Faults and rock mechanics, Phase 2. Geological/geotechnical investigation of faulted limestone with low permeability
Knowledge of fracture occurrence and orientation in chalk is important for optimum field development planning and evaluation of well-bore stability.
The objective of this project is to combine geological description of faults and rock mechanical/geotechnical properties. The project comprises: 1) Improvement of methods to define models of faults in reservoirs. 2) Improvement of methods to predict rock mechanical properties in reservoir zones, faults taken in account. The project will draw up models for chalk reservoirs with low permeability where natural fractures has considerable influence on the recovery of oil and gas. This will make it possible to model fracturing so that natural or created fractures could be part of finite element calculations and reservoir simulations and contribute to an increased understanding of the size and direction of rock mechanical stresses in a reservoir. The coordinated effort including integration of geological and geotechnical results is interdisciplinary research which can contribute to secure an effective exploitation of the Danish oil resources. Close coodination with the oil industry will be assured by a project coordination group from one or more interested oil companies. The project is coupled to recovery and is placed within the areas of characteristics of reservoirs (modelling of fracturing and fracture characteristics) and reservoir models (methods for prediction of stress distributions)
The combined geological and geotechnical studies carried out under the EFP-96 and EFP-98 programmes have investigated the influence of fractures on rock mechanics properties for chalk. Data for quantifying the effect of natural fractures in chalk have been supplied by triaxial testing in normal scale and large scale on samples from three chalk types, namely from Valhall Tor and Tyra Maastrichtian and an outcrop locality at Hillerslev. From the latter locality special big cylindrical specimens were sampled for the large scale triaxial testing (500x500 mm) in order to get a true representation of the natural fracturing in the Hillerslev chalk. By comparing test results from normal scale to large scale on fractured and non fractured specimens it was found that the stiffness of the chalk is dependent on scale while the shear strength generally seems to depend on fracturs and not on scale. Furthermore the studies revealed, that fractures have a significant reducing effect on the shear strength, that characterisation by the Geological Strength Index, GSI, on fractured test specimens give a very good prediction of shear, that shear failure and yield surface characteristics for fractured and intact chalk can be provided using GSI, that up-scaling influence the elastic deformation properties in the low stress regime and that fractures influence the compressibility in the elastic stress regime, but not in the plastic stress regime. Finally, the practical application of some of the results on reservoir chalk has been addressed, especially the up-scaling of strength and deformation properties from normal scale tests to reservoir conditions. The up-scaling laws are relevant for borehole stability problems but not for compaction. Generally, the observatons in the study are relevant for quantifying the effect of fracturing and sample distrubance in conventional rock mechanics tests with low confining pressure
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| De Nationale Geologiske Undersøgelser for Danmark og Grønland | ||
| Danmarks Tekniske Universitet (DTU) |
Contact
Maglebjergvej 1
DK-2800 Lyngby, Denmark
Jørgensen, Morten (afdelingsleder); Projektleder: Havmøller, Ole (civ.ing.), 45884444, ole.havmoeller@geoteknisk.dk
Øvr. Partnere: Danmark og Grønlands Geologiske Undersøgelse (GEUS); Danmarks Tekniske Universitet. Institut for Geologi og Geoteknik
