This is a past event
Please join us for a research seminar to be given by Dr Stephanie Zihms of Heriot-Watt University entitled ‘Crato Formation laminites - a representative geomechanical (pre-salt) analogue?’ The seminar will take place on Thursday, 9th November in Meston Lecture Theatre 2. All are welcome to attend.
Abstract:
With recent hydrocarbon discoveries in the pre-salt layer of the Brazilian marginal basin, lacustrine laminites from the Crato Formation (CF), Araripe Basin, Brazil, have been considered as an analogue for carbonate reservoirs. Easy access to the outcrops and its range of facies (laminar, convolute, concretions) and structural heterogeneities (open-mode fractures, faults) makes it worth investigating. How representative are the CF laminites as an analogue for Barra Velha Formation, Aptian lacustrine carbonate reservoir, Santos Basin, offshore Brazil.
Analysis and characterisation using petrophysical and visual techniques, such as porosity, microscopy, X-ray tomography and photographs of pre- and post-deformation laminites indicate a strong link between laminite facies characteristics and deformation response. Even though different deformation responses were observed the overall increase in porosity due to fracturing for non-faulted laminites was 5.94% (±1.66). Laminites sampled from a small fault zone showed an increase in porosity of 17.11% (±4.48) due to triaxial deformation. This indicates that deformation behaviour for laminar laminites could be predicted based on key properties such as porosity, permeability and mineralogy. However, for faulted samples these properties alone cannot predict deformation behaviour.
How does this change in porosity affect permeability? For low permeability rocks, open fractures have the potential to provide a main flow pathway but determining effective connectivity, and associated single or multi-phase flow characteristics within an intact sample is not simple. Flow tests can provide a value for any one sample but general predictability requires considerably more information. X-ray tomography (XRT) has been used to identify fracture patterns and apertures in 3D. Here, in addition to XRT of experimentally fractured low permeability laminites, neutron beam radiography and tomography have been used to utilise neutron sensitivity to hydrogen presence. Radiography and tomography identify a complex but rational pattern of initial water movement into the matrix laminae that suggests that in unfractured laminite, the fluid front would progress stepwise from one lamina to the next with a relatively fast filling across an entered lamina and relatively slow progression to the overlying lamina. But in the presence of open fractures, when fluid enters the fracture system it moves up and down along a network of connected fractures, progressing across the sample and also moving into most laminae. Front progression is slowed in the central part of the sample where some stylolitisation is provisionally interpreted, and where the fracture network appears less well developed. Once that central part is entered, filling is rapid, suggesting a low permeability boundary to a higher permeability zone. The upper part of the sample has a smaller number of potentially open fractures and once fluid enters these fractures the front travels more rapidly up the sample. When the other fluid is introduced, it follows a similar path but the already wetted rock responds differently, supporting the expectation that multi-phase behaviour will be complex. But note that the two forms of water have a strong tendency to not mix.
In this study, because the sample is intact, we make predictions of the geological characteristics from the flow behaviour, in contrast to the more typical arrangement of predicting flow from observed geological characteristics. However, in common with XRT studies and unlike the destructive analyses, we can subsequently test our geological inferences.
- Speaker
- Dr Stephanie Zihms
- Venue
- Meston Lecture Theatre 2