Performance of Longitudinally Fractured Horizontal Wells in High-Permeability Anisotropic Formations
Authors: Villegas M.E. , Wattenbarger R.A. , Valko P. , Economides M.J. 
Published: December 2007

This work studies the combined behavior of (1) horizontal wells, and (2) high-permeability fracturing. Fractured vertical wells in high-permeability formations always yield finite conductivity fractures which in turn can reduce the benefits of the stimulation. It has been proposed recently that a fracture along a horizontal well poses far smaller pressure drops than those of the fracture intersecting a vertical well. This work uses numerical simulation to corroborate the recently presented semi-analytical solutions for the problem of a longitudinally fractured horizontal well in a homogeneous, isotropic formation. Additionally, this paper addresses the effects of vertical and horizontal anisotropy in the pressure and production performance of such well and compares it to the performance of a fractured vertical well. Finally, the paper presents a set of transformations that allows for a proper well test analysis of the longitudinally fractured well in an anisotropic formation.

Introduction

In the recent past we have shown that two of the most important new developments in petroleum production engineering (horizontal wells and high-permeability hydraulic fracturing) can be combined. The recommended configuration for a relatively high-permeability reservoir (e.g., k&gt;10 md) is a horizontal well drilled in the expected fracture azimuth and, thus, the executed hydraulic fractures would be longitudinal to the well. Of course, such a well completion must meet two criteria:it is economically superior to a fractured vertical well or to an unfractured horizontal well (incremental discounted benefits minus costs) andit is logistically possible. In certain cases, well trajectories along the required maximum horizontal stress direction are cumbersome to impossible.

In other publications we had compared vertical and horizontal fractured well performance (see Fig. 1) and the conclusion has been that in cases where a vertical well would result in a large dimensionless conductivity fracture (i.e., low-permeability reservoirs) the longitudinal fracture configuration for a horizontal well would be unattractive. In the latter case, the likely configuration should be multiple transverse fractures, subjected again, to a demonstrable economic benefit and, contenting with other considerable logistical difficulties for their execution. We have also suggested that unfractured horizontal wells, drilled in reservoirs where vertical wells are fractured, are unlikely to be attractive and therefore, if drilled, such wells should be hydraulically fractured also. Exception to this is the case of extraordinary areal permeability anisotropy.

We have presented an analytical solution for the longitudinally fractured horizontal well, using a similar approach as the one presented by Cinco-Ley and Meng for fractured vertical wells. The solution shows a profound tilting of the finite conductivity fracture response, whose distinguishing feature is the quarter slope straight line on the log-log plot, towards the response of the infinite conductivity fracture (Gringarten et al.) The latter is the well known half-slope straight line, indicative of linear flow. We have implied that a horizontal well, fractured in the longitudinal direction, would serve as an "infinite conductivity streak" in an otherwise finite-conductivity medium. This effect is more pronounced in relatively thin reservoirs, although it is present to varying degrees in all thicknesses. The implications of this finding have considerable economic impact. Much lower-conductivity fractures (i.e., much narrower), executed from a horizontal well longitudinally, can outperform much wider fractures from vertical wells. The latter has been thus far the indicated stimulation design for high permeability-fracturing employing the tip screen-out technique. Much smaller fracture treatments in horizontal wells would suffice.

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