Abstract

Almost every drilling operation is a potential source of damage to well productivity, lost circulation, differential sticking and other related conventional Drilling problems.

This paper re-visits the key damage mechanisims and provides a broad overview on how they occur during various oilfield operations, and their effect on well productivity. Also, lost circulation or fluid invasion potential in high permeability zones, large open fractures, heterogeneous carbonates with massive interconnected vugular porosity, or pressure depleted zones would be a major issue of concern during conventional drilling condition. The worst-case scenario would be a combination of one of these high permeability features with significant pressure depletion.

In order to overcome the above problems while drilling, the industry developed a method to drill with a bottom hole pressure below the pore pressure, called Underbalanced Drilling - UBD

As the majority of hydrocarbons being exploited today are found in existing pressure depleted or complex and lower quality Reservoirs with lots of the conventional drilling problems, this is where Underbalanced Drilling Technology can add value and in some cases reduce development cost. Soon, Underbalanced Drilling will become the standard field development technique, both Onshore and offshore, where the Geology and Reservoir are suitable.

The paper reviews several case histories and real results highlighting the advantages of Underbalanced Drilling Technology in reducing Formation Damage, Lost Circulations and improving well productivity.

Introduction

The Oil and Gas Industry drills thousands of wells worldwide each year. From the Reservoir and Production Engineers perceptions, a successful well is one that achieves its maximum production potential with no, or minimal, formation damage. Unfortunately, this objective is rarely achieved when conventional overbalanced drilling is used where mud solids and mud filtrate invade the reservoir formation and impair the permeability around the well bore. Formation damage can occur during almost any stage of petroleum exploration and production operations. This paper describes in detail the main formation damage mechanisms that occur during conventional drilling operation and how it can be controlled and prevented using Underbalanced drilling technology.

The severity of solids and filtrate invasion depends on mud rheology, the duration of exposure to mud system, over-balanced mud pressure, rock permeability, and mineralogical composition of reservoir rocks. The solids and filtrate invasion causes the so called "skin effect" which may be attributed to different damage mechanisms that can have significant impact on the production and/or injection rates. Therefore understanding the different mechanisms of formation damage is becoming an important task for reservoir engineers in the oil and gas industry, because it is the first step to be taken to prevent and further alleviate this problem.

What is Formation Damage?

Formation Damage was once defined as: "the impairment of the invisible, by the inevitable and uncontrollable, resulting in an indeterminate reduction of the un-quantifiable"1. Luckily, the fact that Formation Damage is inevitable and Uncontroll-able has been replaced by a more positive attitude: Underbalanced Drilling Technology

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