illustration of capillary pressure concept _ nabi mirzaee _ linkedin

I received the following question from Linked-in through Global SubsurfaceProfessionals. The question asks for the Concept of capillary pressure. I tried toshare what I understand from the concept of the subject. Understanding theconcepts is essential in application of different reservoir parameters throughreservoir characterization and simulation.

Question:

Could anyone please explain me deeply what is the capillary pressure in porousmedia because I've not been able to understand it, I've been trying and reading on alot of references.

Answer:

To be able to give a brief but complete answer, my answer is structured in 6consecutive points as follows:

To be able to give a brief but relatively covering all aspects of the concept ofcapillary pressure, the answer is structured in 6 consecutive points as follows:

Point 1

Capillary Pressure appears when two different and immiscible phases and a thirdsolid material such as rock surface exist. Therefore when only one phase systemexists such as oil or gas or water alone capillary pressure may not be addressed.

Point 2

Between a solid surface and fluids there are adhesive forces. When two differentimmiscible phases exist in a container they exert adhesive forces in differentamounts to the container. The one exerting the bigger force is more likely spread onthe solid surface; it more likely wets the surface. This is more wetting phase andthe other is less wetting phase as demonstrated on the following Figure 1[1]:

Point 3

The more wetting fluid tends to intrude into the phase with less wetting fluid. Thereis a competition between the two phases the more wetting phase tries to advance

Illustration of Capillary Pressure Concept | Nabi Mirzaee | LinkedIn https://www.linkedin.com/pulse/20141130081815-156821532-illustration...

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The more wetting fluid tends to intrude into the phase with less wetting fluid. Thereis a competition between the two phases, the more wetting phase tries to advancemore into the less wetting phase and consequently the less wetting phase tries tostop it. This process stops when they reach equilibrium state. If a capillary tube isinserted into the interface of two immiscible fluids such as water and air, the water(more wetting phase) rises up in the tube as demonstrated on the following Figure2[2]:

Point 4

The amount of adhesive forces between the solid surface and the fluids is changedwith the curvature of the solid surface. The smaller the radius of curvature, thebigger is the adhesive force. In competition between the two phases smaller radiusof curvatures give more intrusion force to the more wetting phase than the lesswetting phase. If one puts two capillary tubes of different radius in a big watercontainer, in the smaller tube, water rises higher than the bigger radius tube. Theheight of water rising in each of the two capillary tubes is called CapillaryPressure. In fact the capillary pressure is a pressure needed to force the fluid toreach to the same higher heights.

In such representation, there are the big water container, two different size capillarytubes and two fluids, water in the container and air on top. There is capillarypressure between water and air in the big container. This is very small as the radiusof the curvature of the big container is very big in compare to capillary tubes. Weassume this as zero and take the water surface as Free Water Level. This is usedas the reference depth. The differences of the water level heights in the differentcapillary tubes are capillary pressure between water and air in two different media.This is demonstrated on the following Figure 3[2]:


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Point 5

In real case in reservoir engineering, in a water oil system for example, instead ofcapillary tubes there are variety of different sizes of interconnected pore structure;each having their own capillary pressure. The free water level (FWL), in such caseis the water level standing on the biggest pore size. This depth is a key referencelevel under which only water is mobile. Smaller pore sizes have higher capillarypressure causing the water to stand at higher heights. The height of the waterrelated to the smallest pore size is other key reference level called oil down to(ODT). Above this level, water is not mobile but oil is. Between the two referencelevels; i.e. FWL and ODT is a distance called Transition Zone. This is demonstratedon the following Figure 4:

Point 6

If a well is perforated under FWL it produces oil free water (water only); ifcompleted above ODT it produces water free oil (oil only); but if it is completedbetween the two references, i.e. transition zone it produced mixture of oil and water.

Nabi Mirzaee

PetroWiKi-Capillary Pressure1.

Figures are taken from the following Link: http://www.ux.uis.no/~s-skj/ResTek1-v03/Notater/Tamu.Lecture.Notes/Capillary.Pressure/Lecture_15.ppt.

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illustration of capillary pressure concept _ nabi mirzaee _ linkedin

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