4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved1 Reservoir Connectivity and Fluid Uncertainty Analysis using Fast Geostatistical.

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved1 Reservoir Connectivity and Fluid Uncertainty Analysis using Fast Geostatistical Seismic Inversion Ashley Francis and Graham Hicks* Earthworks Environment & Resources Ltd *BG Group plc

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved2 Relative, Deterministic and Stochastic Inversion Seismic zero phasing and amplitude spectrum shaping Wavelet removal by assuming blocky, layered earth Introduce low frequency model to give absolute impedance estimates Perturb low and high frequency outside wavelet bandwidth to investigate uncertainty – Inversion is unique within seismic bandwidth – Constrained by wells and spatial model outside seismic bandwidth Relative Impedance, Coloured Inversion Deterministic Inversion Stochastic Inversion

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved3 Limitations of Deterministic Inversion Deterministic inversion works well in thick layered systems – Reservoir interval has strong reflectivity – Reservoir layering is relatively thick with well defined units close to the seismic resolution limit – Blocky layering with little vertical variation in reservoir properties so layer average impedance is useful Deterministic inversion can work in thinner intervals where tuning occurs if – Well control is good – Conformal layering with gradual lateral thickness changes

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved4 Deterministic Inversion Strong Reflectivity 9300 10600 Absolute Impedance

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved5 Deterministic Inversion Porosity Mapping 0.20 0.05 0.25 0.05 Porosity - Jurassic Porosity – Rotliegendes/Devonian

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved6 Seismic Property Constraints & Reservoir Models The use of deterministic seismic inversion as an input to reservoir modelling is becoming common Deterministic inversion is unsuitable for this purpose for several reasons – Seismic resolution is at a much coarser scale than the cell size of reservoir models – Deterministic seismic inversion contains a low frequency model derived generally from wells – Reservoir volume may be under or over-estimated, especially for thin intervals – Deterministic inversion connectivity is exaggerated due to the effective smoothing (resolution limitation)

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved7 Scale of Measurements Core Plug to Seismic Zone C Zone B Zone A 20 m 100 m 2 m (x70)

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved8 Reservoir Model Support Reservoir model Scale-up of well logs is generally vertical correction only – Incomplete correction resulting in too high variance – Varying cell thickness or deviated geometry will give arbitrary change of support correction across model Additional correction for horizontal scale-up – Variogram based support correction by fine-scale simulation – Not generally supported by reservoir model packages Seismic vertical support 10 – 20 cells thick

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved9 Reservoir Model Upscaling 200 x 200 x 2 m Cell Size

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved10 Deterministic Inversion 4ms Block size (λ/8) @ 60Hz

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved11 Deterministic Inversion The impedance information at low frequencies is simply an interpolation of the well data and so deterministic inversions should not be used to condition reservoir models Wells Frequency Seismic

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved12 Deterministic Inversion Constraining Porosity SGS Porosity Collocated Porosity Seismic Impedance Seismic property has similar values vertically over zone of reservoir model Collocated co-simulation follows low frequency trends of seismic impedance Low frequency trends are from wells not seismic If trends were valid, constraint should be to vertical average, not cell by cell

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved13 Reservoir Model Conditioning with Seismic Data Conditioning a reservoir model to a deterministic inversion is largely equivalent to conditioning to a map of the wells If model cell thickness seismic resolution (eg λ/4) then we can condition a reservoir model to – seismic attributes – relative or coloured impedances – deterministic inversion filtered to remove low frequencies This makes the assumption of first order stationarity of the mean ie no lateral trends

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved14 Reservoir Modelling with Stochastic Inversion Condition model to realisations of impedance generated through stochastic seismic inversion – Incorporates geophysical uncertainty in the reservoir model – Sample rate closer to required cell thickness Stochastic inversion is – Spatially constrained, typically by a variogram – Mean of 100+ realisations = deterministic inversion – Each realisation honours the well data – Forward convolution of each realisation matches seismic

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved15 Stochastic Inversion and Resolution Common misconception related to resolution – It…allows substantially increased resolution, capturing details well beyond seismic bandwidth. Stochastic seismic inversion can be run at any required output sample rate This does not imply a higher resolution – resolution is controlled by the (limited) frequency content and bandwidth of the seismic conditioning data Stochastic seismic inversion simulates the broad band impedance and so properly represents the uncertainty in the seismic inversion

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved16 When Should You Use Stochastic Inversion? D > /2 D <= /4 Continuous Change Deterministic Inversion Deterministic Inversion Stochastic Inversion Stochastic Inversion Continuous and Conformal layers

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved17 Stochastic Inversion Principal of producing non-unique realisations of impedance rather than an average impedance Condition impedance realisations to seismic trace & wells – SGS plus accept/reject (Haas & Dubrule, 1994) – Other MCMC methods (eg Moyen et al, 2007) – Good reliable methods but relatively slow To be of any use in calculating probabilities and statistics we need to generate many realisations - at least 100+ – Computational runtimes long so fast methods required MPSI UltraFast FFT direct method used here

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved18 Performance Benchmark Underlying Technology Moyen & Doyen (2008) IP/IS stochastic inversion – 35 Million cells x 25 Realisations – 70 Hours on dual CPU 64-bit machine MPSI 32-bit pre-stack batch technology – Approximately 2 hours for same problem & hardware MPSI recent service project (IP/IS pre-stack) – 240 Million cells x 100 Realisations – 27 hours on quad CPU PC – Moyen & Doyen estimated time = 40 days MPSI 3-term simultaneous pre-stack stochastic inversion now available (IP/IS/Rho)

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved19 Deterministic Inversion

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved20 Stochastic Inversion Realisation 1

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved23 Deterministic Inversion Thick Sand

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved24 Deterministic Inversion Thick Sand Geobody

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved25 Stochastic Inversion Thick Sand Geobody 01

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved28 Thick Sand Volume Deterministic Sand Volume For a thick sand the deterministic volume estimate plots close to the mean volume from the realisations For a thick sand the deterministic volume estimate plots close to the mean volume from the realisations

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved29 Deterministic Inversion Thin Sand

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved30 Deterministic Inversion Thin Sand Geobody

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved31 Stochastic Inversion Thin Sand Geobody 08

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved34 Thin Sand Volume Deterministic Sand Volume For a thin sand the deterministic volume estimate is biased (±) For a thin sand the deterministic volume estimate is biased (±)

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved35 Deterministic Inversion Well08 Connected Net Pay Well08

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved36 Stochastic Inversion Multiple Realisations Multiple realisations from stochastic seismic inversion can be used for – Volumetric uncertainty – Connectivity Analysis – Individual realisations reservoir model conditioning – Net Pay estimation: P90 / P50 / P10

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved43 Probability and Connectivity Probability maps by themselves are not always useful as they do not indicate possible connectivity Geobody (connectivity) calculations followed by net pay or probability calculation much more useful – Spatial geometry of pay analysed – Volumetric uncertainty accounted for – Swept/contacted volume uncertainty – Evaluation of risk in well planning

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved49 Summary 1 Significant scale changes are not properly accounted for in reservoir modelling software Deterministic inversion may be useful when: – Reservoir interval has strong reflectivity – Lots of wells and good seismic horizon constraints – Reservoir layering is relatively thick with well defined units close to the seismic resolution limit – Minimal lateral or vertical trends

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved50 Summary 2 Deterministic seismic inversion data is not suitable for conditioning reservoir models – Low frequency model is an artefact of well gridding – Scale change between seismic and model cells – Not suitable for connectivity / volumetric computations Stochastic inversion benefits – What if Scenario analysis eg well track risks – Geophysical uncertainty input to reservoir modelling – Scale change better handled in reservoir models – Connectivity and volumes more reliable Further discussion available at Booth 137

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved52 Example Text Boxes Green is RGB 84,112,79 Orange is RGB 243,91,27 Grey is RGB 200,211,211 Blue line is RGB 0,0,255 Arrows are 5 pt, no shadow Example Text Box Shadow Style 6 Heavy Grey Shadow 1.25 pt border Example Text Box Shadow Style 6 Heavy Grey Shadow 1.25 pt border Alternate Text Box Shadow Style 6 Heavy Black Shadow 3 pt border Alternate Text Box Shadow Style 6 Heavy Black Shadow 3 pt border Alternate Label for diagram No shadow 3 pt border

4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved1 Reservoir Connectivity and Fluid Uncertainty Analysis using Fast Geostatistical.

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4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved1 Reservoir Connectivity and Fluid Uncertainty Analysis using Fast Geostatistical.

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Presentation on theme: "4 June 2014© Earthworks Environment & Resources Ltd. All rights reserved1 Reservoir Connectivity and Fluid Uncertainty Analysis using Fast Geostatistical."— Presentation transcript:

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