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

Abstract

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.