Geophysical Data Home In On Sweet Spot Drilling

Following the success of shale play exploitation in North America, interest has spread to the rest of the world. Rising demand for energy in developing countries and the quest to diversify suppliers have combined to intensify the desire to exploit these vast, newly identified unconventional resources. As shale E&P matures and spreads, interest is focused more and more on targeted sweet spot drilling rather than the statistical drilling employed in the early stages of exploitation.

Improving Success

In addition to applying the lessons learned, current techniques need to be improved. Development needs to be made safer, reducing both the risks involved and the environmental footprint, by drilling and completing fewer, more productive wells. Also, duplicating program success from one play to another is proving difficult because shales vary in composition, depth, thickness, permeability, and fracture characteristics. Understanding reservoir quality and geomechanics is critical to reducing costs and maximizing recovery and financial return on investment (ROI).

A workflow goes from survey evaluation and design through acquisition and processing to detailed reservoir analysis to produce map volumes to identify sweet spots to drill. (Images courtesy of CGGVeritas)

The challenge when choosing the best locations to drill in shale reservoirs is to identify the areas of highest total organic content (TOC), which also will fracture easily to form good flow networks. At the same time, it is important to ensure that the fractures will be limited to the reservoir. With advanced prestack analysis of seismic data, lithological (elastic rock parameters) and geomechanical (rock strength and stress) attributes can be derived – good indicators for TOC and fracturability.

Existing small fractures can aid production as they provide natural pathways that can be connected to the well bore, but more intensive natural fracturing could mean the hydrocarbons already have migrated away. Detailed geomechanical and lithological models of the reservoir allow both potential hazards and sweet spots to be identified so the greatest return on drilling investment can be achieved.

In today’s environment, shale operators would benefit from customized, comprehensive seismic strategies designed to optimize resource recovery and to reduce drilling and production risks. While information from well bores, well logs, and vertical seismic profile data can give a detailed view of the reservoir at the well location, combining this with advanced reservoir analysis from seismic data can provide detail between the wells, critical in these very heterogeneous reservoirs.

CGGVeritas has developed a new workflow to deliver integrated geophysical solutions offering advanced technologies and services individually designed for each project to enhance all stages from exploration to development and production of shales. The complete workflow recently has been applied to data library surveys over the Montney and Haynesville shales. Different elements also have been applied successfully to a number of proprietary datasets over the major shale plays in North America. The results of tying the geomechanical models from these studies with actual gas production figures will be published in the near future.

Data Analysis

The workflow begins with a fundamental understanding of the target through careful analysis of all available geological, petrophysical, and geophysical data. This information is incorporated into a reservoir model that drives the survey design and acquisition parameters. Without the appropriate azimuths, offsets, and receiver/source intervals, it is difficult to extract the necessary lithological, fracture, and geomechanical properties critical for successful shale drilling programs.

Straightforward maps, identifying good (green) and bad (red) areas to drill, can be produced by carefully combining all the elements of the detailed lithological and geomechanical reservoir models.

Broad bandwidth, high-density seismic data are ideal for providing the high spatial and temporal resolution necessary to define the thin beds and spatial heterogeneity of shale reservoirs and create optimal reservoir models. Wide-azimuth data are required for meaningful anisotropy analysis, which is critical for stress and fracture characterization. Once the optimum acquisition parameters have been derived from the survey evaluation and design studies, experienced crews are available worldwide to perform the required high-quality acquisition.

Acquisition

Advanced technologies such as point-source-point-receiver recording and mega-channel-count supercrews using state-of-the-art equipment and customized Vibroseis sweeps can provide the broadband data to enhance shale reservoir characterization. These methods use proprietary software to intelligently tailor Vibroseis sweeps to extend the bandwidth while working within the hydraulic and mechanical constraints of the vibrator and without compromising quality or productivity. They can sweep more than five octaves, making them particularly applicable for high-resolution imaging of shale reservoirs.

Processing

Carefully tailored, reservoir-driven processing flows are applied to the recorded data, which include advanced algorithms such as Gabor deconvolution for increased resolution and 5-D interpolation to provide fully populated common-offset vector volumes. Geomechanical properties are derived from careful measurements of azimuthal anisotropy. Therefore, an amplitude versus offset and azimuth-compliant processing flow is required. Depth imaging is preferable as it avoids any anisotropy in the overburden being imprinted onto the reservoir, and it provides more reliable amplitudes.

Important lithological and geomechanical attributes can be derived from seismic gathers by inverting the data for compressional- and shear-wave velocities and density, including Young’s modulus, Poisson’s ratio, and shear modulus. Azimuthal anisotropy analyses provide information about the existing fractures, while simultaneous prestack azimuthal elastic inversion enables estimates of the various stress fields, such as hoop stress, closure stress, and differential horizontal stress ratio, to be produced. Understanding the stress characteristics provides hydraulic fracturing details, including whether fractures will form, remain open, and be limited to the desired zone. The fracture initiation pressure provides information on how much proppant and pressure will be required to create the fractures.

Reservoir Modeling

Advanced reservoir modeling to provide detailed lithological and geomechanical models is the final component of the workflow, producing a volume that combines all of the derived attributes to identify the sweet spots and to predict well flow rates. This detailed reservoir modeling gives engineers a broader understanding of the entire field, enabling the development of more cost-effective drilling programs that incorporate information about directions to drill, locations of zones of interest, and recommendations for fracture stimulation to achieve the most profitable and environmentally sound production rates.

Microseismic services for monitoring the frac process, using purpose-designed surface layouts and in-field real-time processing, also are of use. Interpretation of these data allows seamless in-field adjustments based on fracture direction, orientation, and penetration in and out of zones of interest to optimize fracing operations. Interpreted data also can be used to validate and refine geomechanical reservoir models. Staying within zone and using the minimum frac stages and fluids to achieve the desired well flow can make the difference between a positive and a negative ROI.

The best lithological and geomechanical attributes are derived from seismic surveys designed to specifically target the reservoir and acquired and processed with this in mind. Detailed reservoir models derived from advanced integrated seismic analysis provide the information engineers need to design the most productive drilling and completion projects and therefore maximize ROI while minimizing the environmental footprint. Integrating microseismic data to fine-tune the geomechanical reservoir models derived from seismic technologies is providing key insights for effective exploration and development of unconventional reservoirs and the foundation for the move from statistical drilling to targeted, sweet-spot production.