An experimental and numerical investigation into hydraulic fracture propagation in naturally fractured shale gas reservoirs


Yildirim B.

Diğer, ss.1-203, 2019

  • Yayın Türü: Diğer Yayınlar / Diğer
  • Basım Tarihi: 2019
  • Sayfa Sayıları: ss.1-203
  • Orta Doğu Teknik Üniversitesi Adresli: Hayır

Özet

Despite the large amount of research conducted to date, the performance of hydraulic
fractures in naturally fractured structures and their effect on hydrocarbon production is
still not well understood. To this end, this research aimed at developing a better
understanding of natural fracture and hydraulic fracture interaction in shale formations by
two- and three-dimensional discrete element modelling (DEM) approaches, results of
which were compared against the large-scale hydraulic fracturing experiments.
The samples used in the experimental research were collected from the Hope Cement
Works shale quarry in Derbyshire, UK. The mineralogy as well as the mechanical, elastic,
and flow properties of samples were obtained through several laboratory sample
characterisation tests. The subsequent true-triaxial hydrofracturing experiments with
acoustic measurements were performed on one homogeneous and one naturally fractured
0.3 × 0.3 m × 0.3 m rock samples, which reflected the temporal information on hydraulic
fracture initiations. For further identification of the location and geometry of hydraulic
and natural fractures, computed tomography (CT) and seismic velocity tomography
analyses were conducted. The preliminary two-dimensional discrete element modelling
research results were obtained using discrete fracture network (DFN) approach in two-
dimensional Particle Flow Code (PFC2D). The two-dimensional model results provided
a fundamental understanding of the effects of certain parameters, in particular the angle
of approach, differential stress, mechanical properties as well as ubiquity and randomness
of natural fractures on fracture interaction mechanisms.
However, in view of the limitations of two-dimensional representation of both the
laboratory and field scale applications, three-dimensional discrete element models were
developed using XSite, results of which were first compared against the findings of true
triaxial hydrofracturing experiments, and then extended through a parametric research.
The effects of mechanical properties of natural fractures and operational parameters on
fracture interaction mechanisms were then analysed using 3D XSite models. A curved
shape hydraulic fracture, which propagated perpendicular to the minimum horizontal
stress direction (x) in the homogeneous sample model, agreed well with the CT scan
analysis and seismic wave velocity tomography results from the laboratory experiments.
Similarly, the natural fracture and hydraulic fracture interaction observed in the second
heterogenous/fractured sample, particularly the arrest by the main natural fracture and the
subsequent crossing with offset mechanisms, were captured well by the developed 3D
numerical models.
Both experimental, and the parametric two- and three-dimensional particle- and lattice-
based discrete element modelling research have demonstrated that the hydraulic fracture
propagation in homogeneous rock with no weakness planes/natural fractures is mainly
controlled by the differential stress, as it is growth is perpendicular to the minimum
horizontal stress with no observed branching/diversion. The presence of natural fractures,
on the other hand, introduced the additional effects of mechanical properties of natural
fractures on observed interaction mechanisms in such a way that the stronger natural
fractures are found to be favouring the crossing mechanism. Importantly, the ubiquity and
randomness of natural fractures, which increased the complexity in hydraulic fracture
growth significantly, have shown that the hydraulic fracture almost always propagates
along the nearest natural fracture plane as the least resistant and shortest path, instead of
being controlled by the differential stress. These findings, indeed, emphasised the
dominating role of natural fractures and their dispersion within the reservoir on hydraulic
fracture propagation and subsequent fracture interaction mechanisms. Regarding the
operational parameters, lower flow rate and low viscosity fluids are found to be leading
to arrest mechanism with increased dilation of natural fractures, while higher flow rate
and high viscosity fluids resulted in direct crossing mechanisms with observable increase
in total stimulated areas.