Oil lives in a nanoscale "bedroom"? Let's take a look at the "close-up" of shale magnified a million times

The Daqing Oilfield core library is filled with neatly arranged gray-black cylindrical stones. They are cylindrical rocks drilled out from the well using special tools. They are important physical materials for understanding and studying underground conditions and mineral conditions, and are also the "home" of Gulong shale oil underground. On one side is the black, almost pore-less, iron-like core, and on the other side is the light yellow, clear oil. What is the connection between them?

In order to uncover the mystery of Gulong Shale, petroleum scientists used high-precision instruments and equipment to open the door to the wonderful microscopic world for us. It turns out that in these black shales that look like iron plates, there are countless tiny pores that cannot be seen by the naked eye, which provide a place for oil to live.

Gulong shale oil lives in a nanoscale "bedroom"

The pores and cracks in rocks provide a place for oil and gas molecules to settle down. Gulong Shale is no exception, but compared with conventional oil and gas reservoirs, the storage space of Gulong Shale is extremely small, and most of the oil "lives" in the underground nano-scale "bedrooms" (pores) and micron-nano-scale "gallery" (cracks).

Do you know how small these pores are? Conventional microscopes cannot observe them. Only with the help of high-resolution electron scanning electron microscopes, which can magnify the image by hundreds of thousands or even millions of times and reach a resolution of 1 nanometer, can we see the true appearance of the pores. In this way, we can have an in-depth "close-up" of the microscopic world and see how the pores in these shales are distributed.

The diameter of the pores in Gulong shale is basically less than 100 nanometers. The diameter of a human hair is usually 70 microns, and the pore size of Gulong shale is only 1/700 or even 1/1000 of a human hair. If a human hair is compared to the size of a football field, the effective pore size of Gulong shale is equivalent to a football in the football field.

Shale oil travel has a "channel"

Since the "bedrooms" are so small, how can oil be extracted from underground? If you imagine shale as a big house, the pores inside are independent "bedrooms", and the throat is the "aisle" connecting the bedrooms. Oil needs to flow from countless "bedrooms" into the "aisle" underground. The "bedroom" plays a decisive role in how much oil can be stored, and the "aisle" plays a key role in whether the oil can flow and be transported.

The study found that although the volume of a single pore in Gulong shale is very small, the total cumulative pore volume is relatively large, which means that there are enough "bedrooms" and a large total space; the only drawback is that there are fewer "aisles", making it difficult to form a continuous flow channel. This is a very unfavorable factor for oil development.

So, how does Gulong shale oil solve this problem? Gulong shale has developed bedding fractures, structural fractures and overpressure fractures, among which bedding fractures are the most developed. Due to the existence of bedding fractures, the rock is very easy to peel along the bedding fractures, forming "thousand-layer pancakes". Bedding fractures of meter-millimeter scale can be seen with the naked eye, while micro-nano-scale bedding fractures need to be identified with the help of scanning electron microscopes. The density of micro-nano-scale bedding fractures is extremely high, reaching hundreds of thousands per meter.

Micro-nano-scale bedding fractures are like fine country roads. Although narrow, they are numerous and widely distributed, and can transport oil from country roads to provincial roads. Small-scale and large-scale bedding fractures are like provincial roads or national roads. Although they are relatively few in number, they are wide and extend over a long distance, and have a stronger transport capacity for vehicles. They improve the seepage capacity between reservoirs and facilitate the flow of oil molecules.

When it comes to structural fractures and overpressure fractures, they further build bridges between different channels in shale, making the "roads" in shale more interconnected. In this way, the "oil baby" can "stride" and flow freely in the shale reservoir.

Under the nano-confinement effect, the "oil doll" is easy to change its face

The "oil dolls" living in micro-nano-scale pores are very naughty, and it is not easy to get them out. Here we have to talk about an effect - the nano-confinement effect. In layman's terms, the nano-confinement effect refers to the fact that in the nano-scale space, due to the limitation of the small space, the material properties will change, and even physical or chemical reactions will occur.

In large-scale pores, the interaction between fluid molecules and pore walls is negligible compared to the interaction between fluid molecules. However, at the nanoscale, the interaction between solid molecules on the pore wall and fluid molecules is large. When the average path length of the collision between fluid molecules and pore walls is comparable to the pore size, the free thermal motion of fluid molecules is significantly affected.

It is like the "grease dolls" playing in the school playground during recess. Because the space is large enough, they can happily chase each other without bumping into each other. When the bell rings, the "grease dolls" rush into the narrow corridor. Because of the pushing and shoving, the resistance to moving forward suddenly becomes greater, and they cannot pass through the corridor for a while.

This confinement effect will also affect the change of fluid phase, bringing difficulties to oil development and exploitation. Under this confinement effect, the phase of Gulong shale oil fluid "jumps back and forth" between the two states of "volatile oil" and "condensate gas". This is like the "oil dolls" turning into "Sun Wukong". When they are in a narrow pore, because the "room" is crowded and stuffy, they "whoosh" into a wisp of smoke; when they enter another pore, due to the change of temperature and pressure, they "whoosh" back into oil beads. The challenges brought by the naughty "oil dolls" are far more than that, and this is also testing the wisdom of petroleum geologists and engineers step by step.

Author: Wu Songtao (China Petroleum Exploration and Development Research Institute), Jiang Xiaohua (China Petroleum Exploration and Development Research Institute), Gao Bo (Daqing Oilfield Co., Ltd.)