Abstract

This paper examines some mechanical aspects of formation damage as they apply uniquely to horizontal wells. First a seldom recognized phenomenon of mechanical damage due to drill string rotation is presented along with a method for estimating the extent of the damaged area. Next, the invaded zone is examined in some simple examples to ascertain qualitatively the geometry of the invaded zone due to varying exposure times of different parts of the horizontal section during drilling. The relative time scales between exposing new hole with the drill bit, the invasion rates into the formation, as well as the changing differential pressure in the horizontal annulus are incorporated into a simple model to determine the effect of changing annular pressure while drilling. Finally an inexpensive means of mechanically achieving zone isolation for damage removal treatments in unconsolidated zones is presented.

Introduction

Horizontal wells are unique in many respects. What has become intuitive in our thinking regarding vertical wells may lead to wrong thinking in horizontal wells. We have examined two mechanical aspects of formation damage here. One has received little or no attention in the literature, and that is the damage caused by the drill string rotating and sliding in a horizontal well. We will derive a method for estimating the extent of this damage under quasistatic conditions.

The second phenomenon we examine is the geometry of the invaded zone during typical drilling operations. Frick and Economides1 have shown the zone to be a right circular cone for a homogeneous formation with a larger radius at the near portion (heel) of the horizontal section and a smaller radius at the far end (toe). Because a number of operators have observed greater production from the heel, contrary to what one might expect, it appeared worth examining in more detail the effects of varying drilling rates and realistic circulating modes to determine their effect on the geometry of the invaded zone in a homogeneous reservoir.

What is now becoming most obvious for operators of horizontal wells is that many reservoirs are quite heterogeneous, and that damage removal and effective reservoir management in horizontal wells is not going to be accomplished without some means of zone isolation. The initial cost of conventional zone isolation in any horizontal well is quite high and there is a natural reluctance to commit to such a high initial completion cost. We describe later in this paper a less expensive alternative to conventional zone isolation for application in unconsolidated sands.

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