Gel Pills for Downhole Pressure Control during Oil and Gas Well Drilling

Mahmoud Khalifeh,Tor Henry Omland,Arild Saasen,Lorents Reinås,Larisa Penkala,Bodil Aase,Knut Taugbøl

doi:10.3390/en13236318

Abstract

During drilling of petroleum or geothermal wells, unforeseen circumstances occasionally happen that require suspension of the operation. When the drilling fluid is left in a static condition, solid material like barite may settle out of the fluid. Consequently, the induced hydrostatic pressure that the fluid exerts onto the formation will be reduced, possibly leading to collapse of the borehole or influx of liquid or gas. A possible mitigation action is placement of a gel pill. This gel pill should preferably be able to let settled barite rest on top of it and still transmit the hydrostatic pressure to the well bottom. A bentonite-based gel pill is developed, preventing flow of higher density drilling fluid placed above it to bypass the gel pill. Its rheological behavior was characterized prior to functional testing. The designed gel pill develops sufficient gel structure to accommodate the settled barite. The performance of the gel was tested at vertical and 40° inclination from vertical. Both conventional settling and the Boycott effect were observed. The gel pill provided its intended functionality while barite was settling out of the drilling fluid on top of this gel pill. The barite was then resting on top of the gel pill. It is demonstrated that a purely viscous pill should not be used for separating a high density fluid from a lighter fluid underneath. However, a bentonite or laponite gel pill can be placed into a well for temporary prevention of such intermixing.

Highlights

Drilling fluids are concentrated fluid–solid mixture whereas high density minerals such as barite with a density of 4.2 s.g. are used to increase and adjust the density of drilling fluid.As density of the heavy minerals is much higher than the fluid density, gravitational force acts on these particles making them segregate and settle out of the fluid
The scope was to construct a gel pill that can act as a resting bed for settled barite particles from the drilling fluid volume placed above the gel pill
The brine is used to increase the density contrast to the model drilling fluid placed above the gel pill and to simulate the presence of a low density drilling fluid volume left in the lower section of the hole

Summary

Introduction

Drilling fluids are concentrated fluid–solid mixture whereas high density minerals such as barite with a density of 4.2 s.g. (specific gravity) are used to increase and adjust the density of drilling fluid.As density of the heavy minerals is much higher than the fluid density, gravitational force acts on these particles making them segregate and settle out of the fluid. Saasen et al [4] showed that barite sag can be more intense in fluids that develop fragile gel structures when left static. They defined a fragile gel structure fluid as a fluid with high gel strength, but when the initial gel structure is ruptured, the remaining gel structures will break. Barite settling is known to cause problems during drilling operations with the most severe consequence leading to well control issues. This is due to the reduction of hydrostatic pressure exerted by the drilling fluid

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