The standard use of the centrifuge is to determine capillary pressure in plugs. What is proposed in this work is to extend its application making it possible to determine the pore-throat size distribution in plugs in addition to the capillary pressure indicated above.
To this end we start with a plug properly cleaned and saturated with the corresponding brine. The plug is then placed in the centrifuge and the collected data consists of the total evacuated brine volume VT(n) as a function of the centrifuge rotation speed n.
A simple model, consisting of capillary tubes running from one end of the plug to the other, is proposed to describe the complex system network of pores and the corresponding interconnecting pore-throats. The corresponding theory, based on treating the capillary pressure as that due to the water-air interphase, is worked out in such a way as to link the capillary size distribution to VT(n) vs. n. The mathematical procedures turn out to be straightforward and the solution is unique.
This original analysis of the centrifuge data was successfully applied to a large number of plugs. As an example the corresponding data and analyses are fully given and described.
In a companion paper these results are compared with those obtained by MICP (Mercury Injection Core Porosimetry), and the agreement found may be considered as excellent.
There is a practical limitation of the method proposed in this work. The capillary pressures that can be reached with the centrifuge are not as high as those that can be reached by MICP. This limitation manifests itself in the fact that pore-throat sizes below 1 mm are poorly detected or not detected at all. However, for pore-throat sizes above that value the agreement is excellent.
The main result obtained in this work is that it is shown that similar information to that produced in a MICP run with two main advantages (1) the centrifuge is a non-destructive experiment, and (2) is a non-contaminating experiment.
Porosity of formation rocks is one of the most relevant petrophysical characteristic parameters. Porosity is made up of pores and pore-throats and their sizes span wide range of values from about 10-2 mm up to 103 mm, and their sizes distribution functions are very important for a proper evaluation and management of an oil field.
There are not many experimental techniques to determine these distribution functions in bulk formation rocks (plugs). They are NMR (Nuclear Magnetic Resonance) and MICP (Mercury Injection (or Intrusion) Core Porosimetry). No one of them provides the information we are looking for. Also, in real formation the pores and the pore-throat network interconnecting them is so complex that it is not always possible to clearly discriminate among pores and pore-throats.
NMR measurements provide an information called the T2-distribution function, which rigorously speaking correspond to the addition of both the pore and the pore-throat sizes distribution functions, and it is not possible to separate them out. One of the most appreciated advantages of NMR is that it is a non-destructive technique.
On the other hand, MICP provides a good description of the pore-throats sizes present in the plug weighed by the fraction of the porosity corresponding to the pores interconnected by that given pore-throat size. Additionally, and unfortunately, MICP experiments are destructive and contaminating.
Thus, neither NMR nor MICP are able to provide the pore and the pore-throat size distributions.
In this work we propose to carry on a new experiment, the WRC (Water Removal by Centrifuge), using the centrifuge whose results resemble those obtained by MICP but with the main difference that is a non-destructive and non-contaminating one.
In the following sections the basic theory is developed, the equipment used (the centrifuge) is described, the results obtained in a standard plug are presented, and the conclusion is that a new method to determine the pore-throat sizes which closely resembles the results obtained by MICP. Limitations and advantages are described.