Process oriented modelling of heterolithic tidal reservoirs Example from Heidrun well

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1 Process oriented modelling of heterolithic tidal reservoirs Example from Heidrun well Why, What and How and some results Kjetil Nordahl, Philip Ringrose & Carsten Elfenbein Statoil Research and Technology FORCE Upscaling Workshop March, NPD, Stavanger

2 Outline Why do we have to model small-scale heterogeneities What is process-oriented modelling How did we apply the modelling method to a well interval Some results Representative volumes Biased sampling Estimation of vertical permeability in the well interval Summary

3 Description of the selected interval ~ 25 m of Lower Tilje Formation, Heidrun Field, Haltenbanken (T 1.1 T 2.1). From prodelta heterolithics to inshore estuarine bars. Many scales of heterogeneity. From Yoshida et al. (2002)

4 Why these reservoir types are difficult to characterize 1 Petrophysical characterization of heterolithic deposits is challenging due to cm-scale variability. Variation in each lithofacies Distal on the Delta same order Lobe as between lithofacies. Fairly well sorted sand Mud Depth (m) XX 2595 XX 2600 XX 2605 XX 2610 XX 2615 XX ,001 0,01 Permeability (md) 0,1 1 Wireline Permeability estimate Porosity estimate ,0 0,1 0,2 0,3 0,4 0,5 Porosity Core Plug Porosity Horizontal permeability Vertical permeability Inshore estuarine bar Delta front Laminated delta front lobe Prodelta LST HST SB

5 Why these reservoir types are difficult to characterize 2 V 1 < V 2 < V 3 σ 1 > σ 2 > σ 3 Probe Core plug Wireline log Sampling bias and scales of heterogeneity ~ 1 m What is the representative value for these zones (e.g. k v /k h )? Integration of core and wireline data (scale transition issues)

6 What is a Process-oriented geomodelling tool -1 Sedimentary bedding models generated by a series of surfaces: t S x, y = Asin( x) + Bsin( y) + C + ( ) e Migration Vector Preserved lamina after migration Multiple migration steps The SBED modelling tool generates a near wellbore, 3D geostatistical representation of smallscale sedimentary structures. 30 cm 30 cm Lower Tilje Formation ~ 10 cm

7 What is a Process-oriented geomodelling tool -2 The volumes between surfaces (i.e. a lamina) are populated with porosity and permeability drawn from a 2D Gaussian field. Mud Silt Sand Mud Silt Sand Frequency Geometry Realization Frequency Ln k (Permeability) Lamina scale Porosity Lamina scale Permeability Realization Porosity Realization Bedding scale Effective permeability (kx, ky, kz) and bulk porosity The near wellbore model has potential to reconcile conflicting well data.

8 The near wellbore (NWB) model Core Bedding models Stacked NWB model Sedimentology realization Permeability realization HCS ~ 1 m Wavy The 3D geostatistical near wellbore model can be evaluated at various scales to better reconcile conflicting well data (e.g. core plugs, wireline logs and well test) HCS Wavy HCS

9 How to create a 3-D near wellbore model from core data - geometry Geometry parameters Focus on sand/mud thickness statistics and spatial distribution (manually measured on core). Mean, StDev Periodic components 2.5 m cm See the possibility for image logs! 30 cm 30 cm

10 How to create a 3-D near wellbore model from core data - petrophysics Porosity and permeability anisotropy from core plugs through an iterative procedure (simulating the core plug process) See the possibility for probe data and NMR measurements

11 Some results: REV and implications for core-log integration Effective Permeability (md) Horizontal Permeability Vertical Permeability Mud Fraction Effective Vertical Permeability (md) REV Sample Volume (cm 3 ) Core plugs are generally inadequate for describing the petrophysical properties (mean and StDev) Quantification of a representative volume scale Functional relationship found between mud fraction and effective, representative k v and k h in tidally influenced systems. A Coefficient of Variation

12 Some results: Biased sampling and representative values Reduction in variance with increasing sample volume Biased core plug data set (k v /k h ratio) Representative estimate of the k v /k h ratio Horizontal permeability k v /k h ratio Permeability (md) Sample volume Sample volume

13 Some results: Forward modelling of vertical permeability 10 realizations Representative scale

14 Some results: Type curves Effective, representative permeability as a function of mud fraction For more details, see the next presentation by Ringrose et al.

15 Summary Small-scale sedimentary heterogeneities do make petrophysical characterization difficult in this reservoir type. A near wellbore model has been built of the lower Tilje Formation based on available core data Biased sampling and Representative volumes have been quantified The near wellbore model reconciles the core data and provides a better estimate of k v /k h ratio (at the scale of interest).