It’s not enough to find oil and/or gas in a reservoir. The next question to answer is how much of this oil and/or gas was found. This is because the volume of hydrocarbons present in the reservoir will determine if to complete the well or if to plug and abandon the well and move on. Porosity is a term that determines how much hydrocarbons a rock can potentially hold. So porosity is a measure of the fluid holding capacity of a rock. This means that it is safe to say that rocks with a greater porosity will potentially hold more hydrocarbons.
Porosity = Pore Volume, Vp / Bulk Volume, Vb --------------------eqn i
What then is the pore volume? It is the volume of the entire rock that is composed of ONLY spaces. Now, see it this way, try stacking 16 soccer balls with balls lying on top of each other. Now, there will be spaces in between balls because of the packing arrangement.
The totality of these pore spaces make up the pore volume. The balls, on the other hand, make up the grain volume. When we consider the volume of the entire stack of balls then we are referring to the bulk volume. So like we said in eqn i above, the porosity of the stack of balls is the pore volume divided by the bulk volume. This is the same with some rocks. Those rocks with this kind of spaces in between the grains are porous rocks.
Understand though, that a rock does not need to hold the fluid (oil, water or gas) in its pores to be considered porous. Take these two examples, samples of clay and loose sand. Clay has pores and will easily allow water to get into the pores, but it will hold onto this water tightly and will not readily allow the water leave its pores. Loose sand on the other hand will also allow the water into its pores but finds it difficult to retain the water in its pores. So as the water comes into the sand, almost all of it will leave if there is no external barrier like a container to hold the water. Remember, that both these rocks are porous. Porosity is only concerned with the volume of space in a rock available for fluids, porosity doesn’t go further to measure the possibility of the fluid to remain in the pores or leave almost immediately after it comes in. To understand how fluids flow in rocks after they get in, we’ll have to consider permeability.
What does it mean when we say a reservoir has a porosity of 35%?
Recall that Porosity = Pore Volume, Vp / Bulk Volume, Vb --------------------eqn i
So the ratio in equation i is normally multiplied by 100 to convert porosity to a percentage. It’s easier to look at it in percentages. When we say the porosity of a reservoir rock is 35%, we mean that 35% of that rock has the potential to contain hydrocarbon fluids in its pores. Notice that we keep saying the rock has a potential to hold hydrocarbon fluids. This is because not all the available pore space in a reservoir rock will contain hydrocarbons. Some pore spaces may simply be empty, like it will contain nothing at all. Other pore spaces in the rock may contain water. So when told that a rock has a porosity of 35%, the question to ask should be “is it the absolute porosity or the effective porosity?”
What is effective porosity?
Now, it is true that a rock could have a porosity of 35%. But, then how much of this 35% actually contains fluids. To be honest, some pores are so surrounded by rock grains cemented together that is impossible for any fluid to migrate into those pores. These kind of pores are isolated from others and really have nothing to do with other pores. They are called isolated pores. They do not contribute to fluid flow and hence, are not part of the effective porosity of that rock. At other times, some pores may even contain water, but the water is bound tightly to the certain clay minerals in the rock matrix, see it like the clay and water have become one and this water will not flow out from the reservoir. Lost. Pores like this one too won’t make the cut; this is because they contain fluids that cannot flow. So they are not part of the effective porosity of the rock. So for a pore to be considered when measuring effective porosity, two conditions must be met. The pore must contain fluids (oil, water or gas) and the fluid should contribute to flow.
What then is absolute porosity?
Porosity obtained from utilizing every available pore space in the rock; isolated plus the ones occupied by clay bound water. Everything. This is why the absolute porosity is usually higher than the effective porosity. If we were told that the 35% porosity was actually the absolute porosity, then we can infer that the effective porosity will be less than 35%. In reality, effective porosity is never equal to absolute porosity, there is always some form of clay bound water or isolated pores scattered all around the reservoir. Absolute porosity tells us the fluid holding potential of that rock, but sadly, some rocks don’t always reach their full potential. So since we are concerned in how much hydrocarbons we can produce from the reservoir rock, we are usually more concerned in the effective porosity.
Porosity is indeed significant to how much fluid a rock can hold. It is the fluid holding capacity of a rock. However, some rocks may not live up to their full potential. This is why effective porosity is always less than absolute porosity. Not every pore space in a rock is available to be occupied by fluids; some are isolated from others by cemented rocks grains and others contain clay that tightly holds fluids onto their surfaces. The effective porosity is an important ingredient when determining the original hydrocarbon in place.