In this opportunity I want to explain in a pleasant, clear and simple way how the resistivity of the formation water can be calculated from the knowledge that the oil industry and engineering have of how to interpret a well log.
It should be noted that the interpretation of the well log is very important to determine the saturation of fluids that are stored in the lithological formations of the subsurface, and thus to estimate the saturation of oil, gas and water present in the producing formation.
As many may imagine the search and exploration of hydrocarbons is a quite complex task, and therefore the objective I pursue through the well logs with the knowledge of petroleum engineering and petrophysical exploration, is to be able to find the resistivity value of the formation water for the correct interpretation of the logs that can be run in the well, and thus be able to know exactly which are the lithological intervals that contain oil and which are the prospective sands to be run in the well to be put into production.
The formation water represents in the well logs an element of primary importance, because when it is possible to measure the resistivity, ie when it is possible to measure the degree of resistivity to the electrical conduction, it can provide data needed for the calculation of saturations of fluids contained in the formation such as oil, water and gas.
The question that arises in the calculation of the resistivity of the formation water is
Is all the water found in the subsoil formations formation water?
The formation water that we are interested in to calculate the resistivity is the innate formation water, that is to say the water that was not able to be contaminated by the drilling fluid during the drilling works, this formation water not contaminated by the drilling fluid has the particularity that saturates the porosity of the rock.
Another question necessary for the calculation of formation water resistivity is
From what sources or elements can information be found regarding the resistivity of formation water?
It can be said that formation resistivity can be obtained many times by searching it in some water catalogs, in which resistivity data for different formation waters obtained from different oil fields and productive horizons are listed.
The origin of how this formation water resistivity data is recorded is mostly water samples that are obtained from when the well is in production, and in other cases from the geophysical records of the well. In these catalogs are also placed resistivity values of formation water from the spontaneous potential (SP) and comparisons made to two physical parameters such as resistivity and porosity.
Also chemical analyses to produced water samples are made, nevertheless in this article one of the main objectives is to give them some alternative forms, mainly centered in the different methods to calculate the resistivity of formation water using some very advisable petrophysical data for the solution of these calculations.
Due to the diversification of known methods to determine the resistivity of formation water, the objective of this article is to present to you the most common and simple forms and methods to calculate the resistivity of formation water.
Determination of formation water resistivity (Rw)
In order to determine the resistivity of the formation water, the first thing we must associate in this determination process is the conditions that the reservoir originally has, that is, before being drilled with a drilling fluid, so many of the parameters used to determine the resistivity of the reservoir water are always referred to conditions of the virgin reservoir.
It is necessary to take into account that drilling activities significantly alter the conditions of the reservoir in the vicinity of the well, resulting in a series of phenomena that affect the resistivity not only of the formation, but affects all resistivity values of the profiles that are run in the well.
When a well is drilled, a mud or drilling fluid is used to penetrate the formation in order to create a plaster, however this mud infiltration into the formation can generate a mud invasion into the formation that alters or causes problems in the profiles that contemplate the resistivity, and in this case the resistivity of the formation water does not escape.
In the graph shown you can see how in the vicinity of the wall of the well almost all the formation water and some of the hydrocarbons have been displaced by the invasive process of drilling mud from the well into the formation by that process of filtering, all that zone of displacement is known as the Wash zone, and when the resistivity in this zone is to be calculated it is known as the mud filtering resistivity (Rmf).
The other thing we can analyze from the graph is that there is a zone that is occupied by the fluids that have been displaced, this zone is called the invaded zone, and it is represented by the oil zone in which the resistivity of the oil (Ro) and the resistivity of the formation water (Rw) are measured.
Finally there is a zone called transition, under which one of its main characteristics is to be very close to the walls of the well, although sporadically and depending on the time can be away from the walls of the well.
Once we have understood the barriers that hinder a correct reading in the resistivity of the formation water, then we can obtain a measurement of resistivity by different means, one of these measurements of the resistivity of formation water can be obtained by direct measurement on a water sample, whether that value is extracted from the catalog of water resistivity values, or can also be obtained from the spontaneous potential curve, which is a very important well logging when providing information for other necessary calculations.
Within some methods of obtaining water formation resistivity (Rw) is had:
Perhaps to obtain a sample of the formation water is a reliable and optimal way to calculate its resistivity, nevertheless of a reliable form we can obtain it of the separator in where the well flows in a high relation water-oil, where it exists guarantee that that water comes from the hydrocarbon producing formation for its later chemical analysis.
When the well is in production, the last joint of the production pipe string can also be removed, and in this way we can check the water being produced from the last production pipe.
 Determine the resistivity of formation water according to its sodium chloride (Nacl) content
There are many chemical compounds and elements found in the different fluids found in the subsoil, however when we refer to the water found in the formation, there is a very common mineral existing in the formation waters of the different oil fields and it is sodium chloride (NaCl).
To know what concentration of sodium chloride has the formation water has to be measured in parts per million (PPM) NaCl concentration, then use a chart of resistivity - salinity, with the value of PPM of the concentration of sodium chloride at a given temperature we can know the value of the resistivity of the formation water.
In the graph of the previous image we can see how at a formation temperature of 75 ºF, the value of Rwe is the resistivity value of the uncorrected formation water, which is why we must enter the graph with the resistivity value of the uncorrected formation water, we cut the curve of the sodium chloride at the concentration in PPM determined and we read the resistivity of the formation water already corrected by the value of the sodium chloride concentration.
In case the water sample contains other chemical elements as shown in the graph, the average correction can be made based on the concentration of all the elements present.
Given the multiplicity of techniques to determine the resistivity of the formation water, I will present as a last method, one that involves the use of a single equation but that in its use depends on the extent that has the porosity record to reach the virgin zone, ie the zone that has not been affected by the invasion of drilling mud. The method is as follows:
Determination of formation water resistivity from resistivity records - porosity
Initially we propose the following equation:
Where: is the true resistivity, i.e. the measurement of the resistivity of the virgin zone, i.e. the resistivity of the zone that was not affected by the drilling mud invasion. To measure this resistivity of the true formation it is required to run a log in the hole called a deep investigation log, as it is necessary that its measurement reaches the virgin zone of the formation.
Now for the factor:
The calculation depends on the interpretation that can be made to the porosity record.
In the case that the measurement of the true resistivity does not reach the virgin zone, the value of (Rt) must be corrected for the elements found in the formation water sample. On the other hand, if the formation is a clean formation, the resistivity of the formation water (Rw) is equal to the resistivity of the true formation (Rt).
To apply this equation it is recommended that the values of Rt and F are measured at intervals of the formation where the resistivity of the formation water does not have significant changes, or that the changes are partially with very small changes. In cases where the production intervals are very deep, the conditions of use of the equation are favorable, since at greater depths the resistivity of the formation water does not present considerable changes, this is because the lithology presents very little permeable rocks and the invasion of drilling mud does not occur as dramatically as in higher intervals.
Importance of electrical resistivity as a fundamental knowledge to understand the fluids that are contained in the formation
It is very common to measure resistivity in some materials that are highly conductive to electrical current, however among these materials we would never imagine any other material than the ones we commonly know.
In the case of the resistivity records of the formation (subsoil) it is fulfilled that for this case the resistivity is calculated in the formation (producing field) in which it is believed that oil exists, because if we focus on the principle of resistivity we are going to realize that when the resistivity is high it is because the conductivity of the electrical current is low, this in terms of finding oil is very favorable, because if a formation in the subsoil has high resistivity it is because the most probable thing is that this formation contains oil.
However, if there is a formation which has a low resistivity it means that it is highly conductive and that there is most likely formation water, one cannot skimp on the fact that there may not be a distribution of fluids such as water, gas and oil in the formation, so the calculation of the saturation of these fluids in the rock is critical to evaluate the feasibility of canyoning in an oil-containing sand.
Conclusions and contributions within the engineering framework
The experience obtained in the study and evaluation of the formation water samples can help us to develop a good work where the chemical elements, like mineral salts can predominate in the formation water of deposits that can be repetitive in different oil fields, this experience together with new representative models that contemplate the type of mud used in relation to the composition of the formation water will help us more and more to develop charts supported in empirical models.
As the logs are run at greater depths the formations will become cleaner, making it easier to use resistivity determination with the equation that contemplates the true resistivity of the formation and the formation factor F, even to the point that if we find a clean formation filtering drilling mud to equal the resistivity of formation water to the resistivity of the formation.
When correction analyses are performed for the different chemical elements present in the formation water, not only sodium chloride, but other elements of which must be corrected by the different existing charts developed by the different well logging service companies, must be considered in the analysis of the formation water sample.
Finally, as a contribution to engineering, through this article it is necessary to have the influential tools to make comparisons at the level of resistivity logs, and thus determine which are the prospective intervals to find hydrocarbons, all this coupled with the fact that hydrocarbons are highly resistive, ie have low electrical conductivity, it is necessary to measure the resistivity of formation water to complement a final decision of the various records run in the well.
The formation water allows resistivity measurement taking into account that it is highly conductive of electricity as long as there are dissolved salts.
Note: The images that do not have image sources is because they are of my authorship, and were developed using the design tools of Microsoft Powerpoint..