How to keep the oil flowing? Build a "highway" for the oil to flow!

"Shale is very dense, and oil cannot be extracted, and even if it is extracted, it cannot be high-yield." This has long been a "conclusion." This "conclusion" is because the crude oil in the shale reservoir is stored in nano-scale pore throats and cannot flow effectively into the wellbore under natural conditions. Therefore, to achieve effective shale oil extraction, it is necessary to "break up" the dense shale reservoir through advanced engineering technology and build a "high-speed channel" between the shale pore throats and the wellbore that allows crude oil to flow. So, how to build such a channel? Hydraulic fracturing technology can take on this task.

"Break up" the reservoir to let the oil and gas flow out

In 1947, the United States conducted the world's first hydraulic fracturing production increase test, which opened the prelude to reservoir production increase and transformation technology. In hydraulic fracturing operations, petroleum engineers use pump trucks to inject fracturing fluid carrying high-strength proppants into the reservoir through the wellbore, causing the formation to break and extend into cracks under high pressure, and use the supporting effect of the proppants to build a "high-speed channel" for oil and gas migration in the reservoir after fracturing.

However, as unconventional oil and gas resources gradually become the main battlefield for exploration and development, although traditional hydraulic fracturing technology can open artificial fractures with certain conductivity in the reservoir, it is still difficult for oil and gas in the matrix pores to flow into the fractures due to the dense reservoir characteristics, and it is impossible to achieve effective oil and gas extraction. Faced with this problem, volume fracturing technology came into being. Its concept is to "break" the reservoir through engineering and technical means, and while forming multiple main fractures, multiple secondary fractures are formed on the side of the main fractures to form a complex fracture network. Compared with conventional hydraulic fracturing, volume fracturing is equivalent to building a "main road" while supporting a multi-level "highway network" such as "town roads" and "village roads". After the oil and gas flow out of the nano-scale "bedroom", they can enter the "highway network" at the shortest distance, greatly improving the traffic efficiency.

The height and length of artificial cracks are not good enough

Based on the preliminary research on Gulong shale oil, some international oil companies concluded that "the risk of obtaining commercial oil flow is very high, and there are no similar lacustrine shale blocks known to have commercial development value." Why is it difficult to mine Gulong shale oil? There are two main reasons.

The first reason is that the clay content of the target layer of other shale oil reservoirs that have been put into large-scale development is generally less than 10%, while the average clay content of Gulong Shale is higher than 35%, which makes the reservoir have extremely strong plasticity (that is, the ability to change its shape arbitrarily without being broken under the action of external force). It's like when we knead plasticine, the shape of the plasticine will change but it will not break; but when we hit the glass, the glass will break, which means that the plasticity of plasticine is stronger than that of glass. When fracturing in the Gulong shale layer, the reservoir rock is relatively easier to deform, but it is difficult to produce cracks that extend far, which greatly limits the range of the reservoir where the cracks communicate.

The second reason is that Gulong shale has extensively developed complex bedding fractures, with a density of hundreds to thousands per meter. It looks like a pile of hard, soft, thick, thin, and uneven cardboard stacked together. Petroleum geologists vividly call this structure "thousand-layer pancakes". To vertically pass through the "thousand-layer pancake" type shale, the fracturing fluid will undoubtedly encounter layers of obstacles. The horizontal bedding fractures become a relatively advantageous channel for the flow of fracturing fluid, but under the combined effect of clay minerals and complex bedding fractures, horizontal fractures are difficult to extend far. Petroleum engineers found through on-site monitoring data that the effective height of the artificial fractures formed by Gulong shale oil is less than 10 meters, the effective length is less than 200 meters, and the effective transformation volume is only 40% of the tight oil reservoir under the same construction scale.

"Control near and expand far" to form a complex crack network

How to solve the problem of fracturing and production increase of Gulong shale oil reservoir? In water conservancy projects, engineers build dams to centrally manage water resources scattered in different tributaries on the main river channel, forming a greater water flow force for power generation and other purposes, thereby improving the utilization of water resources. Petroleum engineers adopt a similar idea and innovatively propose a fracturing design concept with "control near and expand far" as the core.

The so-called "control of proximity" is to let the fracturing fluid concentrate its strength and form a combined force to create a larger-scale fracture "main road". We found that clean water is easier to penetrate into micro-cracks than honey, because honey has a higher viscosity than water. In the fracturing operation, petroleum engineers can also significantly increase the viscosity of the liquid and inhibit the loss of fracturing fluid in micro-cracks by adding additives of certain ingredients to the fracturing fluid.

"Far expansion" is to let the fracturing fluid create more "rural roads" and "village roads" around the "main roads" in the far well area, so that more oil and gas can enter the "highway network" at the shortest distance, and finally converge in the "main roads" and be extracted through the wellbore. To achieve this goal, after using high-viscosity fracturing fluid to create the "main roads", petroleum engineers cleverly use a certain proportion of low-viscosity fracturing fluid to continue injecting it into the formation at a high displacement. These fracturing fluids follow the main fractures to the far well area and further impact the reservoir. Due to their low viscosity, they are more likely to penetrate into the bedding fractures, forming a complex fracture network.

"Slim Oil Flows Long" to Maintain the Flowing Power of Shale Oil

After the hydraulic fracturing operation is completed, the oil well is ready for production. However, the crude oil needs to flow out of the shale pores which are one thousandth of the diameter of a hair (about 70 microns) and cross the multi-level fracture network, so the production is long-term and slow.

Petroleum engineers have been studying how to make crude oil flow more easily since the initial design stage of hydraulic fracturing. Increasing the amount of fracturing fluid or adding crude oil-soluble gas to the fracturing fluid system can displace crude oil, increase formation pressure, and thus increase oil production. Adding surfactants to the fracturing fluid can reduce the surface tension of the oil-water interface and enhance the fluidity of the oil.

After a period of shale oil production, the pressure and output will decrease. If the pressure drops too quickly, it will cause the cracks to close and the high-speed flow channels to close. Therefore, the shale oil development strategy of "controlling pressure production and long-term oil flow" can release the formation energy in an orderly manner, which can not only maintain the driving energy for a long time, but also avoid the premature closure of the flow channel, prolong the stable production period, and improve the final recovery degree and development benefits.

Author: Meng Siwei, Liu Yishan (China Petroleum Exploration and Development Research Institute), Deng Dawei (Daqing Oilfield Exploration and Development Research Institute)