Lost Circulation

Mechanisms and Solutions

Alexandre Lavrov

• Publisher: Gulf Professional Publishing
• Year: 2016
• Print ISBN: 9780128039168
• eBook ISBN: 9780128039410
• Topic: Technology & Engineering
• Subtopic: Environmental Management
• Index: Technology & Engineering

Lost Circulation: Mechanisms and Solutions provides the latest information on a long-existing problem for drilling and cementing engineers that can cause improper drilling conditions, safety risks, and annual losses of millions of wasted dollars for oil and gas companies.While several conferences have convened on the topic, this book is the first reliable reference to provide a well-rounded, unbiased approach on the fundamental causes of lost circulation, how to diagnose it in the well, and how to treat and prevent it in future well planning operations.As today's drilling operations become more complex, and include situations such as sub-salt formations, deepwater wells with losses caused by cooling, and more depleted reservoirs with reduced in-situ stresses, this book provides critical content on the current state of the industry that includes a breakdown of basics on stresses and fractures and how drilling fluids work in the wellbore.The book then covers the more practical issues caused by induced fractures, such as how to understand where the losses are occurring and how to use proven preventative measures such as wellbore strengthening and the effect of base fluid on lost circulation performance.Supported by realistic case studies, this book separates the many myths from the known facts, equipping today's drilling and cementing engineer with a go-to solution for every day well challenges.- Understand the processes, challenges and solutions involved in lost circulation, a critical problem in drilling- Gain a balance between fundamental understanding and practical application through real-world case studies- Succeed in solving lost circulation in today's operations such as wells involving casing drilling, deepwater, and managed pressure drilling

Chapter 1

The Challenge of Lost Circulation

Abstract

Drilling a well is the most common way to access oil and gas resources and geothermal reservoirs. During drilling, a fluid is circulated in the well in order to cool the bit, transport cuttings, and prevent the influx of formation fluids. Lost circulation is a situation where less fluid is returned from the wellbore than is pumped into it. Lost circulation may be caused by different mechanisms in different formations. The negative impact of lost circulation can be severe and result in nonproductive time, loss of drilling fluid, and possible well control issues. The concept of lost-circulation pressure is introduced in this chapter. Knowledge of lost-circulation pressure is crucial for setting up the casing program. Uncertainties involved in prediction of lost-circulation pressure are discussed. Published accounts of lost-circulation incidents are reviewed.

Keywords

Bottomhole pressure; Casing program; Drilling; Drilling fluid; Lost circulation; Mud loss; Well; Wellbore

Drilling a well is the most common way to access oil and gas resources and geothermal reservoirs. During drilling, a fluid is circulated in the well. This fluid (the drilling fluid) cools the drillstring, transports rock cuttings out of the well, and prevents the surrounding formation from collapse. The bottomhole pressure of the drilling fluid is kept within a certain “window.” The lower bound of the wellbore pressure is usually dictated by the formation pore pressure or the minimum pressure obtained from the borehole stability analysis, whichever is greater. If the bottomhole pressure drops below the pore pressure, an influx of formation fluids into the well may occur. If the bottomhole pressure drops below the minimum value obtained from borehole stability analysis, the formation may cave in.

The upper operational bound of the bottomhole pressure is chosen so as to avoid lost circulation. Lost circulation is a situation where less fluid is returned from the wellbore than is pumped into it. When lost circulation occurs, some drilling fluid is lost into the formation. Lost circulation gives rise to nonproductive time spent on regaining circulation. According to Ref. [1], lost circulation was responsible for more than 10% of nonproductive time spent when drilling in the Gulf of Mexico between 1993 and 2003. The inability to cure losses and resume drilling may, in the worst case, necessitate sidetracking or abandoning the well.

The economic impact of lost circulation includes, in addition, the costs of the lost drilling fluid and of the treatment used to cure the problem. According to one estimate, the cost of drilling fluids amounts to 25%–40% of total drilling costs [2]. Given that both regular drilling fluids and lost circulation materials are often quite expensive, the direct economic impact of losing these substances into the formation may be substantial. The cost issue is especially relevant for oil-based muds that are usually more costly than water-based fluids.

In addition to the direct economic impact (cost of expensive drilling fluid and nonproductive time), lost circulation may cause additional drilling problems. In particular, the reduced rate of returns may impair cuttings transport out of the well. This leads to poor hole cleaning, especially in deviated and horizontal wells [3]. Poor hole cleaning may eventually result in pack-offs and stuck pipe.

Losing drilling fluid into the formation in the pay zone increases formation damage as pores and fractures in the reservoir rock become plugged with particles present in the drilling mud (barite, bentonite, cuttings, solids used as lost circulation material, etc.). The formation damage created by lost circulation needs to be removed before production can start, which leads to additional costs.

Severe cases of lost circulation may lead to well control problems. In particular, the mud column disappearing into the formation may reduce the fluid pressure in the well, which will enable the influx of formation fluids, in particular gas, into the well. This may eventually lead to a kick or borehole collapse. Lost circulation in tophole sections may lead to shallow water flow events.

Given the scope of its negative consequences, lost circulation has been identified as “one of the drilling industry's most singular problems” [4]. According to some estimates, the annual cost of lost circulation problems, including the cost of materials and the rig time, is around one billion dollars globally [5,6].

Lost circulation in the overburden can be equally as bad as in the reservoir, even though formation damage is of no concern there. If losses are not treated properly and drilling proceeds without first sealing the thief zone, subsequent cement jobs can be compromised. The quality of well cementing depends crucially on placing the cement column all the way up to the target height. If an unplugged thief zone exists against the annulus to be cemented, cement slurry may escape into this zone during the cement job, and the cemented length of the annulus will be shorter than planned. Remedial cementing can be employed to cure the problem, but this will increase nonproductive time and incur extra costs.

Lost circulation is common in geothermal drilling (Boxes 1.1 and 1.2) [7,8]. Large fracture apertures (on the order of cm) often cause severe or total losses while drilling the overburden or the reservoir. According to Ref. [9], lost circulation problems are responsible for 10% of well costs in mature geothermal fields and often more than 20% of well costs in exploration wells in the United States. In Iceland, an analysis revealed that lost circulation or hole collapse was the primary cause of drilling troubles in 18 out of 24 wells in the Hengill Geothermal Area [10]. These problems may further lead to cement losses into the formation during subsequent well cementing.

Wells drilled in fields with elevated geothermal gradient are often prone to losses caused by cooling. When the relatively cold drilling fluid coming from the surface contacts the much hotter formation, the rock contracts and the hoop stress around the hole becomes smaller—ie, less compressive. The rock is then easier to fracture because of this effect. Ballooning and losses observed in some Gulf of Mexico wells are attributed to this effect [11].

Lost circulation is common in naturally fractured formations. Severe or total losses are common in carbonate rocks in the Middle East [12]. In a naturally fractured carbonate field in Iran, mud losses were reported in 35% of drilled wells [3]. In Saudi Arabia, 32% of wells in the naturally fractured carbonate Khuff Formation experience ballooning, while 10% experience lost circulation [13].

Box 1.1 Lost Circulation in a Geothermal Well in Iceland

Lost circulation is a common problem in geothermal drilling, where it is exacerbated by high temperatures and hard rocks. A sequence of lost circulation events while drilling a geothermal well in the Krafla field in Iceland in 2008–09 was described by Pálsson et al. in their paper “Drilling of the well IDDP-1” (Geothermics, 2014, 49, 23–30).

The well, IDDP-1, was part of the Iceland Deep Drilling Project and was originally designed to reach a 4500-m depth. Severe problems were encountered during drilling, and the well had to be sidetracked every time it approached magma at 2100 m. Mud losses were experienced often, becoming worse as the depth increased. Also, the well became increasingly more unstable and washed out with depth, which affected the hole cleaning and reduced the rate of penetration that already was low due to the hard rock.

Minor mud losses occurred in the first 1000 m while drilling for the intermediate 18-5/8″ casing. The losses were cured with lost circulation material.

Losses of 20 L/s were then experienced at 1432 m. The problem was cured by cementing the thief zone.

Losses in excess of 60 L/s were experienced at 2043 m. The losses could not be cured. The weighted drilling fluid was replaced with water, and drilling continued.

At 2101 m, the bottomhole assembly broke. After unsuccessful fishing, a cement plug was placed, and the well was sidetracked.

Upon the sidetrack, total losses occurred at 2054 m. A cement plug was placed in the well, and the drilling continued from 2060 m. Losses resumed at 2067 m, and total loss of circulation occurred at 2076 m.

Continued problems with stuck pipe and unsuccessful fishing attempts at 2103 m forced a second sidetrack. Mud losses continued after the sidetrack, and total loss of circulation occurred at 2071 m. Drilling was terminated upon reaching magma at 2100 m. The well was then tested and completed.

The best way to deal with lost circulation is to prevent it from happening a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Chapter 1. The Challenge of Lost Circulation
• Chapter 2. Stresses in Rocks
• Chapter 3. Natural Fractures in Rocks
• Chapter 4. Drilling Fluid
• Chapter 5. Mechanisms and Diagnostics of Lost Circulation
• Chapter 6. Preventing Lost Circulation
• Chapter 7. Curing the Losses
• Chapter 8. Knowledge Gaps and Outstanding Issues
• Index