Measurement parameters in Hydrogeological Profiling

Resistivity profiles

Resistivity is the property of materials that describes their ability to resist the passage of electric current. Resistivity profiles measure this ability in order to determine the nature of the fluids contained in the pores of the formation.

Fluids with different salinities are present at certain levels of the wellbore, but in addition, the use of mud or injection as a drag and containment medium during drilling adds a foreign and different fluid to the formation, which is important to take into account when taking measurements in the well. For this reason, resistivity measurement devices with two or more investigation depths are used to measure resistivity in order to reach the zone invaded by mud filtrate independently of the virgin zone containing only formation fluids.

Problems associated with non-focused resistivity tools

Until 1950, electrical resistivity logging consisted of simultaneous short and long normal and lateral runs, obtaining three different depths of investigation with the lateral being the deepest. Despite having been used extensively in the oil industry for almost twenty years, the profiles obtained were often impossible to interpret, even using cumbersome correction charts for wellbore effect, layer thickness and adjacent layer resistivity. Finally, it was found that the readings were completely useless for layer heights less than 1.5 times the spacing. The problem with these devices was that the direction of the measurement current was not controlled, and it took the path of least resistance, favoring conductive muds and adjacent layers that were more conductive than the layers in front of the tool. As a consequence, Lateral and Normal curves were replaced by focused profiles. In the latter, adjacent layer and wellbore effects are minimized and vertical resolution and depth of investigation are improved. Two good examples of focused resistivity devices are electrical induction and the laterolog.

Spontaneous potential:

Another important parameter in determining permeable zones is the spontaneous potential. This is an electric potential of a few thousandths of that which occurs naturally in front of permeable layers and its magnitude is proportional to the salinity contrast between the formation fluid and the drilling mud. Thus, in front of permeable layers, a significant response of the curve with respect to the baseline obtained in front of clays is expected. The curve is then used as an indicator of permeable layers and for calculating the resistivity of the formation water under certain conditions.

Gamma rays:

The gamma ray profile represents the natural gamma ray emission of the formation. Since clays generally have significantly higher gamma emission levels than sands, the gamma ray reading is used as the lithological profile par excellence. Because the nature of the measurement allows the profile to be run in cased holes, it is also used to correlate cased hole readings with the original open hole readings, or to correlate with other adjacent wells.

Conductivity:

Conductivity profiling has been introduced into the standard open-hole RS measurement to provide the driller with periodic quality control of the drilling mud, thereby avoiding the use of excessive conductive injection that may cause contamination of the aquifers to be protected. The other very important application of this open-hole profiling is the detection of aquifers with higher pressure, which may manifest themselves by modifying the conductivity of the mud at the time of the profiling.

Although the injection is designed to contain the formation fluids and create a filter cake that prevents the formation fluids from entering the well, in reality and very often, the most important aquifers manifest themselves subtly by modifying the conductivity of the mud column, thus allowing them to be located in order to achieve a precise isolation design. This only occurs in shallow wells where the hydrostatic pressure is not high enough. In no way would this effect be seen in deeper areas because the well could not maintain equilibrium under such conditions.

In a cased well, the conductivity profile is probably the most powerful tool in the diagnosis of salinized water wells, as it allows the conductivity value of the formation water to be measured in front of each filter and eventually design a well repair that allows the productive areas with poor water quality to be taken out of service and leave the development in optimal operating conditions.

Temperature:

The mud temperature in thermally stabilized wells reflects the formation temperature that under normal conditions follows the thermal gradient of the area, a fundamental data for the study of thermal wells. It is also used in the detection of losses in production pipelines, in total or partial loss of circulation in perforations, for conductivity correction and to determine saturated or salinized horizons.