What are the best practices for determining water saturation using wireline logging methods?

1 Basic concepts

Wireline logging methods involve lowering a tool or a probe into a wellbore and recording various physical properties of the formation. Some of these properties are related to the fluid content and distribution in the pores, such as resistivity, dielectric constant, neutron porosity, and nuclear magnetic resonance. These properties can be used to estimate water saturation, but they are also affected by other factors, such as rock type, clay content, salinity, temperature, and pressure. Therefore, it is important to understand the assumptions and limitations of each method, and to use appropriate models and corrections.

2 Resistivity methods

Resistivity methods are based on the principle that water and hydrocarbons have different electrical resistances. Water is more conductive than hydrocarbons, so a higher resistivity indicates a lower water saturation. Resistivity methods can be divided into two types: direct and indirect. Direct methods measure the resistivity of the formation directly, using electrodes or induction coils. Indirect methods measure the resistivity of the fluid in the pores, using a sampling tool or a formation tester. The best practice for resistivity methods is to use a combination of direct and indirect measurements, and to calibrate them with core data and fluid samples.

3 Dielectric methods

Dielectric methods are based on the principle that water and hydrocarbons have different dielectric constants. Water has a higher dielectric constant than hydrocarbons, so a higher dielectric response indicates a higher water saturation. Dielectric methods measure the dielectric response of the formation using electromagnetic waves or capacitive sensors. The best practice for dielectric methods is to use them in conjunction with resistivity methods, and to account for the effects of rock type, clay content, salinity, and frequency.

4 Neutron methods

Neutron methods are based on the principle that water and hydrocarbons have different neutron interactions. Water has a higher neutron capture cross-section than hydrocarbons, so a lower neutron porosity indicates a lower water saturation. Neutron methods measure the neutron porosity of the formation using a source of fast neutrons and a detector of thermal neutrons. The best practice for neutron methods is to use them in combination with density methods, and to correct for the effects of lithology, borehole, and environmental conditions.

5 NMR methods

NMR methods are based on the principle that water and hydrocarbons have different nuclear magnetic resonance properties. Water has a shorter relaxation time than hydrocarbons, so a shorter NMR signal indicates a higher water saturation. NMR methods measure the NMR signal of the formation using a magnet and a radiofrequency transmitter and receiver. The best practice for NMR methods is to use them in addition to other methods, and to analyze the distribution and mobility of the fluids in the pores.