CHAPTER ONE 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).
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
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.
