首页|Anisotropy in shale and its impact on AVO modelling and prospect de-risk: a case study of a Central North Sea field
Anisotropy in shale and its impact on AVO modelling and prospect de-risk: a case study of a Central North Sea field
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NETL
NSTL
Taylor & Francis
The inherent anisotropy in shale is an attribute associated with its clay minerals platelets alignmentand layering and very common with transversely isotropic (TI) medium. This implies thatvelocity is fastest along the horizontal axis and slowest along the vertical axis, which could affectvelocity signature from sonic tools measurements in deviated wells. This may lead to wrongamplitude variation with offset (AVO) signature, inaccurate depth conversions, seismic-to-welltie difficulty etc. Nineteen (19) wells in Forties field andthirteen (13) in Huntington field wereinvestigated and interpreted for the presence of anisotropy effect within the caprock over thereservoir Forties sands of the Sele (shales) Formation. In order to analyse the Thomsen weakanisotropy parameters (ε, γ and δ) per field, wells with closer Sele depth range were considered.The well inclinations (θ), average compressional (α) and shear (β) velocities per well were utilisedfor forward and inverse modelling of Thomsen anisotropy equations. The estimated anisotropy(VTI) parameters and the P-wave (αo) and S-wave velocities (βo) at zero inclinations in Sele Formationfor the studied wells are: αo = 2448±15.9 m/s, β_o = 1003±34.8 m/s, ε = 0.15±0.02,γ = 0.33±0.10, δ = 0.030±0.015 in the Forties field Sele, while αo = 2775±23.6 m/s, βo =1306±47.9 m/s, ε = 0.22±0.02, γ = 0.43±0.14, δ = 0.022±0.018 in the Huntington field. AVOresponses were compared, assuming both isotropic and anisotropic P–P reflectivity models.Result shows an increase in AVO gradient for all anisotropic case in both fields. AVO signatureswere either completely or averagely masked in both fields using the isotropic assumptions. Thereis also an increase in AVO gradient for all anisotropic cases in both fields. Velocity decreases ofabout 1.2% and 3.5% were observed in the Sele Formation for the Forties and Huntington fieldsrespectively for anisotropy. The corrected two-way traveltime (TWT) should in principle allowfor a better seismic-to-well tie estimation. Similarly, using the faster deviated velocity (i.e. uncorrectedvelocity) in any depth conversion would result in under estimation of the actual depthacross the field.
Anisotropyshalesamplitude variation with offset (AVO)P-wave velocityvertical transverse isotropy (VTI)attributes
Mathew Osaretin Ogieva、Olawale Babatunde Olatinsu
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School of Earth and Environment, University of Leeds, Leeds, UK||Ikon Science Ltd, Surbiton, UK
Department of Physics, Faculty ofScience, University of Lagos, Lagos, Nigeria
NETL
NSTL
Taylor & Francis
