Th or collective redistirbution or collective of this by of any article portion SPECIAL ISSUE FEATURE in published been has is article Oceanography

D/V Chikyu , Volume 4, a quarterly journal of Th 19, Number RISER OPERATIONS AND THE FUTURE or other means reposting, is only photocopy machine, permitted OF SCIENTIFIC OCEAN DRILLING e Oceanography Society. Copyright 2006 by Th Society. e Oceanography

BY DANIEL CUREWITZ AND ASAHIKO TAIRA

Earth science disciplines focused on the geological record. erational means by which core recovery,

investigation of climatic, ecological, or Advances in scientifi c drilling over the core preservation, stability, Th of approval the with tectonic change as recorded in geological last decade have enabled progressively geophysical measurement, and borehole deposits (e.g., paleoceanography, marine better recovery and greater confi dence in monitoring operations are enhanced us- micropaleontology, paleoclimatology, the fi delity and coherence of recovered ing riser drilling (Sawyer, 1996; Kerr and Permission reserved. Society. All rights e Oceanography is gran paleomagnetism) require high-resolu- sections. The resulting interpretations Normile, 1998; Normile and Kerr, 2003). all correspondence to: Society. Send [email protected] e Oceanography or Th tion, continuous, -preserved records and analyses have enhanced the high- for accurate analysis. Recovery of long, resolution short- and long-term records RISER DRILLING BASICS uninterrupted, relatively undisturbed of changes in the Earth system (Brewer Riser drilling involves several steps that sections has long been a primary techni- et al., 2005). are accomplished in a variety of ways de- cal challenge for any drilling operation. The adaptation of riser drilling tech- pending on specifi c technological pack- Recovered cores and geophysical mea- nology for scientifi c ocean drilling pur- ages, water depths, and bottom condi- surements form the backbone for any poses on board the D/V Chikyu (Fig- tions. First, a wide-diameter pilot hole is further data collection and data analysis. ure 1) represents a giant step forward for drilled and cased, then a specialized well- Limitations on core recovery, core condi- the scientifi c ocean drilling community. head designed to anchor a pressure sen- ted in teaching to and research. copy this for use Repu article tion, and sample preservation have long The D/V Chikyu is operated by the Cen- sor and shut-off valve assembly, called a been recognized as serious challenges for ter for Deep Earth Exploration of the Blow Out Preventer (BOP; Figure 2), is any reconstructions or interpretations of Japan Agency for Marine-Earth Science installed. The BOP itself is lowered to the MD 20849-1931, USA. 1931, Rockville, Box PO Society, e Oceanography and Technology. The use of this state-of- wellhead and attached using a Remotely Daniel Curewitz ([email protected]) the-art drilling and coring system as one Operated Vehicle (ROV). Lowering of is Staff Scientist, Center for Deep Earth of the principal platforms in the Inte- the BOP is accomplished by successively Exploration (CDEX), Japan Agency for grated Ocean Drilling Program (IODP) attaching riser pipe (Figure 3) to the top Marine-Earth Science and Technology will advance the science of reconstruct- of the BOP and lowering it to the sea- (JAMSTEC), Yokohama, Kanazawa, Japan. ing past environments and analyzing fl oor. Riser pipe itself comprises wide-di- Asahiko Taira is Director-General, Center Earth system evolution as recorded in ameter, high-strength pipe with external blication, systemmatic reproduction, for Deep Earth Exploration, Japan Agency the sediments and rocks below the ocean conduits for cables and connectors to al- for Marine-Earth Science and Technology, fl oor. In the following discussion, we low control and monitoring of the BOP. Yokohama, Kanazawa, Japan. enumerate the technological and op- Once the BOP is installed at the wellhead

168 Oceanography Vol. 19, No. 4, Dec. 2006 Figure 1. (top left) Deep Sea Drilling Vessel Chikyu. Figure 2. (bottom left) Blow Out Preventer on the deck of D/V Chikyu. Figure 3. (right) Riser pipe sections racked on the deck of D/V Chikyu.

and linked to the drilling vessel via the borehole inside the drillstring. Drilling etrated formations (Figure 4). riser pipes, drilling and coring (as well as mud specifi cally designed for coring op- The drilling mud is fi ltered through a any downhole logging, measurement, or erations, called “coring mud” or “bland graduated series of screens (called “ sampling) operations begin (Brewer et coring mud,” can be used when preserva- shakers”), removing drilling cuttings and al., 2005). tion of porosity/permeability structures other fragments (e.g., borehole-wall col- The drillstring (drill pipe, drill bit, or reduced chemical contamination of lapse fragments, which are distinguished and any attached equipment, sensors, the cored formations is required. Drill- from cuttings based on physical appear- or tools) is lowered into the borehole ing/coring mud exits the drill pipe at the ance). These cuttings are then available entirely inside the riser pipe. Drilling drill bit, and returns through the riser for geological and physical properties ex- mud (a high-density, chemically inert, pipe (outside the drillstring) to the drill- amination and interpretation, including fl uidized mixture tailored to match the ing vessel, carrying with it suspended lithology, geochemistry, paleontology, ambient geological fl uid- and gas-pres- cuttings, fl uids, and gases derived from consolidation, p-wave, and porosity/den- sure conditions) is pumped into the the drilling process and from the pen- sity analysis. Gases and fl uids trapped

Oceanography Vol. 19, No. 4, Dec. 2006 169 in the drilling mud are also collected for chemical analysis for both safety and scientifi c purposes. The fi ltered drilling mud is then cycled back into the drill- ing operation. As drilling progresses, the chemical composition of the drill- ing mud is altered to compensate for increased lithostatic stress, gas pressure, hydrocarbon deposits, fault zones, and/ or fractured intervals (Yano et al., 2001).

RISER DRILLING ADVANTAGES Recirculation of high-density drilling fl uid through the borehole enhances the drilling and coring operations in a variety of ways (Adamson et al., 1998). First, the density of the drilling fl uid is calibrated using preexisting and real- time information about the properties of the penetrated geological formation, including gas and fl uid pressures, frac- ture density, porosity and permeability, and physical characteristics of the rocks. Drilling-fl uid density is tailored to pre- vent borehole collapse, gas , or excessive fl uid fl ow in the borehole, in- creasing the safety and effi ciency of the drilling operation. Additionally, rock fragments produced by drilling and cor- ing are carried away from the drill bit by the high-density fl uid, reducing the risks of jamming the drill bit and producing a cleaner borehole, both for drilling and for later downhole operations. Tem- perature and at the drill bit are reduced, increasing the life span of each Figure 4. Diagrammatic depiction (modifi ed after Sawyer, 1996) of riser drilling op- bit, while increasing potential drilling erations showing the borehole, the wellhead, the BOP, and the riser pipe connecting to the drilling vessel. Drilling mud fl ows down through the drillstring, exits at the speed and enhancing both core recovery drill bit, and is returned in the annular space between the drillstring and the riser and core quality. This combination of pipe walls. advantages allows increased performance throughout the entire drilling operation, discussed below.

170 Oceanography Vol. 19, No. 4, Dec. 2006 Deeper fragments and borehole deformation. Operations in Gas- or Fluid-Rich The circulation of high-density drill- Riser drilling also improves the ability Formations ing fl uid increases borehole stability and to ream and case completed boreholes Riser drilling using high-density drill- reduces the possibility of borehole wall by enabling rapid clearing of fragments, ing fl uid was initially developed in the collapse or constriction. Riser boreholes preventing borehole collapse, and reduc- oil industry to reduce the risks of blow- can potentially be drilled to depths of ing the risks of cavity creation that may out resulting from penetration of over- 7,000 meters or more. Reduced bit wear, occur during hole-opening operations. pressured formations. Ocean Drilling elimination and/or reduction of bit jam- These operations improve the feasibility Program (ODP) operations were, by ming or drillstring constriction, and and success rate of logging operations in necessity, restricted from drilling in areas elimination of loose fragments in the open holes (directly following drilling thought to host gas, oil, or other poten- borehole contribute to this expanded operations), and increase the viability tially pressurized fl uid deposits or con- depth range. Prior experience in scientif- and feasibility of downhole measure- duits. As a result, drilling in active fault ic ocean drilling with non-riser systems ment and monitoring operations using zones, methane-hydrate-rich regions, or indicates that boreholes are limited to either CORK (borehole seal; see Kastner any areas characterized by high possibil- ~2,000-meter depths due to the combi- et al., this issue) or CORK-like equip- ity for hydrocarbon deposits was lim- nation of borehole collapse, constriction, ment packages or downhole sampling ited by requirements for densely spaced, and fragment-related bit jamming. and measurement tools. In addition, high-quality, site-specifi c hazard survey reentry and re-occupation of existing data analyzed and available before any Enhanced Core Recovery boreholes drilled with riser technology drilling could take place. The adoption Reduced friction at the drill bit, reduc- have a greater success rate due to the ini- of oil industry, riser-based technology tion of the number and volume of rock tial high-quality borehole. for scientifi c drilling will permit opera- fragments, prevention of borehole col- tions in areas that were previously off- lapse and constriction, and smoothing of Long-Term Monitoring, Reentry, limits or extremely restricted. the entire drilling process should ensure and Borehole Instrumentation enhanced core recovery, both in terms The stability of the borehole, the quality Operations in Highly Fractured or of percent recovery and in terms of the of the procedure, and the condi- Friable Formations quality of the cores recovered. Riser tions both inside the borehole and in The use of drilling fl uid to clean and drilling enhancements to core recovery the penetrated formations are critical stabilize the borehole during drilling, should be most marked during rotary to the success or failure of operations and to enable rapid and effective ream- coring in consolidated sediments or hard that take place following the completion ing and casing of boreholes will enhance rock (as opposed to hydraulic piston cor- of drilling and coring operations. The the ability to drill, core, log, and moni- ing operations in soft sediments). installation of packers, fl uid pressure tor in fragile, fractured, or friable rock monitoring devices, geochemical sen- formations such as sand-rich turbidite Borehole Preservation and sors, and downhole packages of strain, deposits, fractured basalt layers, and/or Stability tilt, or seismic measurement devices is highly fractured fault zones. Prior ODP Riser drilling provides for enhanced entirely dependent on the condition and operational experience indicated that borehole quality both during drilling preservation of the borehole. Riser drill- non-riser drilling was limited and prone and in post-drilling (e.g., downhole ing increases the probability that a given to being compromised in these kinds measurement) operations. Stabilizing borehole will be suitable for such opera- of rock formations, mostly as a result the borehole by preventing collapse, con- tions by increasing the borehole stability of jamming of drill bits and borehole striction, or fl uid/gas blowout enables over long periods of time. collapse. Drilling with a riser will allow clean and effi cient drilling and coring, more comprehensive and high-resolu- and removes risks associated with rock tion rock and geophysical measurements

Oceanography Vol. 19, No. 4, Dec. 2006 171 in a wide array of formations that were given batch of cuttings can be translated and weight limitations of the riser pipe. previously either un-drillable or were into a rough depth estimate, allowing Ongoing research and development in expensive and risky. the correlation of cuttings to depth to the hydrocarbon industry coupled with within ~1- to 10-meter resolution. Mud, academic efforts and design initiatives Sample Integrity mud-gas, and cuttings analyses thus add within IODP and CDEX are aimed at In addition to the basic physical integrity both safety and analytic components to expanding this envelope to enable riser- of the core samples derived from the in- the scientifi c drilling operation—data drilling operations in waters deeper than creased speed, effi ciency, and quality of from cuttings and gas/fl uid analyses 2,500 meters. Other ongoing initiatives, drilling operations, the core samples and contribute to the understanding of the some adapted from the hydrocarbon sub-samples from cores collected dur- geology and geophysics of the borehole. industry, are aimed at capitalizing on ing riser drilling operations may be bet- While these measurements are defi nitely the advantages gained by the use of riser ter preserved for biological, biochemi- of lower resolution than core or log data, drilling, including advanced long-term cal, and/or geochemical analysis. The they have the potential to fi ll gaps in re- monitoring and telemetry packages, riser hole is isolated from the seafl oor covery, to strengthen correlations made downhole sampling and measurement environment once the BOP is installed between cores and log data, and to add techniques, and incorporation of ad- and sealed, and all circulation within signifi cantly to the understanding of gas vanced techniques such as logging and the borehole is controlled via the ship- and fl uid conditions in the borehole. measuring while drilling. board pumping system. The chemical Together these innovations and tech- composition of the drilling fl uid is also SUMMARY AND FUTURE nical advances will broaden the frontiers known and closely monitored for safety DIRECTIONS of ocean drilling science by opening up and drilling reasons. The result, in terms The addition of riser capability to the new areas for analysis and investigation of chemical or biological investigation, scientifi c drilling toolbox will enhance in a wide array of geological settings. is that contamination of the cores and the ability to drill to greater depths with samples can be better monitored and un- better recovery, produce more stable, REFERENCES derstood, allowing for improved under- long-lived boreholes suitable for reentry Adamson, K., G. Birch, E. Gao, S. Hand, C. Mac- donald, D. Mack, and A. Quadri. 1998. High- standing of geo-bio-chemical systems. and long-term monitoring operations, pressure, high-temperature well construction. and allow drilling, coring, and monitor- Oilfi eld Review 10(2):36–49. Brewer, T., T. Endo, M. Kamata, P.J. Fox, D. Gold- Mud, Cuttings, Mud-Gas, and ing in an array of geological environ- berg, G. Myers, Y. Kawamura, S. Kuramoto, Fluid Logging ments that were previously inaccessible S. Kittredge, S. Mrozewski, and F. Rack. 2005. The return fl ow of drilling fl uid to the to the scientifi c drilling community. The Scientifi c deep-ocean drilling: Revealing the Earth’s secrets. Oilfi eld Review 16(4):24–37. ship is monitored for gas and fl uid con- ability to monitor and sample mud-gas, Kerr, R.A., and D. Normile. 1998. Ocean drill- tamination with respect to the known fl uids, and cuttings adds another layer of ing fl oats ambitious plans for growth. Science 282(5392):1,251–1,252, doi: 10.1126/sci- composition of the original mixture analytical and data collection capabili- ence.282.5392.1251. pumped into the borehole, and is se- ties and will help to enhance our ability Normile, D., and R.A. Kerr. 2003. A Sea Change in Ocean Drilling. Science 300(5618):410–412, doi: quentially screened to smaller and to integrate drilling information, core- 10.1126/science.300.5618.410. smaller clast sizes in order to collect any derived data, and logging data, and may Sawyer, D. 1996. 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