After more than 10 years of research, Jiqing Oilfield Operation Area (Jiqing Oilfield) has developed a comprehensive technical system for continental shale oil exploration and development.

"We have been able to drill vertically and horizontally thousands of meters underground, which has greatly improved the exploration rate of shale oil," said Guo Xuguang, general manager of the Exploration Division of the Xinjiang Oilfield Branch, China National Petroleum Corporation (CNPC).

Finding the 'sweet spots'

Shale oil refers to oil-bearing shales, or underground rock formations that contain trapped petroleum, which is difficult to extract. Previously, only the U.S. had the ability to extract shale oil and gas.

China is dominated by continental shale oil, which is mainly distributed in Xinjiang Junggar Basin. Because of its small continuous distribution area, low maturity and high clay content, the drilling is extremely difficult.

CNPC took up the challenge. In 2011, the Xinjiang Oilfield Branch deployed the first vertical well in Jimsar Depression, and managed to extract shale oil. Experts estimated that there are more than one billion tons of shale oil reserves in this area, which shocked the entire oil industry worldwide.

"But after the discovery of high production wells, we found that the oil production varied widely," said He Jixiang, an engineer at the Research Institute of Petroleum Exploration and Development of Xinjiang Oilfield Branch, adding that subsequent drilling operations had to be suspended because the standards for stable production had not been met.

Generally speaking, the favorable parts of shale oil with good oil content and high exploitation value are called "sweet spots". "We had not found the 'sweet spot', so the exploitation result was far from what we expected," He told Science and Technology Daily.

The research team was determined to try again. In a warehouse with tens of thousands of core samples, they used microscopes to look for clues of "sweet spots" in the pores and cracks of samples. Finally, the team characterized the stratigraphic features of the continental shale oil and identified the "sweet spots" distribution pattern.

Probability of penetration improved

The exploration of continental shale oil is difficult because of the depth of the oil reservoir and the thinness of the "sweet spot".

The average depth of the reservoir in Jiqing Oilfield is 3800 meters, in which the "sweet spot" is a wave-like distribution, with an average thinness of only 1.5 meters, undoubtedly posing a huge challenge for exploration.

"The exploration drill had just hit the inside of the 'sweet spot', and then it drilled out of oil reservoir within dozens of meters, just like a man driving on a country lane without lights at night, who will drive off the road accidentally," said Yue Hongxing, an engineer of the Jiqing Oilfield Geological Research Center.

"The probability of penetration in oil exploitation means the probability of a drill bit finding an oil layer. In 2020, the domestic index of the probability of penetration was stuck at less than 50 percent, meaning that half of the pipes in the rock formations were non-oil producing," said Yue.

In the same year, Jiqing Oilfield was approved to build the first national shale oil demonstration zone. To achieve commercial scale production, improving the probability of penetration is critical.

The composition of shale oil in Jiqing Oilfield is very complicated, so more precise petrographic analysis was needed. The team decided to use carbonate content to determine the location of the oil reservoir.

"We added petrographic analysis at the drilling site. If the carbonate content is too high, we will adjust the drill bit until the carbonate level is back to normal," said Liang Chenggang, senior expert of Jiqing Oilfield, adding that this method will increase the probability of penetration to more than 80 percent.

'Nodding Donkey' replaced

Conventional oil fields only need vertical wells to produce oil, while the exploitation of shale oil requires the coordination of vertical wells and horizontal wells, through the method of fracturing.

In 2013, foreign fracturing technology was introduced to Jiqing Oilfield, which was not suitable for local conditions. The high rate of drill bit and power line loss, coupled with the high price of fracturing materials, has caused the cost of extraction skyrocketing.

Therefore, Jiqing Oilfield began to explore horizontal well volume fracturing technology, which uses high-density cutting, high-strength modifications and low-cost materials. After adopting this technology, Jiqing Oilfield completed the fracturing operation of No.8 well on Platform 58 in less than 50 days. Previously, it took 50 to 60 days to drill a well," said Liang.

At present, rodless pumps in the small containerized rooms have replaced the "nodding donkey", traditional pumping equipment transporting the oil to the ground through an up-and-down reciprocating motion. Relying on the Internet of Things technology, the temperature, pressure and other parameters of the rodless pump are automatically transmitted to the central control room.

This year, Jiqing Oilfield will complete 100 shale oil drillings, realizing the new production capacity of more than 600,000 tons, which means a well can be drilled in just over three days on average.