An initial feasibility study (Pilisi and Whitney, 2011) of drilling through the Mohorovičić discontinuity (Moho) into the oceanic mantle specifically focused on future requirements for planning, drilling and coring a hole 500 m into the oceanic mantle from three candidate locations in the Pacific Ocean (Cocos Plate, Baja California, and offshore Hawaii). The study points out some of the critical issues that need to be resolved before embarking upon such a challengingproject. It was conducted on the basis of data provided by the Integrated Ocean Drilling Program–Management International (IODP-MI), the Center for Deep Earth Exploration (CDEX) operating the drilling vessel Chikyu within IODP, public domain information, and past experience that Blade Energy Partners (hereafter mentioned as “Blade”; http://www.blade-energy.com/) has had with frontier projects in the offshore deepwater oil and gas and geothermal industries

Bill Whitney

Nicolas Pilisi

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	Initial Feasibility Study to Drill and Core the Ocean Mantle by Nicolas Pilisi and Bill Whitneydoi:10.2204/iodp.sd.12.05.201146  Scientific Drilling, No. 12, September 2011	IntroductionAn initial feasibility study (Pilisi and Whitney, 2011) of drilling through the Mohorovii discontinuity (Moho) into the oceanic mantle speciﬁcally focused on future require-ments for planning, drilling and coring a hole 500 m into the oceanic mantle from three candidate locations in the Paciﬁc Ocean (Cocos Plate, Baja California, and offshore Hawaii).   The study points out some of the critical issues that need   to be resolved before embarking upon such a challenging project. It was conducted on the basis of data provided   by the Integrated Ocean Drilling Program–Management International (IODP-MI), the Center for Deep Earth Exploration (CDEX) operating the drilling vessel Chikyu within IODP, public domain information, and past experi-ence that Blade Energy Partners (hereafter mentioned as “Blade”; http://www.blade-energy.com/) has had with fron-tier projects in the offshore deepwater oil and gas and geo-thermal industries.Project ChallengesThe challenges associated with drilling/coring a hole through the Moho and into the mantle have been understood for some time and have been comprehensively discussed in previous IODP and legacy program reports and documents. Blade’s focus was therefore not to re-examine the issues, but rather to evaluate them in terms of current and trending technologies in oilﬁeld and geothermal industries to deter-mine how difﬁcult it will be to resolve the issues. The obvious constraints for this project versus “normal” offshore drilling operations are the extreme water depths where operations need to be conducted, the extreme high temperatures present in very hard igneous rocks that push the limit of   all the drilling, and coring tools that are routinely used in less demanding environments. The main challenges that need to be overcome include the following:1. Riser drilling in ultra-deepwater environments with water depths around 4000 m (which will set a new world record)2.  Drilling and coring in very high temperature igneous rocks with bottom-hole temperatures that are esti-mated to be as high as 250°C3. Drilling and coring a very deep hole having a total drilled/cored interval of up to 6700 m in the oceanic crust below the Paciﬁc Ocean seaﬂoorFeasibility Study ResultsThe study (Pilisi and Whitney, 2011) included a detailed analysis of the drilling riser design requirements for the environmental conditions expected at the three candidate locations (Fig. 1). The anal-ysis looked at the operational envelope of the current riser conﬁguration on riser equipped drillship Chikyu (Fig. 2), as well as alterna-tive designs for ﬁve other possible riser con-ﬁguration options (current riser with lighter buoyancy modules, titanium riser, slim riser, hybrid riser, current riser with two addi-tional riser tensioners). The analysis con-cluded that there are existing available tech-nologies, equipment, and materials in the ultra-deepwater industry that should enable the  Chikyu to conduct operations in the expected water depth range (3650–4300 m) at the candidate locations.The study also looked at different well design scenarios in order to further deﬁne the key operational and design issues that will need to be resolved prior to drilling  into 	 WAVE LOADINGCHIKYU DRILLING VESSELMean Sea Level (MSL) = 3650–4300 (m)EXTREME PROFILECURRENT LOADINGBUOYANT RISEROPERATINGPROFILEBARE RISERCASINGLMRP / BOPMUDLINECASINGFigure 1. Drilling riser configuration and environmental loading (LMRP = Lower Marine Riser Package; BOP = Blowout Preventer)Scientific Drilling, No. 12, September 2011  47the mantle well. Most of the information about the downhole conditions is presently unknown. Therefore, assumptions had to be made about the fundamental downhole conditions that impact well design. For example, the assumed downhole tempera-ture proﬁles for the candidate locations were based on previous models of forma-tion burial depth and age provided by IODP-MI and from operations conducted   at the Cocos location in the IODP Hole 1256D (Alt et al., 2007). In addition, a downhole pressure estimate was developed in order to develop a casing program. In most deepwater wells the presence of abnormal pressure is a fundamental cri-terion for determining casing points and the drilling mud density required to reach total depth. Since the Moho wellbore is expected to be entirely normally pressured (1.03 speciﬁc gravity [SG]), abnormal pres-sure is not an issue. Therefore, the selec-tion of casing points and mud weights will be based on wellbore stability considera-tions. For example, if the mud weight is too low, the hole will essentially collapse due to a compressive shear failure in the rock. However, a trade-off must be made be-tween the allowable mud weights and the number of casing strings that are used. There are only so many casing strings that can ﬁt in a well; running multiple strings is time-consuming and costly, and it compli-cates the geometry of the well. It is cer-tainly advantageous to minimize the num-ber of casing strings used for the Moho well, if only to minimize the sizes of the rotary core barrel bits that would need to be developed. This effort resulted in the development of a base case wellbore conﬁguration that assumes that signiﬁcant parts of the well-bore will need to be cased off in order to reach total depth (Fig. 3). With the well-bore thus deﬁned, the expected downhole circulating temperature proﬁles could be modeled, and the issues around the design of a drill string could be examined. It was determined that the circulating tempera-tures exceed the temperature ratings of most downhole tools that are commercially available today. Increasing the downhole tools’ temperature rating is a signiﬁcant issue that will need to be addressed. On the other hand, it was determined that the Moho wells could be drilled today with a Figure 2. Riser tension requirements vs. mud weight for the current Chikyu drilling riser at the Hawaii location (Chikyu Spec. 2008.pdf).IODP - Project Mohole - Minimum Tension - Current RiserHawaii - Water Depth = 4050 mMaximum Tensioner System Capacity at Mid-StrokeAPI Maximum Allowable (90%)Maximum Tension if 1 Tensioner is lostMinimum Tension Required During Opeartoins (ton)Figure 3. Base case wellbore configuration.Base Case Wellbore SchematicRig: ChikyuWater Depth: 3650 mTD = 9,900 m MD / TVDMax Mwt = 1.52 SGBHT = ± 250°C Base Case Wellbore SchematicSedimentsShoe Mwt Hole PP36"22"1.03 1.03 3711 m 61 m penetrationSet 15 m above base of MudlineLava__3900 m__4550 mh = 250h = 6501.0317.5"4206 mDikesTextures __5350 m 5700 m h = 800 13-3/8"1.281.33 1.03 1.03Set 15 m above base of DikesFoliated Gabbros__5700 m__6400 mh = 350h = 70012.25"Layered Gabbros7400 m 11-3/4" 1.28 1.03 1.571.52Moho __9400 m h = 30009-7/8"Mantleh = 500 9900 m 1.03 1.52 BHT = 250°CDepths are measured from the rig floor (DRF)GabbrossedimentsSet 1000 m into theLayered Gabbros1.03 1.031.06 20"48  Scientific Drilling, No. 12, September 2011	combination of readily available oilﬁeld drill pipe and the Chikyu’s existing drill pipe. Therefore a non-standard, special high strength drill string is not needed.Operational time estimates for four differ-ent scientiﬁc drilling cases were developed for each of the candidate locations (Table 1). Case 1 assumed that that the hole is contin-uously cored to total depth. Case 2 assumed long sections of continuous core are taken across the major lithologic and geophysical transition intervals of key sections. Case 3 assumed that only spot coring is done during the last 10 m of hole before each bit trip. And , Case 4 assumed that the hole is drilled to   the Moho and that only the mantle is cored. Operations time curves were developed for each different case and location (Fig. 4). It was determined from this work that the time spent tripping for a new drill bit was as much as 40% of the overall operational time. The development of drill bits with enhanced operation hours will therefore have a tremendous impact on the time and cost it will tak e to drill/core the Moho well.ConclusionsThe results of this study show that drilling/coring a scien-tiﬁc hole into the upper mantle is certainly feasible, and that existing solutions are currently available to many of the tech-nological challenges based on work being done in the oilﬁeld and geothermal industries. In addition, technologies and techniques are continuously advancing and can be expected to continue to close the gap between what is required for the “Moho Project” and what is currently possible.ReferencesAlt, J.C., Teagle, D.A.H., Umino, S., Miyashita, S., Banerjee, N.R., Wilson, D.S., the IODP Expeditions 309 and 312 Scientists, and the ODP Leg 206 Scientiﬁc Party, 2007. IODP Expeditions 309 and 312 drill an intact section of upper   oceanic basement into gabbros. Sci. Drill., 4:4–10. doi:10.2204/iodp.sd.4.01.2007 Pilisi, N., and Whitney, B., 2011. IODP - Project Mohole - Initial Blade Feasibility Study. Report to IODP by Blade Energy Partners, Frisco, Texas, 7 June 2011. Available online at http://www.iodp.org/weblinks/Featured-Publications-HOME-PAGE/ IODP-Project-MoHole-Initial-Feasibility-Study-PDF-For full references, see the full report at http://www.iodp.org/weblinks/Featured-Publications-HOME-PAGE/I ODP-Project-MoHole-Initial-Feasibility-Study-PDF- AuthorsNicolas Pilisi and Bill Whitney, Blade Energy Partners, 2600 Network Boulevard, #550, Frisco, TX 75034-6036, U.S.A., e-mail: bwhitney@blade-energy.com, npilisi@blade-energy.com.Table 1. Operational time estimates (based on current technologies).CandidateLocationWaterDepth (m)Total Depth Below (m):Drill/CoreTime (days)Total ProjectTime (days)Rig FloorSea FloorCocos LocationCase 1 3650 9900 6250 696 756Case 2 3650 9900 6250 564 617Case 3 3650 9900 6250 433 480Case 4 3650 9900 6250 374 418Baja LocationCase 1 4300 10,400 6100 807 866Case 2 4300 10,400 6100 642 693Case 3 4300 10,400 6100 405 445Case 4 4300 10,400 6100 386 425Hawaii LocationCase 1 4050 10,750 6700 876 934Case 2 4050 10,750 6700 688 737Case 3 4050 10,750 6700 448 485Case 4 4050 10,750 6700 422 443Figure 4. Depth vs. duration of operations. Example for Baja location comparing the four scientific drilling cases05001000150020002500300035004000450050005500600065007000750080008500900095001000010500110000 100 200 300 400 500 600 700 800 900Measured Depth (m)Proje t DaysBaja California Lo a onDrilling Curve ComparisonCase 1Case 2Case 3Case 4MSLMoho

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Initial Feasibility Study to Drill and Core the Ocean Mantle

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Initial Feasibility Study to Drill and Core the Ocean Mantle

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