A shear experiment over the Natih field in Oman: pilot seismic and borehole data

Hake, J.H.; Gevers, E.C.A.; Kolk, C.M. Van Der; Tichelaar, Bart W.

doi:10.1046/j.1365-2478.1998.00111.x

Cited by 14 publications

(17 citation statements)

References 6 publications

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“…(1989 ) for a smaller survey acquired over a comparable fractured carbonate field in Wyoming. It should also be pointed out that our anisotropy measurements are in agreement with the complementary VSP measurements reported by Hake et al . (1998 ).…”

Section: Discussionsupporting

confidence: 93%

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The 3D shear experiment over the Natih field in Oman. Reservoir geology, data acquisition and anisotropy analysis

Potters¹,

Groenendaal

Oates

et al. 1999

Geophysical Prospecting

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This paper describes a large‐scale reservoir characterization experiment carried out in Oman in 1991 which comprised the acquisition, processing and interpretation of a 28.4 km2 3D multicomponent seismic experiment over the Natih field. The objective of the survey was to obtain information on the fracture network present in the Natih carbonates from shear‐wave anisotropy. Shear‐wave anisotropy in excess of 20% time splitting was encountered over a large part of the survey. The seismic results are confirmed by geological and well data but provide additional qualitative information on fracturing where this was not available before. Regions of stronger and weaker shear‐wave anisotropy appear to be fault‐bounded. The average well flow rates (which are fracture‐dominated) within such blocks correlate with the average anisotropy of the blocks. The further observation that the anisotropy is largest in the fracture gas cap of the reservoir suggests that shear waves can provide a direct hydrocarbon indicator for fractured rock.

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Section: Discussionsupporting

confidence: 93%

“…Initially a pilot experiment was carried out to investigate acquisition parameters ( Hake et al . 1998 ).…”

Section: Screening Of Candidate Fieldsmentioning

confidence: 99%

The 3D shear experiment over the Natih field in Oman. Reservoir geology, data acquisition and anisotropy analysis

Potters¹,

Groenendaal

Oates

et al. 1999

Geophysical Prospecting

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“…The static corrections applied to the data also reveal no such pattern and are of much smaller magnitude than the observed time anomaly. The low shear‐wave velocity zone is not seen in the stacking velocities, but the Fiqa shales exhibit strong transverse isotropy (Hake et al . 1998) and stacking velocities are more sensitive to the horizontal than the vertical velocity (both δ and ε are large).…”

Section: Quantitative Interpretation Of the Fiqa Shear Anomalymentioning

confidence: 99%

The 3D shear experiment over the Natih field in Oman: the effect of fracture‐filling fluids on shear propagation

Kolk

Guest

Potters

2001

Geophysical Prospecting

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This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three‐component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear‐wave velocities. One is that the shear‐wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear‐wave velocity which senses both the fractures and the fracture‐filling fluid. Using a new effective‐medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear‐wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear‐wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P‐wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective‐medium modelling and the geology. With this new effective‐medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional‐wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S‐wave data, as well as the conventional P‐wave data, as a direct hydrocarbon indicator.

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“…As a support for the above picture for S-waves, Hake et al 7 describe an example where an S-wave splitting anomaly was associated with the gas cap for a reservoir being explored in Oman by Shell. Thus, contrary to the conventional wisdom, S-waves can be sensitive to the presence of fluids within fractured reservoirs.…”

Section: Asymmetric Dry Rock: Tight Matrix Example -Introducing S-wavmentioning

confidence: 90%

Seismic Determination of Saturation in Fractured Reservoirs

Brown¹,

Wiggins²,

Gupta³

2001

Proceedings of SPE Production and Operations Symposium

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe determination of reservoir saturation using the seismic response is an important element of predicting the performance of fractured reservoirs. One of the primary saturation problems the seismic signal can address is the location of the gas cap with respect to the water or oil leg. The classical Gassmann (1951) approach to predicting the effects of saturation upon the seismic signal is not valid for fractured reservoirs. Instead, the results of Brown and Korringa (1975) that account for the anisotropy of the dry rock are used to predict the effects of saturation upon the seismic signals. For a single set of vertical, aligned fractures the Pwaves are most sensitive to the saturation when they are nearly perpendicular to the fractures. For S-waves, a classical result is obtained: the S-waves are not sensitive to the saturation within the vertically aligned fractures. However, when the elastic symmetry of the dry rock differs from the case of vertical fractures, the S-waves become sensitive to the saturation. This can happen when either the fractures are tilted slightly away from the vertical or when the fractures are best described as rough surfaces. In these cases, S-waves are most sensitive to the saturation when propagating parallel to the cracks with a polarity perpendicular to the cracks. These results indicate that an S-wave "bright spot" or region of increased S-wave splitting can be used to predict gas versus brine or oil-saturated portions of the reservoir. Since P-waves have the maximum sensitivity to the saturation of a fractured reservoir when they propagate perpendicular to the fractures, they require long-offset surface seismic methods that may be difficult to implement in practice. However, since vertically propagating S-waves with different polarizations can be used to study the effects of the saturation, they offer the most potential for mapping saturation within fractured reservoirs.

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A shear experiment over the Natih field in Oman: pilot seismic and borehole data

Cited by 14 publications

References 6 publications

The 3D shear experiment over the Natih field in Oman. Reservoir geology, data acquisition and anisotropy analysis

The 3D shear experiment over the Natih field in Oman. Reservoir geology, data acquisition and anisotropy analysis

The 3D shear experiment over the Natih field in Oman: the effect of fracture‐filling fluids on shear propagation

Seismic Determination of Saturation in Fractured Reservoirs

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