Physical model for the downhole orbital vibrator (DOV) - I. Acoustic and borehole seismic radiation

Abstract

S U M M A R YThe downhole orbital vibrator (DOV) applies the Vibroseis principle to borehole seismic sourcing. Accelerations by an internal rotating eccentric mass excite cylindrical pressure waves converted at the wellbore to seismic waves essentially within 20 • -30 • of the plane normal to the borehole. DOV pressure waves in open water are quantitatively described by a rotating point force radiating acoustic waves with displacement amplitude u(r ) ≈ 1/2 u 0 γ R 1 /r , where R 1 = π 2 ρ dov /ρ water a 2 /λ 2 ≈ 1 mm is the DOV effective size for DOV radius a ≈ 5 cm, length ≈ 1 m and acoustic radiation wavelength λ ≈ 10 m, and u 0 ≈ 1 µm and γ x are, respectively, the frequency-independent DOV displacement amplitude and the direction cosine of the observer relative to the instantaneous point-force axis in the plane of the rotating point force. Crosswell seismic radiation amplitude, spectrum and angular dependence are quantitatively described by acoustic wave diffraction at a slit with DOV axial cross-section 2a , followed by conversion to seismic waves at the wellbore. Seismic wave displacement amplitudes u(r ) ≈u 0 γ R 2 /r scale with effective radius R 2 ≈ 2a /λ ≈ 1 cm. The frequency dependence of R 2 is observed as linear frequency enhancement of the seismic wavelet spectrum relative to the source wavelet spectrum. DOV borehole P-and S-wave production peaks strongly in the plane of DOV rotation, with converted S waves both parallel to and transverse to the borehole axis. The small effective source sizes R 1 ≈ 1 mm and R 2 ≈ 1 cm at operational frequencies 50-350 Hz imply that DOV motion in a borehole is dynamically decoupled from the borehole wall. Dynamic decoupling allows DOV borehole seismic correlation wavelets to be quantitatively modelled in terms of a stable kinematic relation between source and sensor motion. Acoustic and seismic data rule out dipole-source action associated with claims for shear traction and primary S-wave radiation from boreholes. The stable linear kinetics of DOV acoustic action in borehole fluids produces (i) useful crosswell seismic signals at offsets to 650-800 m, (ii) negligible tube waves and (iii) stable seismic wavelets suited to in situ time-lapse seismic imaging of fluid migration fronts in crustal reservoirs.

A Second Opinion on "Operational Earthquake Forecasting: Some Thoughts on Why and How," by Thomas H. Jordan and Lucile M. Jones

“…The prototype SMS in northern Iceland recorded SWS propagating horizontally at 500 m depth between two boreholes offset 350 m. The shear-wave source, the downhole orbital vibrator (DOV) (Leary and Walter 2005) was pulsed 40,000 times, two to four times each minute in a two weeks' experiment, without changing the waveform or damaging the borehole wall (Crampin et al 2003). The source-to-receiver geometry was non-optimal, but the recordings showed exceptional sensitivity to seismic-induced changes of stress associated with low-level seismicity at a distance of 70 km on a neighboring transform fault with equivalent energy to one M = ~3.5 earthquake (Crampin et al 2003).…”

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

“…The downhole orbital vibrator (DOV) achieves low‐impact, high‐stability borehole seismic sourcing by adapting the Vibroseis principle (Waters 1981) to rotary motion. In a companion paper, Leary & Walter (2005) show that DOV action in a borehole is consistent with the dynamics of a rotating point force acting on the borehole fluid. The mechanical equivalent size of the DOV can be expressed in terms of its physical radius a ≈ 5 cm and length ℓ≈ 1 m, and the radiation wavelength ≈10 m. DOV pressure fields in open water are consistent with a dynamic source with effective radius R 1 =π 2 ρ dov /ρ a 2 ℓ/λ 2 ≈ 1 mm.…”

“…Instantaneous motion () can be directly observed in open water for sensors close to the DOV (Leary & Walter 2005). In seismic media, however, instantaneous DOV sourced motion described by is harder to observe.…”

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

“…The prospects of time lapse seismic is conditioned by source wavelet stability and by signal strength. Extensive monitoring observations with the downhole orbital vibrator seismic source used to acquire the Liaohe crosswell seismic data yields an estimate of the time lapse traveltime source stability equivalent to traveltime sensitivity of 50–100 μs (Leary & Walter 2005a,b). Waveguide energy travelling between a source and sensor over distance d comprising length d 1 of water saturated rock with effective wave speed v and length d 2 of oil saturated rock with effective wave speed v +Δ v has travel time t = d 1 / v + d 2 /( v +Δ v ).…”

“…In response to the surface seismic‐imaging deficiency at the Liaohe oil field, Geospace Engineering Resources International (GERI) was contracted to conduct a series of exploratory crosswell surveys in existing production wells at offsets between 600 and 850 m. Higher‐frequency and lower‐power (300 versus 30 Hz, 500 W versus 500 KW) seismic‐wave generation made possible by downhole reservoir‐proximate sourcing and sensing (Leary et al 2003; Walter et al 2003; Leary & Walter 2005a,b) offer an option to surface seismic imaging if far‐offset crosswell seismic signals can be detected and accurately interpreted in terms of reservoir‐scale heterogeneity structures lying below the resolution of surface seismic imaging.…”