Underbalanced Drilling: Limits and Extremes

Bill Rehm, Arash Haghshenas, Amir Saman Paknejad, Abdullah Al-Yami, Jim Hughes, Bill Rehm, Arash Haghshenas, Amir Saman Paknejad, Abdullah Al-Yami, Jim Hughes

• Publisher: Gulf Publishing Company
• Year: 2013
• ISBN:
  9780127999807
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

The present crude oil and natural gas reservoirs around the world have depleted conventional production levels. To continue enhancing productivity for the remaining mature reservoirs, drilling decision-makers could no longer rely on traditional balanced or overbalanced methods of drilling. Derived from conventional air drilling, underbalanced drilling is increasingly necessary to meet today's energy and drilling needs. While more costly and extreme, underbalanced drilling can minimize pressure within the formation, increase drilling rate of penetration, reduce formation damage and lost circulation, making mature reservoirs once again viable and more productive. To further explain this essential drilling procedure, Bill Rehm, an experienced legend in drilling along with his co-editors, has compiled a handbook perfect for the drilling supervisor.

Underbalanced Drilling: Limits and Extremes, written under the auspices of the IADC Technical Publications Committee, contain many great features and contributions including:

• Real case studies shared by major service companies to give the reader guidelines on what might happen in actual operations
• Questions and answers at the end of the chapters for upcoming engineers to test their knowledge
• Common procedures, typical and special equipment involved, and most importantly, the limits and challenges that still surround this technology

CHAPTER 1

Introduction

Bill Rehm, Drilling Consultant

1.1 Book Description

As a matter of convenience and organization, each of the various underbalanced drilling (UBD) techniques is presented as a discrete operation in a single chapter. There are some theoretical and practical limits to the various techniques. The primary part of each chapter will describe the procedure and point to these limits. In some cases, there is a chapter section with a theoretical development of a complex issue. Finally, a number of descriptions or case studies of complex or extreme operations are presented to give the reader guidelines on what might happen in actual operations.

Since the initial plan will often be field modified, the discrete operation can become more complex, and the clear lines between the various techniques often become blurred. The interface between underbalanced operations and managed pressure concepts is often not clear because they may grade into each other. Many long horizontal holes are overbalanced, at balance, and underbalanced because of the effect of Circulating Pressure Loss or Annular Pressure Loss (APL).

This chapter provides a definition of underbalanced drilling (UBD), the advantages and constraints to UBD, and briefly outlines the various underbalanced techniques. Section 2, Lessons in Underbalanced Drilling, includes descriptions of practices that are common to all underbalanced operations. Within these discussions, enough of the basic operating ideas involved in UBD are presented to lead a reader to an understanding of the processes and to point the way for further reading.

Best practices based on present technology are a shifting target. Underbalanced drilling has its place, but other concepts, such as wellbore strengthening, liner and casing drilling, and expandable liners, shift the need or the use of the underbalanced concept to new horizons.

1.2 Other Options

Other technology can provide options to certain drilling problems without using underbalanced drilling techniques:

• New drilling fluids additives can strengthen the wellbore (Wellbore Strengthening).

• New lost circulation materials have improved lost circulation control.

• Drilling fluids that cause less skin damage.

• Expandable casing or liners provide “steel filter cake” for lost circulation or unstable zones.

• Casing and liner drilling.

• Motors increase drill rate (but do even better underbalanced).

• Bit technology has improved drill rate.

However, many new drilling technologies benefit from keeping the wellbore pressure at or below the pore pressure.

1.3 Introduction to Underbalance Drilling

Cable tool drilling, the natural form of underbalanced drilling, gradually lost out to rotary drilling tools because rotary was faster and controlled pressures downhole with drilling fluid density. With the advent of the rotary drill bit and oil booms, the apparent advantage of drilling the reservoir underbalanced was lost to the need for faster and deeper drilling. The old American Petroleum Institute (API) records in 1920 discuss wells drilled underbalanced in Trinidad, Montana, and California. The records note that in 1921, a well in the Panuco District of Mexico was drilled with 600 psi pressure on the well head.

1.4 Drilling Fluid Density

Drilling fluid density, or the density of the mud column, is normally the first barrier against a well kick and often contributes to wellbore stability.

There are three general regimes of drilling fluid density:

• Normal overbalanced drilling uses a fluid density that produces about 150 psi overbalance against the bottom hole formation pressure. This may also be also expressed as ½ ppg (.06 kg/L) overbalance. The overbalance is based on experience and prudent drilling practices; however, in some cases it is required by statute. Another requirement for normal overbalance is that the density of the fluid column must be adequate to limit the pressure against the base of the last casing string that is based on a certain kick size (kick tolerance).

• Balanced pressure drilling is the province of Managed Pressure Drilling where the fluid column, either static or circulating, is balanced against formation pressure with the aid of an impressed surface pressure.

• Underbalanced operations, which are the province of this book, are where the fluid column is deliberately kept below the formation pressure (pore pressure). This may include drilling with air or gas, drilling with a light single-phase fluid column, or drilling with a two-phase fluid column that has been made less dense by the addition of a gas.

1.5 Advantages to UBD

The drilling industry today recognizes that while drilling, a wellbore pressure lower than the formation pressure may be advantageous since lower pressure:

• Increases instantaneous drill rate

• Prevents the drilling fluid from entering the reservoir and thus limits skin damage

• Reveals hidden productive formations

• Avoids lost circulation

• Avoids differential sticking

• May allow earlier production

• Provides a condition where reservoir flow measurements may be taken during the drilling operation

One of the driving forces behind many of the underbalanced drilling operations in this decade is the preservation of oil mud. Oil mud, or oil invert emulsions, often start at upwards of USD 150/bbl.

Loss of a thousand bbl of oil mud to lost circulation and the ensuing rig time as well as transportation costs can be an AFE (expenditure) disaster.

1.6 Challenges with UBD

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