Wells – Submerged well – Testing
Reexamination Certificate
2000-12-06
2002-12-31
Bagnell, David (Department: 3672)
Wells
Submerged well
Testing
C166S250080, C073S152270
Reexamination Certificate
active
06499540
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates generally to methods and procedures for maintaining well control during drilling operations. More specifically, the invention relates to methods and procedures where “riserless” drilling systems are used.
2. Background Art
Exploration companies are continually searching for methods to make deep water drilling commercially viable and more efficient. Conventional drilling techniques are not feasible in water depths of over several thousand feet. Deep water drilling produces unique challenges for drilling aspects such as well pressure control and wellbore stability.
Deep Water Drilling
Deep water drilling techniques have, in the past, typically relied on the use of a large diameter marine riser to connect drilling equipment on a floating vessel or a drilling platform to a blowout preventer stack on a subsea wellhead disposed on the seafloor. The primary functions of the marine riser are to guide a drill string and other tools from the floating vessel to the subsea wellhead and to conduct drilling mud and earth cuttings from a subsea well back to the floating vessel. In deeper waters, conventional marine riser technology encounters severe difficulties. For example, if a deep water marine riser is filled with drilling mud, the drilling mud in the riser may account for a majority of the drilling mud in the circulation system. As water depth increases, the drilling mud volume increases. The large volume of drilling mud requires an excessively large circulation system and drilling vessel. Moreover, an extended length riser may experience high loads from ocean currents and waves. The energy from the currents and waves may be transmitted to the drilling vessel and may damage both the riser and the vessel.
In order to overcome problems associated with deep water drilling, a technique known as “riserless” drilling has been developed. Not all riserless techniques operate without a marine riser. The marine riser may still be used for the purpose of guiding the drill string to the wellbore and for protecting the drill string and other lines that run to and from the wellbore. When marine risers are used, however, they typically are filled with seawater rather than drilling mud. The seawater has a density that may be substantially less than that of the drilling mud, substantially reducing the hydrostatic pressure in the drilling system.
An example of a riserless drilling system is shown in U.S. Pat. No. 4,813,495 issued to Leach and assigned to the assignee of the present invention. A riserless drilling system
10
of the '495 patent is shown in FIG.
1
and comprises a drill string
12
including drill bit
20
and positive displacement mud motor
30
. The drill string
12
is used to drill a wellbore
13
. The system
10
also includes blowout preventer stack
40
, upper stack package
60
, mud return system
80
, and drilling platform
90
. As drilling is initiated, drilling mud is pumped down through the drill string
12
through drilling mud line
98
by a pump which forms a portion of mud processing unit
96
. The drilling mud flow operates mud motor
30
and is forced through the bit
20
. The drilling mud is forced up a wellbore annulus
13
A and is then pumped to the surface through mud return system
80
, mud return line
82
, and subsea mudlift pump
81
. This process differs from conventional drilling operations because the drilling mud is not forced upward to the surface through a marine riser annulus.
The blowout preventer stack
40
includes first and second pairs of ram preventers
42
and
44
and annular blowout preventer
46
. The blowout preventers (“BOP”s) may be used to seal the wellbore
13
and prevent drilling mud from travelling up the annulus
13
A. The ram preventers
42
and
44
include pairs of rams (not shown) that may seal around or shear the drill string
12
in order to seal the wellbore
13
. The annular preventer
46
includes an annular elastomeric member that may be activated to sealingly engage the drill string
12
and seal the wellbore
13
. The blowout preventer stack
40
also includes a choke/kill line
48
with an adjustable choke
50
. The choke/kill line
48
provides a flow path for drilling mud and formation fluids to return to the drilling platform
90
when one or more of the BOPs (
42
,
44
, and
46
) have been closed.
The upper end of the BOP stack
40
may be connected to the upper stack package
60
as shown in FIG.
1
. The upper stack package
60
may be a separate unit that is attached to the blowout preventer stack
40
, or it may be the uppermost element of the blowout preventer stack
40
. The upper stack package
60
includes a connecting point
62
to which mud return line
82
is connected. The upper stack package
60
may also include a rotating head
70
. The rotating head
70
may be a subsea rotating diverter (“SRD”) that has an internal opening permitting passage of the drill string
12
through the SRD. The SRD forms a seal around the drill string
12
so that the drilling mud filled annulus
13
A of the wellbore
13
is hydraulically separated from the seawater. The rotating head
70
typically includes both stationary elements that attach to the upper stack package
40
and rotating elements that sealingly engage and rotate with the drill string
12
. There may be some slippage between rotating elements of the rotating head
70
and the drill string
12
, but the hydraulic seal is maintained. During drill pipe “trips” to change the bit
20
, the rotating head
70
is typically tripped into the hole on the drill string
12
before fixedly and sealingly engaging the upper stack package
60
that is connected to the BOP stack
40
.
The lower end of the BOP stack
40
may be connected to a casing string
41
that is connected to other elements (such as casing head flange
43
and template
47
) that form part of a subsea wellhead assembly
99
. The subsea wellhead assembly
99
is typically attached to conductor casing that may be cemented in the first portion of the wellbore
13
that is drilled in the seafloor
45
. Other portions of the wellbore
13
, including additional casing strings, well liners, and open hole sections extend below the conductor casing.
The mud return system
80
includes the subsea mudlift pump
81
that is positioned in the mud return line
82
adjacent to the upper stack package
60
. The subsea mudlift pump
81
in the '495 patent is shown as a centrifugal pump that is powered by a seawater driven turbine
83
that is, in turn, driven by a seawater transmitting powerfluid line
84
. The mud return system
80
boosts the flow of drilling mud from the seafloor
45
to the drilling mud processing unit
96
located on the drilling platform
90
. Drilling mud is then cleaned of cuttings and debris and recirculated through the drill string
12
through drilling mud line
98
.
Subsea Well Control
When drilling a well, particularly an oil or gas well, there exists the danger of drilling into a formation that contains fluids at pressures that are greater than the hydrostatic fluid pressure in the wellbore. When this occurs, the higher pressure formation fluids flow into the well and increase the fluid volume and fluid pressure in the wellbore. The influx of formation fluids may displace the drilling mud and cause the drilling mud to flow up the wellbore toward the surface. The formation fluid influx and the flow of drilling and formation fluids toward the surface is known as a “kick.” If the kick is not subsequently controlled, the result may be a “blowout” in which the influx of formation fluids (which, for example, may be in the form of gas bubbles that expand near the surface because of the reduced hydrostatic pressure) blows the drill string out of the well or otherwise destroys a drilling apparatus. An important consideration in deep water drilling is controlling the influx of formation fluid from subsurface formations into the well to control kicks and prevent blowouts from occurring.
Drilling operations typically involve
Alexander Carmon H.
Choe Jonggeun
Juvkam-Wold Hans C.
Schubert Jerome J.
Weddle, III Curtis E.
Bagnell David
Conoco Inc.
Rosenthal & Osha L.L.P.
Stephenson Daniel P
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