Because many fundamental questions concerning the dynamics of the Earth and itsstructure remain unanswered, the Integrated Ocean Drilling Program (IODP) hasrecently completed a feasibility study for drilling and coring a hole 500meters (1,640 feet) through the Mohorovicic seismic discontinuity into theupper mantle of the oceanic crust from three candidate locations in the PacificOcean (Cocos Plate, Baja California, and offshore Hawaii).
The main challenges discussed in this paper are threefold. First, drilling withriser in ultra-deepwater environments with water depths around 4,000 meters(13,120 feet) which will set a new world record. Secondly, drilling and coringin very high temperature igneous rocks with bottom-hole temperatures that areestimated to be as high as 250°C (480°F). Finally, drilling and coring a verydeep hole with a total drilled and/or cored interval around 6,000 meters(19,685 feet) in the oceanic crust below the Pacific Ocean seafloor in order toreach the upper mantle which will constitute a major achievement for theworldwide scientific community.
This paper presents detailed analyses and several discussions concerning marinedrilling riser options by first reviewing the capabilities of the current riserconfiguration that is onboard the IODP scientific drilling drill-ship Chikyuand then evaluating alternative designs such as titanium riser, hybridtitanium-steel riser, slim-riser and lighter buoyancy modules. Furthermore, thedeepwater subsea equipment, drill-pipe design, wellbore design, down-holetools, drilling fluids, circulating temperature, cementing methods and variousadvanced technologies that would be required for this type of operation arealso reviewed. In addition, operational time and cost estimations for differentscientific drilling cases are provided (borehole continuously cored to totaldepth, continuous cores only across the major lithologic and geophysicaltransition intervals, spot coring and when only the mantle section iscored).
Finally, this study helps evaluate critical issues in terms of current andtrending technologies in oilfield and geothermal industries that need to beresolved before embarking upon such a challenging project. The results of thiswork show that drilling to the mantle is certainly feasible, and that there areexisting solutions to many of the technological challenges based on work beingdone in the oilfield, offshore and geothermal industries.
The design of a conductor casing is significantly different than that of the other tubulars in a well. Conductors are drag-dominated structures that enable conducting drilling operations from offshore fixed platforms or jack-up rigs through the seawater column. To date, the majority of technical papers concerning conductor design have focused on a particular aspect of the conductor pipe but very few of them have detailed the design of conductors in response to the combination of hydrodynamic loads, external and internal loads acting on this casing string as well as the soil-conductor interaction.
This paper discusses the design of both platform conductors and jack-up supported conductors used in open water by first providing the essential steps (i.e. deriving and presenting equations) that need to be followed to ensure the structural integrity of the conductor casing along its entire length (i.e. from the Texas Deck to the its setting depth hundreds of feet below the mudline). Then, in order to illustrate the methodology, four case studies were performed to investigate the performance of well conductors at different shallow water locations (the Gulf of Mexico, South America, the North Sea and South-east Asia).
For all analyses, the conductor pipe has been checked for three failure modes: strength, stability and fatigue and finite element analyses have been computed to solve the hydrodynamic loading, the statically indeterminate problem and the complex soil-structure interaction. The results show that, in general, stability controls the conductor design which means that the conductor will buckle before it yields; and therefore, there is a requirement to apply tension at the Texas Deck. Also, in some instances (i.e. rougher water environments) or with smaller size conductors (13.375-in. and 20-in.), the fatigue analysis may become the most important failure mode for both the conductor pipe body and the connectors.
Shallow Water Exploration
It is only in 1947 that the first offshore drilling/production platform was installed, 10 miles offshore the coasts of Louisiana in 20 feet of water to conduct exploration activities for crude oil and natural gas. Since then, numerous drilling campaigns using either fixed platform structures or jack-up rigs have been launched in various regions across the globe to extract hydrocarbons from different shallow water areas.
When injected in deep saline aquifers or depleted oil and gas reservoirs, supercritical CO2 remains mobile and can, therefore, migrate through any conduits or fractures. In addition, public opinion, regulations and the lack of space for CO2 injection in some densely populated regions of the world such as the Japanese archipelago encourage investigating other alternatives such as carbon dioxide sequestration in deepwater sub-seabed formations.
This paper intends to present a technical feasibility study of CO2 sequestration in deepwater sediments offshore Japan. The main processes, technical requirements, technologies and structures that are currently available to transport and inject liquid CO2 successfully in sub-seabed formations below 9,000 feet of water (˜2,750 meters) are first discussed. Then, three storage sites situated offshore Japan; one located in the Northwest Pacific Ocean near the island of Shikoku; another located in the Sea of Japan near the island of Honshu; and the last one located farther in the Northwest Pacific Ocean in ultra-deepwater (18,000 feet); are selected to conduct reservoir simulations.
From this study, it appears that CO2 capturing technologies and transporting means seem to be at a mature stage. Also, current and newest 5th and 6th generation drilling vessels are estimated to be capable of drilling very shallow wells in water depths greater than 9,000 feet and even in ocean waters as deep as 18,000 feet if new materials such as titanium or composite for riser systems were to be deployed for both the drilling and CO2 injection operations. However, CO2 storing and injection facilities are not available yet to unload large quantities of CO2 offshore. As a result, some concepts should be designed, qualified and tested for these large scale operations within the next decade to demonstrate through pilot projects the technical feasibility of CO2 sequestration in sub-seabed geological formations.
Additionally, the main findings from this comparative study and reservoir simulations conducted at three different sites located offshore Japan confirm that a significant part of ultra-deepwater regions with at least 9,000 feet of ocean water and planar seafloor are appropriate for CO2 storage. Secondly, reservoir models confirm that due to high pressures and low temperatures reigning at water depths greater than 9,000 feet, the liquid CO2 injected in the first few hundred feet of sediments has a higher density than the surrounding formation pore-fluid and therefore remains buoyantly trapped under certain condition of geothermal gradient, sediments permeability and formation pressure and; hence constitute a valid and safe CO2 storage candidate.
CO2 injection and storage in deepwater sediments under water depths greater than 9,000 feet (˜2,750 meters) where high pressures and low temperatures result in the CO2 being denser than seawater and therefore being buoyantly trapped in the sediments pore-fluid, could provide an attractive sequestration option for countries and regions densely populated and emitting large quantities of anthropogenic CO2 such as East and West Coasts of the United States of America, Japan, the East Coast of China and Western Europe. In these places, public opinion, government regulatory agencies, a lack of space for CO2 injection sites and few depleted oil and gas fields available necessitate the application of alternative technologies to sequester CO2 in order to mitigate a significant part of the 30 billion tons of CO2 annually released in the Earth's atmosphere.
This paper presents the results of multiple reservoir simulations and parametric studies for different types of deepwater sediments located in various regions of the globe (Pacific Ocean, Atlantic Ocean, Japan Sea and Gulf of Mexico). Since not all regions and sediments deposited below 9,000 feet of ocean waters seem to be viable to permanently store CO2, this study focuses on the critical parameters that need to be considered to successfully inject and permanently store liquid CO2 in deepwater sub-seabed sediments.
In fact, when injecting liquid CO2 through an ultra-deepwater conduit (injection pressurized riser) within the first few hundreds of sediments, several uncertain variables such as temperature, sediment type, sediment thickness, permeability, porosity and CO2 injectability greatly influence the overall integrity of the buoyant trap. Very long-time reservoir simulations (e.g. 250 years) have been used to assess the effects of different decision and uncertain variables on the behavior and the evolution of the CO2 plume within the sediments. Also, experimental design and response surface methodologies have been used to quantify the risk associated with each of the critical parameters and to determine the optimal conditions for deepwater sediments CO2 storage. Finally, the essential findings of the paper provide the offshore and carbon sequestration industries with a high-level mapping of the world's oceans and deep seas best candidates for CO2 storage in deepwater sediments.
Because supercritical CO2, when injected onshore or in shallow water depths offshore, is mobile and can, therefore, migrate through any conduits or fractures, there is a need for proper physical trapping and also a necessity to monitor the CO2 migration in the injected zone. In addition, public opinion, government regulatory agencies and the lack of space for CO2 injection sites in some of the largest CO2 emitting regions of the world encourage investigating other alternatives such as CO2 sequestration in deepwater sub-seabed formations.
Furthermore, at the high pressures and low temperatures reigning in deepwater sediments where water depths are greater than 9,000 feet (˜2,750 meters), scientists have proposed that the CO2 should become denser than seawater and therefore would remain buoyantly trapped when liquid CO2 is injected within the first few hundred feet of sediments even in the absence of geological seals and traps. Besides, the bulk of the studies and technical papers concerning CO2 sequestration in deepwater sediments have focused on showing the potential and the feasibility of the concept but very little has been published to demonstrate the viability of the injection and long-term storage of CO2 in deepwater sub-seabed formations.
This paper presents the results of several case studies located in the Gulf of Mexico, the Pacific Ocean, the North Atlantic Ocean and the Sea of Japan. Large time-scale reservoir simulations have been conducted for up to 250 years and show that injected liquid CO2 can remain trapped in deepwater sediments under certain sediment physical properties. Therefore, CO2 sequestration in deepwater sediments provide another attractive technical solution when applied under certain conditions of pressure, temperature, sediment type, thickness, permeability and porosity notably for regions where there are few depleted oil and gas fields available for storage or limited space accessible onshore.