-
The present invention relates in general to a brine-based drilling fluid and methods for
storing fluids on drilling vessels, and more specifically to storing brine-based drilling fluids in
drilling vessel ballast tanks.
-
For many years petroleum companies concentrated on developing oil and gas fields on
land. But the world's appetite for energy sources, coupled with diminishing returns from land
drilling, has driven petroleum companies to develop offshore reserves.
-
Sub-sea geologic sediments and structures are often similar and in some cases superior to
geologic conditions that have proven highly productive on land. In fact, offshore reserves have
been estimated at 21% of the world's proven reserves, with estimates that 40% to 50% of all
future resources will come from offshore reserves.
-
A need exists for a method to store sufficient quantities of drilling fluids on a drilling
vessel to reduce the dependency of a drilling operation on supplies brought in by work boats,
thereby ensuring uninterrupted drilling in the event of inclement weather.
-
A further need exists for drilling fluid compositions suitable for storage on a drilling
vessel.
-
Drilling offshore wells in deep water, greater than 1000 feet (304.8 m) in depth, creates
its own set of problems. When drilling on the edge of the continental shelf, quite frequently
pressured shallow depth sands, of apparently artesian flow, are encountered. The depth of these
sands and the pressures that they exhibit create a unique well design situation.
-
The unique well design is the result of being unable to hydrostatically control the
shallow water flows (SWF) by the conventional method of returning the drilling fluid to the
drilling rig. The hydrostatic head generated by returning the fluid to the rig exceeds the fracture
gradients of the rock above the SWF. Therefore, the well is designed in a manner that a fluid of
the proper density returns only to the sea floor, riserless drilling.
-
In a riserless drilling situation, large volumes of drilling fluid are required due to the fact
that the fluid is not returned to the rig and reused. Depending upon the depth of SWF, volumes
from 10,000 to over 30,000 bbls (1590 to 4770 m3) of drilling fluid could be required. The
surface mixing equipment of existing rigs is insufficient to store or prepare the large volumes of
fluids required to drill riserless. To date, riserless drilling operations have been dependent upon
work boats and barges to store and transport the required fluids that were prepared at land based
facilities. Often, bad weather has interrupted the supply of work boats and therefore the supply
of drilling fluid, causing the termination of drilling operations.
-
The invention contemplates a system for storing, mixing and pumping drilling fluids on
drilling vessels such as deep water rigs.
-
According to a first aspect of the invention, there is provided a method of drilling a sub-sea
well comprising: (a) preparing a drilling fluid suitable for storage in a ballast tank, wherein
said drilling fluid contains little or no particulate material and fluid density is provided at least in
part by dissolved solids; (b) transporting the drilling fluid to a floating drilling vessel having at
least one ballast tank; (c) pumping a quantity of the drilling fluid into said ballast tank or tanks
of said drilling vessel while monitoring said pumping and distribution of said fluid into said tank
or tanks so as to maintain balance of said vessel; and (d) pumping the drilling fluid from said
ballast tank or tanks into the wellbore as it is being drilled while monitoring said pumping of
said fluid from said tank so as to maintain balance of said vessel.
-
According to a second aspect of the invention, there is provided a process for storing
drilling fluids on a drilling vessel including preparing a drilling fluid suitable for ballast tank
storage, transporting the drilling fluid to a drilling vessel, pumping a quantity of the drilling
fluid into at least one ballast tank compartment of the drilling vessel, and trimming the drilling
vessel during the addition of the drilling fluid.
-
In an embodiment, the drilling fluid provides a biostatic environment in the ballast tank
compartment.
-
In another embodiment, the drilling fluid pumped into the ballast tank contains little of
no particulate material.
-
In another embodiment, at least 10,000 bbls of drilling fluid are stored in the ballast tank.
-
According to a third aspect of the invention, there is provided a method for drilling a
sub-sea well comprising (a) preparing a drilling fluid suitable for storage in at least one ballast
tank; (b) transporting the drilling fluid to a drilling vessel; (c) pumping a quantity of the drilling
fluid into a ballast tank compartment of a drilling vessel: and (d) pumping the drilling fluid into
the wellbore as it is being drilled.
-
In an embodiment, the method further comprises designing the drilling fluid based on an
analysis of the geologic information gathered at the drilling site.
-
In another embodiment, the drilling fluid provides a biostatic environment in the ballast
tank compartment.
-
In another embodiment about 10,000 to 30,000 bbls are pumped into at least one ballast
tank compartment.
-
In a further embodiment the method further comprises removing an amount of the
drilling fluid from the ballast tank compartment and mixing the drilling fluid with a particulate
material before pumping the drilling fluid into the wellbore.
-
In a further embodiment, the method further comprises trimming the drilling vessel
during the addition and removal of the drilling fluid from the ballast tank compartment.
-
According to a fourth aspect of the invention, there is provided a method of drilling a
sub-sea well comprising: (a) gathering geologic information about the drilling site; (b) preparing
a drilling fluid based on the geologic information gathered about the drilling site, the drilling
fluid being suitable for storage in the ballast tank compartment; (c) pumping at least 10,000 bbls
of the drilling fluid into at least one ballast tank compartment of a drilling vessel; (d) removing
an amount of the drilling fluid from that ballast tank compartment; (e) trimming the drilling
vessel during the addition and removal of the drilling fluid from the ballast tank compartment;
(f) admixing the drilling fluid with a particulate material; and (g) pumping the mixture of
drilling fluid and particulate material into the wellbore as it is being drilled.
-
According to a fifth aspect of the invention, a drilling fluid is prepared, transported to a
drilling vessel, pumped into a ballast tank compartment of the drilling vessel for storage until
the drilling operation begins, removed from the ballast tank compartment, mixed with solid
particulate matter and pumped into the wellbore during drilling.
-
The stored drilling fluid will be designed (1) to contain no undissolved solids, (2) to be
rheologically stable, (3) to be biostatic, (4) to be capable of suspending particulate matter that is
added in the drilling operation, and (5) to provide density through dissolved solids.
-
The foregoing has outlined, rather broadly, aspects of the present invention in order that
the detailed description of the invention that follows may be better understood. Additional
features and advantages of the system for storing, mixing and pumping drilling fluids on a
drilling vessel will be described hereinafter which forms the subject of the claims of the
invention. It should be appreciated by those skilled in the art that the concept and the specific
embodiment disclosed may be readily utilized as a basis for modifying or designing other
processes or compositions for carrying out the same purpose of the present invention. It should
also be realized by those skilled in the art that such equivalent constructions do not depart from
the spirit and scope of the invention as set forth in the appended claims.
-
The present invention provides a process for storing drilling fluids in ballast tanks of
drilling vessels and drilling fluid formulations suitable for ballast tank storage.
-
As petroleum companies have turned to developing offshore oil and gas reserves, they
have been faced with a number of problems. For example, a number or offshore wells have been
lost due to shallow water flows.
-
The use of weighted drilling fluids during the drilling of offshore wells would be helpful
in controlling shallow water flows. A drilling fluid is a liquid circulated through the wellbore
during rotary drilling operations. In addition to its function of bringing cuttings to the surface,
drilling fluid cools and lubricates the bit and drill stem, protects against blowouts by holding
back subsurface pressures, and deposits a mud cake on the wall of the borehole to prevent loss
of fluids to the formation. Drilling fluids are formulated to maintain the hydrostatic pressure
within the wellbore necessary to prevent shallow water flows into the wellbore.
-
Drilling fluids are used throughout the drilling process. A drilling operation requires a
large quantity of drilling fluid (10,000 to 30,000 barrels (1590 to 4770 m3)) to complete the
operation. Such large quantities of drilling fluid present a problem for offshore drilling
operations, since the drilling fluid is typically supplied by work boats or barges bringing the
drilling fluid from land out to the drilling vessel. However, bad weather can interrupt the supply
of work boats and therefore the supply of drilling fluid to the drilling vessel.
-
Whenever the supply of drilling fluid is terminated, the drilling must cease until the
drilling fluid supply is once again available. Interrupted drilling operations require a larger
overall quantity of drilling fluid, than uninterrupted drilling operations and such interruptions
can put the well at risk of shallow water flows. The present invention addresses this problem by
storing sufficient drilling fluid on the drilling vessel to reduce the dependency of a drilling
operation on supplies brought in by work boats, thereby ensuring uninterrupted drilling in the
event of inclement weather.
-
A number of drilling vessels (such as floating mini-tension leg platforms like the
SeaStar™, floating production systems with semi-submersible drilling and production
equipment, tension leg platforms, and SPAR™ platforms, and drillships) are designed with
ballast tanks that are filled with fluid to provide platform stability. The ballast tanks are typically
filled with sea water and the water level raised or lowered as necessary to trim the platform.
-
One embodiment of the present invention utilizes ballast tanks of drilling vessels to store
large quantities of drilling fluids in order to reduce the dependency of drilling vessels on the
supply of work boats during the drilling operation. Drilling fluid formulations are based on an
analysis of geologic information gathered about or at the drilling site. Thus drilling fluids with
the desired characteristics can be prepared for storage in the ballast compartment of a drilling
vessel.
-
Once the drilling fluid is prepared, a period of predicted good weather is selected,
preferably a period of at least two days of predicted good weather is selected. The drilling fluid
is loaded on work boats and transported to the drilling vessel where the drilling fluid is pumped
into the vessel's ballast tank compartments for storage. Keeping the platform balanced or trim
during this operation is important and requires a careful monitoring of pump action and drilling
fluid distribution.
-
Drilling vessel ballast tanks typically have multiple compartments on each side of the
drilling vessel. Individual compartments are emptied of fluid and refilled with drilling fluid in a
sequence and pattern to keep the drilling vessel balanced or trim.
-
When the drilling is ready to begin, the drilling fluid is pumped from the ballast tanks and
mixed with optional ingredients, such as sized solid particulate material like calcium carbonate or
barium sulfate, in the mixing tanks of the drilling vessel. Once again it is important that all the
pumping operations be planned to keep the platform trim throughout the operation. The final
drilling fluid formulation is then ready to be circulated through the wellbore during the drilling
operation.
-
Drilling fluids are formulated to meet the requirements of the well site. For example, the
density of the drilling fluid is designed to maintain the hydrostatic pressure within the wellbore
to prevent shallow water flows. Fluid density is provided by dissolved solids, including without
limitation the solid salts of sodium, potassium, calcium and zinc and the organic acetate and
formate salts of sodium, potassium and cesium. A particular salt is selected to adjust the density
of the drilling fluid based on environmental considerations, the required density, cost, and the
freezing point of the required solution (highly concentrated solutions of certain salts have a high
enough freezing point that they are subject to freezing in colder waters).
-
Furthermore, the drilling fluid should have sufficient carrying capacity to remove the bit
cuttings from the wellbore. Materials used to adjust the carrying capacity of the drilling fluid
include without limitation hydroxyethyl cellulose, welan gum, guar gum, xanthum gum,
polyacrylamide/polyacrylate, or carboxymethyl cellulose.
-
Drillers often encounter zones that accept large volumes of drilling fluid due to fractures,
coarse sand, gravel, or other formations. Severe losses in drilling fluid can be controlled by
circulating high concentrations of sized solids suspended in viscous fluids or gels. Such mixtures
are referred to as lost circulation materials. The lost circulation materials are designed to bridge
and seal very permeable formations and to prevent fractures from growing. Appropriate water
soluble viscosifiers or suspension agents for drilling fluids are xanthan gum and N-VIS™ HB
(available from Barold Drilling Fluids, Houston, Texas). Suitable sized solid particulates include
barium sulfate, calcium carbonate, iron carbonate, and hematite. Additional fluid loss control can
be provided by starch derivatives, polyacrylates, amps polymers, and lignin based materials.
-
One major concern for drilling fluid that is to be stored in ballast tanks and used on a
drilling vessel, is that any additional components that must be added to the drilling fluid during
the drilling operation must be kept at a minimum. Since drilling vessels generally have limited
mixing capacity, it is important to minimize the need for mixing additional materials. However,
it is also important that drilling fluids to be stored in ballast tanks should not contain particulate
material that could settle out of the drilling fluid, any sized solid particulate material such as
barium sulfate, calcium carbonate, iron carbonate, or hematite must be mixed with the fluid on the
drilling vessel before it is used during the drilling operation.
-
Yet another desirable feature of drilling fluid, suitable for ballast tank storage and use on
a drilling vessel, is that the fluid be rheologically stable and remain in a homogenous state while
being stored. For example, the drilling fluid should provide a biostatic environment that would
inhibit bacterial growth and the bacterial breakdown of certain drilling fluid components.
Examples of brine-based drilling fluids suitable for ballast tank storage are set forth below. The
examples given below are meant to be illustrative and not limiting.
Example 1
-
Potassium Chloride Based Drilling Fluid
|
Ingredients
|
Per barrel
|
9.7 lb/gal saturated KCl brine |
1 bbl. |
N-VIS™ |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristics of the Drilling Fluid in Example 1
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
14 |
11 |
Yield point, lb/100 ft. |
28 |
20 |
10 Sec gel, lb/100 ft. |
9 |
7 |
10 Min gel, lb/100 ft. |
12 |
11 |
pH |
7.6 |
API filtrate, ml |
6.8 |
Fann 35 dial readings |
|
600 rpm |
56 |
42 |
300 rpm |
42 |
31 |
200 rpm |
35 |
26 |
100 rpm |
26 |
20 |
6 rpm |
10 |
8 |
3 rpm |
9 |
7 |
Example 2
-
Sodium Chloride Based Drilling Fluid
|
Ingredients
|
Per barrel
|
10.0 lb/gal saturated NaCl brine |
1 bbl. |
N-VIS™ |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristics of the Drilling Fluid in Example 2
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
18 |
14 |
Yield point, lb/100 ft. |
28 |
22 |
10 Sec gel, lb/100 ft. |
9 |
7 |
10 Min gel, lb/100 ft. |
11 |
11 |
pH |
7.3 |
API filtrate, ml |
6.4 |
Fann 35 dial readings |
|
600 rpm |
64 |
50 |
300 rpm |
46 |
36 |
200 rpm |
38 |
29 |
100 rpm |
28 |
23 |
6 rpm |
10 |
9 |
3 rpm |
9 |
7 |
Example 3
-
Calcium Chloride Based Drilling Fluid
|
Ingredients
|
Per barrel
|
10.0 lb/gal saturated CaCl2 brine |
1 bbl. |
N-VIS™* HB |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristics of the Drilling Fluid in Example 3
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
38 |
27 |
Yield point, lb/100 ft. |
22 |
18 |
10 Sec gel, lb/100 ft. |
6 |
6 |
10 Min gel, lb/100 ft. |
9 |
8 |
pH |
5.8 |
API filtrate, ml |
3.8 |
Fann 35 dial readings |
|
600 rpm |
98 |
72 |
300 rpm |
60 |
45 |
200 rpm |
45 |
34 |
100 rpm |
28 |
22 |
6 rpm |
7 |
7 |
3 rpm |
6 |
6 |
Example 4
-
Calcium Bromide Based Drilling Fluid
|
Ingredients
|
Per barrel
|
14.2 lb/gal CaBr2, brine |
1 bbl. |
N-VIS™ HB |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristics of the Drilling Fluid in Example 4
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
29 |
23 |
Yield point, lb/100 ft. |
14 |
12 |
10 Sec gel, lb/100 ft. |
2 |
2 |
10 Min gel, lb/100 ft. |
4 |
3 |
pH |
5.2 |
API filtrate, ml |
6.2 |
Fann 35 dial readings |
|
600 rpm |
72 |
58 |
300 rpm |
43 |
35 |
200 rpm |
32 |
25 |
100 rpm |
19 |
15 |
6 rpm |
4 |
3 |
3 rpm |
3 |
2 |
Example 5
-
Sodium Formate Brine Based Drilling Fluid
|
Ingredients
|
Per barrel
|
11.1 lb/gal saturated sodium format brine |
1 bbl. |
N-VIS™ |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
Sodium hydroxide |
0.1 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristics of the Drilling Fluid in Example 5
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
26 |
17 |
Yield point, lb/100 ft. |
28 |
20 |
10 Sec gel, lb/100 ft. |
6 |
5 |
10 Min gel, lb/100 ft. |
8 |
7 |
pH |
10.7 |
API filtrate, ml |
4.6 |
Fann 35 dial readings |
|
600 rpm |
80 |
54 |
300 rpm |
54 |
37 |
200 rpm |
40 |
28 |
100 rpm |
25 |
20 |
6 rpm |
7 |
6 |
3 rpm |
6 |
5 |
Example 6
-
Potassium Formate Brine Based Drilling Fluid
|
Ingredients
|
Per barrel
|
13.1 lb/gal saturated Potassium formate brine |
1 bbl. |
N-VIS™ HB |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
Potassium hydroxide |
0.1 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristics of the Drilling Fluid in Example 6
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
23 |
17 |
Yield point, Ib/100 ft. |
12 |
12 |
10 Sec gel, lb/100 ft. |
5 |
3 |
10 Min gel, lb/100 ft. |
7 |
5 |
pH |
10.6 |
API filtrate, ml |
4.2 |
Fann 35 dial readings |
|
600 rpm |
58 |
46 |
300 rpm |
35 |
29 |
200 rpm |
27 |
21 |
100 rpm |
17 |
14 |
6 rpm |
6 |
4 |
3 rpm |
5 |
3 |
Example 7
-
Sodium Bromide Based Drilling Fluid
|
Ingredients
|
Per barrel
|
12.7 lb/gal saturated NaBr brine |
1 bbl. |
N-VIS™ |
1 lb. |
N-DRIL HT PLUS™ |
4 lb. |
LIQUI-VIS EP™ |
0.5 lb. |
Characteristic of the Drilling Fluid in Example 7
|
Characteristics Measured
|
Measurement
|
|
Test 1
|
Test 2
|
Stirred, min |
30 |
30 |
Temperature, °F |
72 |
120 |
Plastic viscosity, cP |
18 |
13 |
Yield point, lb/100 ft. |
27 |
21 |
10 Sec gel, lb/100 ft. |
6 |
5 |
10 Min gel, lb/100 ft. |
8 |
7 |
pH |
6.2 |
API filtrate, ml |
3.6 |
Fann 35 dial readings |
|
600 rpm |
63 |
47 |
300 rpm |
45 |
34 |
200 rpm |
36 |
26 |
100 rpm |
25 |
19 |
6 rpm |
7 |
6 |
3 rpm |
6 |
5 |
-
Numerous modifications and variations in the process for storing, mixing and using
drilling fluids on a drilling vessel and in the composition of the drilling fluid composition are
possible in light of the above teachings. It is therefore understood that within the scope of the
appended claims, the invention may be practiced other than as specifically described.