Paul Hatchell

Rocks under strain: Strain-induced time-lapse time shifts are observed for depleting reservoirs

Pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir. Seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields. The cumulative change in two-way traveltimes with depth is referred to as time-lapse time shift.

Monitoring primary depletion reservoirs using amplitudes and time shifts from high-repeat seismic surveys

In the high-porosity, poorly consolidated turbidites of the deepwater Gulf of Mexico, production-induced compaction is the main production-drive mechanism when aquifer support is weak and prior to pressure support by secondary recovery water injection. Time-lapse (4D) seismic monitoring of this class of reservoirs has provided several new learning opportunities. The time-lapse amplitude response of these fields can be complicated due to saturation changes (water replacing oil) inside the reservoir, rock compaction causing density and velocity changes inside the reservoir, stress relief and associated deformation of the rock outside the reservoir, and changes in reservoir fluid pressures due to pore-pressure decrease. Methods that rely on time-lapse amplitude changes with offset to discriminate pressure and saturation changes can help separate and thus simplify the interpretation of some of these effects (Tura and Lumley, 1999; Landro, 2001).

Whole Earth 4D: Reservoir Monitoring Geomechanics.

Time-lapse (4D) seismic monitoring of pressure-induced changes in depleting gas fields reveals that detectable differences in seismic arrival times are observed above the reservoir interval. Geomechanical models of depleting reservoirs predict that as a result of reservoir compaction due to pressure depletion, changes in the long-wavelength stress and strain fields occur in the rocks bounding the reservoir. Models incorporating the geomechanical stress and strain field changes predict changes in the two-way arrival times that are compared with actual time-shift observations at a depleting gas field in the North Sea. The geomechanical-based predictions are in good agreement with the observations. Detecting geomechanical changes in the overand underburden rocks opens up new ways of using 4D data, especially in places where the signal from the reservoir rocks is small.

Fault whispers: Transmission distortions on prestack seismic reflection data

Transmission distortions are observed on prestack seismic data at two locations in the Gulf of Mexico. These distortions produce anomalous amplitude versus offset (AVO) signatures. The locations of the distortion zones are determined using acquisition geometry and ray tracing. No obvious reflection events, such as shallow gas zones, are observed at the predicted locations of the distortion zones. Instead, the distortion zones correlate with buried faults and unconformities. It is postulated that the distortions are produced by velocity changes across buried faults and unconformities. The distortions result from an interference pattern resulting from seismic waves arriving from different sides of the faults. A simple model is developed to explain many of the characteristics of the distortion pattern.

Apparent horizontal displacements in time‐lapse seismic images

In addition to vertical time shifts commonly observed in time-lapse seismic images, horizontal displacements are apparent as well. These apparent horizontal displacements may be small relatively to seismic wavelengths, perhaps only 5 m at depths of 5 km, but they consistently suggest an outward lateral expansion of images away from a compacting reservoir. It is well known that apparent vertical displacements are caused mostly by a decrease in seismic wave velocities above compacting reservoirs. Those same velocity changes contribute to horizontal displacements. This contribution can be computed from the velocity changes that, in turn, can be estimated from measured vertical displacements. Horizontal displacements computed in this way are similar to those measured, and this similarity suggests that horizontal as well as vertical displacements may be largely due to velocity changes.

Inversion of 4-C borehole flexural waves to determine anisotropy in a fractured carbonate reservoir

Observations of anisotropy in fractured carbonate reservoirs can yield important information such as the orientation of aligned crack systems and crack density. A new generation of borehole acoustic tools provides the data necessary to observe anisotropy around the borehole. The flexural waves, excited by two dipole sources operating perpendicularly to each other in these tools, are recorded in two perpendicular directions, resulting in four-component flexural wave recordings. Superposition of sources and rotation of receivers are the basic elements for the inversion of the waveforms to determine fast or slow mode orientations. Since the two sources are not necessarily of the same strength, a reciprocity-based source equalization is carried out as part of the inversion procedure. We have inverted four-component borehole flexural waves, recorded in a vertical well, penetrating three lithologic units of a fractured carbonate reservoir in northern Oman. For the lower unit, a northeast-southwest azimuth of flexural wave anisotropy is well resolved, which is parallel to the strike of open fractures observed in the borehole, preferential flow directions, and to the regional present-day in-situ maximum horizontal stress. Furthermore, the northeast-southwest direction corresponds with polarization directions of the fast shear wave observed in a multicomponent surface seismic survey over the reservoir. The middle unit is most likely isotropic, except for a depth interval of low gamma readings, where a northeast-southwest orientation is found for the fast mode. For the upper unit, a north-south fast mode orientation is found, which does not correspond to the expected northeast-southwest direction. Borehole geometry data indicate that the shape of the hole is circular for the middle unit but elongated for the upper and lower unit. Caliper orientations track the flexural wave anisotropy azimuths in these two units. Because of the elongation of the borehole, the observed flexural mode orientations, as well as the magnitude of flexural wave anisotropy, cannot be related to formation anisotropy directly, which obscures the determination of fracture orientation and fracture density from four-component borehole flexural waves.

Ocean Bottom Seismic (OBS) timing drift correction using passive seismic data

Summary The ocean contains low-frequency passive acoustic waves that are readily detected on seismic recording devices. One source of this ambient noise is from wind and wave action at the sea surface that creates passive sound waves that propagate in all directions. Passive energy recorded during an ocean bottom seismic (OBS) survey in 1000 m of water is analyzed using seismic interferometry and a lowfrequency dispersive arrival is observed that is interpreted using the normal mode theory of water waves. The interferometry arrivals allow the detection of clock drifts on individual sensors by analyzing the time asymmetry between the causal and acausal arrivals. The clock drifts are found to increase with time in a manner consistent with other observations. A second method for clock drift detection is developed that takes advantage of the long wavelengths in the passive energy that are typically many times the separation distance between OBS sensors. A synthetic co-located node is created at the location of an existing sensor by interpolation of the data from neighboring sensors. Comparing the arrival times on the synthetic and real data allows for the timing drift error identification. This technique produces results consistent with the interferometry observations. Both of these techniques are useful for improving the timing fidelity of OBS data. In existing OBS node designs, up to 50% of the battery energy is used to power the ovenheated clocks that maintain a stable known temperature. The new methods open up the possibility of significantly extending node life by using less energy-intensive clocks.

Time‐lapse seismic makes a significant business impact at Holstein

acquisition over the Holstein field (undershoot polygon in green). NRMS is estimated in a 1s window centered at 2.5s. An average NRMS value of 0.23 on final migrated data and an average shot+receiver repeat error of 75m is achieved. Time-lapse seismic makes a significant business impact at Holstein Hesham Ebaid*, Mosab Nasser, Paul Hatchell, Shell Exploration and Production Co, Darrell Stanley, BP America Inc.

First dual-vessel high-repeat GoM 4D survey shows development options at Holstein Field

In the Gulf of Mexico (GoM), loop and eddy currents can cause large errors in 4D shot and receiver locations between baseline and repeat streamer surveys, which invariably lead to poor data quality. In a recent 4D acquisition, a dual-vessel 3D acquisition method addressed the repeatability problem and showed reliable time-lapse measurements over Holstein Field. The time-lapse seismic data show time shifts up to 6 ms over depleting sands and amplitude changes over swept and compacted sands. This 4D information has improved understanding of the field and can support optimal placement of injection and production wells.

Quantitative Comparison Between a Dipole Log And VSP In Anisotropic Rocks From Cymric Oil Field, California

Multicomponent VSP and seismic measurements are traditionally used to measure shear-wave birefringence in anisotropic rocks. Crossed-dipole shear logs measure flexural mode birefringence which, at low frequencies, should be identical to shear-wave birefringence in the absence of borehole related phenomena such as alteration, stress-relief, ellipticity, etc. A quantitative comparison between a multicomponent VSP and crossed-dipole logs in rocks previously reported to exhibit large shear-wave birefringence and depth-variable fast shear-wave polarization azimuths is made. It is found that the crossed-dipole measurements agree quantitatively with the VSP measurements with respect to the fast and slow shear-wave transit times and the fast shear-wave polarization direction. The crossed-dipole measurements reveal significant vertical variability of the anisotropy parameters.