reservoirs, this provides the best understanding of the subtle interplay of structure, facies, texture, and stress regime, and natural fractures. These fractures can be exported as part of the geo-cellular grid for use reservoir simulators such as Eclipse® and CMG®, allowing users to forecast well- and field-scale production.

Oil and gas operators are looking for insight about how to drill and complete their wells and maximize reservoir potential and long-term operational efficiency across the life of the asset. Smart use of fit-for-purpose technologies in a Real-Time Dynamic Modeling workflow gives E&P companies the power to implement the best solution and enhance production efforts, reduce drilling and completion costs, and drive profitability. Armed with better understanding, asset teams can make the optimal selection for drilling targets, completion design, and field development in fractured reservoirs.