Should Shale Gas Exploitation in Southern Sichuan be directly responsible for frequent earthquakes in Rongxian County?

 Should Shale Gas Exploitation in Southern Sichuan be directly responsible for frequent earthquakes in Rongxian County?

After three earthquakes of magnitude 4 or above occurred in two days, Rongxian County, Zigong City, Sichuan Province, called a halt to local shale gas production operations. Will exploration and exploitation of shale gas increase seismic risk? The answer is yes. This view has been basically unified in academia. However, as far as the three recent earthquakes in Rongxian County are concerned, there are no sufficient reasons and data to draw the conclusion that the development of shale gas leads to.

The reason why shale gas development is easy to induce earthquakes should start with its special exploitation mode.

As the name implies, shale gas is a kind of unconventional natural gas, which is extracted from shale formation and is mainly located in dark shale or high carbon shale. After hundreds of millions of years of sedimentary evolution, natural gas is stored in adsorption or free state and confined in the pore of shale. Shale is not only the source rock of natural gas generation, but also the reservoir and caprock of natural gas accumulation and preservation.

In order to extrude oil and gas concealed in crevices, hydraulic fracturing technology must be adopted. Shale resources are usually exploited by drilling straight wells to several kilometers underground, then drilling hundreds to thousands of meters horizontally, and injecting a large amount of high-pressure water mixed with chemical substances into shale formation for hydraulic fragmentation, which creates crevices and prizes rock strata artificially, thus allowing oil or natural resources. Gas flows down the wellbore to the surface. For decades, the popularity of hydraulic fracturing in North America has boosted North Americas shale oil and gas prosperity, resulting in a huge amount of shale gas, which lowers North Americas gas prices and makes the United States a net exporter of natural gas. This technology has also been applied to shale gas production in Sichuan.

Sichuan Basin is one of the three most abundant natural gas basins in China. The shale gas burial depth in this area is relatively deep, generally between 2000 meters and 3000 meters. Rongxian area is located in the Changning-Rongxian-Weiyuan National Shale Gas Industry Demonstration Zone, which is the best reservoir area of shale gas in Sichuan Basin.

According to Sichuan Daily, a spokesman for Rongxian Development Bureau introduced the development project of shale gas in Rongxian at a conference held on the evening of February 25. Up to now, 15 new platforms and 39 wells have been drilled in Rongxian, of which 25 have been drilled and 10 have been put into operation. The early development is mainly in Xuyang Town, Shuangshi Town, Wangjia Town, Guoshui Town, Gaoshan Town and Lede Town of Rongxian County, which are within 5-10 kilometers of the main urban area of Rongxian County. The depth of the well is 3.4 kilometers vertical and the horizontal direction is 1.7 kilometers.

On February 24 and 25, three earthquakes occurred successively in Rongxian County, with magnitudes of 4.7, 4.3 and 4.9 respectively, and focal depths of 5 km. From the focal depth, it is not consistent with the official shale gas exploitation depth. From this point of view alone, it is difficult to judge that there is a direct relationship between earthquake and shale gas exploitation, and the correlation between them still needs detailed data support.

Rongxian requires shale gas development enterprises to suspend mining operations, not because the acquiescence of mining led to earthquakes, but due to seismic safety reasons and safety production needs.

The relationship between hydraulic fracturing and earthquakes, as well as other potential risks of this technology, has long been studied in academia. The most famous case is the increasing frequency of earthquakes in Oklahoma, the major shale oil and gas producing area in the United States.

Since records began, from 1975 to 2008, Oklahoma has experienced up to three earthquakes of magnitude 3.0 or more each year. Since 2009, the number of earthquakes with magnitude 3 or above has been increasing rapidly. There were 20 in 2009, 35 in 2010 and 64 in 2011. It fell back to 35 in 2012, but rebounded to 109 in 2013 and reached 585 in 2014. In September 2016, a magnitude 5.6 earthquake occurred in the north-central part of the state, which is the strongest earthquake ever recorded in the state.

At that time, researchers at Stanford University reported in the American Journal of Progress in Science that Oklahoma had become earthquake-prone mainly because local companies injected large amounts of salt-bearing wastewater from hydraulic fracturing to deep deposits of the earth.

Yu Tingting and others from PetroChina Southwest Oil and Gas Field Company published in 2013 Environmental Protection Challenges and Suggestions for Shale Gas Development that large-scale exploitation of shale gas causes fault activation, which may lead to geological disasters such as earthquakes and landslides. In April 2012, the U.S. Geological Survey released a report saying that earthquakes in the Midwest of the United States from Alabama to the northern Rocky Mountains have been almost certainly man-made in the past decade, in which oil and gas drilling activities, especially the hydraulic fracturing method used in shale gas development, and the waste water treatment wells involved in the method are all related to the increase in the number of earthquakes.

For example, a study by seismologists at the University of Texas and Southern Methodist Union University shows that the 2008-2009 earthquake in the town of Cluben, Texas, has no decisive relationship with hydraulic fracturing during the development of Barnett shale, but injecting large amounts of salt-bearing formation water from gas well production into injection wells may trigger nearby earthquakes. Activities.

In some areas of Western Canada, hydraulic fracturing during oil and gas exploitation has induced many small earthquakes and moderate earthquakes. In November 2016, Professor David Eaton and postdoctoral Bao Xuewei of the University of Calgary in Canada published a new study entitled Fault activation by hydraulic fracturing in Western Canada in the top academic journal Science, explaining the mechanism of earthquake induced by hydraulic fracturing. Two researchers tracked the small and medium earthquakes and six hydraulic fracturing sites near Altera Fox Creekfu in Western Canada from December 2014 to March 2015, and analyzed the existing fault structures near the hydraulic fracturing sites in high resolution. Two different processes of fracturing activation faults in horizontal wells were found: (1) Fracturing fluid infiltrating nearby faults increased their porosity. Pressure; (2) In-situ stress changes caused by hydraulic fracturing itself.

So, can the above conclusions deduce that hydraulic fracturing and water injection directly triggered nearly three earthquakes in Rongxian County, Sichuan Province? It can only be said that they are not excluded, but is shale gas exploitation the decisive factor in recent earthquakes? It is still uncertain.

After the three earthquakes, experts from China Seismic Network Center and Sichuan Earthquake Prediction Research Center analyzed that there are two main reasons for the successive earthquakes in Rongxian County: one is that Rongxian County belongs to the medium seismicity area in Sichuan Province. There are 9 small-scale faults within the range of 100 km near the epicenter. The closest one is Changshanzhen fault, which is only about 15 km away from the epicenter. Since the record of Sichuan Seismic Network in 1970, five earthquakes with magnitude 4.0 or above occurred within 50 km of the epicenter, the largest being the Zigong earthquake with magnitude 4.8 on March 29, 1985. Second, the recent period belongs to the relatively active period of Sichuan earthquake. The seismicity in Sichuan has a certain regularity in time. At present, the seismicity in Sichuan is in a relatively frequent and strong period. Small earthquakes have also been active in Rongxian area in recent years.

Experts from Sichuan Seismological Bureau explained that the relationship between earthquakes and local industrial mining could not be determined according to seismic records and focal mechanism solutions. According to the current seismic wave analysis, earthquakes are related to anticlinal structures in this area, especially to some small faults. More and more in-depth studies are needed to determine the relationship between human activities and earthquakes. From the current waveform records and focal mechanism solutions, the Rongxian earthquake belongs to tectonic earthquake. Tectonic earthquakes, also known as fault earthquakes, are caused by faults in the crust (or lithosphere, with a few occurring below the crust in the upper mantle of the lithosphere). The crust (or lithosphere) is deformed in tectonic movement. When the deformation exceeds the rocks bearing capacity, the rock will break. The energy accumulated for a long time in tectonic movement is released rapidly, resulting in rock vibration, thus forming an earthquake.

In addition, Yunwukongcheng, a general author of Geology of Fruit Shell Network, believes that three earthquakes with similar magnitudes in two days are quite normal. In addition to the types of one main earthquake plus several small aftershocks, there are also two main-shock earthquakes and swarm earthquakes. The earthquakes that occurred in and around Zigong in the last two or three days are essentially an earthquake.

Finally, since the development of shale gas hydraulic fracturing has potential environmental risks and high production cost, why should we choose shale gas? In 2018, China surpassed Japan to become the worlds largest natural gas importer, with its dependence on foreign gas rising to about 45%, and will continue to rise this year. In order to achieve the goal of striving to achieve more than 200 billion cubic meters of domestic natural gas production by the end of 2020, which matches the growth prospects of Chinas natural gas consumption, unconventional natural gas is an essential component, and its development can not be ignored.

Source: Peng Mei News Responsible Editor: Qian Juanxiao_NBJ10675