Crosswell seismic waveguide phenomenology of reservoir sands & shales at offsets >600 m, Liaohe Oil Field, NE China

Abstract

S U M M A R YCrosswell seismic data recorded at 620-650 m offsets in an oil-bearing sand/shale reservoir formation at the Liaohe Oil Field, northeast China, provide robust evidence for waveguide action by low-velocity reservoir layers. Crosswell-section velocity models derived from survey-well sonic logs and further constrained by observed waveguide seismic wavegroup amplitudes and phases yield plausible evidence for interwell reservoir-sand continuity and discontinuity. A pair of back-to-back Liaohe crosswell vector-seismic surveys were conducted using a source well between two sensor wells at 650 and 620 m offsets along a 200-m-thick reservoir formation dipping 7 • down-to-east between depths of 2.5 and 3 km. A downhole orbital vibrator generated seismic correlation wavelets with frequency range 50-350 Hz and signal/noise ratio up to 5:1 over local downhole ambient noise. The sensor wells were instrumented with a mobile 12-to 16-level string of clamped vector-motion sensor modules at 5 m intervals. Using 5 m source depth increments, crosswell Surveys 1 and 2 cover source/sensor well intervals above and through the reservoir of, respectively, 600 m/600 m (13 000 vector traces in 9 common sensor fans) and 300 m/560 m (7000 vector traces in 7 common sensor fans). Survey 1 common sensor gathers show clear, consistent high-amplitude 20 ms waveletgroup lags behind the first-arrival traveltime envelope. Such arrivals are diagnostic of seismic lowvelocity waveguides connecting the source and sensor wells. Observed Survey 1 retarded wavegroup depths tally with source and sensor depths in low-velocity layers identified in sonic well logs. Finite-difference acoustic model wavefields computed for waveguide acoustic layers constrained by well-log sonic velocity data match the observed waveguide traveltime and amplitude systematics. Model waveforms duplicate the observed m-scale and ms-scale sensitivity of waveguide spatio-temporal energy localization. Survey 2 crosswell data, in contrast, provide no comparable evidence for waveguide action despite a sensor-well sonic well log similar to that of Survey 1. Instead, acoustic wavefield modelling of Survey 2 data clearly favours an interpreted waveguide model with 10 • downdip interrupted by a 75-100 m throw downfault near the sensor well. The absence of clear waveguide arrivals is adequately explained by dispersal of waveguide energy at the fault discontinuity. Auxiliary well sonic velocity and lithologic logs confirm the model-implied 75-100 m of down-throw faulting near the sensor well.

Wave guiding in fractured layered media

“…Compressional waves guided by impedance contrasts have been used in cross-well studies in oil-bearing sand -shale reservoirs (Leary et al 2005), coal seams (Buchanan et al 1983) and sandstone-shale formations (Parra et al 2002) for distances greater than 600 m for a frequency range of 50-350 Hz. To apply these techniques to fractured carbonate reservoir characterization requires an understanding that fractures can produce wave guiding, and the behaviour of the guided modes depends on contributions from the mechanical properties of both the fracture and the matrix.…”

Physical model for the downhole orbital vibrator (DOV) - II. Rotary correlation wavelets and time-lapse seismics

“…Consider the case that oil/gas is produced from a reservoir formation intersected by both the source and sensor wells. As in the seismic waveguide reservoir structure in the Liaohe oilfield in China (Leary et al 2005), the formation may be either continuous or discontinuous between the source and sensor well. Crosswell traveltime monitoring can in principle decide between reservoir continuity and discontinuity; and, if the reservoir formation hydrocarbons are draining via a specific well, downhole monitoring can track the spatial progress of the fluid interface as water moves up‐dip within the formation.…”

“…More generally, time‐lapse crosswell seismic surveys at offsets 600–800 m conducted between multiple wells over a range of azimuths can be expected to accurately distinguish reservoir zones producing hydrocarbons from reservoir zones not producing hydrocarbons, and hence fix production models that typically have little or no constraint on in situ flow or well connectivity. Crosswell seismic traveltime monitoring capability is particularly apt for layered reservoirs supporting waveguide seismic wave transmission as observed in the Liaohe oilfield, China (Leary et al 2005).…”

“…Borehole seismic sourcing and sensing is an area of potentially rapid progress. Borehole‐based seismic imaging is capable of making use of the usually quiet, stable and reservoir‐proximate downhole environment to lower equipment costs and raise the spatial and temporal resolution (Leary 2002a; Leary et al 2005). Problems in groundwater aquifer structure relevant to nuclear waste isolation, carbon sequestration and contaminant transport can also be addressed by high‐resolution, high‐stability crosswell seismic time‐lapse imaging (Bear et al 1993; Haszeldine & Smythe 1996; Daley & Cox 2001; Daley et al 2003; Davis et al 2003).…”

Visco-acoustic data modeling using optimum layer approximation technique: greenwood oil field example, USA

“…Understanding the reservoir pore structures at this unique scale is critical to optimizing primary production from reservoirs, choice of injection fluid, and optimal hydrocarbon recovery. On one hand, surface seismic profiling and generalized velocity modelling are often insufficient to illuminate reservoir-scale flow-discontinuity structures in deep-seated compartmentalized reservoirs (Leary et al 2005). On the other hand, information from the core data and well logs has insufficient coverage to reflect the spatial attitude of large structures that could inhibit free flow of reservoir fluid.…”