01.politecnico di torino drilling fundamentals 2010

01. DRILLING FUNDAMENTALS

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CHAPTER 01

DRILLING FUNDAMENTALS


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The Exploration and Production of Hydrocarbons involves many technical disciplines, which are strictly connected together, each contributing to solve a specific aspect of the overall problem. These technical disciplines are:

Geophysics

Geology

Drilling Engineering

Production Engineering

Reservoir Engineering

Engineering

HSE

1. INTRODUCTION


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1.1. ROLE OF GEOPHYSICS IN DRILLING

The concept on which seismic exploration is based is simple.

Acoustic waves are generated (with frequencies typically ranging from about 5 Hz to just over 100 Hz) using specific energy sources. As these sound waves travel downwards into the Earth, they encounter changes in the earth's geological layering, which cause their:

reflections

refractions

diffractions.


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The reflections travel upwards to the surface where electromechanical transducers (geophones or hydrophones) detect the echoes arriving on the surface and convert them into electrical signals, which are then amplified, filtered, processed, digitized and recorded.

The recorder, consequently, measures the time the wave employs to travel from the source to the reflector and from the reflector to the receiver (this time is called Two Way Time) and the characteristics of the signal (frequency, amplitude). Different values of the Two Way Times are due to different reflection points. The result is: a vertical 2D section; a 3D volume.

After data has been acquired and processed with the transformation of Two Way Times into depths, the next two steps involve interpretation and mapping, which allow to decide if: the rocks might contain valuable resources; it is opportune to drill a well.

1. ROLE OF GEOPHYSICS IN DRILLING


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Two different types of seismic data acquisition: 2D (left) and 3D (right)



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Seismic Data InterpretationTime/Depth Conversion



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1.2. ROLE OF GEOLOGY IN DRILLING

Geology studies the composition, the structure and the evolution of the Earth.

It also studies the type, duration and sequence of the chemical, physical, biologic events which characterized the history of our Planet from its origin to now.

Modern Geology employs several specialized disciplines, the Earth Sciencesthe Earth Sciences, which comprehend:: Petrography Sedimentology Palaeontology Stratigraphy Structural Geology Geochemistry


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Petrography: it studies the chemical and mineralogical composition, the structure, the texture and the origin of rocks. Sedimentology: it studies the characters (structure, texture and composition) of sedimentary rocks in order to reconstruct origin, transportation processes and deposition environments of the sediments. Palaeontology: it studies the organisms which lived in the past and which are contained (as fossils) in the sedimentary rocks. It allows to determine the age of sedimentary rocks. Stratigraphy: it studies the chronological succession of sedimentary rocks, their geographical distribution, depositional environment, the age of the sedimentary and tectonic events which occurred during the geological evolution of the Earth. Structural Geology: it studies the deformations of the rocks. Its aims is to define the structural asset of the rocks and their geometrical conformation Geochemistry: it studies the distribution and quantity of the chemical elements in the Earth and the chemical-physical phenomena occurring in rocks, minerals, soil, atmosphere.



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Geologists, using as basis what derived from seismic data interpretation, complete the information needed by Drilling Engineers providing data on aspects such as:

lithology (especially of sedimentary rocks);

stratigraphy;

thickness of the various levels;

their mineralization (water, gases, oil);

expected hole problems (fractured formations, shallow gas);

rig-site data acquisition and processing systems (Mud Logging Unit, MWD, LWD, Seismic-While Drilling);

reference wells.



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Among the items seen before, of particular importance for the Drilling Engineers is the description of the rocks present in the well, because from them depend decisions on several aspects of the drilling process:

expected penetration rates;

expected hole problems;

configuration of the well (vertical, deviated, horizontal);

expected drilling time;

cost of the well.



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Among the different type of rocks (igneous. metamorphic, sedimentary), Sedimentary Rocks are the most frequently encountered in drilling, because hydrocarbons are normally found in sedimentary basins

METAMORPHIC

IGNEOUS INTRUSIVE

EXTRUSIVE GRAVEL

SAND

SILT

CLAY

SEDIMENTARY

TERRIGENOUS

CHEMICAL-ORGANOG.

SALT

ANHYDRITE

DOLOMITE

LIMESTONE

EVAPORITES

LIMESTONE

DOLOMITE NON-EVAP.



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MAIN TYPES OF SEDIMENTARY ROCKS

- Sandstones- Sandstones- Shale- Shale- Limestone- Limestone- Conglomerate- Conglomerate

- Dolomite- Dolomite- GypsumGypsum- AnhydriteAnhydrite- Gravel- Gravel- Marl- Marl

- Sand- Sand- Salt- Salt- Silt- Silt- Siltstone- Siltstone



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The Geophysicists and Geologists tell to Drilling Engineers: where to drill a well, providing its coordinates (surface, bottom-hole). According to the values of surface and bottom-hole coordinates, the well will be: vertical, if the two sets of coordinates coincide; deviated or horizontal, if they are different; where are located the targets (presence of possible hydrocarbon bearing levels); the depth of the various formations present in the well and their lithological and stratigraphic sequences; the final depth the well shall reach; the other wells that could be correlated with the well to be drilled (reference or offset wells).

They provide the Drilling Engineer with a sketch of the well, as that shown in the Figure of the next slide.

1.3. ROLE OF GEOPHYSICS AND GEOLOGY IN DRILLING


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Seismic Data Interpretation Example

Cross-correlation between “A” field data andSeismic Line XX9

1.3. ROLE OF GEOPHYSICS AND GEOLOGY IN DRILLING


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Oil wells can be drilled onshore or offshore with different rig types, depending on the environment in which they will be called to operate. Normally two basic design of rigs are used: the derrick (A) and the mast (B), both with a similar shape but with some constructive differences which makes the second type easier to install and transport and, therefore, more suitable for onshore operations than the first type, used generally offshore.

2. DRILLING RIGS

A. DERRICK B. MAST


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Depending on the environment where they operate, drilling rigs can be classified as follows:

2. DRILLING RIGS


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TYPES OF ONSHORE DRILLING RIGS

CONVENTIONAL LAND RIG WINTERIZED LAND RIG

2.1. ONSHORE DRILLING RIGS


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TYPICAL LAYOUT OF A LAND DRILLING RIG


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LAND RIG FOR DESERT AREAS



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LAND RIG FOR SLIM HOLE WELLS HELI-TRANSPORTED LAND RIG



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LAND RIGS ON ARTIFICIAL ISLANDS IN THE CASPIAN SEA



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2.2. OFFSHORE DRILLING RIGS

3000 m

When drilling offshore, several different types of rigs are available, whose choice depends usually on water depth and operations to perform.

They can be classified as: submersibles; jack-ups; semisubmersibles; drilling ships; fixed platforms


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TYPES OF OFFSHORE DRILLING RIGSAn offshore drilling rig has to create the same working conditions as for onshore rigs which can move from one place to another without any difficulty. It must therefore be a mobile unit equipped to contain an autonomous drilling site, including the derrick, the technical personnel and all the service equipment. This can be done with a supporting structure (or platform) which rests on the seabed and rises above sea level, or with a floating structure, kept vertical above the well by means of anchors or with dynamic positioning systems.

From an operational point of view, offshore drilling may be subdivided into two main categories, depending on the water depth: Drilling with the rig standing on the seabed: the safety equipment is located permanently above sea level and is accessible from the supporting structure; in this case drilling operations are practically identical to those carried out in onshore drilling.

Drilling with floating rigs: the wellhead and the safety equipment (BOPs) are placed on the seabed and are not, therefore, directly accessible from the supporting structure. In this case, a number of sequences of the drilling operations differ from those onshore, as the plant is not immobile in relation to the wellhead but, because it floats, is subject to the action of the wind, currents and waves, which cause it to make small horizontal and vertical movements.



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.1. SUBMERSIBLE DRILLING PONTOONSSubmersible drilling pontoons were designed in the Thirties in Louisiana, where they were used for drilling wells in the swampy areas of the Mississippi delta, not accessible from ordinary roads. In concept, the first pontoons consisted of an ordinary rig on a suitably adapted barge, which was transported to the site along channels dredged for the purpose. The barge was then filled with water and the pontoon came to rest on the seabed, and was held firm by means of driven piles. Today drilling pontoons consist of a shallow-draft hull (usually 2-3 m deep), divided into compartments which can be flooded to enable the pontoon to rest on the seabed and emptied at the end of operations so as to refloat the pontoon and enable it to move.

The hull is covered by one or two decks; in the case of two decks, the engine-room, the mud circulation pumps, the storage area for chemical products and the cementing unit are situated on the lower deck, while the offices, the accommodation, the deposit for tubular materials and the rig are on the upper deck, the derrick usually being located in the stern.



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.1. SUBMERSIBLE DRILLING PONTOONSAfter drilling in swampy areas, the next step was to conquer actual delta areas, characterized by periodic variations in the sea level, which made it necessary to raise the level of the deck for the rig and the various items of equipment.

To be able to operate in these conditions, a pontoon was designed with a hull that could be flooded, with the main decks raised over the hull using a series of posts. In a working position, the pontoon was transformed into a sort of pile-work structure, which allowed drilling to take place in up to 8-10 m of water. The typical posts used for raising the deck gave their name to this particular type of vessel (posted barge), usable only in extremely calm waters (Fig.). They are still in use today, and the drilling techniques applied are identical to those of onshore wells.

Subsequently this type of rig was further modified, to be able to operate in deeper and deeper waters. The piling was transformed into a steel reticular structure, formed by big pipes welded together. The outside piles, consisting of large-diameter pipes, were enlarged to enable them to be flooded and then emptied to make the structure float, and allow it to be moved. The platform placed over the reticular structure, on which the whole of the drill rig was housed, was called a submersible bottle platform because of its design.



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2.2.1.1. SUBMERSIBLE DRILLING PONTOONS

Drilling Pontoon of the Posted-Barge Type



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.2. JACK-UP DRILLING PLATFORMSOffshore drilling continued to be performed in deeper and deeper waters, and to do this it was necessary to use a different type of plant. To limit the high costs necessary to construct higher and higher submersible drilling pontoons, drilling platforms commonly known as jack-ups were devised (Fig.). Jack-ups are triangular or rectangular floating hulls fitted with long mobile legs (usually 3 or 4) at the corners of the hull, which are able to move vertically up and down. Thanks to lifting systems using jacks, or rack-and-pinion mechanisms, it is possible to rest the feet (also known as spud cans) on the seabed and thus to lift the hull above sea level.

The first jack-up was constructed in 1954, and its design made it an immediate success, thanks to its stability and efficiency. In fact, the whole structure rests firmly on the seabed, and the big platform can house even the most complex drill rigs without difficulty.

In the ensuing decades these plants were considerably improved and enlarged, this applying in particular to the size and length of the legs. The largest jack-ups can weigh up to 26,000 t and operate at considerable water depths: some of them have legs 150 m in length and are able to drill in water depths of 90-110 m, the maximum depth depending on the mechanical strength of the seabed. All modern jack-ups are also provided with a side platform for the use of helicopters.



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2.2.1.2. JACK-UPS

Jack-Up Drilling Platforms



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.2. JACK-UP DRILLING PLATFORMSWhen moving from one well to another, the legs are raised and the rig floats on the surface. Once on site, the legs are lowered until the spud cans touch the seabed. By continuing to lower the legs, the spud cans slowly penetrate the bed, compacting it so that it can bear the weight of the entire hull. When the hull has been raised about one metre above sea level, preloading takes place, using the seawater as a ballast for the hull, so as to simulate the maximum loads foreseeable in the operative phases. In this way the spud cans are further embedded, giving stability to the whole structure until drilling operations have been completed. Finally, the hull is raised some 10-15 m above sea level, according to the maximum wave height foreseen in case of storm. The height that the hull is raised has to be such that its underside cannot be reached by the crest of the waves, which could destabilize the structure.

Jack-ups can have independent legs or legs that are connected together at the base by a loading plate which replaces the single spud cans (mat-supported rigs). This configuration enables the supporting area to be increased, reducing the specific weight acting on the seabed. This is necessary where the seabed has little bearing capacity.



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.2. JACK-UP DRILLING PLATFORMSTo be moved for short distances, jack-ups are floated and towed by tugs (although some jack-ups have their own means of propulsion).

For longer distances, ships equipped with a submersible loading deck are used: after the jack-up has been positioned on the deck, with its legs raised, the ship proceeds normally, transporting the rig even from one continent to another in a relatively short time.

The drilling technique used on jack-ups is the same as that used onshore. Ordinary BOPs are used, located on the conductor pipe, which must be self-supporting. This is possible for depths of around 50-60 m, beyond which a fixed structure has to be installed on the seabed, to be able to withstand both the lateral stresses generated by the sea currents, and the vertical loads due to the dead weight of the conductor pipe and of the BOPs. The conductor pipe is embedded with the use of a pile driver, if the seabed is sufficiently soft. Otherwise the bedrock has to be drilled with a drill string fitted with a bit, using seawater circulation. In this case, the conductor pipe has to be completely cemented.



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.3. FIXED PLATFORMSIf a series of development wells has to be drilled offshore in not excessive water depth (approximately 500 m), a fixed platform is built and positioned above the reservoir to exploit.

These platforms usually consist in a metal pipe structure, the jacket, whose legs (usually 4) are fixed to the seabed by means of piles; the jacket is topped by the deck, which houses the derrick, drilling, treatment and production equipment, tanks and living-quarters for the crew. Jacket and deck are normally built at a yard on land, loaded onto barges, transported to the selected location offshore and, then, installed using a derrick barge with a crane capable of lifting thousands of tons.

Fixed platforms represent the most economical solution for the development of both small-sized and large fields, since more than 30 directional wells can be drilled from a single platform. Normally, all the development wells are drilled and, then, they are put into production, though more recently, drilling and production phases can take place simultaneously (early production) to accelerate the return on investments; in this case great care is devoted to guarantee the highest safety conditions for the personnel and the rig.



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.3. FIXED PLATFORMSThe “legs” are constructed with concrete or steel, extending down from the platform, and fixed to the seafloor with piles.

With some concrete structures, the weight of the legs and seafloor platform is so great, that they do not have to be physically attached to the seafloor, but instead simply rest on their own mass. There are many possible designs for these fixed, permanent platforms.

The main advantages of these types of platforms are their stability, as they are attached to the sea floor there is limited exposure to movement due to wind and water forces. However, these platforms cannot be used in extremely deep water, simply because it is not economical to build legs that long.



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.3. FIXED PLATFORMS



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.3. FIXED PLATFORMS



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2.2.1. RIGS STANDING ON THE SEABED2.2.1.4. OTHER TYPES OF FIXED PLATFORMSDuring the years, several designs of platforms have been proposed with the intention of increasing the water depth where they could be used and decreasing their cost (Fig.). Some platforms must possess, also, huge storage capacity of the hydrocarbon produced, especially when marginal fields have to be exploited.



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2.2.1. RIGS STANDING ON THE SEABEDCompliant TowersCompliant towers are very similar to fixed platforms. They consist of a narrow tower, attached to a foundation on the seafloor and extending up to the platform. This tower is flexible, as opposed to the relatively rigid legs of a fixed platform. This flexibility allows it to operate in much deeper water, as it can “absorb” much of the stresses exerted on it by winds and sea, and is strong enough to withstand hurricane conditions.



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2.2.1. RIGS STANDING ON THE SEABEDSeastar PlatformsSeastar platforms are like miniature tension leg platforms. The platform consists of a floating rig, much like the semisubmersible type. A lower hull is filled with water when drilling, which increases the stability of the platform against wind and water movement. In addition to this semisubmersible rig, however, Seastar platforms also incorporate the tension leg system employed in larger platforms. Tension legs are long, hollow tendons that extend from the seafloor to the floating platform. These legs are kept under constant tension, and do not allow for any up or down movement of the platform. However, their flexibility does allow for side-to-side motion, which makes possible the platform to withstand the force of the ocean and wind, without breaking the legs off. Seastar platforms are typically used for smaller deep-water reservoirs (e.g. in the Gulf of Mexico), when it is not economical to build a larger platform. They can operate in water depths of up to 3,500 feet (1100 m).



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2.2.1. RIGS STANDING ON THE SEABEDSpar PlatformsSpar platforms are among the largest offshore platforms in use. These huge platforms consist of a large cylinder supporting a typical fixed rig platform. The cylinder however does not extend all the way to the seafloor, but instead is tethered to the bottom by a series of cables and lines. The large cylinder serves to stabilize the platform in the water, and allows for movement to absorb the force of potential hurricanes. The first Spar platform in the Gulf of Mexico was installed in September of 1996. Its cylinder was 770 ft (235 m) long and 70 ft (21 m) in diameter; the platform operated in 1,930 ft (588 m) of water.



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2.2.1. RIGS STANDING ON THE SEABEDTension Leg PlatformsTension leg platforms are larger versions of the Seastar platform. The long, flexible legs are attached to the seafloor, and run up to the platform itself. As with the Seastar platform, these legs allow for significant side to side movement (up to 20 ft or 6 m), with little vertical movement. Tension leg platforms can operate as deep as 7,000 ft. (2130 m).



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2.2.1. RIGS STANDING ON THE SEABEDFloating Production SystemsFloating production systems are essentially semisubmersible drilling rigs, except that they contain petroleum production equipment, as well as drilling equipment. Ships can also be used as floating production systems. The platforms can be kept in place through large, heavy anchors, or through the dynamic positioning system used by drillships. With a floating production system, once the drilling has been completed, the wellhead is actually attached to the seafloor, instead of up on the platform. The extracted petroleum is transported via risers from this wellhead to the production facilities on the semisubmersible platform. These production systems can operate in water depths of up to 6,000 ft (1800 m).



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2.2.1. RIGS STANDING ON THE SEABEDFloating Production Storage and Offloading SystemsThis unit is a floating vessel used for the processing and storage of oil and gas.The FPSO vessel is designed to receive oil or gas produced from nearby platforms or subsea template, process it, and store it until the oil or gas can be offloaded onto a tanker or transported through a pipeline.FPSOs can be a conversion of an oil tanker or can be a vessel built specially for the application.FPSOs are preferred in frontier offshore regions as they are easy to install, and do not require a local pipeline infrastructure for exporting the oil and gas.A vessel that is used for oil storage purposes only is designated a Floating Storage Unit (formerly FSU, now FPSO).



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2.2.2. FLOATING RIGSThe drilling of exploratory wells offshore is strongly conditioned by the water depth: beyond about 100 m, the use of rigs standing on the seabed is no longer possible, and it is therefore necessary to use floating units, i.e. buoyant structures on which a drill rig is installed.

Such structures are designed to be kept in position as firmly as possible above the well being drilled, by means of anchoring or dynamic positioning systems. The main problem in such operations is obtaining a sufficiently rigid connection between the seabed and the floating unit, thus enabling the drilling equipment to be lowered into the well and guaranteeing hydraulic continuity for circulation of the drilling fluid, which has to return to the rig. The connecting element between the floating unit and the wellhead (through submarine BOPs) is a special pipe called the marine riser. On account of the movement of the sea, wind and tides, floating units, not being rigidly connected with the seabed, can move vertically and horizontally in relation to the well axis: these movements, although very modest compared with the water depth, must never exceed the limits imposed by the rig design conditions,. As a rule, the admissible horizontal movement during drilling is about 3-5% of the water depth. During operations, the movement or displacement has to be constantly monitored so as to prevent the occurrence of excessive stresses on the structures connecting the subsea wellhead with the floating unit; if the weather and marine conditions cause the displacement to exceed the safety limits, the structures have to be disconnected.



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MARINE RISER examples



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2.2.2. FLOATING RIGSFloating drill rigs can be divided into two main classes: semi-submersible rigs; drilling ships.

In both cases, they are basically vessels constructed to contain an autonomous drilling site, a platform for helicopters, quarters for all the personnel and spaces for the materials and equipment. Floating rigs are proper vessels and therefore they have a captain and a crew of seamen.

Both types of rig, not being firmly connected with the seabed, need to use far more complex wellheads and submarine BOPs than those used in onshore drilling operations.



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2.2.2.1. SEMI-SUBMERSIBLE RIGS Semi-submersibles consist of a large triangular, rectangular or pentangular platform, connected with submerged hulls by means of large columns which vary in number from 3 to 8, according to the shape of the vessel; they are kept vertical over the site by means of mooring or dynamic positioning systems.

When being moved from one site to another, the submerged hulls are emptied and the rig becomes a floating unit, similar to an ordinary vessel. Some semisubmersible rigs have to be towed by tugs, while others have an autonomous propulsion system. In its working position, the height of the platform above sea level can be regulated by filling the hulls and the columns with seawater as ballast. By appropriately regulating the amount of ballast water, the draft of the vessel is varied, optimizing its stability during drilling operations. Moreover, when the state of the sea becomes particularly severe, the safety of the vessel can be improved by increasing the ballast, which lowers the vessel’s centre of gravity.

Semi-submersible rigs are constructed with a natural period of rolling and pitching different from the period of the waves normally encountered in the open sea, and they thus have considerable stability, which is little affected by the wave motion, and permits comfortable working conditions. In fact, as a large part of the mass of the vessel is submerged, it is hardly subject to rolling or pitching.



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2.2.2.1. SEMI-SUBMERSIBLE RIGS Anchored semi-submersible rigs are used for drilling in water depths of up to about 1,000 m. At greater depths dynamic positioning systems are required.

Semisubmersible Rigs



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2.2.2.2. DRILLING SHIPS The first drilling ships were usually old colliers, whalers or cruisers, with their hulls suitably adapted to make an opening, still known as the ‘moon pool’, vertically above the centre of gravity. The derrick was installed above this, together with the relevant equipment. The deck was organized to accommodate the tubular materials, while the pumps and the mud treatment plant were housed in the hold. Modern drilling ships are designed and built specifically to act as drilling sites and they are equipped with particularly complex technological systems.

Drilling ships are used for operating in deep waters, often under extreme environmental conditions, such as drilling in Arctic areas. To this day it is the best means of drilling exploratory wells in remote areas, far removed from supply points, as it can carry all the material necessary for drilling even a particularly difficult well. Just as for semi-submersible rigs, drilling ships are kept in a vertical position over the well by means of mooring or dynamic positioning systems (Fig.). These ships when moored can be used for drilling in depths of up to about 1,000 m, while for greater depths dynamic positioning systems must be used, and with these the ship is capable of operating in 3,000 m of water. In this case, the depth limit depends only on the weight and the mechanical strength of the connecting system with the subsea wellhead.



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2.2.2.2. DRILLING SHIPS

Drilling Ships



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2.2.2.2. DRILLING SHIPS


Drilling Ships


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3. THE DRILLING PROCESS

The technique used by the Oil Industry to drill wells is called rotary drilling, where a drilling bit, attached to a system of hollow tubular rods, the drill string, is rotated by the rotary table, a part of the drilling rig. In modern rigs, the drill string and the bit are rotated by a tool called top drive.

The drill string is composed by 9 m long pipes, which are added one above the other, permitting to reach the final planned depth of the well (down to 7000 m and more).

The rock excavated by the bit is transported to the surface by a fluid, called drilling fluid or mud, which is circulated down the hole through the hollow drill string, the opening of the bit (nozzles) and then returns to the surface passing through the annulus between the wellbore and the exterior of the drill string. The circuit followed by the drilling fluid is termed hydraulic circuit.

The drill string is very important in determining the well’s geometry and path; the rigidity and stabilization of the drill string is realized by mounting just above the bit special joints called drill collars, of a diameter and weight much higher than that of the drill pipes, and special devices, called stabilizers, which have a diameter very close to that of the bit. The drill string can also have installed other equipment, such as jars, bumpers, shock absorbers used in emergency situations.


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DRILLING RIG

DRAWWORKS

TRAVELLING BLOCK AND HOOK



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ROTARY TABLE

DRILL BIT

TOP DRIVE



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THE HYDRAULIC CIRCUIT



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Drill Pipes

Heavy Wate (intermediate stiffness pipes)

X-over

Drill Collars

Jar (Shock tool to be activated while drilling string stuck)

Drill Collars Stabilizer (for hole reaming)

Drill CollarShock absorber (vibrations damper) Stabilizer

Drill Collar (Short Drill Collar)

Near bitDrilling bit

DRILL STRING COMPOSITION



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DRILL

COLLARSTABILIZERJAR

HEAVY

WEIGHT

DRILL

PIPE BIT



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Once a planned depth is reached, the bit and the drill string are pulled out of the hole, wireline logs are recorded to obtain information on lithology and fluids inside the pores of the rock and the hole is lined up with pipes, called casings, having a size slightly smaller than the hole in which it is run. The casing is anchored to the wellbore walls by pumping in the annulus between the exterior of the casing and the wellbore walls a cement slurry (mixture of cement and water), whose main task is to isolate the formations so far drilled from the surface.

At this point drilling is resumed with a bit having a size smaller than the interior of the casing string and is continued to a new planned depth where the operations seen above are repeated (logs recording, casing running, cement slurry placement).

This sequence of operations continues several times (2-4), always running equipment smaller in diameter with respect to the previous one, until the final depth is reached. Now two possibilities exist:

- the well is dry: in this case the well is plugged and abandoned;

- the well is mineralized: the well is completed and production tests are carried out. The well is, then, given to Production



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The well is made by a series of concentric holes of decreasing diameter drilled down to certain planned depths.

The sequence is as below indicated.

1. CONDUCTOR PIPE

The first hole is usually drilled down to 30-40 m with a big size bit. When this depth is reached the bit and the drill string are pulled out of the hole and the hole is lined with pipes, called casing, having a diameter slightly smaller than the size of the hole drilled before (for instance a 30” casing is run in a 36” hole). Very often this first casing, called conductor pipe, is hammered down to the planned depth.

If the hole has been drilled, once the casing has been run in hole, a slurry, made by cement and water, is pumped in the annulus to anchor the casing to the formations drilled, isolating them from the surface.

Its main function is to permit the circulation of the drilling fluid without causing the caving in and erosion of the unconsolidated shallow formations.



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2. SURFACE CASING

Once the Conductor Pipe has been set, a hole of smaller diameter (normally 26”) than the first one is drilled down to a certain depth (usually some hundred meters). Once reached this depth, the drill string is pulled up and a second casing having a diameter of 20” is run in hole and cemented up to the surface.

Its main function is to allow the installation of the safety equipment (Blowout Preventers, BOP) and their closure in case of a kick without fracturing the formations.



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3. INTERMEDIATE CASINGS

After the surface casing has been set, a new hole smaller than the previous one (usually 17 ½” in diameter) is drilled down to the new planned depth and a new smaller casing, having a 13 3/8” diameter, is run and cemented. This third casing is called intermediate casing.

The intermediate casings can be more than one, normally from 2 to 3 and have always decreasing sizes as the holes where they have been run in. They are all cemented up to a certain depth or the surface.

Their function is to isolate formations which can create potential hole problems (circulation losses, abnormal pressures, instability, etc.) and to permit the installation of higher pressure rated safety equipment.



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4. PRODUCTION CASING

When the total planned depth is reached and in case the well results to be productive, the last casing string is run in hole with the purpose to allow the production of the well; this casing string is called production casing.

It allows the installation of the production equipment and must be capable to withstand all the stresses foreseen during the producing life of the well.

In case the well is dry, this last casing is not run and the well is abandoned.

Next slide shows a typical sequence of holes and casings used to drill and case a well.



61

1 61

30" CP

20" csg

9 ⅝” csg

7“ csg

13 ⅜“ csg

STANDARD CASING PROFILESTANDARD CASING PROFILE

26" Hole

17 1/2“ Hole

12 ¼” Hole

8 ½” Hole

Cement

Drilling Mud


01.politecnico di torino drilling fundamentals 2010

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