Characterization of naturally fractured Arbuckle Group in the Wellington Field, Kansas, using S-wave amplitude variation with offset

Abstract

S-wave amplitude variation with offset (AVO) analysis is sensitive to the presence of fractures and can provide a high-resolution seismic-based fracture characterization as compared with traditionally used traveltime-based methods. To determine viable attributes for estimation of properties such as spatial density and fluid fill of fractures, S-wave AVO modeling and analysis is carried out in the Wellington Field, Kansas, where 9C-2D seismic data have been acquired. Analysis is performed on the Ordovician fractured-carbonate interval called the Arbuckle Group, which is being considered for [Formula: see text] sequestration. AVO modeling of the Arbuckle interval indicates that differences in AVO intercepts of different S-wave polarizations can estimate S-wave anisotropy parameter [Formula: see text], which gives an estimate of fracture density. In addition, modeling suggests that AVO gradients of [Formula: see text] and [Formula: see text] waves can be used to derive a seismic attribute to discriminate fluid fill in fractures, provided good-quality S-wave gathers are available. The intercept anisotropy (IA) attribute obtained from AVO intercepts of S-waves provides fracture density estimates within the Arbuckle Group. These estimates are consistent with the field-wide, low-frequency observations from seismic velocities and spatially limited, high-frequency estimates obtained from drill cores and sonic and borehole-image logs. The IA attribute highlights possible high-permeability zones in the Upper and Lower Arbuckle suitable for [Formula: see text] injection. The Middle Arbuckle indicates low fracture density, potentially acting as a baffle to vertical flow and providing a seal for the Lower Arbuckle. The gradient anisotropy attribute obtained from the AVO gradient of S-waves suggests that most fractures in the Arbuckle are brine saturated. This attribute has a potential application in monitoring the movement of a [Formula: see text] plume in the Arbuckle Group when time-lapse data become available. These results demonstrate that S-wave AVO attributes can supplement the P-wave derived subsurface properties and significantly reduce uncertainties in subsurface fracture characterization.

Injection‐Induced Earthquakes Near Milan, Kansas, Controlled by Karstic Networks

“…Open fractures are present in the lower units of the Arbuckle Group with measured maximum fracture permeability of 1·10 −12 m 2 (Gupta et al, 2017; Morgan & Murray, 2015). Importantly, larger dissolution features identified in a 60‐cm core (Figure 1b) show dissolved apertures ranging from 0.04 to 1.80 cm (Cansler & Carr, 2001; Gupta et al, 2017; Holubnyak et al, 2017; Kansas Geological Survey, 2018; Nissen & Sullivan, 2006). Although permeability is a scale‐dependent parameter and core measurements are unlikely to represent deep confining pressures, the mineralization evidences water flowing and the absence of fill material suggests that these centimeter‐wide apertures remain open about 1,500 m deep.…”

“…The Arbuckle Group is a fractured-karstic saline aquifer composed of spatially complex heterogeneous carbonates underlying about 1,300 m of Paleozoic carbonates. It directly contacts the Precambrian granitic basement in the Milan area (Carr et al, 1986;Franseen et al, 2003;Gupta et al, 2017;Keeling, 2016;Puckette & Al-Shaieb, 2003;Schwab et al, 2017). The Arbuckle Group divides into three subunits (upper, middle, and lower) defined by the order of magnitude of the permeability (Figure 1b) (Hearn et al, 2018;Holubnyak et al, 2013Holubnyak et al, , 2016Morgan & Murray, 2015;Scheffer, 2012;Watney, 2012).…”

“…Open fractures are present in the lower units of the Arbuckle Group with measured maximum fracture permeability of 1•10 −12 m 2 (Gupta et al, 2017;Morgan & Murray, 2015). Importantly, larger dissolution features identified in a 60-cm core (Figure 1b) show dissolved apertures ranging from 0.04 to 1.80 cm (Cansler & Carr, 2001;Gupta et al, 2017;Holubnyak et al, 2017;Kansas Geological Survey, 2018;Nissen & Sullivan, 2006).…”

Application of Seismic Radial Anisotropy for Near-Surface Fractures Identification

“…Seismic P-wave anisotropy has been used to characterize fractures in oil and gas reservoirs via some methodologies such as Amplitude-Variation with Offset and Azimuth (AVOA) (Hall and Kendall 2003), Velocity Versus Azimuth (VVAZ) (Crampin et al 1980) and well-log data (Hornby et al 2003); these methods take advantage of the difference in velocity depending on the direction of the P-wave propagation. There are also studies of S-wave anisotropy based on the polarization of the S-wave in an anisotropic medium where Swave splits into two polarized shear waves (Vecsey et al 2008, Aki andRichards 1980), and studies that make use of the variation of the S-wave velocity with azimuth (Azimuthal anisotropy) (Gupta et al 2016, Li 1997, DeVault et al 2002, Mueller 1991, Lynn and Thomsen 1990, Martin and Davis 1987.…”