Abstract

A unique oil-based drilling fluid system weighted with treated micronized barite (TMB) slurries has been developed and successfully introduced to the field. The utilization of this weight material provides the fluid system with low viscosity, reduced torque values, superior sag stability thus giving a fluid with low Equivalent Circulating Density (ECD) contribution and excellent hydraulics performance. These exceptional fluid characteristics make the fluid system an excellent solution for drilling sections with narrow mud-weight windows, coiled tubing operations, managed pressure drilling and extended reach drilling. Many of these drilling challenges are encountered in high-temperature, high-pressure (HTHP) and ultra-deepwater field developments and in depleted, mature fields.

Much of the early fluid system development focused on design, the system's physics and chemistry, and the optimization of mineralogy of the weighting agent. Also of concern was the variability of results seen both from return permeability as well as from standard fluid-loss experiments. On this basis a comprehensive study was undertaken to identify and understand the damage mechanisms operating in the formation and filter cake. During this period the fluid system was used in a number of operations in the North Sea such that the current available database includes 5 different types of field applications.

The paper presents the findings of the formation damage study including relevant productivity data from the various field applications. The results demonstrate that while invasion of the formation by treated micronized barite can occur, it does not necessarily lead to permanent productivity impairment. Furthermore, the micronized barite does not interfere with the added fluid-loss-control material over a wide range of fluid densities and formation permeabilities. The authors discuss the processes observed relating them to current field experience describing why the formation damage mechanisms do not concur with previous preconceptions and moreover describes where the limitations of the system occur.

Introduction

The paper summarizes field experiences and a collection of formation damage studies to identify and understand the damage mechanisms that may arise from the use of an oilbased drilling fluid weighted with Oil-Based Treated Micronized Barite (OB TMB).

Typically productivity impairment by oil-based drilling fluids arises due to poor fluid-loss control whereby drilling fluid filtrate and occasionally fine drill solids, emulsifiers and other additives that may modify wettability or that may be incompatible with the reservoir fluids enter the near wellbore formation and reduce the permeability. In many of these cases, the poor fluid-loss control is a result of the suboptimal design of the bridging material that allows ingress of fluid through an unnecessarily permeable filter cake.

In the case of treated micronized barite, the weighting material has a particle size distribution of 0.01 - 5µm. This means that the barite particles are so fine that they act as part of the fluid filtrate rather than as a separate solids phase. As such, if the filter cake is insufficiently impermeable then the micron-sized particles may penetrate with the fluid filtrate and enter into the formation. This then naturally poses the question of whether this leads to permanent damage and impairment of permeability.

To address this issue, the paper presents field experience from a number of North Sea field operations in combination with the results from relevant laboratory formation damage studies. Together the data is used to identify and describe potential damage mechanisms, how they occur, and how they can be avoided through good fluid engineering design.

Treated Micronized Barite Technology (TMB)

A unique oil-based drilling fluid system weighted with treated micronized barite (TMB) has been developed and successfully introduced to the field. The specially treated barite weighting material has a particle size distribution of 0.01 - 5µm with a mean value less than 2 µm. It is typically supplied in the form of a 2.3-sg (19-lb/gal) liquid concentrate (slurry) and is blended into the base oil to give the required mud weight.

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