Faults and fractures are common features in many well-known reservoirs. In many of these reservoirs, horizontal wells are drilled to intersect a large number of fractures, particularly in low-matrix-permeability formations. In addition, the application of horizontal wells intersecting multiple hydraulic fractures has been widespread to allow shale gas and oil reservoirs, some of which are also naturally fractured, to produce economically.
In this paper, we investigate the pressure transient behavior of horizontal wells in continuously and discretely naturally fractured reservoirs (NFR)s using semi-analytical boundary element solutions. These solutions have the advantages of the absence of grids and reduced dimensionality. Furthermore, they provide continuous rather than discrete solutions. The solutions are sufficiently general to be applied to many different well geometries and reservoir geological settings, where the spatial domain may include arbitrary fracture and/or fault distributions with different types of outer boundaries. A number of solutions have been published in the literature for horizontal wells in NFRs using the conventional dual-porosity models that are not applicable to many of these reservoirs. Most of these published solutions ignore the wellbore and the unfractured sections of the horizontal well. Therefore, they cannot capture the true early-time response, such as fracture radial flow. They may also yield incorrect damage skin values. Our solutions take these effects into account.
Our solutions can also be applied to shale gas and oil reservoirs without shale-gas transport nonlinearities when the average reservoir pressure is above the desorption pressure. Our solutions for horizontal wells in fractured reservoirs can contain any spatial distribution of finite or infinite conductivity fractures with arbitrary length and orientation. The number and type of fractures (hydraulic or natural) intersecting the wellbore and with each other are not limited in both homogeneous and naturally fractured reservoirs. We present a number of examples to show different flow regimes that a horizontal well with multiple fractures exhibits, and to show that the conventional dual-porosity models simply do not work and can be deceptive. In our solutions, continuous and discrete conductive and nonconductive fractures are treated explicitly. The exact treatment of the uniform wellbore pressure condition, and the inclusion of the wellbore and the unfractured sections of the horizontal well have led to identification of new flow regimes that were not apparent from the existing solutions. Consequently, our solutions capture the true pressure transient behavior of the system, such as fracture radial flow. In this paper, a new classification of wellbore and fracture skin damage is given, and their effects on the pressure transient behavior are investigated.
There are many factors that dominate the pressure transient behavior of horizontal wells intersected by multiple hydraulic fractures in naturally fractured reservoirs, such as fracture conductivities, lengths, and distributions, as well as whether or not fractures intersect the wellbore. Diagnostic derivatives plots are presented for a variety of horizontal wells with multiple fractures in homogenous and NFRs. It is shown that these reservoirs exhibit many different flow regimes. A multistage-fractured horizontal well in a very low-permeability shale reservoir example is also presented. Finally, we have presented two field buildup test examples from NFRs