Rock heterogeneity at the centimeter scale, proxies for interfacial weakness, and rock strength-stress interplay from downhole ultrasonic measurements

Abstract

Analysis of data acquired with an ultrasonic rotating device lowered inside a vertical well of a highly laminated, kerogen-rich, carbonate source formation reveals centimeter-scale rock heterogeneities. The high-frequency (50–600 kHz) measurement used consists of a pulse-echo modality that yields a high-resolution (millimeter-scale) borehole shape and a pitch-catch modality with one transmitting and two receiving transducers that provide compressional (P) and shear (S) slownesses (depth-versus-azimuth) images estimated from signals propagating in the near-wellbore region as compressional head waves and pseudo-Rayleigh surface waves. The slownesses are compared with their counterparts estimated from a lower-frequency (1–15 kHz) sonic measurement logged in the same well interval. The sonic P and S logs are seen to average the centimeter-scale slowness spatial variation between compliant and stiff laminates at a scale larger than 30 cm (1 ft). This is also accompanied by a reduction in resolution of the slowness contrast, which markedly reduces the spectrum of rock-mechanical property variations that are estimated from the ultrasonic data. Further, ultrasonic images of the P and S slownesses and borehole acoustic caliper reveal a host of features associated with rock geomechanics in the near-wellbore and that inform on a first-order basis the interplay between rock strength and local stress regime, as affected by the lamination. The features include breakouts present in the stiffer limestone layers and their arrest at the intersection with the compliant siltstone layers, as well as azimuthal P and S slowness variations indicative of azimuthal stress concentrations, but without the appearance of breakouts. The ultrasonic borehole shape data also identify compliant thin layers that retract into the formation by amounts that are commensurate with their Young’s modulus, suggesting a proxy for detecting and characterizing thin-layer weakness in situ.