Resistivity Profiling

Resistivity logging measures the electrical resistivity of subsurface rocks. Resistivity is the resistance that a material presents to the passage of an electric current. This helps differentiate between formations containing salt water (good conductors of electricity) and those saturated with hydrocarbons (poor conductors of electricity). Resistivity and porosity measurements are used to calculate the water saturation of the reservoir. Resistivity is expressed in ohmmeter, and is often plotted on a logarithmic scale as a function of depth due to the wide range of resistivity values. The distance from the wall of the well penetrated by the electrical current to the interior of the geological formation varies depending on the tool, from a few centimeters to several meters. This distance is known as investigation depth and is radial.

well images

The term “wellbore imaging” refers to the logging and data processing methods used to produce centimeter-scale images of the borehole wall and its component rocks. The context is therefore that of an open hole, but some of the tools are closely related to their cased hole equivalents. Wellbore imaging has been one of the most rapidly advancing technologies in wireline well logging. Applications range from detailed reservoir description to reservoir performance and enhanced hydrocarbon recovery. Specific applications are the identification of fractures,[5]​ analysis of small-scale sedimentological characteristics, evaluation of net oil thickness (net pay) in thin-strata formations and the identification of ruptures (irregularities in the borehole wall that are aligned with the minimum horizontal stress and appear where the stresses around the wellbore exceed the compressive strength of the rock).[6]​ The subject area can be classified into four parts:.

Porosity profiles

Porosity profiles measure the fraction or volume percentage of pores in a volume of rock. Most porosity peerfilages use acoustic or nuclear technology. Acoustic logging measures the characteristics of sound waves propagating through the wellbore environment. Nuclear logging uses nuclear reactions that take place in the downhole logging instrument or in the formation. Nuclear surveys include density surveys and neutron surveys, as well as gamma ray logs that are used for correlation.[7]​.

The basic principle behind the use of nuclear technology is that a neutron source placed close to the formation whose porosity is being measured will result in neutrons being scattered by hydrogen atoms, primarily those present in the liquid existing in the formation. Since there is little difference in the neutrons scattered by hydrocarbons or water, the measured porosity gives a figure close to the true physical porosity, while the value obtained by electrical resistivity measurements is that due to the conductive fluid in the formation. The difference between neutron porosity and electrical porosity measurements therefore indicates the presence of hydrocarbons in the formation fluid.

Density logging measures the apparent density of a formation by bombarding it with a radioactive source and measuring the resulting gamma ray count resulting from Compton scattering and photoelectric absorption effects. This bulk density can be used to determine porosity.

Neutron porosity profiling works by bombarding a formation with high-energy epithermal neutrons (which lose energy through elastic dispersion) to near-thermal levels before being absorbed by the nuclei of the atoms in the formation. Depending on the particular type of neutron logging tool, either the capture gamma ray, thermal scattered neutrons, or higher energy scattered epithermal neutrons are detected.[8] Neutron porosity logging is predominantly sensitive to the number of hydrogen atoms present in a particular formation, which generally corresponds to the porosity of the rock.

Boron is known to cause abnormally low neutron count rates because it has a high capture cross section for thermal neutron absorption.[9] An increase in the hydrogen concentration in clay minerals has a similar effect on the count rate.

Coring is the process of obtaining an actual sample of a rock formation from the borehole. There are two main types of coring: “full coring,” in which a rock sample is obtained using a specialized bit when the well first penetrates the formation, and “sidewall coring,” in which multiple samples are obtained from the side of the well after it has traversed a formation. The main advantage of sidewall coring over full coring is that it is cheaper (no need to stop drilling) and multiple samples can be easily acquired, the main disadvantages being that there may be uncertainty in the depth at which the sample was taken and the tool may fail and not acquire the sample.[10]​[11]​.

Resistivity Profiling

Resistivity logging measures the electrical resistivity of subsurface rocks. Resistivity is the resistance that a material presents to the passage of an electric current. This helps differentiate between formations containing salt water (good conductors of electricity) and those saturated with hydrocarbons (poor conductors of electricity). Resistivity and porosity measurements are used to calculate the water saturation of the reservoir. Resistivity is expressed in ohmmeter, and is often plotted on a logarithmic scale as a function of depth due to the wide range of resistivity values. The distance from the wall of the well penetrated by the electrical current to the interior of the geological formation varies depending on the tool, from a few centimeters to several meters. This distance is known as investigation depth and is radial.

well images

The term “wellbore imaging” refers to the logging and data processing methods used to produce centimeter-scale images of the borehole wall and its component rocks. The context is therefore that of an open hole, but some of the tools are closely related to their cased hole equivalents. Wellbore imaging has been one of the most rapidly advancing technologies in wireline well logging. Applications range from detailed reservoir description to reservoir performance and enhanced hydrocarbon recovery. Specific applications are the identification of fractures,[5]​ analysis of small-scale sedimentological characteristics, evaluation of net oil thickness (net pay) in thin-strata formations and the identification of ruptures (irregularities in the borehole wall that are aligned with the minimum horizontal stress and appear where the stresses around the wellbore exceed the compressive strength of the rock).[6]​ The subject area can be classified into four parts:.

Porosity profiles

Porosity profiles measure the fraction or volume percentage of pores in a volume of rock. Most porosity peerfilages use acoustic or nuclear technology. Acoustic logging measures the characteristics of sound waves propagating through the wellbore environment. Nuclear logging uses nuclear reactions that take place in the downhole logging instrument or in the formation. Nuclear surveys include density surveys and neutron surveys, as well as gamma ray logs that are used for correlation.[7]​.

The basic principle behind the use of nuclear technology is that a neutron source placed close to the formation whose porosity is being measured will result in neutrons being scattered by hydrogen atoms, primarily those present in the liquid existing in the formation. Since there is little difference in the neutrons scattered by hydrocarbons or water, the measured porosity gives a figure close to the true physical porosity, while the value obtained by electrical resistivity measurements is that due to the conductive fluid in the formation. The difference between neutron porosity and electrical porosity measurements therefore indicates the presence of hydrocarbons in the formation fluid.

Density logging measures the apparent density of a formation by bombarding it with a radioactive source and measuring the resulting gamma ray count resulting from Compton scattering and photoelectric absorption effects. This bulk density can be used to determine porosity.

Neutron porosity profiling works by bombarding a formation with high-energy epithermal neutrons (which lose energy through elastic dispersion) to near-thermal levels before being absorbed by the nuclei of the atoms in the formation. Depending on the particular type of neutron logging tool, either the capture gamma ray, thermal scattered neutrons, or higher energy scattered epithermal neutrons are detected.[8] Neutron porosity logging is predominantly sensitive to the number of hydrogen atoms present in a particular formation, which generally corresponds to the porosity of the rock.

Boron is known to cause abnormally low neutron count rates because it has a high capture cross section for thermal neutron absorption.[9] An increase in the hydrogen concentration in clay minerals has a similar effect on the count rate.

Coring is the process of obtaining an actual sample of a rock formation from the borehole. There are two main types of coring: “full coring,” in which a rock sample is obtained using a specialized bit when the well first penetrates the formation, and “sidewall coring,” in which multiple samples are obtained from the side of the well after it has traversed a formation. The main advantage of sidewall coring over full coring is that it is cheaper (no need to stop drilling) and multiple samples can be easily acquired, the main disadvantages being that there may be uncertainty in the depth at which the sample was taken and the tool may fail and not acquire the sample.[10]​[11]​.