Riserless drilling fluids are redefining interval limitations in deepwater operations.

In offshore areas with young sedimentary deposits, there is a narrow margin between formation pore pressure and fracture pressure.
The traditional approach of running an additional casing string may not be viable in deepwater wells due to engineering or cost constraints. The alternative is riserless drilling, which decouples the hydrostatic pressures above and below the mud line, thereby enabling deeper casing setting depths.
Significant cost savings have been realized by reducing the number of work boats used and the storage area required for kill and pad fluids. The success rates for running casing to bottom and cementing it properly on the first attempt have improved, and fewer casing strings have been needed to achieve the desired total depth.
Dynamic kill drilling fluids
The upper hole sections are drilled in riserless mode using Dynamic Kill Drilling (DKD) fluids, weighted, preformulated fluids that are mixed with seawater. DKD methodology was developed to address shallow water flows, a high-profile problem in the deepwater Gulf of Mexico. However, this technology also reduced the size and number of casing runs, as shown in Figure 1. The DKD process offers similar advantages to the dual-gradient industry initiatives but is only applicable in the absence of a riser.
The process employs a dual-gradient concept, consisting of the seawater hydrostatic gradient above the mud line and the ability to vary the hydrostatic below the mud line through mud weight variations. Unlike dual-gradient drilling, the DKD process has no annular restriction capability to vary downhole pressure. The mud density required to maintain borehole stability is calculated with consideration of bottomhole pressure requirements, the seawater contribution and depth below mud line. The seawater gradient and mud gradient below the mud line are cumulative to bottomhole pressure as shown in
(Figure 2).
How it is done
To push 20-in. casing past shallow water flow hazards and eliminate a pipe string, very large fluid volumes possessing the appropriate mud properties must be pumped at the precise density. Factors to be considered in the planning process include fluid type, required fluid volumes, logistics and the availability of a reliable mixing apparatus.
Appropriate fluid types include heavy brine and water-based drilling fluids. When choosing between brine or a blended water-based system, engineers must consider existing drilling fluids issues such as drillstring balling, hole quality and fluid cost. Fluid volumes are calculated based on pump rates, rate of penetration (ROP), hole size, interval length, drilling and cementing practices and mud weight. The mixing apparatus should incorporate a metering device for blending fluids of different types and weights so that the end result is an instantaneously homogenous fluid.
Logistical issues include the amount of weighted fluid needed to drill the interval, the rig's storage capacity, boat capacity, distance from the supply base and weather. A typical well using a weighted slurry fluid system will require 8,000 bbl to 20,000 bbl of heavyweight fluid. After blending these heavy fluids with seawater, final mud volumes will be between 16,000 bbl and 40,000 bbl of mixed fluid. Only the industry's newest drilling vessels are equipped to handle these volumes efficiently. Therefore, it becomes imperative to minimize the volume of fluid stored on the drilling vessel and have a system capable of delivering reliable, accurate, high mixing rates while allowing mud weight flexibility.
DKD process and equipment
To meet these mixing requirements, the DKD process incorporates planning software, the mixing manifold (IMM) and real-time surveillance software. The IMM consists of flexible quick-connect hoses, ball valves, magnetic flow meters and a mixing chamber. The equipment was developed during a 6-year period and represents third-generation technology.
The IMM can be shipped to location in a toolbox and rigged up in 4 to 6 hours. The mixing chamber incorporates specifically engineered flow tube extenders for increased meter accuracy, and it can blend three fluids simultaneously.
ECD management
Management of equivalent circulating density (ECD) has always been an important planning component, especially for ultradeepwater wells - those drilled below 20,000 ft (6,100 m). Such wells have smaller casing, smaller drillstring and hole sizes, higher mud weights and a much higher ECD at total depth.
In many cases, the DKD drilling process can impact these small hole and ECD problems by eliminating a casing string early in the well construction process, gaining one hole size at the bottom. Landing the 20-in. casing in a higher pore pressure environment permits the operator to set subsequent casing strings at greater depth.
Fluid costs
Because the DKD process uses thousands of barrels of drilling fluids, drilling fluid composition must be economical while meeting the many criteria established at the outset of the process. A densified fluid must be formulated to maintain filtration control and viscosity when blended with seawater. This can be accomplished using a low-salinity water and mud slurry. On occasion, higher density, more shale-inhibitive calcium chloride brines are used due to storage and weather concerns. These fluids can be more technically challenging to formulate and may cost substantially more per barrel due to the limited products developed to build viscosity and filtration control in a concentrated divalent environment. However, these costs are more than made up by eliminating a casing string and having a larger diameter wellbore penetrating the reservoir.
Case history No. 1
For one prospect in 2,167-ft (661-m) waters, the well's casing program was designed in anticipation of a rapidly increasing pore pressure profile. The desire was to reach total depth with a minimum hole size of 97/8 in. This goal required either pushing the 20-in. shoe deeper than normally possible with seawater and viscous sweeps or using a big-bore well configuration.
The decision was made to use the DKD process to push the 20-in. casing an additional 430 ft (131 m). This would require drilling with an 11.0-lb/gal fluid over this interval. The DKD equipment was installed on the mud pits. Prior to drilling, the blending system was tested to determine maximum flow rates on the seawater and mud lines. Pilot tests blended the 16.0-lb/gal mud with sea water to the required yield point of the fluid. The tests showed that a cutback of 2 bbl of sea water to 1 bbl of 16.0-lb/gal fluid would produce adequate rheology and would not require additional polymer treatments. However, additional monitoring of the yield point was planned while drilling the riserless interval.
Next, the boats containing the 16.0-lb/gal base mud were tested to determine the maximum pump rate to the rig. The test indicated the boats could pump fluids in a quantity sufficient to sustain the anticipated flow rates.
The 30-in. casing was jetted 320 ft (98 m) below the mud line, pumping prehydrated bentonite sweeps as needed without any problems. A 26-in. drilling assembly was run in the hole, and drilling began with 100-bbl prehydrated bentonite sweeps pumped at every connection.
Drilling continued to 3,600 ft (1,098 m), at which point the DKD process was initiated using an 11.0-lb/gal mud system.
Drilling progressed, averaging 1,100 gal/min to 1,200 gal/min downhole while pumping base fluid from the boats, feeding the mixing manifold and blending seawater to a final density of 11.0 lb/gal. Rheology was monitored, and no polymer additions were required to maintain an adequate yield point. At 4,030-ft (1,229 m) TD, bottoms-up was circulated, and the bit pulled to the shoe. The well was flow-checked, and the bit was run to bottom with no fill. Bottoms-up was circulated again with an 11.0-lb/gal mud. The well was displaced to 11.5 lb/gal within 8 minutes without shutting down the mud pumps. Once the hole was displaced to 11.5 lb/gal, the drilling assembly was pulled into the 30-in. casing and the well monitored for 15 minutes. The bit then was pulled to the surface and the rig prepared to run the 20-in. casing. The 20-in. casing was picked up, run to bottom and cemented without incident. The shoe test was 1.0 lb/gal higher than anticipated.
Case history No. 2
At another well in 4,746-ft waters (1,448-m), no shallowwater flows were expected. Initial plans were to initiate the DKD process at the normal 20-in. shoe depth about 2,200 ft (671 m) below the mud line and extend the casing shoe an additional 1,000 ft (305 m).
As planned, seawater and viscous sweeps were used to 2,200 ft (671 m), with average penetration rates ranging from 100 ft/hr to 150 ft/hr.
The DKD process was initiated at 2,200 ft (671 m) due to indications of hole closure. Starting with a 10.0-lb/gal mud, weight was increased to a maximum of 11.0 lb/gal. Operations progressed smoothly through this portion of the well, so it was decided to push the shoe farther than originally planned. The push ended up totaling 1,334 ft (407 m) and was stopped finally due to decreased ROP and insufficient mud volume to continue. A 13.0-lb/gal pad fluid was spotted prior to running pipe and cementing. Using the DKD process, there were no indications of hole closure, and no cement squeezes were required.
Case history No. 3
A major operator in the Gulf of Mexico identified a potential shallow water flow hazard 1,418 ft (432 m) below the mud line on a prospect in 6,200-ft (1,891-m) waters. The operator determined substantial time and costs could be saved if the hazard were drilled in riserless mode below the structure pipe. If accomplished, a 20-in. casing string could be run across the hazard, eliminating the need for a 26-in. casing string. Pore pressure data indicated a 12.0-lb/gal fluid would be needed to control flow within the 1,480-ft (451-m) hazard zone. After a great deal of planning, structure pipe was washed down, and 1,418 ft (432 m) of 26-in. hole was drilled using seawater and sweeps. The downhole pressure tool confirmed a suspected shallow water flow at this depth, requiring the initiation of the DKD process.
A 16.0-lb/gal fluid (at a rate of 850 gal/min) was blended with seawater (at a rate of 730 gal/min) to yield a 12.0-lb/gal fluid. This mud weight held constant over the entire hazard section with no indication of water flow by visual means or downhole pressure tool measurements.
Slight variations in the weight of the mud blended with seawater necessitated minor adjustments to the blend. This was accomplished easily by adjusting the meters controlling the flow tubes. Penetration rates ranged from 90 ft/hr to 130 ft/hr (27 m/hr to 40 m/hr) for the first 900 ft (275 m) of the DKD operation. ROP decreased slowly over the balance of the interval, reaching 20 ft/hr (6 m/hr) at total depth. The flow rate was reduced to match the lower ROP and ensure sufficient supplies of weighted fluid were available to reach the projected total depth. At total depth, pipe was short-tripped, bottoms-up circulated and a 13.0-lb/gal pad mud was spotted in the open hole by adjusting the metering system prior to tripping out for casing. The 20-in. casing was run to bottom and cemented. The 20-in. shoe did not require squeezing, and a higher than anticipated leakoff test was recorded.
Case history No. 4
A major Gulf of Mexico operator scheduled a nine-well batch of 20-in. pipe set in 5,423-ft (1,654-m) waters. Planning initiatives identified weighted pad mud as a potential bottleneck. Each of the nine wells would require 2,800 bbl of 12.5-lb/gal fluid with only 3 days between each well. The project's drilling rig possessed a total fluid capacity of 3,200 bbl. Supply vessels could meet these requirements, but that would require flawless execution and no weather delays.
The DKD process was identified as a means to mitigate risk associated with the project's batch-setting phase. A volume of 2,200 bbl of 16.0-lb/gal fluid could be preloaded on the rig, yielding 3,955 bbl of 12.5-lb/gal fluid. In this manner, enough weighted fluid could be preloaded on the rig to service the needs of 1.5 wells and allow several days before resupply of the drilling vessel was required.
The plan was modified to further reduce risk by loading the supply vessel with 6,000 bbl of 16.0-lb/gal weighted fluid that would yield 10,788 bbl of 12.5-lb/gal fluid. The boat would maintain the extra 3,800 bbl of 16.0-lb/gal fluid as reserve volume. The batch setting of 20-in. casing on all nine wells was accomplished using 17,000 bbl of base fluid supplied by two workboats. The drilling phase was completed with zero hours of nonproductive time relating to fluid supply. The drilling and cementing
of the nine batch-set wells was completed in fewer days than planned and at a reduced cost.
Cost savings
Significant advances have been made to the processes and equipment necessary to address shallow hazard challenges in deep water. The third-generation equipment designs are compact and can be mobilized quickly while offering needed reliability.
While first developed as a tool to mitigate shallow hazards, the DKD process has been extended to push 20-in. casing to new depths faster and more safely with lower drilling costs. In some instances, savings were due to the cumulative cost associated with eliminating a casing string, hole-opening run and cementing. In other cases, savings were realized by eliminating the need to use a big-bore well configuration. In still others, cost savings were attributed to reaching deeper horizons of interest with a larger hole size.
All of the wells employing the DKD process realized fewer logistical problems and maximized borehole stability. The wells that have pushed 20-in. casing have obtained the necessary fracture gradient without the need to squeeze.
Acknowledgment
This paper originally was presented as SPE 71752 at the 2001 SPE Annual Technical Conference and Exhibition in New Orleans, La., Sept. 30 to Oct. 3, 2001.

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