China's first domestically developed submarine oil drilling equipment – subsea Christmas (Xmas) tree. It is a core piece of equipment used in subsea production systems in the petroleum industry. Photo: Courtesy of CNOOC

China's domestically developed submarine oil drilling equipment - a subsea Christmas (Xmas) tree system - was put into operation on Tuesday near Zhanjiang, South China's Guangdong Province, said China National Offshore Oil Corporation (CNOOC), the developer of the system. It can efficiently increase oil output in mature oilfields.

The operation of the new system shows the safety and reliability of domestically made equipment, which is significant for promoting the development of offshore oil and gas-related industries, CNOOC said in a statement sent to the Global Times on Tuesday.

The subsea Xmas tree is a core piece of equipment used in subsea production systems in the petroleum industry. It is used to connect oil from deep in the formation to external oil and gas transport pipelines. By injecting filtered sea water into the reservoir, the pressure will pump oil out of deep layers.

The subsea Xmas tree put into operation in Zhanjiang's Weizhou Oilfield is 3.5 meters long, 3.2 meters wide and 3 meters high, with an overall weight of nearly 22 tons. The subsea oil extraction system is expected to increase output by 50,000 tons annually, said CNOOC.

At present, only a few companies in the world have mastered the design and manufacture of subsea Xmas trees, said Yan Bangchuan, a project manager at CNOOC's Zhanjiang branch.

"CNOOC's Xmas tree is 40 percent lighter compared with foreign products of the same type. It is also 60 percent less costly to use and it can be applied in shallow waters," Yan said.

CNOOC launched its first self-developed subsea Xmas tree system in North China's Bohai Sea in July 2022. The oil and gas extraction system can help to extract more oil and gas reserves in the area and lift the annual oil and gas output by over 40 million tons.

Only 27 percent of the world's offshore oil and gas fields are in the primary oil recovery stage, and up to 67 percent are in the secondary oil recovery stage. The primary recovery stage of petroleum reservoirs is the production period when flow to wells relies on the natural energy of the reservoir or before a secondary or tertiary recovery approach is implemented.

Most oil and gas fields in shallow waters are "mature" oil and gas fields with a long history of exploitation. Due to insufficient pressure, it is necessary to rely on water injection and other exploitation methods to improve recovery.

For example, the oil recovery rate in the Beibu Gulf in South China is averaged at 20-30 percent.

After using the subsea Xmas tree system, the oil recovery rate can be raised by 10 percent, said Huang Yi, deputy chief engineer at CNOOC's Zhanjiang branch.

Global Times