Two Methods for Achieving a Dual Gradient in Deepwater

Abstract

The advantages of a dual gradient mud column have been well documented. Significant work has been done on the riserless drilling systems by several different companies. This paper will propose two methods to achieve the same dual gradient.Nitrogen Injection: Building from proven air drilling procedures and underbalanced techniques nitrogen can be used to cut the mud weight back in the riser above the seafloor or the cut can be made deeper by combination with a concentric riser.Floating Mud Cap: A dual activity rig can use a casing riser linked to the adjacent marine drilling riser. A submersible pump in the casing riser regulates returns that will set the mud cap level in the drilling riser. A pressure sensor in the subsea BOP allows monitoring the effective hydrostatic pressure. This approach again combines field proven procedures and hardware in the deepwater environment. Both methods will be presented with emphasis on the hardware and operational procedures required to successfully implement them. Both of the methods employ existing equipment and procedures. Introduction One of the major challenges of drilling in deepwater in the Gulf of Mexico is due to low fracture gradients and shallow abnormal pressure. The margin between the fracture gradient and required mud is often less than 1 ppg.In young formations, fracture pressures are almost equal to the overburden1,2,3. Because much of the overburden is only due to the weight of the seawater the fracture pressures are only slightly greater than ‘normal pressure’. Unfortunately, there is not a corresponding drop in deepwater pore pressures due to the immaturity of the deposits typically encountered. The water confined in the rapidly deposited clay is in part pressurized by the weight of the overlying sediments trying to wring the water out, which creates a narrow margin between the pore pressures and fracture pressures. As a result the pore pressure and fracture gradients curves almost lie atop each other. A dual gradient drilling approach will result in the effective mud weight at the previous casing shoe being less than the effective mud weight at the drilling depth. The industry's effort for riserless drilling4 is designed to take advantage of this benefit. If casing points are not limited by the mud weight of the previous casing shoe, it is possible to eliminate casing points from the well program. Figure 1 shows a common goal of the dual gradient drilling. Previous efforts only brought the mud weight back to sea water equivalent at the seabed. Even better results can be achieved by cutting the mud to less than seawater at the seabed or at the shoe. By doing this it is possible to find a combination of the two mud weights that cause the heavier mud gradient to always lie between the pore pressure and fracture pressure lines. The pressures may be underbalanced inside the cased hole or riser but the open hole is overbalanced. This can be achieved by reducing the density in the upper mud section to less than seawater or by adjusting the cut point to below the seabed. This paper proposes to achieve the dual gradient by two methods. Both of which use existing equipment and employ known drilling techniques. The first proposal is nitrogen injection where normal gasification drilling methods are applied to the riser section. 1) Nitrogen injection option This paper will outline how nitrogen injected subsea can effectively create a dual gradient by gas lifting the mud in the riser. Another operator had considered a similar approach, but had proposed to sweep the entire 21" marine drilling riser with gas. This paper proposes to combine nitrogen injection with a high pressure concentric casing riser5. This reduces greatly the gas required to cut the mud. Figure 2 illustrates an example of the nitrogen being injected into a subsea BOP stack.

Managed-Pressure Drilling; Techniques and Options for Improving Efficiency, Operability, and Well Safety in Subsea TTRD

“…(A schematic diagram of the different methods is included in Figure 7 in appendix) There will be other methods or combinations of the methods listed above. Most of these methods are described and discussed in different papers included in the reference list [11][12][13][14][15][16][17][18][19][20][21][22][23] .…”

Evaluation of MPD Methods for Compensation of Surge-and-Swab Pressures in Floating Drilling Operations

“…However, a rapid system response is required. Figure 8 illustrates a dual gradient system using subsea mud lift pump and riser 13,14 . The subsea pump controlls the sea water / mud interface at desired level.…”

Opening New Exploration Frontiers With The Micro-Flux Control Method For Well Design

“…In the last 5 years the industry has focused on developing alternative drilling methods for various onshore and offshore situations [8][9][10][11][12][13][14][15][16][17][18][19][20] . The deepwater environment is a good and topical example; here, dual gradient drilling was thought to be the only way wells could be drilled in basins with high pore pressure [8][9][10][11][12] .…”

“…The deepwater environment is a good and topical example; here, dual gradient drilling was thought to be the only way wells could be drilled in basins with high pore pressure [8][9][10][11][12] . But, even in this scenario, the critical conditions originally thought to restrict drilling did not materialize and the need for such expensive and complex methods now appear unfounded.…”