We present a method of improving microseismic source depth locations using reflected phase arrivals identified on a single vertical receiver array. We demonstrate the technique using data collected while monitoring CO2 enhanced oil recovery and concomitant salt water disposal (SWD) in the Aneth oil field, Utah. A 60-level, 900-m-long geophone array was cemented into a monitoring well with the deepest sonde placed 27 m above the top of the oil reservoir. During the first year of monitoring approximately 3,800 microseismic events of moment magnitude from −1.2 to 0.8 were detected within about 5 km of the geophone array. The events delineate two distinct structures active on opposite flanks of the Aneth field. More than 96% of the detected events occurred along a NW-SE-trending fracture zone, 1,500 m from the monitor well at its closest edge. This structure was also more than 1,500 m long and was initially located about 335 m beneath the oil reservoir within a salt layer when using direct P- and S-wave arrivals. We identified two multiple reflected phases and, through a series of trial depth positions, matched their arrivals to ray-trace arrivals from two distinct velocity boundaries while also honoring the direct P- and S-wave arrivals. We then verified the observed reflected arrivals with synthetic waveform modeling. The final source locations moved about 200 m downward within the Leadville Limestone, corresponding to the interval where high-volume SWD injection occurs.
Single-well microseismic monitoring provides a practical solution for mapping hydraulic fracture geometry (Maxwell et al., 2010). However, the placement of distant monitor wells or short arrays presents challenging conditions for determining accurate source depths. In this paper we present a method of using reflected phases and direct arrivals to improve microseismic source depth estimates. In general, using reflected phases that are generated above or below a vertical array effectively increases the array's aperture because the reflection points can be considered image receivers. We demonstrate the technique using microseismic data collected while monitoring CO2 enhanced oil recovery and concomitant salt water disposal (SWD) in the Aneth oil field, Utah. Our method is implemented in a relative mapping technique where a subset of high signal-to-noise ratio (SNR) events are more accurately mapped by including the reflected phases. These high-SNR events are treated as master events. We tie the lower SNR events to the master events by waveform correlation to obtain consistent arrival-time picks and then precisely locate the weaker events relative to the master subset. The method has practical applications in areas where one or more strong seismic reflectors are present above or below the target reservoir, a condition commonly found in shale-gas reservoirs.