注入井可在數公里之外的地方引發地震

注入井可在數公里之外的地方引發地震

研究發現流體注入沉積岩中比注入下方基岩所產生的地震還要更大，距離也更遠

By Tim Stephens

當人類把流體注入地底深處可能會產生地震。一項有關於此的研究得出了意料之外的模式，顯示現今對於水力壓裂法、廢水處置和地熱井開鑿的建議可能需要重新審視。

此示意圖顯示了一座正在運作的注入井。藍色和紅色區塊分別為注入流體到基岩(藍)和上覆沉積岩(紅)中而誘發的地震發生範圍。灰色線段代表斷層網絡。下方圖表則顯示了兩者對應的地震發生機率如何隨著距井遠近而變化。(圖片來源：Goebel and Brodsky, Science, 2018)

加州大學聖克魯茲分校的研究人員彙整並分析世界各地明顯和注入井有關的地震資料。他們發現單獨一口注水井可在10公里以外的地方引發地震。此外，他們還發現總體來說把流體注入到沉積岩中，比注入到下方基岩中引發的地震規模更大且距離更遠。

加州大學聖克魯茲分校的地球和行星科學教授Emily Brodsky表示：「這會造成問題，因為現有建議是偏向於把流體注入到理論上較為安全的沉積岩序列當中，而不是基岩。」

博士後研究員Thomas Goebel表示這項議題的關鍵是誘發地震在注入井周圍的空間分布。他說：「這並非代表基岩就是安全的，因為在基岩中仍有機率遇到會造成大地震的斷層；但是誘發地震在基岩中的分布範圍較小，使它遇到這類斷層的機率也比較低。」

在8月31日發表於《科學》(Science)的論文中，Goebel和Brodsky描述了兩種截然不同的誘發地震分布模式，分別和作用在基岩與沉積岩中的不同物理機制有關。在第一種模式下注入井達到碁岩之中，其誘發的地震傾向於集中在井的周圍，離井越遠地震的發生機率也急遽下降。在另一種發生於沉積岩的模式中，誘發地震的發生機率則是離井越遠而緩慢下降，使得地震發生的位置也離井更遠。

過往研究認為注入井誘發地震的物理機制是岩石孔隙的液體壓力上升的直接結果，這使得斷層更容易滑動。Goebel表示此機制確實可以解釋基岩中的注入井造成的誘發地震分布模式。但是，在沉積岩中看到的分布模式卻顯示出有另一種機制大力參與其中。此機制稱為「孔彈性耦合(poroelastic coupling)」，其決定了某種岩石把內部液體壓力傳遞至岩石固體基質的能力。

Brodsky說：「當人類把水注入地下，水會擠壓周圍的岩石而在岩石內部產生彈性應力，這股力量造成水雖然沒有流入遠方斷層，但斷層面上的壓力還是會上升。因此，如果誘發地震以孔彈性為主要機制，最後造成的地震範圍就會更大，因為它對鄰近斷層造成壓力的範圍比孔隙水壓增加的範圍還大。」

Goebel表示由結晶岩組成的基岩相較於沉積岩較為堅硬且孔隙也較少。他說：「因此，孔隙水壓增加的區域僅限於注入井周圍的獨立區塊，而孔隙水壓伴隨產生的總應力場強度也較低。」

奧克拉荷馬州之類的區域因為開採油氣而有許多注入井，Goebel表示他們的發現有助於解釋這些區域的誘發地震活動程度。奧克拉荷馬州自2010年開始地震的數目便持續激增，到現在每年發生的地震數量甚至比加州還多。不過Goebel和Brodsky並未將奧克拉荷馬州列入他們的研究範圍，因為這裡有太多注入井，使得他們無法分離出單一井位造成的影響。

「奧克拉荷馬州的注入井是把液體注入到基岩上方高孔隙率的沉積岩層，但在這類岩層當中的彈性應力可以傳遞到很遠的地方，使得10公里外岩盤當中的大型斷層有可能因此而活動。」Goebel表示，「這或許解釋了我們在奧克拉荷馬州之類的地方所觀察到的現象。」

Injection wells can induce earthquakes miles away from the well

Study finds injecting fluid into sedimentary rock can produce bigger, more distant earthquakes than injecting into the underlying basement rock

A study of earthquakes induced by injecting fluids deep underground has revealed surprising patterns, suggesting that current recommendations for hydraulic fracturing, wastewater disposal, and geothermal wells may need to be revised.

Researchers at UC Santa Cruz compiled and analyzed data from around the world for earthquakes clearly associated with injection wells. They found that a single injection well can cause earthquakes at distances more than 6 miles (10 kilometers) from the well. They also found that, in general, injecting fluids into sedimentary rock can cause larger, more distant earthquakes than injecting into the underlying basement rock.

"This is problematic, since the current advice is to preferentially inject into the sedimentary sequence as a theoretically safer alternative to the basement rock," said Emily Brodsky, professor of Earth and planetary sciences at UC Santa Cruz.

Postdoctoral researcher Thomas Goebel said the key issue is the spatial footprint of induced seismicity around the injection well. "It's not that the basement rock is safe, because there is still the possibility of encountering a fault in the basement rock that can cause a large earthquake, but the probability is reduced because the spatial footprint is smaller," he said.

In a paper published August 31 in Science, Goebel and Brodsky described two distinct patterns of induced seismicity, which they associated with different physical mechanisms acting in basement rock and sedimentary rock. In the first pattern, associated with injection into basement rock, earthquakes tend to occur in a compact cluster around the well, with a steep decline in earthquakes farther from the well. In the other pattern, associated with sedimentary rock, induced earthquakes decline gradually with distance from the well and occur at much greater distances.

The physical mechanism by which injection wells induce earthquakes was thought to be a direct result of increased fluid pressure in the pores of the rock, causing faults to slip more easily. This mechanism can account for the spatial pattern of seismicity seen with injection into basement rock, Goebel said. But the pattern seen with injection into sedimentary rock suggests a different mechanism resulting from efficient "poroelastic coupling," which controls the ability of the rock to transmit fluid stresses into the solid rock matrix.

"When you inject water into the ground, it pushes on the surrounding rock and creates elastic stress in the rock, which can put pressure on faults at a distance without putting water into those faults. So if poroelasticity is dominant, you end up with a larger footprint because it's loading neighboring faults beyond the area of increased pore pressure," Brodsky said.

According to Goebel, the crystalline basement rock is stiffer and has lower porosity than sedimentary rock. "Therefore, the increase in pore pressure is limited to isolated pockets around the well, and the coupling of that with the overall stress field is low," he said.

Goebel said their findings help explain the extent of induced seismicity in regions such as Oklahoma where there are many injection sites in oil and gas fields. Oklahoma has seen a dramatic surge in earthquakes since 2010, to the extent that there are now more earthquakes each year in Oklahoma than in California. Goebel and Brodsky did not include sites in Oklahoma in their study, however, because there are so many injection wells they couldn't isolate the effects of individual wells.

"In Oklahoma, they are injecting into the high-porosity sedimentary unit above the basement, but these elastic stresses can be transmitted over a large distance, so you could activate a large basement fault at a distance of 10 kilometers," Goebel said. "That may be what we're seeing in places like Oklahoma."

原始論文：Thomas H. W. Goebel, Emily E. Brodsky. The spatial footprint of injection wells in a global compilation of induced earthquake sequences. Science, 2018; 361 (6405): 899 DOI: 10.1126/science.aat5449

引用自：University of California - Santa Cruz. “Injection wells can induce earthquakes miles away from the well.”

原文網址：https://news.ucsc.edu/2018/08/induced-earthquakes.html