| dc.description.abstract | Natural fractures play an important role in the hydrocarbon production from tight reservoirs.
The need for fracture network pathways by fraccing matters particularly for shale gas
prospects, due to their nano to micro darcies matrix permeabilities. The study of natural
fractures from outcrops helps to better understand network connectivity and possibility of
reactivating pre-existing planes of weakness, induced by hydraulic stimulation.
Microseismicity also show that natural fractures are reactivated during fraccing in tight gas
reservoirs and influence the success of the stimulation. An accurate understanding of natural
fracture networks can help in predicting the development of fracture networks. In this
research we analyze an outcrop analogue, the Whitby Mustone Formation (WMF), in terms of
its horizontal fracture network. The WMF is the time equivalent of the Posidonia Shale
Formation (PSF), which on itself is the main shale gas prospect in the Dutch subsurface.
The fracture network of the WMF is characterized by a system of steep dipping joints with
two dominant directions with N-S and E-W strike. The network was digitized from bird-view
imagery of the pavement with a spatial extent of ~100 m at sub-cm resolution. The imagery is
interpreted in terms of orientation and length distributions, intensity and fractal dimensions.
Samples from the field were analyzed for rock strength and sample mineralogy.
The results indicate that the fracture networks differ per bed. Observed differences are for
example; the geometry of the fracture network, its cumulative length distribution, the fracture
intensity, the fracture length vs its orientation and the fractal dimension. All these parameters
greatly influence fracture network connectivity, the probability that longer fractures exist
within the pavement and whether the network is more prone to clustering or scattering. Apart
from the differences, the networks display a fairly similar orthogonal arrangement with
dominant large (> 5-10 m) N-S striking fractures and smaller E-W striking cross-joints (< 2-3
m). A nested network arrangement is indicated by some smaller-scale N-S fractures abutting
against the E-W striking ones. Furthermore, abutment relations provide some constraints on
relative time. Timing indications with respect to burial-exhumation are difficult to establish.
Some joints are cemented and measurable from the high-resolution imagery. The vein
measurements helped establishing a first order relation between the fracture aperture with
respect to their length and confirm that longer fractures have a wider aperture.
The above stated parameters and results all prove to be very valuable information which can
help predict the geometries of the different fracture networks present within the PSF. It is
important to understand the possible mechanisms which can cause these differences in
fracture network characteristics. Bulk lithological variations between beds are minor, beds are
mainly consisting of clay minerals. Furthermore, some quartz and pyrite is present in all
samples and TOC is present in variable amounts. However, the occurrence of concretions up
to 0.5m in size and differences in layer thickness correlates makes notable variations in
distinct network arrangement. Therefore it appears that the presence of these concretions and
variations in layer thickness alters the overall strength of the rock, hence the fracture network
geometry. | |
