Rapid Discrete Fracture Analysis of Hydraulic Fracture Development in Naturally Fractured Reservoirs

Abstract

Hydraulic fracture simulation is essential to safe, efficient, and productive development of many unconventional resources, including shale gas and oil, tight sands, and increasingly, even fractured carbonates. Hydraulic fracture propagation geometry, and the geometry of both inflated and reactivated (sheared) natural fractures, can be difficult to predict due to their complex interaction with the natural faults and fractures, the mechanical properties of surrounding formations, and the in situ stress field. This paper presents a Discrete Fracture Network (DFN) algorithm, which provides a full, three dimensional assessment of hydraulic fracture and reactivated natural fracture geometry, including both the interaction with the pre-existing geologic setting, and also the interactions which occur between adjacent or simultaneous hydraulic fractures. This approach is rapid, flexible, and convenient, and can be used directly for hydraulic fracture design, resource assessment, and permitting. The approach is limited by the use of semi-empirical algorithms. However, these algorithms can be tuned and verified based on calibration to in situ measurements from production, microseismic imaging, and tomographic fracture imaging. In addition, the algorithms can be refined by comparison to more computationally intensive distinct element simulation methods. Introduction Unconventional oil and gas resources frequently stimulated through large scale hydraulic fracturing. These induced hydraulic fractures provide permeable, connected pathways to deliver oil and gas to production wells, and can dramatically increase improve reservoir economics. Hydraulic fracturing add permeable flow pathways and connected surface areas corresponding to both the induced hydraulic fracture, and in some reservoirs, also inflated and reactivated (“critically stressed”) natural fractures. It is widely accepted and has been proven that natural fracturing heavily influences hydraulic stimulation characteristics (Olson 2010). It follows that an understanding of natural fracturing surrounding the treatment zone, and in the interval between this zone and a potential geohazard, is extremely important to allow assessment of hydraulic fracture characteristics. The Discrete Fracture Network (DFN) approach (Dershowitz et al. 1996) is a proven method in characterising the three dimensional spatial pattern of the fracturing existing within a naturally fractured reservoir. Rogers et al. (2010) describes a Discrete Fracture Network (DFN) approach to better understand the geometry, connectivity, and hydraulic properties of induced hydraulic fractures, and inflated and reactivated natural fractures, and how they influence reservoir production. The DFN Approach has the advantage that it realistically represents discrete fractures, stratigraphy, and geomechanics using a combined continuum and discrete feature approach, which provides a more geomechanically realistic basis for analysis. A typical DFN model for a fractured rock formation is given in Figure 1. URTeC 2013 Page 2341