The economic feasibility of the exploitation of unconventional resources is highly dependent on the ability of the operator to maximize individual well productivity, making hydraulic fracture design and implementation the defining factor for a successful field development in most cases. Some unconventional reservoirs, as shallow coal bed methane and over-pressured oil and gas shale formations, commonly present the minimum principal stress in the vertical direction, resulting in the occurrence of horizontal hydraulic fractures. Models for the transient flow and pressure behavior of horizontal fractures emanating from vertical wells exist and clearly show distinct performance from those for vertical fractures. This suggests that the widely accepted unified fracture design (UFD) approach to maximize well productivity for vertical and horizontal wells with vertical hydraulic fractures cannot be used for horizontal fractures. Thereafter, the necessity for guidelines to model and design horizontal fractures becomes evident.
This investigation begins by presenting a new set of equations for horizontal fracture design based on the UFD approach, which allows the direct calculation of fracture width, half-length and conductivity for a given proppant number. Later, a reservoir numerical simulator is used to model well productivity behavior for horizontal fractures in homogeneous formations, with or without vertical to horizontal permeability anisotropy and for different aspect ratios as a function of suitably-defined proppant number, dimensionless fracture conductivity, and fracture penetration index parameters.
The findings of this work reveal a complex behavior for horizontal fractures that prohibits the extrapolation of previous generalizations between proppant number, penetration index and dimensionless fracture conductivity established for vertical fractures. For a number of scenarios, new relationships among these variables are provided to guide horizontal fracture design. Anisotropy and reservoir aspect ratio were also found to significantly impact fracture performance. Additionally, a set of multi-variable functions that permit the estimation of maximum achievable productivity index for the horizontal fracture has been fitted, based on commonly known reservoir parameters and the proppant number. This investigation provides a comprehensive framework to assist the design of optimal horizontal fracture geometry that maximizes productivity for a given mass of proppant.