Let’s go, “group” to find the shale “underground palace”

In ancient times, people conducted hydrogeological surveys and dug wells to obtain water; today, people look for shale "sweet spots" and drill wells to extract underground shale oil. At the end of 2020, the factory-based well construction project of Platform 1 in the Daqing Gulong Shale Oil Experimental Area kicked off. Seven steel giant drilling rigs were deployed near seven oil wells, and seven underground passages advanced into the shale oil layer. Why are the towering derricks gathered here? Where do the underground passages lead to? What "magic weapons" are used in the drilling process?

Find the "sweet spot" and build a "golden target"

When you open a watermelon, the center of the flesh has the highest sugar content, is the sweetest, and is the most delicious. This part is the "dessert" of the watermelon. Shale oil reservoirs buried deep underground also have similar "dessert" areas.

Gulong shale oil has a low enrichment level and high mining cost. In order to achieve commercial profitability of shale oil development, the core of well construction is to find the "sweet spot" in the formation. How to find the "sweet spot" in such a large Gulong Sag?

Gulong shale oil is mainly found in the pores and bedding fractures of shale matrix. The Gulong shale reservoir seems to be a continuous and uniform whole, but there are still certain differences in the geological conditions of different layers and regions. Some areas are richer in oil and gas, the abundant original organic matter is fully converted, and the free hydrocarbon content is high; in some areas, the larger diameter pores provide larger oil storage space, and oil and gas are easier to flow; some areas are of suitable maturity, and organic matter produces a large amount of oil and is stored in situ. Such areas are usually called geological "sweet spots". Some areas have good rock brittleness, low fracture pressure, and are easy to be fractured and transformed, which are called engineering "sweet spots". The area with both of the above characteristics is the "sweet spot" area of ​​Gulong shale oil.

As exploration progresses, although the geological conditions of oil wells in different regions and layers are basically similar, the production of oil wells varies greatly when different fracturing and transformation processes are used. Using the idea of ​​​​geology and engineering integration, with the help of seismic detection, geophysical logging, laboratory analysis and other methods, a three-dimensional geological model is established, and the "golden target" for drilling is further constructed in the "sweet spot" to ensure accurate discovery of shale oil and efficient development.

Make good use of the three tools for well construction to quickly open a channel to attract the "oil dragon"

After the "golden target" is constructed, how to connect it to the ground to achieve "remote target shooting"? According to the wellbore trajectory design on site, a special control tool is used to make the drill bit drill horizontally in a certain section of the well, forming a wellbore similar to an "L"-shaped trajectory.

The average drilling distance of Gulong shale oil horizontal wells is generally 5 kilometers. The horizontal drilling length in the Qingshankou Formation, which has a complex structure and is prone to collapse and leakage, exceeds 2.3 kilometers, which has caused many difficulties in increasing the speed and efficiency of drilling. How can we successfully enter the "target" and make the well drilled quickly and well?

In the drilling engineering industry, it is often said that "there will be no oil and gas without the drill bit". The drill bit is the most core tool for drilling. Driven by rotary power equipment such as the drilling rig turntable and top drive, the drill bit continuously breaks the formation rock and eventually forms a wellbore. The most economical and efficient polycrystalline diamond composite (PDC) drill bit is used in Gulong shale oil drilling. It is inlaid with multiple "self-sharpening" PDC cutting teeth. This high-end drill bit has the high hardness, high wear resistance and thermal conductivity of diamond, as well as the strength and impact toughness of cemented carbide. It can cope with the challenges of high temperature and soft and hard interlayers in the underground Gulong shale more calmly, and is a pioneer in fast drilling of shale oil wells.

How to guide the drill bit to move in the "golden target"? The PDC drill bit is "blind" and can only move forward along the prescribed route with the help of the logging while drilling and guidance system. The logging while drilling sub is a "lookout". By measuring and transmitting underground parameters in real time, people on the ground can "see" various information about the drill bit and the wellbore, and guide the drill bit to reach the target layer and head for the "sweet spot". The directional drilling tool is a "steering wheel". Combined with the ground control rig rotation system, it can achieve "straight movement" or "turning" in the wellbore, allowing the drill bit to drill wherever it points.

After the original stress balance of the drilled Qingshankou Formation is destroyed, problems such as well wall peeling and collapse are prone to occur. At this time, the drilling fluid, known as the "blood of drilling", starts the stabilization mode. As an "exclusively customized" anti-collapse plugging oil-based drilling fluid, the oil-based component can lubricate the drill tool, inhibit shale hydration and formation slurrying, and facilitate rapid drilling; the specific density provides a liquid column pressure to balance the bottom hole pressure and support the well wall; a variety of plugging materials are combined to form a dense and complete plugging belt to seal the well wall. As a result, the drill bit can drill in the reinforced "copper wall and iron wall", and rapid "target shooting" is smoother.

Build a well factory assembly line to make mining more efficient

A well factory is a "factory" that quickly builds oil wells. This technology originated in North America and is a revolutionary technology that has enabled the United States to achieve the "shale gas revolution." Well factory drilling refers to the centralized deployment of a group of oil wells in the same area, using standardized equipment or services to drill in a manner similar to factory production or assembly lines, thereby improving development efficiency and reducing construction and operation management costs.

The Gulong shale oil well factory designs a three-dimensional platform according to the principle of cluster wells, and "assembles" and "lines up" the wellheads of 2 to 8 horizontal wells on the same drilling platform to form a well group. The bottom of each well extends to different depths and directions, covering a larger reservoir area underground with a smaller footprint on the ground. According to the well site specifications, wellhead location, and number of well groups, engineers design the wellhead spacing and horizontal section spacing to facilitate the simultaneous construction of multiple drilling rigs. Mobile drilling rigs can also save time for transportation and installation, opening and switching, and reduce drilling costs.

The Gulong shale oil well factory has a "small factory" where one drilling rig works continuously, and a "large factory" where multiple drilling rigs work simultaneously. The well factory model uniformly allocates resources, has a shorter construction and production cycle, lower costs, and better results than the single well construction model, which has taken the Gulong shale oil operation method a big step forward.

Author: Wang Chunjiao and Cheng Xinyang (Daqing Oilfield Co., Ltd.)