What Is a Resistivity Log?

Resistivity logging is a method of measuring the resistivity of rocks (including fluids in them) by using power supply electrodes and measuring electrodes arranged at different locations in the borehole. Resistivity logging includes common resistivity logging, lateral logging, and induction logging. ^[1]

Logging methods for measuring dielectric resistivity include: ordinary resistivity logging, micro-electrode logging, micro-lateral logging, spherical focus logging, adjacent lateral logging, dual lateral logging, and induction logging. .

Ordinary resistivity logging is to put a common electrode system (composed of three electrodes) into the well and measure the resistivity curve of the rock in the well. When measuring formation resistivity, it is affected by factors such as well diameter, mud resistivity, upper and lower surrounding rocks, and electrode distance. The measured parameter is not equal to the true resistivity of the formation, but is called the apparent resistivity of the formation. Therefore, ordinary resistivity logging is also called apparent resistivity logging.

The underground resistivity of a reservoir is a physical quantity that can neither be directly observed nor directly measured, and can only be measured indirectly when a current passes through it. Therefore, when measuring the resistivity, a certain current must be passed into the rock formation, and then the influence of different rock resistivities on the electric field distribution must be studied to further find out the relationship between the potential and the resistivity.

An electrode system is a group of electrodes arranged in a certain order. It consists of power supply electrodes A, B and measurement electrodes M, N. Of the three electrodes of the electrode system, two are in the same line C power supply line or measurement line, called pair electrodes or electrodes of the same name, and the other is in the same line (measurement line or power supply line) as the ground electrode, which is not called. Counter electrode or single electrode. It can be divided into gradient electrode system and potential electrode system according to the relative position of the electrodes.

In order to evaluate the oil-bearing property, the resistivity of the formation must be obtained more accurately. In the case where the thickness of the formation is large and the difference between the formation resistivity and the mud resistivity is not very different, ordinary electrode system logging can be used to obtain the formation resistivity. However, in the case of thin layers, the resistivity is very high, or in the case of salt cement slurry, because the mud resistivity is very low, most of the current flowing out of the electrode flows in the well and the surrounding rock, and the current into the measurement layer is very small. Therefore, the measured apparent resistivity curve changes gently and cannot be used to divide the stratum and judge the lithology. In addition, in the sand-mudstone interactive layer area, the high-resistance front layer has a great influence on the shielding of ordinary electrode systems, making it difficult to obtain the true resistivity of the formation.

In order to solve the above problems, a lateral well with a focusing electrode has emerged, which can make a flat current beam with a main current of a certain thickness enter the formation vertically, so that the shunting effect of the well and the influence of surrounding rocks are greatly reduced. Lateral logging began as three lateral logging, and later seven laterals have been developed. Now dual lateral logging has been developed. Dual lateral logging-microsphere focusing logging has become a salt cement slurry and high resistivity formation profile. Must test items.

Generally, the resistivity logging method requires a conductive liquid in the well, so that the current of the power supply electrode passes through it into the formation, forms a DC electric field in the formation around the well, and then measures the distribution of the electric field to obtain the resistivity of the formation. These methods can only be used in muds with good conductivity, but sometimes in order to obtain the original oil saturation data of the formation, in individual wells, oil-based muds are used for drilling, and sometimes air drilling is used. Under these conditions, Without conductive media, direct current logging cannot be used. In order to solve this problem, based on the principle of electromagnetic induction, induction logging was proposed.

During the well logging, an accidental error occurred in the project, and the power supply electrode did not supply power, but the change curve of the potential with the depth of the well was still measured. Since this potential is generated by natural potential, it is called natural potential and is expressed by SP.

The causes of the natural potential in the well are complex. For oil wells, there are two main reasons: (1) The salinity of the formation water is different from that of the mud. (2) Formation pressure is different from mud column pressure.

Practice has proved that the natural potential of an oil well is mainly caused by diffusion. Only when the pressure difference between the mud column and the formation is large, the filtering effect becomes a more important factor. ^[1]