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The well Through the rock layers with the drill bit

Finding oil and gas is not easy – but bringing these two natural resources to the surface is even harder: most reservoirs are deep underground, and to get to them you have to make your way through layers and layers of hard rock. To reach them, you have to drill your way down. But even before it is possible to begin producing the oil and gas, exploration and appraisal wells are necessary first.

Even if the geological analyses and geophysical surveys indicate that it is very likely that there is oil or gas reservoir, the prospect has to be confirmed first. An exploration well is conducted for this purpose.

This type of well brings core samples from the target formation to the surface. These cores contain an entire cross section of the rock layers in the possible reservoir and give the geologist important information from the earth’s interior – especially, whether or not oil and gas are really present.

 

Frode Nygård
Senior Drilling Supervisor

If the results of the exploration well confirm the discovery, appraisal wells are then drilled. The aim of these wells is to determine the size of the reservoir more precisely.

After all, it’s not just the question of whether oil and gas are present that is crucial for the production planning; the volume of the natural mineral resources available is also of key importance. The larger and more complex the reservoir, the more appraisal wells are needed.

When we drill, it is very rare that the wells are drilled vertically into the ground. Most wells are sidetracked and even run horizontally, with the
drill bits reaching depths up to seven to eight thousand metres – not an easy task! Numerous
safety measures enable us, however, to prevent
oil and gas from escaping uncontrollably.

Frode Nygård
Senior drilling supervisor

From production to injection A well for every purpose

When the Wintershall experts have confirmed the discovery of a reservoir and have collected precise information about the size and play of the reservoir, the drilling team begins to drill production wells: the reservoir is drilled in such a way that commercial production can begin.

Now oil and gas are finally flowing to the surface. To make sure they do not escape from the borehole in an uncontrolled manner, a so-called “Christmas tree” fitted with valves and pressure gauges is installed.

However, the production well is not the only well necessary for producing hydrocarbons. When oil is being produced, saline reservoir water is also brought to the surface. This water is separated from the oil and pumped back into the reservoir.

Hence, disposal wells are drilled for this purpose. But these wells are also needed for the secondary and tertiary phases of oil production.

For example, the production specialists at Wintershall pump water or hot steam into the reservoir through these wells in order to force the oil to the borehole or to make it runnier by warming it.

Rat hole

A rat hole is something that nobody would want in their own home or garden.

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Drilling rig the heart of a well

Above the ground you can recognize a well primarily from the drilling rig, or derrick, which is visible for miles around. The drill string is inserted from here; at the tip of the drill string the drill bit bores its way through the ground.

The individual pipes of the production string are made of extremely strong steel and are about nine meters long – once the drill bit has made this distance a new pipe must be screwed onto the string.

To replace the drill bit once it is worn down, the entire drill string must be pulled out of the borehole.

 

To allow the recovery and subsequent installation to proceed rapidly, three pipes always remain connected and are placed inside the drilling rig together – that is the reason for its height.

At sea drilling platforms or drilling vessels are necessary for drilling the wells. Cutting-edge drilling vessels even have two drilling rigs that work in parallel. During the drilling the drilling specialists pump a drilling fluid through the string to the drill bit at the bottom. The drilling fluid cools and lubricates the bit. Afterwards it flows back up at the outside of the string to the top and transports the cuttings – the fragments of rock cut by the drill bit – to the surface. The drilling fluid is filtered at the drilling rig and the cuttings are analyzed by geologists.

This provides them with information about the rock during the drilling process.

The right movement counts Various methods for deep drilling

In the widely used “rotary drilling process” the drill bit is set in motion from the drilling rig. A diesel or electric engine rotates the entire drill string including the drill bit. With this rotary motion and the enormous weight of the drill pipe, the drill bit bores through the rock centimeter by centimeter.

An alternative to the rotary process is turbine drilling. The drill bit is powered by a turbine installed directly above it. The drilling fluid, which is injected into the borehole at high pressure, is responsible for rotating the turbine and drill bit. Turbodrilling is not limited to vertical wells – thanks to an electronic control unit, the well can be deviated into all directions. The control unit determines the angle and direction of the well and supplies data on the rock that has just been drilled through. Thanks to cutting-edge technology, today it is also possible to develop reservoirs that are far away from the drilling rig.

Notably, extended reach wells can be drilled with turbodrilling – a technique which is now standard internationally. With so-called “horizontal drilling”, drilling within a reservoir even takes place horizontally. However, there is one thing that has remained the same since the beginning of oil and gas production through all the changes in drilling technology: the drilling rig is and remains the landmark of every well.

The drill bit Heavy weight for hard rock

The drill bit is the most important tool for exploring and developing a reservoir. And different types of drill bits are used – after all, not every well has the same aim. Roller bits, which grind their way through the ground, are used most often. If the rock is very hard, diamond or PDC (polycrystalline diamond compact, synthetic diamond) bits are used. They last longer than roller bits but are also much more expensive: a drill bit of this kind costs up to 100,000 euros.

Core drills are a special kind of drill bit – they are used to recover cores during exploratory drilling. The drill bits are hollow in the middle so that these cores are not ground down.

Hence, they provide a rod-shaped cross section of the ground from the very first meter of the section. Thanks to these core samples, the geologists can see for the first time exactly  the composition of the rock underground and the properties of the rock: is the rock as porous as they thought and do the pores really contain the precious hydrocarbons? The experts also collect knowledge to be used for later projects. That’s also why Wintershall keeps the core samples in storage, so that it can use this resource in future too.

Safe drilling Our top priority

Wintershall works with the highest standards, especially when it comes to safety and environmental protection. Naturally, this also applies to the wells it drills. The casing is central to the safety of every well. The borehole is supported by the casing and sealed off from the surrounding rock layers. This prevents liquids from escaping from the well to the surrounding area and prevents the outside pressure on the borehole from making it cave in.

As soon as the drill bit has drilled its way through a certain section, the drilling team pulls out the drill pipe with the drill bit at the tip and then builds the casing. The casing is made up of several individual pipes about 10 meters long which are screwed together.

A layer of cement is laid round the casing: the cement is forced upwards from the bottom end of the casing along the outside. The cement layer stretches from the end of the casing to the surface – so nothing can escape from the borehole. Drilling only continues when the casing is anchored firmly in the ground. Since the casing and the cement layer fill out part of the borehole, a drill bit with a smaller diameter must be used for the next drilling stretch.

Hence, several casings are fitted together like a telescope, which means the well becomes narrower with each new casing. How many casings are fitted, and in which sizes, depends on the type of well, the well depth and the conditions on the ground.

The casing A safe sealing reaching all the way down

The conductor pipe

The conductor pipe is the first casing of a well. It prevents the borehole from caving in because of the loose earth at the surface and it prevents the foundations of the well site from being softened by the drilling fluid. It also separates the borehole from the groundwater layers that run near the surface.

Surface casing

Surface casing is installed to seal off the borehole from the underlying groundwater layers and to stabilize the well.

The surface casing also serves as a base for the blowout preventer, a device equipped with safety valves and weighing several tons. It is installed above the borehole and prevents oil and gas from leaking if the pressure in the borehole is not sufficient to keep the mineral resources down.

Intermediate casing

Intermediate casings prevent the difference in pressure between the rock and the borehole from becoming too great along the entire drilled section, and they prevent the borehole from caving in or drilling fluid from escaping.

Depending on the depth of the well and the geological conditions, it may be possible to do without intermediate casing. Alternatively, several of these casings may be needed.

Production casing

When the well has reached the reservoir the production casing is inserted. It separates the surrounding rock layers from the production area.

The tubing is set in the production casing. The oil or gas actually arrive at the surface through this tubing. Hence, the production casing therefore seals off every production well.

DCI – Drilling Complexity Index

The Drilling Complexity Index (DCI) developed by Wintershall indicates the level of complexity of a well on a scale of 1 to 10 – the more complex the well, the higher this value.

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