OA12493A

OA12493A - Liquid lift method for drilling risers.

Info

Publication number: OA12493A
Application number: OA1200300202A
Authority: OA
Inventors: Charles Rapier Dawson; Yuzh-Hwang Tsao; Sandra Nowland Hopko
Original assignee: Exxonmobil Upstream Res Co
Priority date: 2001-02-23
Filing date: 2002-02-21
Publication date: 2006-05-24
Also published as: CA2438885A1; BR0207407B1; AU2002245482B2; NO20033720L; GB2391572A; GB2391572B; BR0207407A; GB0321638D0; CA2438885C; NO325188B1; MXPA03007387A; NO20033720D0; WO2002068795A1

Abstract

A method for drilling a well below a body of water as disclosed which includes injecting into the well, at a depth below the water surface, a liquid (74) having a lower density than a density of a drilling mud (76) producing a mixture of drilling mud (76) and low-density liquid (74) in the well. The mixture of drilling mud (76) and low-density liquid (74) is withdrawn from an upper end of the well. The drilling mud (76) and the low-density liquid (74) are separated, with at least a portion of the separated low-density liquid (74) returned to the depth below the water surface and at least a portion of the separated drilling mud (76) returned to an upper end of the drill string (60).

Description

012493

LIQUID LIFT METHOD FOR DRILLING RISERS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application daims priority benefit from U.S. provisional applicationnumber 60/271,304 filed on February 23,2001.

FIELD OF THE INVENTION 5 The invention relates generally to offshore drilling Systems. More particularly, the invention relates to a dual-gradient offshore drilling System usinglow-density liquid lift for drilling risers.

BACKGROUND OF THE INVENTION

The search for crade oil and natural gas in deep and ultra-deep water has10 resulted in greater use of floating drilling vessels. These vessels may be moored ordynamically-positioned at the drill site. Deep water drilling typically involves the useof marine risers. A riser is formed by joining sections of casing or pipe. The riser isdeployed between the drilling vessel and wellhead equipment located on the sea floorand it is used to guide drill pipe and tubing to the wellhead and to conduct a drilling 15 fluid and earth-cuttings from a subsea wellbore back to the floating vessel. A drillstring is enclosed within the riser pipe. The drill string includes a drilling~assemblythat cames a drill bit. A suitable drilling fluid (commonly called “drilling mud” or “mud”) is 20 supplied or pumped under pressure from the drilling vessel. This drilling muddischarges at the bottom of the drill bit. Mud lubricates and cools the bit, and liftsdrill cuttings out of the wellbore. In conventional offshore drilling, drilling mud iscirculated down the drill string and up through an annulus between the drill string andthe wellbore below the mudline (sea floor), and from the mudline to the surface 25 through the riser/drill string annulus.

Drilling mud is very important in the drilling process. It serves as: (1) a 2 012493 lubrication and heat transfer agent; (2) a medium to cany away and dislodge pièces ofthe formation eut by the drill bit; and (3) a fluid seal for crucial well control puiposes.To maintain well control, drilling operators attempt to carefully control the muddensity at the surface of the well to avoid many potential problems. One potential 5 problem is “lost circulation” when a column of drilling mud exerts excess hydrostaticpressure, which propagates a fracture in the formation. Formation fluids may enterthe wellbore unexpectedly when the hydrostatic pressure falls below the formationpressure. Such an event is called “taking a kick.” A blowout occurs when theformation fluid enters the wellbore in an uncontrolled manner. Both of these 10 problems become even more difficult to overcome in deep water. In a conventionaldrilling System, the relative density of the drilliftg mud over that of the seawater,along the length of the riser in deep water, combined with a low overburden pressure,results in excess hydrostatic pressure in the riser/drill string annulus and thewellbore/drill string annulus. 15

Because of the narrow margins between pore pressure (formation fluidpressure) and fracture pressures (leak-offTlost circulation pressures), équivalentcirculating density (ECD) is tightly controlled by balancing hole cleaningrequirements and circulation rates. The wellbore is also cased off at frequent intervals 20 tojmaintain well control.

One solution to these problems known in the art is dual-gradient drilling.Dual-gradient drilling is an area of technology that is primarily used to overcome thenarrow pore pressure/fracture gradient margins found in abnormally pressured, ultra- 25 deepwater wells. As an enabling technology, dual-gradient drilling permits drilling indeep and ultra deep water using fewer casing strings than possible using conventionaldrilling Systems. Because there are fewer casing strings used, there is potential fordrilling dual-gradient wells faster than conventionally drilled wells. Dual-gradientdrilling can also enhance extended-reach drilling by reducing the influence of 30 circulating pressure losses on bottom-hole pressure. Dual-gradient drilling can beused to drill a wellbore with a larger diameter hole at the bottom of the wellbore,resulting in lower pressure drop per unit length than a smaller diameter wellbore. ( 3 012493

Forms of dual-gradient drilling technology being developed include pump-lifted and gas-lifted drilling risers. Pump-lift Systems use pumps positioned near thesea floor to pump the heavy mud/drilling retums from the mud line to the drilling 5 vessel to reduce the hydrostatic pressure at the mud line, generally to that whichwould resuit from a sea water gradient. Illustrative of the pump-lift Systems is U.S.Patent 4,813,495 to Leach that discloses a method and apparatus for drilling subseawells in water depths exceeding 3000 feet (915 meters) (preferably exceeding 4000feet (1220 meters)) where drilling mud retums are taken at the seafloor and pumped to 10 the surface by a centrifugal pump that is powered by a seawater driven turbine. Seealso U.S. Patent No. 4,149,603 to Arnold and published PCT applicationWO9915758. Limitations with the pump-lift Systems include wear and equipmentreliability for the subsea pumps and motors. Also, the ability of the pump-lift Systemto handle dissolved and entrained gas is potentially very poor. 15

Gas-lift Systems use air or nitrogen to “lift” the drilling retums, effectivelylowering the riser hydrostatic pressure to a seawater pressure gradient. For example,U.S. Patent No. 4,099,583 to Maus discloses an offshore drilling method andapparatus which are useful in preventing formation fracture caused by excessive 20 hydrostatic pressure of the drilling fluid in a drilling riser. One or more flow Unes are used to withdraw drilling fluid from the upper portion of the riser pipe. Gas injectedinto the flow lines substantially reduces the density of the drilling fluid and helpsprovide the lift necessary to retum the drilling fluid to the surface. The rate of gasinjection and drilling fluid withdrawal can be contre lied to maintain the hydrostatic 25 pressure of the drilling flûid remaining in the riser and wellbore below the fracturepressure of the formation. See also U.S. Patent No. 3,815,673 to Bruce, et al., U.S.Patent No. 4,063,602 to Howell, et al. and U.S. Patent No. 4,091,881 to Maus.Limitations with the gas-lift system include inefficient or ineffective cuttingstransport, dealing with pressurized equipment on the drilling vessel, and détection of 30 fluid influx from the formation to the well bore (kick détection). 012493 r

SUMMARY OF THE INVENTION

Generally, the invention is a method of drilling a well below a body of waterusing a drill string that starts by injecting into the well, at a depth below the watersurface, a liquid having a lower density than a density of a drilling mud. This 5 produces a mixture of drilling mud and low-density liquid in the well. The low-density liquid may be miscible or immiscible with the drilling mud. The mixture ofdrilling mud and low-density liquid is withdrawn from an upper end of the well. Atleast a portion of the low-density liquid is separated from the mixture of drilling mudand low-density liquid, with at least a portion of the separated low-density liquid 10 retumed to the depth below the water surface and at least a portion of the drilling muddepleted of low-density liquid being retumed to an upper end of the drill string.

An embodiment of the invention includes controlling the injection rate of theliquid. First, the rate of the liquid injected can be selected so the cuttings within the 15 riser pipe hâve an upward velocity in excess of the settling rate of the cuttings in theriser pipe. Secondly, the rate of the liquid injected can be selected so the liquid liftmaintains a bottom-hole pressure that is below the fracture pressure of the earthformation and above the pore pressure of the formation. 20 Other aspects and advantages of the invention will be apparent from the following description and the appended daims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrâtes an offshore drilling System configured for dual gradientriser drilling. 25

Figure 2 illustrâtes a liquid lift System for drilling risers in accordance withone embodiment of the présent invention.

Figure 3 illustrâtes mud processing in a liquid lift System for drilling risers in 30 accordance with one embodiment of the présent invention. 5 012493

Figure 4 depicts a flowchart of miscible liquid lift in accordance with oneembodiment of the présent invention. 5 Figure 5 depicts a flowchart of immiscible liquid lift in accordance with one embodiment of the présent invention.

DETAILED DESCRIPTION OF THE INVENTION

Spécifie embodiments of the invention, will now be described in detail with10 reference to the accompanying figures. Like éléments in the various figures are denoted by like reference numerals for consistency.

Fig. 1 illustrâtes one type of offshore drilling System (10) where a drillingvessel (12) floats on a body of water (14) which overlays a pre-selected earth 15 formation (17A). A drilling rig (20) is positioned in the middle of the drilling vessel(12), above a moon pool (22). The moon pool (22) is a walled opening that extendsthrough the drilling vessel (12) and through which drilling tools are lowered from thedrilling vessel (12) to the sea floor or mudline (17). At the mudline (17), a structuralpipe (32) extends into a wellbore (30). A conductor housing (33) is attached to the 20 upper end of the conductor pipe (32). A guide structure (34) is installed arôund theconductor housing (33) and adjacent a blowout preventor (38) before the" conductorhousing (33) is run to the mudline (17). A wellhead (35) is attached to the upper endof a conductor pipe (36) that extends through the structural pipe (32) into the wellbore(30). The wellhead (35) is of conventional design and provides a facility for hanging 25 additional casing strings in the wellbore (30). A riser System like the one depicted in Fig. 1 typically includes one or more auxiliary Unes (well-control lines 53 and boost line 68) on the outside of a riser (52).

Well control lines (53) provide a high-pressure conduit for fluid flow between a BOP 30 (38) and a drilling rig (20). A boost line (68) supplies drilling fluid to the bottom of a riser (52) to enhance the removal of drill cuttings. 6 012493 A drill string (60) extends from a derrick (62) on the drilling rig (20) into thewellbore (30) through a riser (52) which extends generally from the blowoutpreventor (38) back to the drilling vessel (12). Attached to the end of the drill string 5 (60) is a bottom hole assembly (63), which typically includes a drill bit (64) and one or more drill collars (65). The bottom hole assembly (63) may also includestabilizers, mud motor, and other selected components required to drill a wellbore(30) along a planned trajectory, as is well known in the art. The end resuit is thecréation of a well that extends from above the water surface to below the mudline (17) 10 into the earth formation (17A). During conventional drilling operations, drilling mudis pumped down the bore of the drill string (60) by a surface pump (not shown) and isforced out of the nozzles (not shown) of the drill bit (64) into the bottom of thewellbore (30). Cuttings resulting from the drilling become entrained in the mud at thebottom of the wellbore (30) and the mud laden with cuttings rises up the wellbore 15 annulus (66) and into the riser/drill string annulus (54 in Fig. 3), and to the surface fortreatment in mud cleaning facilities (îlot shown). The passage of the mud from thebottom of the wellbore to the surface of the body of water may be referred to as aretum flow System. 20 The présent invention is not limited to any particular retum flow System. In one embodiment, the retum flow System may comprise a fîrst annular spacê betweenthe drill string (60) and the wall of the wellbore (30), and a second annular spacebetween the drill string (60) and the inner surface of casing (36) positioned in thewellbore, and a third annular space between the drill string (60) and the riser (52) 25 extending between the cased wellbore and the surface of the body of water (14). A liquid-lift drilling riser System, as shown in Fig. 2, uses a lightweightmiscible or immiscible fluid to reduce the density of a drilling mud to as low as thatof seawater. A surface pump (not shown) pumps a low-density liquid (74) through a 30 riser boost line (68). The low-density liquid (74) is directed to the riser (52)approximately at the mud line (17) via the riser boosFline (68). During normaldrilling, the low-density liquid (74) will mix with the high-density mud (76) retuming 012493 from the bottom of the well. This mixture (80) will retum to the surface andflow over shale shakers (not shown). Once through the shale shakers (not shown), themixture (80) will be separated and treaied into its original low-density liquid (74) andhigh-density mud (76) components. The high-density mud (76) (preferablysubstantially ail of the high-density mud which is depleted of low-density liquid 21)will again be pumped down the drill string (60) and the low-density liquid (74)(preferably substantially ali of the separated low-density liquid 74) will again bepumped down the riser boost line (68) back to the bottom of the riser (52). Properséparation provides a closed loop System with low fluid losses.

Fig. 3 shows an alternative configuration for a liquid lift drilling System. Alightweight miscible or itmniscible fluid is used to reduce the density of a drillingmudto as low as that of seawater. A surface pump (not shown) pumps a low-densityliquid (74) through a fluid injection line (72). The low-density liquid (74) is directedto a position below the mud line (17) via a parasite string (71) installed in the casedwellbore (37). The parasite string thereby placing the low-density liquid 74 in anannular space between the drilling string 60 and the inner wall of casing 36 . Duringnormal drilling, the low-density liquid (74) will mix with the high-density mud (76)retuming from the bottom of the well. This mixture (80) will retum to the surface andflow over shale shakers (not shown). Once through the shale shakers (not shown), themixture (80) will be substantially separated and treaîed into its original low-densityliquid (74) and high-density mud (76) components. The high-density mud (76) willagain be pumped down the drill string (60) and the low-density liquid (74) will againbe pumped down the fluid injection line (72) through the parasite string (7Ï) to thecased wellbore (37).

In one embodiment, a miscible liquid-lift system uses a miscible liquid such asseawater to be injected into a water-based mud. For lifting a water-based drillingmud, seawater is injected into the riser boost line (68) to dilute the mud, effectivelyreducing mud density (weight). A portion of a retum fluid is discarded at surface, andthe water-based drilling mud is rebuilt with necessary additives needed to regain thedesired mud weight. 8 012493

For lifting a weighted mud, or if drilling with a synthetic or an oil-based mud,it may not be economical or environmentally acceptable to discard diluted drillingmud at surface. In such a case, the miscible liquid-lift System can comprise a base 5 fluid common to both the low-density liquid (74) and the high-density mud (76). Thehigh-density mud (76) generally contains barite, hématite and/or other suitableweighting agents and is directed down the drill string (60) as previously explained.The low-density liquid (74) may contain one or more density-reducing agents, such aslow-density particulate materials, including, for example, hollow glass 10 beads/microspheres or other density-reducing additive. As previously explained, thelow-density liquid (74) is directed to the riser (52) at the mud line (17) via the riserboost line (68 in Fig. 2), or is directed into the wellbore (37 in Fig. 3) via a parasitestring (71 in Fig. 3). The fluid mixture (80) retuming up the riser pipe (52) containsboth weighting agents and weight-reducing agents (if any). 15

Referring to Fig. 4, drill solids are removed from the retum fluid mixture (80)using one or more standard rig solids control devices (116). The resulting fluid (82)then travels to one or more séparation devices (112), such as mechanical separators,gravity separators, centrifuges, or other similar equipment. The one or more 20 séparation devices (112) separate the fluid (82) into the low-density liquid (74) andweighting agent (114). The low-density liquid (74) is moved to mud pits (H0) beforebeing redirected into the riser annulus (54 in Fig. 2) above the BOP (38 in Fig. 2) orinto wellbore annulus (37 in Fig. 3) below the mud line (17 in Fig. 3). The high-density mud (76) is re-formulated at (106) by combining the weighting agent (114) 25 and a portion (83) of unprocessed fluid (82). Then, the re-formulated high-densitymud (76) may be moved to mud pits (111) for temporary storage before beingredirected into the wellbore (30 in Fig. 2). The miscible liquid-lift System can be usedfor any type of drilling fluid, and this embodiment of the liquid-lift System can beused to drill part or ail of the well. 30

Another embodiment is an immiscible liquid-lift System. Referring to Fig. 5,an immiscible System uses a low-density boost liquid (74) that is substantially 9 ο 12 4 9 3 ί immiscible with the high-density mud (76) to lighten the retuming drill fluid. Anexample of this is to drill with a weighted water-based mud and boost with alightweight, immiscible synthetic fluid, such as an ester, olefin or glycol. The low-density liquid (74) is introduced into the retuming drill fluid at the base of the riser 5 (52 in Fig. 2) or down the fluid injection string (72 in Fig. 3) or both the base of the riser (52 in Fig. 2) and down the injection string (72 in Fig. 3) simultaneously. Theresulting fluid (80) is a stable, two-phase fluid of lower density than the mud (76).Referring to Fig. 5, one or more conventional séparation devices (81), such as a three-phase centrifuge, can be used to separate the fluid mixture (80) on the drilling vessel 10 (12 in Fig. 1), where the fluids (74, 76) can be re-circulated. First, the fluid mixture (80) can be processed using standard solids control equipment (120), such as course-screen shakers, to remove part or substantiallyall of the drill solids. Next, theresulting fluid (82) is separated in oil-water separator (81), such as a three-phasecentrifuge, to produce drill solids (86), low-density liquid (74), and drill fluid (122). 15 The drill solids (86) may be discarded in any environmentally suitable manner. Thelow-density liquid (74) may be moved tô mud pits (110) for temporary storage. Thedrilling fluid (122) in this embodiment may pass through additional standard rig solidscontrol devices (116), and then moved to mud pits (111) for temporary storage ashigh-density mud (76). 20

Another embodiment of the liquid lift System uses a combination fluid, such aslow-density glass beads (or a density-reducing agent) in a miscible low-density liquidslurry. By using miscible low-density liquid slurry instead of the low-density mudwithout the slurry, the volume of low-density liquid needed for producing a 25 significant mud weight change in the riser (52 in Fig. 2) may be reduced. Thedensity-reducing agent may be recovered at the surface before discarding the excessvolume of fluid, if any. The resuit is a stable, homogeneous fluid of lower densitythan the mud pumped down the drill string (60 in Fig. 1). 30 Referring to Fig. 2, controlling the rate of the low-density liquid (74) injected into the riser (52) at or near the mud line (17) via the riser boost line (68), or directedinto the cased wellbore (37 in Fig. 3) via the fluid injection string (72 in Fig. 3) has r 10 012493 two primary puiposes in the liquid-lift system. First, the rate of the liquid injected canbe controlled so the cuttings within the riser pipe annulus (54) hâve an upwardvelocity in excess of the settling raté of the cuttings in the riser pipe (52). Secondly,the rate of the low-density liquid (74) injected can be controlled to maintain a bottom- 5 hole pressure that is below the fracture pressure of the earth formation and above thepore pressure of the formation.

The liquid-lift system has several advantages over pump-lift and gas-liftSystems. The liquid-lift system can use conventional solids control equipment and rig 10 pumps to produce a simpler, more reliable dual-gradient drilling system than a pump-lift system. Cuttings transport is conventional, kick détection is conventional,circulation can be stopped (remain static) without adverse conséquences, and there islittle or no additional subsea equipment to break down, thereby creating a need for ariser trip to repair. 15

The liquid-lift system also allows the switching of drilling from dual-gradientto conventional, single-gradient merely by ceasing the injection of the low-densityboost fluid to the riser (52 in Fig. 2). The liquid-lift system also allows for additionalinjection/lift points than just the mud line. The use of a parasite string (71 in Fig. 3) 20 to inject lift fluid below the mud line (17 in Fig. 3) increases the effectiveness of theliquid-lift system and provides incentive for use of dual-gradient drilling in shalîowwater or on land. Additionally, by using the parasite string to inject the lift fluidbelow the mudline (17 in Fig. 3), the volume of lift fluid necessary to create lift in theriser (52 in Fig. 3) can be reduced. 25

While the invention has been described with respect to a limited number ofembodiments, those skilled in the art will appreciate that other embodiments can bedevised which do not départ from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by the attached daims.

Claims

012493 11 ( CLAIMS What is claimed is:

1. A method of drilling a well below a body of water in which a drill bit isrotated at the end of a drill string, comprising: (a) injecting into the well at a depth below the water surface a liquidhaving a lower density than a density of a drilling mud producing a 5 mixture of drilling mud and low-density liquid in the well; (b) withdrawing the mixture of drilling mud and low-density liquidfrom an upper end of the well; (c) separating at least a portion of low-density liquid from the mixtureof drilling mud and low-density liquid, thereby producing a drilling 10 mud depleted of low-density liquid; (d) retuming at least a portion of the separated low-density liquid tothe depth below the water surface; and (e) retuming at least a portion of the drilling mud depleted of low-density liquid to an upper end of the drill string.
2. The method of claim 1, wherein the low-density liquid is immiscible with the drilling mud.
3. The method of claim 2, wherein the drilling fluid is water-based and the low-density liquid is at least one of oil-based, synthetic and non-aqueous liquid.
4. The method of claim 2, wherein the low-density liquid comprises density- 20 reducing particulate material.
5. The method of claim 1, wherein the low-density liquid is miscible with thedrilling mud.
6. The method of claim 1, wherein separating comprises: at least one of mechanical séparation, gravity séparation and25 centrifugal séparation.
7. The method of claim 1, forther comprising: 12 012493 controlling a rate of the liquid injecting so that a bottom-hole pressure in the well is below a fracture pressure of an earth formation and above a pore pressure of the formation.
8. The method of claim 1, further comprising: 5 controlling a rate of the liquid injecting into the lower end of riser so the cuttings within the riser pipe hâve an upward velocity in excess ofthe settling rate of the cuttings in the riser pipe.
9. The method of claim 1, wherein the low-density liquid comprises density-reducing particulate material.
10. The method of claim 1, further comprising: ceasing the injection of the low-density fluid into the well at a depthbelow the water surface to switch from dual-gradient drilling toconventional drilling.
11. The method of claim 1, wherein substantially ail of the separated low-density 15 liquid is retumed to the depth below the water surface, and substantially ail of the separated drilling mud is retumed to the upper end of the drill string in aclosed System.
12. The method of claim 1, wherein the depth below the water surface is betweenthe drill string and the wellbore at a position below a wellhead.
13. The method of claim 1, wherein the depth below the water surface is at a lower end of a riser pipe that extends from a drilling vessel on the surface ofthe océan downwardly to wellhead equipment on the sea floor.
14. The method of claim 1, wherein the low-density liquid is injected via aparasite string into an annular space between the drill string and a casing's 25 inner wall at a position below a wellhead. 13 012493
15. The method of claim 1, wherein the low-density liquid is injected into a lower end of a riser pipe that extends from a drilling vessel on the surface of thebody of water downwardly to wellhead equipment on the floor of the body ofwater.
16. The method of claim 9, wherein the particulate material comprises low-density glass beads.
17. The method of claim 9, wherein the particulate material comprises low-densitymicrospheres.
18. A method of treating a drilling fluid used in drilling a wellbore in a earth 10 formation below a body of water in which a drill string extends from a water- surface drilling facility into the wellbore and the drilling fluid passes throughthe drill string and flows from the drill string into the wellbore wherebycuttings resulting from the drilling becomes entrained in the drilling fluid andthe drilling fluid with the entrained cuttings retums to the surface of the body 15 of water by means of a retum flow System, comprising (a) injecting into the retum flow System at a depth below the surface of the body of water a liquid having a density lower than a density of the drillingfluid, thereby producing in a retum flow System a mixture of drilling fluidand a low-density liquid; 20 (b) withdrawing the mixture of drilling fluid and low-density liquid from an upper end of the retum flow System; (c) separating at least a portion of the low-density liquid from the mixture ofdrilling fluid and low-density liquid, thereby producing a drilling fluiddepleted of low-density liquid; 25 (d) retuming at least a portion of the separated low-density liquid to the retum flow system to the depth below the water surface; and (e) retuming at least a portion of the drilling fluid depleted of low-densityliquid to the drill string. 14 012493 r
19. The method of claim 18 in which the retum flow System comprises a firstannular space between the drill string and the wall of the wellbore, and asecond annular space between the drill string and the inner wall of a casingpositioned in the wellbore, and a third annular space between the drill string 5 and a riser extending between the cased wellbore and the surface of the body . of water, wherein the retum of the separated low-density liquid of step (d) is to the annular space at the lower end of the third annular space.
20. The method of claim 18 in which the retum flow System comprises a first 10 annular space between the drill string and the wall of the wellbore, a second annular space between the drill string and the inner wall of a casing positionedin the wellbore, and a third annular space between the drill string and a riserextending between the cased wellbore and the surface of the body of water,wherein the retum of the separated low-density liquid of step (d) is to the 15 second annular space.