Fluid Selection and Planning for Drilling Unconsolidated Formations
Authors: Stephens M.P. , Bruton J.R. 
Published: October 2008

Extended-reach and horizontal drilling activity has increased dramatically over the last several years. With advancements in horizontal drilling technology, many wells have been drilled successfully and economically. In fact, in many areas these drilling projects have essentially become routine. Drilling fluid research has primarily focused on drill cuttings transport in horizontal wells for both water-based and oil-based muds. Although hole-cleaning considerations are important and necessary, other factors are equally important when drilling unconsolidated zones.

Control of drilling fluid properties is essential when encountering unconsolidated formations in horizontal wells. These properties include fluid density and resulting wellbore pressure, type and quality of filter cake to be deposited on the unconsolidated zone, the effects of mud chemistry and mud hydraulics on hole erosion, and the compatibilityy of the fluid with regard to formation damage if the the unconsolidated zone is a potential producer.

This paper describes guidelines for the mud selection process when drilling unconsolidated formations in extended reach and horizontal wells. Selection of the proper drilling fluid should be based on these guidelines at the planning stages of the well.

SURVEY OF PROBLEMS

In nearly all offshore drilling areas, soft unconsolidated sand formations are penetrated at some time during drilling operations. These unconsolidated formations present the drilling operation with a number of difficulties in maintaining a mechanically stable wellbore, transporting drill cuttings with non-erosive flow, and preventing formation damage. These problems all become more severe when highly deviated or horizontal wells are drilled through unconsolidated formations. Drilling fluid composition and properties play an important role in minimizing the problems presented by unconsolidated sands.

CAUSES OF WELLBORE INSTABILITY

Unconsolidated sands are composed of sand grains that do not have any inherent cohesion between one another. They differ, on one hand, from clays wherein attractive force between clay particles give the sediment a weak natural cohesion, and, on the other hand, from sandstones and limestones wherein natural mineral cementation between grains gives these rock materials a considerable degree of natural cohesion. The lack of natural cohesion between the grains of unconsolidated sands gives these formations mechanical properties that lead to several types of wellbore instability.

Confining stress on unconsolidated sands plays an Important role in determining their strength in the subsurface environment. In addition to the pore pressure of fluids in the sediment, the mechanical state of the formation is defined by a mechanical stress field. 1 For the purpose of defining the stress field for granular permeable materials, effective stress is used. 2 The effective stress is simply the absolute stress minus the pore pressure, PP. The effective stress field in an unconsolidated sand in the subsurface can be characterized by three mutually perpendicular principal effective stress components: (available in full paper)