Abstract
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INTRODUCTION
DGD is a technology that makes use of separate fluids with different densities in the wellbore.
The lighter fluid floats on top of the heavy fluid in the riser. The lighter fluid is only used for
inducing pressure and is otherwise inactive. However the heavy fluid is used for the same
purpose as used in the conventional drilling procedures. This helps to adjust the bottom hole
pressure (BHP) in a shorter time, and make it able to adjust the well bore pressure curves with
the formation pressure curves. The attractions that DGD highlights are the reduction in the cost
of drilling and an increase in the production rate after well completion ( Gaup, 2014).
The development work on the DGD was accelerated during the 1990s when a joint industry
project was undertaken with the aim to utilize such technology to be used in the high pressure,
low fracture gradient in ultra-deep waters. Even though sufficient investments have been made
on drilling rigs which can operate in depths greater than 8000ft, the resources present at these
reservoirs cannot be extracted unless new procedures are developed to lower hydrostatic mud
pressures to avoid fractures in the shallow zones. The problems faced in ultra-deep drilling
include shallow water flowing, lost circulation and loss of well control. If any of these problems
occur, they will prevent the completion of the well to be achieved.
Multiple casing strings are used to avoid such problems. This means that the production string is
quite small for a high production well and also for horizontal and multilateral completions in
order to make the project economically viable. Pumps are used to reduce the hydrostatic head
from the mud-line to the surface in DGD techniques. This is the reason why there is no balanced
u-tube present in DGD as compared to the conventional drilling ( Kennedy 2001).
The primary component that enables the DGD operations is the Mud Lift Pump (MLP). With the
help of diaphragm pumps powered by the seawater, it pumps the drilling fluid and cutting back
to the rig floor. The Subsea Rotating Device (SRD) maintains the boundary between the sea
water density fluid in the drilling riser and the drilling fluid and redirects the mud through the
MLP through the Solids Processing Unit (SPU). SPU is used to decrease the size of the drill
cuttings which can be managed by the MLP (Ganpatye et al. 2013).
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The principal objective of this study is to help the Drilling Industry to enhance the reliability and
improve the cost effectiveness with minimal maintenance. To address the issues, Dual Gradient
Drilling (DGD) is seen as the most economically viable option. DGD is currently providing
solutions for the problems associated with the depleted offshore and deep-water reservoirs.
In the past, the oil and gas industry has typically used the single gradient system to drill wells
offshore. With this system the bottom hole pressure was controlled by a mud column
extending from the drilling rig to the bottom of the wellbore. This mud column was used to
achieve the required bottom hole pressure. Because of the narrow margin between the pore and
fracture pressures it is somewhat difficult to reach total depth with the single gradient system.
This led to the invention of the dual gradient system. In the dual gradient method, heavy density
fluid runs from the bottom hole to the mud line and a low density fluid from the mud line to the
rig floor so as to maintain the bottom hole pressure. Dual gradient Drilling is an unconventional
method of drilling in which a relatively small diameter return line is used to circulate drill fluids
and cuttings from the sea floor to the rig’s surface mud system. During DGD, the rig’s marine
riser is kept full of seawater. A rotating diverter, which is similar to a rotating control head,
separates the wellbore and its contained fluids from the seawater in the marine riser. During well
kill operations, the return line is utilized as the choke line in conventional riser drilling.
Drilling technology has been in a continuous developing process. As early as in the 1890s, oil
wells were drilled in water, from land connected platforms in lakes and along the coastline, and
in the late 1940s wells were drilled from platforms in the open sea. Today, wells drilled in water
depths of more than 3’000m are not unusual, and offshore wells with a measured depth of more
than 10’000m have been drilled. This line of developing new ways of reaching the hydrocarbons
in the ground has not come to an end, and further technological improvements are still needed to
reach the hydrocarbons the world will need in the years to come.
To drill oil wells safe and problem free in ultra-deep (a greater depth than 1500m) waters,
accurate pressure control is required. The main topic in this research, Dual Gradient Drilling
(DGD), is a drilling technology that separates from conventional drilling by simultaneously
utilizing two different fluids with different densities in the wellbore. This enables both a quicker
way of adjusting the bottom hole pressure (BHP), and the ability to make the wellbore pressure
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curves fit better with the formation pressure curves. DGD is ideal for use in ultra-deep waters,
and primarily by reducing drilling cost, but also by increasing the production rate when the well
is completed, the use of DGD can add great value to a prospect. Both details and challenges for
the DGD technology, and possible benefits will be discussed in more detail in this report. To
cover the world’s future consume of hydrocarbons, technological improvements are needed,
turning currently unreachable and unprofitable reservoirs into the opposite. The main focus of
this thesis, Dual Gradient Drilling (DGD), is a drilling technology that is ideal for drilling in
ultra-deep waters, and could prove vital in drilling the reservoirs that are currently undrillable.
This will increase the possible supply of hydrocarbons available for consumption.
DGD is not fully accepted in the drilling industry, and DGD is still considered unconventional.
At present the deep sea drilling utilizes conventional drilling fluids and enormous number of
casings which significantly affects the fluid density. The well productivity is directly affected by
the inculcation/leakage of mud in low producing zones. The cost of deep sea water exploration
rapidly increases and at the same time imposes technical boundaries on the depths of the well
that can be reached thereby affecting the productivity. Dual Gradient Drilling (DGD) is the
establishment of multiple pressure gradients within the selected sections of the annulus for
managing the annular pressure profile.
Figure 1.1 shows the different pressure profile of conventional and DGD while Figure 1.2 shows
the principle difference between conventional drilling and DGD.
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Figure 1.1 Conventional vs. DGD Drilling procedure with pressure profile
(Drillingcontractor.org 2014)
Figure 1.1: Principle difference: Conventional and DGD
1.1 Problem statement
The concept of Dual Gradient Drilling is a new concept within the exploration and production
industry. It allows access to reservoirs present in ultra-deep waters in a cost effective fashion
with improved safety by reducing the issues that occur because of the narrow margin between
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the pore pressure and the fracture pressure gradients. This system reduces the hydrostatic
pressure of the drilling mud from inside the riser and hence the bottom hole pressure is reduced.
The basic objective for a DGD system is to discover the deep water environment and drill wells
in ultra-deep waters. This procedure reduces the operational costs which is the main incentive for
companies to adopt this process.
1.2 Project aim
To critically review and analyze the benefits involved in using dual gradient drilling as compared
to conventional drilling.
1.3 Project objectives
? Evaluation of the key challenges to successful execution of dual gradient drilling.
? Review of the advantages and limitations of the implementation of the dual gradient drilling
procedures with the conventional drilling platforms.
? Evaluation of the reliability of the dual gradient drilling.
1.4 Organization of study
This research aims to investigate the benefit of DGD technology. This work will also involve a
detailed work comparing the conventional drilling and dual gradient drilling. This research is
classed into five chapters. Chapter one covers the introductory phase of the research. The second
chapter covers the literature review which examines the background of the technology based on
the objectives. The third chapter covers the methodology which is based on the collection and
analysis of data from literature. While the results, recommendation and discussion covered the
fourth chapter. The final chapter covered the conclusion of this research.