Shale Reservoirs: Improved Production from Stimulation of Sweet Spots
Khaled Hashmy¹ and Ashok Bhatnagar¹
Recently introduced azimuthal LWD measurements combined with well-site measurement of parameters on drill cuttings have proven to be a game changer in the quest for improved productivity in laterals drilled in shale reservoirs.
While Pyrolysis, XRF, & XRD measurements on drill cuttings at the well site provide mineralogical and organic content information in near real-time, of particular interest are the LWD Azimuthal Spectral Gamma Ray and the LWD Azimuthal Sonic devices, both of which gather data in sixteen azimuthally-fixed bins.
The LWD Azimuthal Spectral Gamma Ray furnishes clay types and identifies zones with high TOC based on the response of the Uranium measurement.
A unipole configuration with a single, directionally focused transmitter and one array of six directional receivers that are azimuthally aligned with the transmitter is used in the LWD Azimuthal Sonic device. The azimuthally focused sensors can differentiate the slowness of the refracted compressional and shear waves emanating from different azimuthal directions around the borehole. The sonic waveforms received in 16 azimuthally fixed-orientation bins, are processed to yield 16 independent, azimuthally oriented compressional and refracted shear slowness curves, which in turn, combined with the LWD density, provide 16 curves of Young's Modulus, Poisson's Ratio and rock Brittleness Index, each of which can also be used to generate the corresponding 360° borehole image along the length of the lateral.
The combination of mineralogy, abundance of organic material and accurate brittleness coefficient along the length of the lateral is often sufficient to define the sweet spots that exhibit enhanced reservoir properties and are amenable to stimulation and fracking.
The LWD Azimuthal Sonic in horizontal wells provides compressional and shear slowness in the vertical and horizontal directions, and as is common in anisotropic shale reservoirs, the shear velocity in the horizontal plane is often greater than that in the vertical plane. Combined with the cross-dipole measurements in a vertical well, an accurate orthorhombic velocity model may be generated. This will lead to greater precision in tracing the topography of the shale pay on the seismic section and hence lead to more precise definition of the well trajectory. This will facilitate in confining the well path to the targeted pay zone.
Not the least of the benefits of determining the shear anisotropy is that it could be factored into the stimulation design to achieve more effective hydraulic fracturing.
AAPG Search and Discovery Article #90202 © AAPG/STGS Geoscience Technology Workshop, Eagle Ford plus Adjacent Plays and Extensions Workshop, February 24-26, 2014, San Antonio, Texas