Seismic surveying objectives used to be focused primarily on providing images of the subsurface to identify likely structural traps for drilling. In unconventional plays, while structure remains important, factors such as stratigraphy and stress also control the success of drilling and completions campaigns. Continuous advances in acquisition and processing technology have led to ever-improving quality and reliability of seismic data, and these data now can be trusted to provide an increasing range of valuable information about lithology and other important reservoir conditions.
In unconventional plays the desired contributions from seismic data are accurate depth images, high-resolution seismic velocities, and calibrated prestack inversion products. The inversion products are especially important in workflows that integrate the seismic data with well logs, core data, and other measurements. The purpose of that integration is for improvement in identifying lateral and vertical variation within the reservoir compartments and characterizing reservoir quality and completion quality to optimize well positioning and hydraulic fracturing strategies.
Factors contributing to good reservoir quality include high gas saturation and kerogen content, high matrix permeability and porosity, and high pore pressure. Factors contributing to good completion quality include strong fracture containment, fracturable formations, and mineralogy – in particular detecting clays with high rock-fluid sensitivity. While seismic data alone cannot confirm all of these factors, when combined with other measurements, they can provide an indication of the most prospective well locations. Hurdles that must be addressed in the seismic contribution include suppressing noise, comprehending anisotropy, and resolving heterogeneity.
Modern point-source and high channel-count point-receiver land seismic acquisition systems are able to reduce noise in the seismic signal and improve the temporal and spatial resolution of the resulting images. Resolution is further improved by using broadband vibroseis source techniques and recording with high-fidelity geophone acceler ometers. Broad bandwidth, especially low-frequency acquisition, is essential for accurate inversion to derive rock properties. Improvements in processing include surface wave inversion, which enables improved correction for near-surface effects that can degrade the seismic image, which is of particular importance when performing prestack depth migration. Full-azimuth acquisition, combined with anisotropy-comprehending amplitude-vs.-offset inversion, can indicate local stress directions and help optimize well orientations and fracturing efficiency.
The azimuths of minimum and maximum stress directly impact the optimum direction for a horizontal well and its completion. The process of high-pressure hydraulic fracturing is constrained by the in situ stresses in the rock. Fracture geometry in unconventional reservoirs is controlled by both the magnitude and direction of maximum stress, the contrast between maximum and minimum stresses, and the complex heterogeneous rock fabric itself. Horizontal wells are normally drilled in the direction of minimum stress to create multiple hydraulic fractures transverse to the wellbore azimuth, as reservoir simulation generally indicates optimum reservoir performance under these conditions. Drilling performance, including ROP and wellbore stability, also is impacted significantly.
The stress regime can be characterized from a description of the azimuthal anisotropy of seismic velocities. Faster velocities can correspond to the maximum stress direction and/or the orientation of natural fractures. Several seismic studies have shown that, regardless of regional stress regimes, local stresses vary considerably in magnitude and direction.
In an example from the Fayetteville shale (SPE 147226) a reservoir model was developed integrating petrophysical, sonic, image, core, stimulation, production, micro-seismic, and processed surface seismic data. This dynamic reservoir model was used to history-match the short-term and long-term production performance and its variations across the exploration area. The reservoir characterization and dual-porosity simulation model was found to be consistent with variations in the gas production history of different perforation clusters, providing confidence in the workflows.
Further advances in acquisition and processing technology, coupled with integrated workflows combining multiple types of measurements, will make high-quality seismic data an increasingly valuable tool for maximizing production in unconventional plays.
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