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.
S U M M A R YThe downhole orbital vibrator (DOV) acts as a rotating acoustic point force coupled to the borehole fluid. Operated as a Vibroseis source modulated over rotational frequencies 50-350 Hz, the DOV can generate crosswell seismic signals at well separations up to 800 m in oilfield sediments. Source-wavelet stability and crosswell seismic traveltime resolution are estimated from an ensemble of 13 000 wavelets recorded at a crosswell seismic facility with 300 m source-sensor separation. DOV auto-correlation source wavelets S i cross-correlated against the ensemble mean wavelet S mn give correlation coefficients γ i = S i S mn having mean γ mn ≈ 99.97 per cent and standard deviation γ rms ≈ 0.03 per cent. S wavelets give observed traveltimes τ i with normalized standard deviation τ rms /τ mn ≈ 0.03 per cent. This level of seismic monitoring source stability and crosswell traveltime resolution offers considerable promise for accurate, cost-effective time-lapse seismic imaging of active geofluid reservoirs.Application of stable DOV waveform production to time-lapse seismic imaging is, however, affected by the dual-wavelet nature of rotary motion cross-correlations. In general, DOV cross-correlation signals mix time-symmetric (even) wavelets χ cc (t) ∼ = cos(ω(t)t) cos(ω(t)t) ∼ = χ cc (−t) and time-antisymmetric (odd) wavelets χ cs (t) ∼ = cos(ω(t)t) sin(ω(t)t) ∼ = − χ cs (−t), where denotes correlation, ω(t) describes the modulation of rotational frequency, and time-reversal t ↔ −t equates to interchanging clockwise/counter-clockwise (cw/ccw) DOV rotations. Cross-correlating sensor motion along source-sensor axis x with source motion along axis g mixes wavelet symmetries as χ g (t) ∼ = cos φχ cc (t) + sin φχ cs (t) where cos φ = g · x and angle φ orients the DOV relative to the source-sensor axis.DOV orientation uncertainty φ affects the sensor wavelet apparent traveltime as τ ≈ 1.3( φ/2π)T min , T min the period of peak modulation frequency. Since source orientation uncertainty can generate apparent traveltime uncertainty as large as 1-2 ms, time-lapse seismics cannot effectively ignore DOV orientation. Traveltime resolution can be improved to order 0.1 ms without knowing DOV orientation if traveltimes are computed using even/odd wavelets composed by summing/differencing cw/ccw wavelets. Prospects for time-lapse resolution improve if observers can orient the DOV. A DOV equipped with a collar of rotation-phase point detectors surrounding the rotating mass permits observer selection of the monitor sensor, hence controlling effective source orientation. DOV orientation can be guided by an on-board biaxial tiltmeter (for borehole tilt δ along an axis making angle φ with DOV sensor orientation, biaxial tilts T 1 and T 2 fix φ as T 1 = −cos φtan δ and T 2 = sin φtan δ).Numerical simulation of full-sensitivity time-lapse reservoir monitoring suggests it is feasible to resolve migrating oil/water substitution volumes of characteristic dimension 20 m with crosswell transmission data over ≈600-800 m offsets, or in bac...
The ability to estimate the stimulated reservoir volume and to enhance the effectiveness of hydraulic fracturing operation is fundamental in field development planning and critical to optimizing well completion in unconventional gas exploration and production. Microseismic Monitoring (MSM) survey provides essential observations study how induced rock volume responds seismically under field hydraulic fracture stimulation. Recent advanced microseismic technology with further development can provide real-time monitoring and interpretation which will improve hydraulic fracturing efficiency and better EUR, stop induced fracture development toward fault zones, and avoid connecting water-bearing layers. Advanced processing can enhance weaker microseismic signal, which provides a more complete Microseismically-Stimulated Reservoir Volume (M-SRV). M-SRV is essential to build the 3D understanding of the induced fracture network under various stimulation designs in volumes and in patterns from each fracturing stage.
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