Макарейко Роман Сергеевич

DRILLING TECHNIQUE FOR GAS HYDRATE EXPLORATORY DRILLING Drilling shallow stratigraphic boreholes in deep water is an important component leading to reduction of exploratory costs. New offshore area investigation for oil and gas or other deposits consists of several stages that include geophysical survey, sampling and coring with soil investigation, logging and (probably) testing the hole. Every stage needs a certain technology and type of vessel to conduct the work. So-called stratigrafic boreholes can be drilled with relatively small drilling vessels (Bavenit, Bucentaur, Norskald) normally used for geotechnical purposes..

ALUMINUM DRILL PIPES: A solution to this is the usage of a lightweight aluminum drill string. It allows expanding vessels operating area up to the water depth of 1400-1600 meters and considerable costs reducing. Russian pipes of 164-mm diameter made of 1953 alloy were being successfully employed to drill from the above-mentioned vessels. The lightweight ADP drill string allowed to realize multiply offshore drilling projects in deep water during 1993-1999, from the geotechnical vessel with displacement of ~5000 tones only. Drilling operations were performed in areas of Atlantic Ocean and Pacific Ocean at water depth of up to 1,616m. The maximum drill string length of 1,981.5 meters was achieved with the drill rig lifting capacity up to 40 tons only..

CORING TECHNOLOGIES: Continuous coring systems are worldwide known and extensively used in geological exploratory drilling both for rock minerals and for oil and gas, as well as in special types of drilling (scientific, geotechnical etc.).
ODP CORING SYSTEMS: The highest peaks in modern coring technique with retrievable core barrels development were reached, when realizing programs of deep sea scientific drilling: DSDP (1968-1983) and ODP (since 1985). The most advanced ODP system in the sense of penetration through different types of rocks by coring without tripping of the drill string is APC/XCB/MDCB, where APC-Advanced Piston Corer, XCB-Extended Core Barrel, MDCB-Motor Driven Core Barrel.

"BAIKAL-2" CORING SYSTEM: Since 1993 the Baikal Lake scientific drilling project is implementing in Russia. The "Baikal-2" coring system was developed by Aquatic for this purpose. It includes several retrievable and changeable corers/samplers: v Rapid Piston Sampler (RPS) - the modified and simplified APC version v Hydropercussion Sampler (HS) - uses the hydraulic hammer (HH) for sample pipe driving v Pilot Corer (PC) - simplified XCB option v Vibro-Corer (VC) - short PC uploaded by the HH v conventional Rotary Corer (RC) v Center Bit assembly (CB) for full-face drilling. 1890 meters were cored in 8 holes at water depth up to 1000 meters with 92% average core recovery.

PIGGY-BACK SYSTEM: Experts designing continuous coring systems know very well, that the best results may be obtained when drilling with diamond crowns at high RPM. ODP's Diamond Coring System (DCB) and the "piggy-back" system were successful attempts to apply standard mining drilling technology and coring tools to offshore operations. A small rig was displaced on the power swivel to drive the mining style drill string inside the main one. As the outer drill string is heave compensated, the piggy-back frame with the coring machine is disconnected from the ship's motion, and the coring can be carried out as on land. Such coring technology normally allows excellent core recovery from consolidated formation. Unfortunately a heavy secondary rig reduces an effective hook load capacity. Another limitations are: inner drill string durability; necessity of a special seabed frame with a clamp arranging. It makes such system inapplicable in deep-water conditions.

COMPLETE CORING SYSTEM (CCS) : CSS were formulated on analyzing the results of design, test and commercial use of above-mentioned systems, along with the experience of Russian continental scientific drilling program.
(1) Aluminium drill string of enlarged diameter: 164-168 mm OD and 146 mm ID clearance is results in dubbing the maximum string length with the same rig capacity; increasing the core diameter; increased durability and reliability of retrievable tools, deployment of a powerful down hole motor. Usage of one housing for several drilling systems: piston sampling; rotary drilling and coring including hydrohammer; drilling and coring by down hole motor.
3) Usage of different types of retrievable core barrels with a series of core heads for all rock types. The drill pipes ID size allows to develop drag, diamond and cone type heads having efficient durability and reliability even in conditions of serious load variations. Durable core barrels and drill heads fully realizes the idea of pilot coring with subsequent hole reaming and an optimal strategy of hole deepening.
(2) Cone retractable bit with down hole motor application. This is the key principle, because it opens large possibilities to combine continuous drilling or coring in all the types of rocks. Change of tool from one operation to another does not require drill string tripping. This version does not oppose to usual rotary drilling. In this case a reamer at the lower end of the BHA and a center bit are used. In the event high wear of the reamer is expected a retractable bit is used for subsequent hole deepening. Retractable bit and down hole motor give an unique possibility to drill holes without seabed frame in case of difficult terrain and hard bare rocks. Drill string ID size allows design of reliable bit for hole diameters 216-220 mm.
CCS, designed using above mentioned principles, allows realizing optimal strategy of stratigraphic hole drilling from the small drill ship in changing environment. The CCS system is in commercial use since 1993.

PRESSURE CORING: The special requirements for gas hydrate (GH) deposits exploration are connected with it physical properties. Decomposition and vaporizing of GH because of core decompression and heating during it transporting from the hole bottom to laboratory table leads to loosing the information and involves real fire hazard on the deck. Shortening the time requires for core tripping (it depends on sand winch speed, CB serviceability etc.) and gas-analyzer monitoring service during on-deck core moving allows partial solving this problem. But to receive complete qualitative and quantitative information regarding the MH contents in the soil (rock) a in-situ conditions coring (sampling) tool and technology requires. Pressure Core Barrels (PCB) allow coring, core catching from the bottom, sealing the core barrel both in top and bottom sections and transporting the core on the deck under in-situ pressure.
The rotary PCB types are classified depending on the bottom valve design principle. The most known are the Carter (oven door valve type) and Christensen (ball valve type) PCB. At present time the Christensen type PSBs only have been in commercial use in petroleum industry. Christensen built the same type wireline PCB for ODP. The basic disadvantage of the ball valve type PSB - the ball can be blocked by the core/cutting material.
Russian experience in PCB designing refers to 60-70's. One of the most successful PCB for coal/mining industries uses an original fully-mechanical Sealing principle. Unlike traditional schemes intended for the outer core barrel sealing, it allows to seal inner core barrel only. It ensures better core preventing of the drill fluids influence, easy core analyzing and better safety. The basic limitation of this scheme - it uses the hole walls/bottom resistance to generate the valve pre-compression, so it can be used in consolidated of rocks only. The disadvantage of all above-mentioned rotary PSB's is too small core diameter (because too much space requires for valve displacement. Normally it leads to considerable reduction in the core recovery in comparing with standard CB. Another original wireline Pressure Sampler was built by Fugro. It allowed slow pushing the sample tube into the soil and sample sealing by ball valve. Such technology allows excellent recovery the undisturbed sample from unlitified soils. Unfortunately pushing the heavy-walled (to displace the valve) sample pipe is possible in very soft sediments only. Another limitation - this technology requires a special seabed frame with a clamp arranging.
Generally the pressure coring/sampling requires complicated down hole and laboratory equipment, high-qualified personnel services and involve much expenses.