Stress analysis of Faults by 3D Finite Element Modelling David A. Spencer, Eivind-Swensson Aarseth, Lars Grande, Hugo Harstad, Einar Sverdrup, Jon Vold Saga Petroleum ASA, Norway Rolf Bratli Applied Rock Mechanics LLC, United Arab Emirates Trond Svartvatn, Mohsen Pourjavad FEM Engineering AS, Norway

A key to understanding hydrocarbon reservoir fault geometry is the 3-D detection of sub-seismic faults

With knowledge of 3-D sub-seismic faults, we have: Constraints on ‘unknown’ geometry Understand better the flow effects within a Simulation Grid Detection of ‘Damage Zones’ However, analysis must be done on the major faults with the same stress orientations as the tectonic event that formed the faults

However……. Areas that are close to frictional failure in a stress field are most likely to produce faults It is not simply the orientation of faults that is important. The understanding of the stress magnitude and the use of the Mohr circle are (today) the best method of analysing fracture networks.

Conversion of an IRAP RMS simulation

Geometry file model containing inclined fault blocks converted from Eclipse via GeoTool Frame is removed Different colors can have different geo-mechanical properties

Project Purposes Objectives: Develop a Finite Element Model (Stress Map) grid through conversion by GeoTools (developed by FEM Engineering) Identify and predict the areas of most likely sub-seismic faults and more accurately model faults in the reservoir model

Project Area East-Central Fault Block of Snorre Study area (IRAP RMS Grid) 2 km

Imported Geometry Model 2 km

However…….. How do you know that your grid has no ‘inherent’ properties that will provide misleading results?

Stratigraphic layers with their geo-mechanical Properties Note: Faults can not be modeled from IRAP RMS Format

Boundary conditions Overburden and horizontal principal stresses are added as compressive pressures on to the top surface (BCU) and to the sides of the model The bottom surface, in this analysis, has no movement in any directions. The bottom surface is fixed in the following orientations: X (120 / 330), Y (030 / 210) and Z (up / down) A constant effective stress corresponding to a 500 meter depth is used in this model

Result presentation Stress difference magnitude (1 - 3) Scale of magnitudes for all layers except LL

Faults Section D 2 km

Result, Lunde 08 Section D 2 km Lunde 08 is Pink color Area of Enlargement

Result, Lunde 08 Section D Minimum Horizontal Stress = Red line

Result, Middle Lunde, Section D 2 km Middle Lunde is Grey color

Imported Geometry Model Can you see the boundary problem? 2 km

Cubic test-model White layer is used on test-results

Inclined test-model White layer is used on test-results

Calculation of fault directions in the histograms

Histogram, Lunde 08 Section D

Possible way to evaluate the Mohr-Coulomb criterion

Be very careful about boundary effects Summary Be very careful about boundary effects Start with a very small model Once the data has been loaded, it is difficult to tell what is ‘real’ and what is ‘artifact’ All data should be quality controlled first by removing all properties that could ‘hide’ boundary condition effects Stress direction and magnitude is not enough - You need to know which ones are critically stressed (use the Mohr circle) IRAP RMS grids are not appropriate to use for Finite Element Modeling if you want to model fault properties (use Eclipse) But Eclipse grids are not appropriate input to Havana because of their coarseness.

Ultimate Goals Reservoir Production optimization Understand injector/producer conformance Improve understanding of Reservoir characterisation by Faults Predict by-passed oil Well Placement Improve with knowledge of faults and stress