Unconventional Reservoir Overview

This post is an elaboration of the "Basic Logging Interpretation" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Basic Logging Interpretation" slide at page 6

Basic: Why the Logging Results Should Show The Permeability?

Because the permeability of the rock represent the capacity of the rock to flow the fluid through it, it is very common that a wellbore be checked by logging tools to measure the permeability of some interesting place of the zone. For more detailed technical and scientific information about permeability, see my previous post here. So since a low permeability rock will hardly flow fluid through them and high permeability rock will flow fluid easily, people will check the logging result whether they can find high permeability zone with low resistivity.

So, People Will Look For High Permeability Zone Which Contains Hydrocarbon!

Exactly! Permeability is usually expressed in millidarcys and people will commonly look for zones which permeability is about ten to several hundred millidarcys as the oil reservoir. It will be categorized as conventional reservoir which procedure, technology, and knowledge have been widely used. Also the economical return is still profitable to the company.

Unfortunately, the party is over! As oil getting harder and harder to be searched, people start to consider gas as one of the main power source. conventional oil resources are estimated to be around 70 billion barrels, while the conventional natural gas is about 590 trilion SCF! Gas is cheaper because it is harder to be handled and less popular, so when the price of oil is getting higher, the price of gas becomes more interesting than years before.

WAIT, YOU ARE WRONG! The oil price is lower now than years before!

As I say higher price, it means as supply and demand relation. That means, as unrenewable energy, the more oil demand now will make the less supply in the future, and when the demand in the future still high although we only have few supply, the price will be high. The low oil price now is the impact of political decision and oversupply from the unconventional gas. We may enjoy the oversupply now, but when we realize that we have depended on oil and gas too much, the price may be much higher than before when the supply decrease abruptly.

So there’s conventional gas reservoir and unconventional gas reservoir? What is the definition of unconventional?

Yes there are differences of conventional gas and unconventional gas. The differences of gas and oil are about the properties of the fluid, e.g. the percent of methane, the phase of the fluid. In other hand, conventional and unconventional is all about how easy we may produce handle, and sell it from its reservoir. So the unconventional oil reservoir will contain extra heavy oil, bitumen (a little bit solid), or shale oil (oil from shale, which is hard to flow); while unconventional gas will be about low quality gas (high percentage of CO2 and sulfur), tight sand gas (low permeability), and gas hydrates (from the permafrost of ocean)

Why searching unconventional reservoirs when there are plenty conventional gas reservoirs?

The conventional gas reservoir resources were estimated around 590 trillion SCF, but it doesn’t mean they are widely spread all over the world! Arabian countries have a lot of proven reserves in their land that they could produce oil easily and sell them cheaply. In America, such oil is very low in number, but they have a lot of gas in their shale, which is tight in permeability and harder to produce, but the revenue will be very big. It is very important to depend on every country’s own power source, instead expensively buy from other countries.  Just imagine, Venezuela would be the largest oil reserves in the world if their 225 billion barrels of extra heavy oil considered as recoverable.

Back to the picture, how they could get oil/gas from low permeability zone?

Hydraulic Fracturing (“fracking”) is the technology to create artificial permeability to the zone. Advances in fracking techniques create  a tremendous increase in shale oil and gas production.  The permeability will be increased by artificially introducing small fractures into the formation. The pressure from the fluid used in fracturing will create fracture to the rock. Additional pressure injected from the surface will make the fluid penetrate deeper to the formation, and create longer fracture. This fluid contain many “granules”  named proppant which will support the fracture to stay opened when the pressure released. This way, the permeability of the zone has been increased.

First Fluid which is at first in hydraulic fracturing is called "pad". The pressure applied to get the pressure at which the rock break, called the breakdown pressure. Then fracturing fluids is used to a wide fracture, wide enough for the proppant to flow in without bridging or settling. That is why the fracturing fluid should have high viscosity.

Proppant will make high permeability flowing channel into the well. The permeability  contrast between fracture and the formation will determine the increase in the production of a fracturing project. The more fracture permeability contrasted with the real permeability, greater productivity will be gained. But it does not mean that we always use the biggest proppant,we still look for the most economical way, in the selection of size and types of proppant. When proppant stressed  more than its strength, it will be crushed and it is a disadvantage in terms of productivity. There are many things could be used as proppant, such as sand, ceramic, and resin coated proppant.

How about the unconventional reservoir permeability measurement? Will we use the same log?

We also use log to know the permeability of the reservoir. But an important complication is there is different resolution between the logs relative to the heterogeneity often exhibited at the core scale. Core measurements are often performed on a small sample while the log will represent the data for feet depth. Some attempt to scale up the core measurements should be done to allow for calibration of log to core data. Also the problem of the calibration is the heterogeneity of the zone is very high that scale up might be hard. These heterogeinity also makes the unconventional reservoir characterization become harder than the conventional one.

The unconventional reservoir studies are very important nowadays since it is the future of our energy sources, which is agreed by many consultants. The unconventional reservoir engineering and training are developing to create more economic method of production, with the prime safety consideration. Also, young engineers should enrich themself in unconventional reservoir since it might be the future of world's energy


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