Week 11

This week’s lectures were on the topic of drilling and how engineers and geologists work on finding the properties of the subsurface, in order to determine the presence of hydrocarbons and their depth and qualities.

The drilling rig has several systems that work together to make the drilling process possible, which are:

- Power system

- Hoisting system

- Rotary system

- Circulating system

- Well Control system

As for the drilling process, it has several steps, starting with drilling a surface hole from the starter hole. Then the drill bit and collar are placed into it. The Kelly and turntable are attached to provide the rotation energy. As the drilling proceeds, drilling mud is circulated through the pipes to carry out the cuttings that result from drilling. Once the target depth is reached, the bit, collar and pipes are removed.

Casing pipes are now put into the hole to prevent collapsing. A cement slurry is pumped between the hole and the casing, and once it is hardened enough, the process is repeated by drilling further and casing the subsequent holes.

So, what exactly is drilling mud?

It is a mixture of clay, water, and chemicals with specific properties that is used to carry out drill cuttings, control formation pressure, cooling the bit, and maintaining the well stability.

In order to study the subsurface, samples must be taken and for that purpose we have coring. In coring we extract cylindrical reservoir materials and bring them up to the surface for testing.

There are three types of coring:

1- Conventional

2- Wireline

3- Diamond

There is also side wall coring in which smaller samples are obtained, called “bullets”. It works well in consolidated formations but not in unconsolidated ones.

Well logs are an important source of information for scientists and engineers. They are basically a record of formation data verses depth. Different logging tools are used for different types of logs, which we will be looking at shortly.

First we will look at the invasion profile, which depicts the zones surrounding the borehole and the path of the drilling mud and mud filtrate through the bedding.

There are four types of logs, which are:

1- Lithologic logs:

Spontaneous Potential

Gamma ray

2- Porosity logs:

Density

Neutron

Sonic

3- Resistivity logs:

Resistivity

Induction

4- Other:

Dipmeter

Caliper

As an example, the figure below shows the process for resistivity logging:

There are two types of well-logging, open-hole and cased-hole, as shown in the figure below:

In general, the tools for well logging are divided to active and passive tools.

Gamma Ray Logs

These logs are used to measure naturally occurring gamma radiation to characterize the rock or sediment in a borehole or drill hole.

Different types of rock emit different amounts and different spectra of natural gamma radiation.

It is a common and inexpensive method, and particularly helpful because shales and sandstones typically have different gamma ray signatures that can be correlated readily between wells.

For interpreting these logs, it must be noted that Evaporites (NaCl salt, anhydrites) and coals typically have low levels. In other rocks, the general trend toward higher radioactivity with increased shale content is apparent. At the high radioactivity extreme are organic-rich shales and potash (KCl).

It is thus useful for differentiating between reservoir and non-reservoir rocks, for quantitating the shaliness of the formation, and for well-to-well correlation.

Spontaneous Potential Logs

This log measures the difference in electrical potential inside the formation. The log works by measuring small electric potentials (measured in millivolts) between depths in the borehole and a grounded voltage at the surface. Conductive fluids are necessary in bore hole to create a SP response, so the SP log cannot be used in nonconductive drilling muds.

The relevant features of the SP curve are its shape and the size of its departure from the shale baseline. The deflection may be either to the left (negative) or to the right (positive), depending on the relative salinities of the formation water and the mud filtrate.

The movement of ions, essential to develop an SP, is possible only in formations with some permeability, SP measuring circuits are sensitive and therefore prone to recording spurious electrical noise superimposed on the SP curve.

Occasionally, the source of noise cannot be eliminated during logging, and a noisy log is recorded. However, this does not always render the log unusable.

It is simple to obtain and therefore one of the more common ones.

Density Logs

Density logging tools rely on gamma-gamma scattering or on photoelectric (PE) absorption.

The density log belongs to the group of active nuclear tools, which contains a radioactive source and two detectors. The radioactive source is applied to the well bore wall in a shielded sidewall skid and emits medium gamma rays into the formation.

Mudcake adsorption is a source of error.

The density reading is a function of both porosity and rock type. If the rock type is known, then porosity can be calculated. If there is a second log, whose readings are affected by both the porosity and rock type, then both variables can be solved.

Apart from calculating porosity, it is used for gas detection and lithology identification.

Neutron Logs

It is an important tool for measuring porosity. It is used to calculate the hydrogen index, which is defined as the ratio of the concentration of hydrogen atoms per cm3 in the material, to that of pure water at 75 °F. As hydrogen atoms are present in both water and oil filled reservoirs, measurement of the amount allows estimation of the amount of liquid-filled porosity.

Its working mechanism is as follows: A reaction between radioactive elements emits fast neutrons, which collide with the nuclei of other atoms, most importantly hydrogen nuclei. Detectors count the slowed neutrons deflected back to the tool.

An apparent neutron porosity can then be obtained based on the hydrogen index.

For interpretation:

=> High Porosity = High hydrogen content = Low GR counts,

=> Low Porosity = Low hydrogen content = High GR counts

Generally, the neutron and density logs are run together.