Predicting Fractured Well Performance in Naturally Fractured Reservoirs
Authors: Hwang Yunsuk , Lin Jiajing , Zhu Ding , Schechter David 
Published: March 2015

Multiple hydraulic fracture treatments in the reservoirs with natural fractures create complex fracture networks. Predicting well performance in such a complex fracture network system is an extreme challenge. The statistical nature of natural fracture networks changes the flow characteristics from that of a single linear fracture. Simply using single linear fracture model for individual fractures, and then summing the flow from each fracture as the total flow rate for the network could introduce a significant error. In this paper we present a semi-analytical model by a source method to estimate well performance in a complex fracture network system. The method solves the problem by considering the fractures as a combined series of slab sources and by superposing the sources under several boundary or flow conditions. The method simulates complex fracture systems in a more reasonable approach. To reflect heterogeneous nature of natural fractures, a stochastic method of generating discrete fracture networks is adopted. The fractal discrete fracture network model (FDFN) incorporates the various scale-dependent data, such as outcrops, logs and cores; and creates more realistic natural fracture networks. FDFN model is combined with the slab source model to build fracture networks first, and then the flow problem in the complex fracture systems is solved. After generating the complex fracture network, each fracture, the analytical solution of superposed slab sources, is applied to predict the overall flow from all of the fractures in the system by considering the effects between the fractures through the superposition principle. The fluid inside the natural fractures flows into the hydraulic fractures, and the fluid of the hydraulic fractures from both the reservoir and the natural fractures flows to the wellbore. Because of the flexibility of the source method, non-orthogonally intersecting fractures are allowed in the system to simulate the geostatistically distributed fracture systems. The non-orthogonal fractures are approximated as a series of either vertical or horizontal sub-fractures, depending on the intersecting angle of the fractures.

Simplified example of combined geometries of hydraulic fractures and natural fractures is presented. The methodology developed in this study captures the nature of multiple stage fractured horizontal wells in naturally fractured formations, and can be used to predict fractured well performance. It is relatively simple to apply compared with reservoir simulations.

Introduction

Since the successful development of Barnett shale in early 2000s due to the implementation of hydraulic fracturing and horizontal well drilling, unconventional reservoirs such as shale and tight sand have become an important additional resource of hydrocarbon energy. As new technology becomes available, multiple-stage fracturing in horizontal wells is now a primary stimulation method to bring economical and technical benefits of unconventional gas wells. Extended fracture network can be created in shale formations by multi-stage fracturing to improve volumetric transmissibility of nano-Darcy reservoirs. Because the fractures created in multi-stage treatments express a stochastic nature, strongly depending on the natural fracture characteristics of the formation, such a fracture system is hard to describe precisely, posting an extreme challenge in modeling of well performances.