Borehole Acoustic Reflection Survey in Horizontal Wells for Evaluation of Fractured Reservoirs

Yurubcheno-Tokhomskoe oilfield in east Siberia comprised of very low porosity fractured dolostone reservoir, which is being developed by horizontal wells. Formation evaluation program includes ultrasonic-image and acoustic logs to characterize fracture network and identify permeable fractures. Up to several thousand individual fractures might be identified over horizontal well section, however production logs demonstrate that producing intervals are discrete, short and not evenly spaced. Stoneley wave permeability indicators are not consistently successful in identifying producing intervals due to plugging off permeable fractures by LCM material. To overcome this limitation, we modified acquisition program to include borehole acoustic reflection survey (BARS) that allows to identify big fractures extending far from a wellbore. A novel processing technique has been devised to image fractures oblique and perpendicular to the borehole axis. Production logs confirmed flowing fractures identified on BARS and demonstrated a complex pattern of oil, water and gas influx from specific fracture types. BARS and PLT data allow localization of fractured interval requiring acid stimulation to clean off plugging material. Theory of the method and a case study of BARS data interpretation in horizontal well are discussed.