Abstract

Reducing the invasion of drilling, completion and workover fluids to as low a level as possible will alleviate many problems of mechanically unstable formations, differential sticking and lost circulation. A properly formulated ultra-low invasion fluid (ULIF) will also reduce formation damage and thereby often increase well productivity. In addition, certain classes of ULIF will provide wellbore strengthening.

This paper describes the technology used to produce a class of ULIF and will review the benefits that limiting fluid and pressure invasion of fluid brings to rock integrity, drilling performance and well productivity. The ULIF additives described are biodegradable, environmentally acceptable and can be used in water based, oil based and synthetic based fluids. They are easy to add to any mud system and the ultra-low invasion properties of the fluids are measured and maintained in the field by using a simple sand bed invasion test.

The paper focuses mainly on case histories from several regions of the world where ULIFs have been used. In North, Central and South America alone, over 120 wells have used the ULIF additives in the past 3 years. The products have also been used in India, the Middle East, Europe, Africa and the Far East. Log data are presented that clearly demonstrate the ability of a ULIF to reduce wellbore instability and fluid invasion. Evidence of wellbore strengthening is also presented along with several cases where well productivity has been increased. In addition to showing the enhanced technical performance obtained with ULIFs, we also demonstrate the economic benefits of using these fluids.

The same technology used to formulate Ultra-low Invasion Drilling Fluids can also be used very successfully - sometimes with modification - in completion fluids, workover fluids and cementing operations to reduce or eliminate the same problems.

Introduction

The ability to reduce the invasion of drilling, completion and workover fluids to the lowest possible level brings many benefits, including less formation damage, fewer differential sticking events and the prevention of some types of mechanical wellbore instability. Also, if transmission of the mud overbalance pressure to the pore pressure is prevented or greatly reduced, the effective stress is not reduced and compressional failure of the rock may be avoided.

If rock can be strengthened (or, more accurately, if the fracture gradient can be increased) by some mechanism, higher fluid densities can be used without resulting in induced losses.

The obvious rewards from being able to reduce invasion to the lowest possible level mean that a considerable amount of effort is being expended in seeking "zero fluid loss fluids" as well as fluids that strengthen the wellbore by stopping pressure invasion, consolidating rock or modifying the stress distribution.

In this paper we review a class of additives that is designed to limit both fluid and pressure invasion. These Ultra-low Invasion Fluid (ULIF) additives can be used in drilling, completion or workover fluids and they also form the basis of a class of cementing spacers. The additives are equally effective in water based, oil based and synthetic based fluids.

Laboratory tests show that, after a brief period of invasion, the ULIF products form an extremely low permeability seal that virtually eliminates further fluid ingress: once this happens, the formation is effectively protected from the consequences of further pressure or fluid invasion. An important feature is that this seal is formed extremely rapidly - in a matter of seconds after the rock has been exposed to the fluid. This rapid shutoff of invasion, depicted in Figure 1, is a key factor in achieving the excellent results seen in the field with the ULIF products.

In this paper we briefly discuss the technology behind this class of Ultra-low Invasion Fluids. We then describe several field examples where the muds have successfully reduced wellbore instability and reduced or eliminated induced losses. We will also provide examples of much reduced fluid invasion into permeable sands: in the reservoir, this can help limit formation damage and hence improve well productivity1,2.

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