Real time fluid analysis during drilling of the Wenchuan Earthquake Fault Scientific Drilling Project and its responding features

Tectonophysics 619–620 (2014) 70–78

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Real time fluid analysis during drilling of the Wenchuan Earthquake Fault Scientific Drilling Project and its responding features Lijun Tang a,⁎, Liqiang Luo a, Changling Lao a, Guang Wang a, Jian Wang a, Yao Huang b a b

National Research Centre for Geoanalysis, No.26, Baiwanzhuang Road, Beijing, 100037, China No.6 Brigade of Jiangsu Geology & Mineral Resources Bureau, Lianyungang, Jiangsu, 222300, China

a r t i c l e

i n f o

Article history: Received 26 February 2013 Received in revised form 3 August 2013 Accepted 19 August 2013 Available online 23 August 2013 Keywords: Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) On-site laboratory Real time fluid analysis Principal Slip Zone (PSZ) Aftershock Responding feature

a b s t r a c t The Wenchuan Earthquake Fault Scientific Drilling project was established shortly after the Wenchuan Earthquake. Several on-site laboratories were built to perform the real time fluid analysis during drilling simultaneously. The concentrations of argon, methane, hydrogen, carbon dioxide, helium, nitrogen, oxygen and radon in drilling mud gas were determined during the entirely process of drilling. The setup for real time fluid analysis was stability for long time. The mud gas such as methane and radon yielded low concentrations above the Principal Slip Zone (PSZ), whereas yielded high concentrations under the PSZ. The real time fluid data might provide the real time information for identifying and validating of the PSZ in the deep fault zone. The gas concentration showed abnormal fluctuation during the Ms 4.0 earthquake on April 27, 2010. The abnormality occurred one hour before the earthquake, and ended half an hour after the earthquake. The real time fluid analysis during drilling might have captured the signal of the Ms 4.0 earthquake at a quarter past six on April 27, one strong and nearest aftershock. © 2013 Elsevier B.V. All rights reserved.

1. Introduction On May 12, 2008, Ms 8.0 Wenchuan Earthquake happened in the Longmenshan region, Sichuan Province, China, along the Yingxiu– Beichuan and Anxian–Guanxian Faults (Li et al., 2008a,b, 2009a,b,c, 2010a; Xu et al., 2007, 2008a,b). The Wenchuan Fault Scientific Drilling (WFSD) project (Dong et al., 2008, 2009; Li et al., 2013; Xu et al., 2008b; Zhang et al., 2009) was funded for better understanding of the mechanism of the Wenchuan Earthquake, for monitoring the changing information of underground fluid and seismic stress in the region (Becken et al., 2008; Brodsky et al., 2009; Dong and Li, 2009; Li et al., 2013; Tang et al., 2010; Xu et al., 2008b). The exact locations of on-site laboratories related to our research are about several tens of meters away from WFSD-1 and WFSD-2 (Fig. 1a, b). The on-site laboratories for real time fluid analysis during drilling were established reliably in WFSD. The real time monitoring of mud gas was performed from oil and gas drilling for safety reasons. However, it was used for scientifical purposes on the German Continental Deep Drilling Program (Erzinger et al., 2006), the San Andreas Fault Observatory at Depth (SAFOD) (Erzinger et al., 2004, 2006), and the Chinese Continental Scientific Drilling project (CCSD) (Luo et al., 2004b, c; Sun et al., 2005, 2006). The drilling mud gas, which contains H2, N2, O2, He and other gasses composition, was degassed under slight negative pressure, then transported to an on-site laboratory continuously ⁎ Corresponding author. Tel.: +86 1068999559; fax: +86 1068998605. E-mail address: [email protected] (L. Tang). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.08.026

(Erzinger et al., 2006; Luo et al., 2004b,c; Tang et al., 2010, 2011). Because the mud gas reflects origin of fluid or interaction in fluid channel, the real time mud gas analysis would be helpful to understand the deep tectonic activity (Erzinger et al., 2006; Li et al., 2005, 2006; Luo et al., 2004a; Sun et al., 2005; Tang et al., 2011) in the Longmenshan region, which provides an opportunity to discuss how the underground fluid respond to the rocks, faults and the aftershocks. For example, the profile of He concentration corresponds to the lithological changes at SAFOD (Erzinger et al., 2006; Wiersberg and Erzinger, 2007, 2008, 2011). Hydrocarbon concentrations coincide with the occurrence of shale (Wiersberg and Erzinger, 2011). The mud gas monitoring during drilling of the SAFOD Pilot Hole shows that the sedimentary rocks accompany with the highest CH4, CO2, and Rn concentrations and low He. Contrast to the sedimentary rocks, the granite rocks occur with much lower CH4, Rn, and high level of He concurrently. Shear zones are also recognized by increased mud gas concentrations (Erzinger et al., 2004, 2006). Gas results permit to speculate the fluid channel for the shear zone (Erzinger et al., 2004, 2006). At the Unzen drilling project, He concentration in monitored gasses shows clear peaks at the dikes (Kajiwara et al., 2004; Tretner et al., 2008). Gas composition and isotopic ratios from circulating drilling mud in the SAFOD III and SAFOD MH helped to interpret the gas migration processes and the permeability structure of the San Andreas Fault around two actively deforming zones at 3194 m and 3301 m borehole depth. The elevated Ra values between both fault strands may reflect either episodic or continuous flow of mantle-derived fluids, suggestive of some limited permeability parallel to the fault direction. Based on the short term anomalies of gas composition in the mud fluid from the CCSD drilling

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Fig. 1. (a) The Longmenshan region lies at the eastern margin of the Tibetan Plateau, China. (b) Geological structures around WFSD-1 and WFSD-2 and their drilling sites location. F1: Wenchuan–Maoxian Fault; F2: Yingxiu–Beichuan Fault; F3: Guanxian–Anxian Fault (Li et al., 2013). (c) Drilling platform of WFSD-2. (d) Drilling platform of WFSD-1. (e) Analytical instrument of on-site laboratory. (f) On-site laboratory of WFSD-2.

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hole, the November 14 2001, West Pass of Kunlun mountains earthquake with Ms 8.1, the December 26th 2004, Sumatra–Andeman Earthquake with Ms 9.3 (Li et al., 2006; Sun et al., 2005) and some other relative weak earthquakes were recognized (Li et al., 2005). We would like to report here some results of real time gas analysis and discuss the relationship between fault and the earthquake. Drilling in the Longmenshan active fault zones, especially after the strong Wenchuan earthquake, provides a fairly good opportunity to better understand the mechanism of the Wenchuan Earthquake, the changes of the underground fluid, seismic stresses and so on (Dong and Li, 2009; Dong et al., 2009; Li et al., 2013; Tang et al., 2010; Xu et al., 2008b), because the aftershock of large earthquake would still occur frequently around this region. On the other hand, the drilling mud gas would be more possibly used to study on the tectonic activities, because the drilling mud gas was circulated from the underground hole (Faulkner et al., 2010; Italiano et al., 2010; Li et al., 2005; Sun et al., 2005). In this paper, the data and samples of drilling mud gas from various holes were attained (Li et al., 2008a,b; Luo et al., 2004b,c; Tang et al., 2010, 2011), especially a great many of fluid anomalies were detected. Therefore, these would possibly provide more evidence to discuss the relationship between fluid anomalies and the occurrence, intensity and distance of the aftershocks, especially during the period when drilling cores would not be collected normally. 2. Experimental setup and data collection The various on-site laboratories for real time fluid analysis were established in the WFSD project. The natural conditions and surrounding environment are extremely unfavorable (Luo et al., 2004b,c; Tang et al., 2010; Xu et al., 2008b; Zhang et al., 2009) (Fig. 1c, d), for example, the drilling station was located in the extra narrow valley, where there was nearly no flat terrain for drilling, the on-site laboratory had to consider the advantage condition around the drilling position, such as the simple and temporal room as laboratory space (Fig. 1f). The details of on-site laboratory were showed and the exact locations of laboratory were described in the previous publications (Li et al., 2010b, 2013; Tang et al., 2010, 2011; Wang et al., 2010; Zhang and Jia, 2009; Zhang et al., 2009). To perform the mud gas analysis, the following steps had to complete in advance. The mud circulation system was reconstructed for better gas extraction firstly, and the gas separator was installed in the mud pit directly at the outlet of the mudflow line to minimize air contamination. The drilling mud gas was then extracted and pumped through a polytetrafluoroethylene (PTFE) tube into the on-site laboratory for determination (Erzinger et al., 2006; Luo et al., 2004b,c; Tang et al., 2006). 2.1. Reconstruction of the mud circulation system The depth of holes drilled by the WFSD project are different, which are caused by the fault zone, the position and orientation of the drilling hole, so the equipment of drilling and the flow line of the mud circulation during drilling are both different (Fan et al., 2009; Tang et al., 2006; Zhang and Jia, 2009; Zhang et al., 2009). In order to analyze the concentration of drilling mud gas, the system of mud circulation had to be reconstructed based on the position and height of the drilling platform and the diameter of the drilling mud pipeline to meet the requirement that the gas extracted from drilling mud should not be contaminated by air on the ground, which might imply the underground information. 2.2. Gas extraction from drilling mud and the gas flow line The equipment for gas extraction from the drilling mud is composed of a steel cylinder (30 cm diameter, variable height), a low-speed electrical motor (on top of the cylinder, drives the impellers) and a set of stirring impeller (several groups of impellers at different heights around axis inside the cylinder, stirring the drilling mud). The gas separator is

normally installed in the drilling mud pipeline (which have been reconstructed as a mud pit) before the shaker screens as close as possible to the outlet of drilling mudflow line to minimize air contamination. The electrical motor, which drives the stirring impellers, needs long term stability and good air tightness in order to stir the mud efficiently. The papers have described the gas separator and gas flow line in detail (Erzinger et al., 2006; Luo et al., 2004b,c; Tang et al., 2006, 2010; Tretner et al., 2008). The gas extracted from the drilling mud would be pumped through the PTFE tubes into the laboratory after the adjustment of air balance, which could prevent drilling mud from condensing within the gas flow line and maintain constant gas flow. Water vapor is removed from the mud gas with a small refrigerator at about 1–4 °C, and the gas is fed into a gas sampling system with glass cylinders devised by our group for subsequent isotope investigation. The gas samples collected are controlled mainly by the mass spectrometer. In the mud gas line from refrigerator to collection system, each analytical device (mass spectrometer, radon detector) individually takes its necessary amount of gas for determination. 2.3. Fluid analysis during the drilling process The requirement of instrument in on-site laboratory is highstandard (Erzinger et al., 2006; Luo et al., 2004b,c; Tang et al., 2006, 2010), including a fairly pretty long term stability and small instrument drift of analytical devices, easy to make the setup and the calibration of analytical method and the maintenance of instrument, meanwhile, free to transport and store calibration materials. Due to some of the characters above, the mass spectrometer and Rn detector are adopted for real time fluid analysis in the on-site laboratory of WFSD. 2.3.1. Mass spectrometer The concentrations of Ar, CH4, H2, CO2, He, N2 and O2 in the drilling mud gas are real time determined by an OmniStar type quadrupole mass spectrometer (Pfeiffer Vacuum, Germany) (Fig. 1e). The mass spectrometer adopts the mode of continuous sampled by capillary at a sampling interval of 5 s (Erzinger et al., 2006; Tang et al., 2006, 2010; Tretner et al., 2008), which is a slightly different from the period used in other drilling projects. In order to verify the accuracy of the analytical method and the long term stability of the analytical devices, the mass spectrometer was tested at regular intervals and re-calibrated if necessary with air, pure CO2 and certified gas mixtures (Regularly including the components of Ar, CH4, H2, CO2, and He with concentrations of 1% (v/v) and N2 as balance matrix). The reliability of the mass spectrometer was checked regularly through the determination of certified gas mixtures with 1% for many times (Erzinger et al., 2006; Tang et al., 2010). The determined result of 20 times by mass spectrometer is showed in Table 1, including the reference value of certified gas mixtures, the minimum value, maximum value, average value and RSD of determination. We could see from this table that the RSD values of all the components are better than 0.25%. The accuracy and precision of the mass spectrometer are well enough to meet the requirements of real time analysis and satisfied for field laboratory. 2.3.2. Rn detector The concentration of Rn in the drilling mud gas is real time determined by a RAD-7 type detector (Durridge, America). The RAD-7 detector has some advantages including good sensitivity, wide range of concentration determined and firmness to be used, which makes it suitable for on-site analysis in poor conditions (Tang et al., 2010, 2011). The RAD-7 is calibrated off-site and keeps in good shape for a long time, which adopts the mode of continuous monitoring at an interval of 5 min for real time determination of drilling mud gas. The highest concentration for calibration of Rn is 8780 Bq/m−3 and the lowest value is 2150 Bq/m−3. Therefore, the average factor corrected is 0.99 and the relative inaccuracy is 5.7%. Until now, the maximum concentration of

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Table 1 The accuracy and precision of mass spectrometer in on-site laboratory. Component

Ar (10−6 v/v)

CH4 (10−6 v/v)

CO2 (10−6 v/v)

H2 (10−6 v/v)

He (10−6 v/v)

N2 (% v/v)

Ref. value Min. value Max. value Ave. value RSD (%)

10,000 9534 9564 9550 0.106

10,100 10,153 10,195 10,174 0.114

10,200 9625 9669 9646 0.147

10,200 11,484 11,562 11,523 0.202

10,100 12,164 12,245 12,213 0.185

94.94 94.60 94.61 94.60 0.00284

Rn in the drilling mud gas determined in WFSD is less than 5600 Bq/m−3, which is within the range of the concentration for calibration and then indicate that it is reliable of the Rn value determined at the interval of 5 min. Meanwhile, the time, temperature and humidity are all recorded automatically with Rn value together. 3. Results and discussion 3.1. The concentration and possible source of gas components in drilling mud The on-site laboratory started the real time analysis of drilling mud gas nearly from the beginning of WFSD project, among which, the gas analysis of WFSD-1 started from November 23, 2008, until July 11, 2009, and that of WFSD-2 was from October 14, 2009, until April 5, 2012. Moreover, we utilized the data statistics function of mass spectrometer analytical software to calculate the maximum, minimum and mean values of the uninterrupted measurement result every day (the period is about 24 h, in general from nine o'clock of previous day to the same o'clock of the present day). Then according to the minimum, maximum and mean concentration value per day, the ratio curves of the minimum and mean value, the maximum and mean value over time are showed (Fig. 2). Due to a few actual reasons, such as no mud circulation, no drilling or instrument drift, the ratio data of some days are not showed in this figure. The on-site laboratory analyzed the drilling mud gas every day for about 24 h, including non-drilling process. Therefore, the measurement values of the drilling mud gas usually would be relatively stable and used as the relative background value, because the drilling mud would stop the circulation and keep in a static state during the non-drilling process. According to the process of real time analysis, whether or not the drilling mud is circulating and the project is drilling, it will

intensively affect the measurements values of drilling mud gas. Usually when the non-underground influence is excluded, we could accurately determine the important section and time of the actual underground fluid invasion and obtain the important information from underground. The abnormal concentrations of multi-components in the drilling mud gas were detected in WFSD, the components of which were mostly He, CH4, CO2, Ar, O2, and N2 (Erzinger et al., 2006; Luo et al., 2004a; Sun et al., 2006; Zhan et al., 2006). The abnormal appearance is that those components have high frequency of abnormity and strong intensity of abnormal change, both showed in Fig. 3. For example, the concentration change line of He in the drilling mud gas has at least three obvious peaks during only one day (the orange line in Fig. 3), and the ratio of maximum value and minimum value of He concentration per day is 1.74, which is larger than that of CCSD (1.38). Generally, He, like all noble gasses, is not affected by alteration through the drilling mud or by chemical reactions. Unlike Ar, the relative concentration of He in the atmosphere is low, so its abnormity is mainly caused by the deepen hole (Luo et al., 2004a,b; Wiersberg and Erzinger, 2007, 2008, 2011; Zhan et al., 2006). The concentrations of CH4 and CO2 are more sensitive to environment and the circulation of the drilling mud. A very small percentage of CH4 and CO2 comes from the air. Besides air, which is carried down the hole by the circulation of the drilling mud, mud additives are another artifact present in the samples. Organic mud additives are usually added to meet the demands of drilling engineering. When these additives decompose as the mud ferments, CH4 and CO2 are produced (Luo et al., 2004a,b). Other sources of CH4 and CO2 might be related to the tectonic characteristic and lithology of the drilling core, for example, carbonic shale would be more easily to produce CH4 and CO2 during drilling (Erzinger et al., 2006; Wiersberg and Erzinger, 2011). The main components of drilling mud gas are N2, O2, and Ar, but in most cases, all of them are from the air. Therefore, it is difficult to distinguish the deep fluid in the earth just from the concentrations of N2, O2,

Fig. 2. The concentration ratio curves of multi components in drilling mud gas per day during the period of WFSD-1.

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be utilized as a way to predict the earthquake and identify the fault zones. 3.2. Mud gas composition cross the Principal Slip Zone (PSZ) of drilling core in WFSD-1

Fig. 3. Real time analysis value of drilling mud gas by mass spectrometer, when WFSD-1 was drilled one day in 2009. The change lines were drew by the concentration value per minute (N2, Dark Yellow; O2, Blue; Ar, Purple; CO2, cyan; CH4, Green; H2, Black; He, Magenta).

and Ar. Rn usually derives from the radioactive decay of U and Th. Due to its low mobility, Rn migration is mostly controlled by active permeable strata, such as fault zones. During the activity of tectonic structure, Rn not only accumulates, but also shows abnormal changes (Tang et al., 2011; Wiersberg and Erzinger, 2008, 2011), which means that it could

The WFSD-1 is located in Bajiaomiao Village, Hongkou County in Sichuan province. Based the observation at the outcrop, the PSZ of the Wenchuan Earthquake was estimated to lie at 759 m-depth in drilling core. However, when the drilling tools were stuck at about 586 mdepth, it seemed that the PSZ of the Wenchuan Earthquake was appeared earlier than expected. The drilling core is composed of a continuous fault zone with fault gouges of different scales and fault breccia. The fault zones in the drilling core are foliated, and the gouge width ranges from several millimeters to a few meters that is rare, indicating a multiple cores model for the Yingxiu–Beichuan Fault (Li et al., 2009a, b; Z.J. Li et al., 2009, Li et al., 2010b, 2013; Wang et al., 2010). After detailed observation of the fault gouge in the drilling core, the conventional logging curve and the measurement curves of borehole temperature, the PSZ could be concluded the exact location of about 590 m-depth. Meanwhile the on-site laboratory with real time fluid analysis, has timely recorded the PSZ in the WFSD-1 core (Fig. 4). The concentrations of many components in the drilling mud gas are abnormal change and the trend of abnormal change is consistent with the thickness of fault gouge in the drilling core (Li et al., 2013; Wang et al., 2010; Zhang et al., 2009), which might reflect the source of fluid from the underground. Through comparison of the concentration abnormal change of different components with two analytical devices of different types, the corresponding feature of concentration change at the PSZ in the drilling core was acquired, which would be helpful for location of PSZ in the drilling core and account for the important role of real time fluid analysis during drilling in WFSD. However, due to the WFSD-1 without drilling logging, we are unable to obtain the exact correspondence between the drilling mud and the drilling depth. The on-site laboratory had to use the drilling speed to calculate the corresponding relationship between the drilling depth and the exact time, and then use the real time result of the drilling mud gas based on the exact time, thus the corresponding relationship between the drilling depth and the real time result of drilling mud gas would be obtained. However, the period of the drilling mud circulating from drilling hole to the ground would be directly affected by the flow rate of the drilling mud,

Fig. 4. Profile of concentration value of drilling mud gas around the PSZ of WFSD-1; Lithology chart (original rock) and fault rocks of WFSD-1 cores from 490 to 625 m-depth.

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the drilling depth and the distance from the drilling mud pipeline to the laboratory, so the real time results of the drilling mud gas will lag behind the drilling depth. The performance of the hysteresis phenomenon is that the real time result of the drilling mud gas is shallower than the corresponding drilling depth in Fig. 4, in the other hand, the real time results of drilling mud gas caused by the actual drilling depth would be reflected in the deepen section than the corresponding drilling depth in Fig. 4. Li and Wang (Li et al., 2010b, 2013; Wang et al., 2010) described the lithology of the drilling core in WFSD-1 in detail. At the PSZ of the drilling core in WFSD-1, an abundance of abnormal concentration change of multi components in the drilling mud gas were detected by on-site laboratory with real time fluid analysis. In addition, the abnormal appearance is that the concentrations of multi-components are abnormal change simultaneously, the abnormal change occurred with the variety of lithology in the drilling core and the intensity of abnormal change are great. Especially, the concentrations of H2, He, CH4 and Rn are appeared the obviously maximum positive values (Fig. 4) and the concentrations of N2 and Ar are appeared the low positive values, while the concentrations of CO2 and O2 are appeared the minimum negative values. Regardless of whether the abnormal change is positive or negative, usually the abnormal changes go like this regulation, such as the normal value-high abnormal value-normal value-low abnormal value-normal value. However, the concentrations of various gasses above and below the PSZ show a slight fluctuation and are close to each other, for example, the concentration of He above the PSZ is 23 ∗ 10−6 (v/v), and that below the PSZ is 20 ∗ 10−6 (v/v), the two values are close to each other. Another concentration of CH4 above the PSZ is about several tens of 10−6 (v/v), and that below the PSZ is the similar. Nevertheless, the concentration of He is up to 54 ∗ 10−6 (v/v) and that of CH4 is several thousands of 10−6 (v/v) at the PSZ, both of them are much higher than those values above and below the PSZ. Within the PSZ, the abnormal concentration of multi-components in the drilling mud correlates with the thickness of the fault gouge. The thicker of the fault gouge, the spanning time of the abnormality of the mud gas is longer. The fault gouge at the shallow section of drilling core is thick, gas abnormality from the drilling mud lasts long, while the fault gouge at the deep section is thin, so the lasting period is short. Between the different fault gouge layers in the drilling core, the lithology of core section and the concentration change of drilling mud gas are both normal and change in the same rule.

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Table 2 Information of aftershock in detail (From April 24 to 30, 2010). Date Time Lati. (YYYYMMDD) (HH:MM)

Long.

Distance to Ms Depth Place WFSD-2 (km) County (In Sichuan) (km)

20100424 20100424 20100424 20100425 20100426 20100427 20100427 20100427 20100428 20100429 20100430

105.12 105.10 103.37 103.47 104.78 103.43 104.52 104.50 105.12 104.15 104.45

3.0 3.0 3.1 3.1 3.0 4.0 4.2 3.8 4.3 4.0 3.3

2:06 2:06 13:22 19:09 5:41 6:15 13:21 15:30 15:12 22:22 7:19

32.40 32.38 31.37 31.23 32.38 31.17 32.00 32.00 32.52 31.87 32.10

17 13 15 8 14 19 22 20 20 24 14

QingChuan QingChuan WenChuan WenChuan PingWu WenChuan BeiChuan BeiChuan QingChuan BeiChuan PingWu

193.2 190.8 39.0 23.3 171.2 24.7 122.3 121.3 202.6 90.4 127.3

3.3. Responding relation to the aftershock of earthquake with fluid change Changes in the pressure, flow rate, color, taste, smell and chemical composition of surface and subsurface water, oil and gas have been termed hydrologic precursors to earthquakes. Many studies concerning gas precursors such as H2, He, and CO2, have been carried out in seismically active zones (Claesson et al., 2004; Hirose et al., 2011; Italiano et al., 2010; Zimmer and Erzinger, 2003). The real time analysis of mud gas in WFSD, which is located in a tectonically active region, could reveal a local link between seismic activity and the variations in the gas components. During the drilling of WFSD2, the intense aftershocks occurred much more regularly near WFSD-2, so this period was paid more attention to investigate the relationship. The number change of aftershocks occurred daily more than Ms 2.0 and the ratio curves of gas concentration are showed in Fig. 5. During such period from March 23 to May 25, 2010, the number of aftershocks more than Ms 5.0 is one, between Ms 4.0 and Ms 5.0 is four and between Ms 3.0 and Ms 4.0 is 53 near this region. In the end, we could find that the maximum number of aftershocks occurred more than Ms 2.0 appears just on April 27, 2010, moreover, the ratio curves of multi components change in large amplitude fluctuations (Fig. 5). In Table 2, we list the characters of the aftershocks from April 24 to 30, which was from three days before to after April 27. In addition, the details of swarms of the aftershocks more than Ms 3.0 are fully listed in the table, such as the exactly Ms, date, time, latitude, longitude,

Fig. 5. The number change of aftershocks occurred around WFSD-1 and WFSD-2 and ratio curves daily (From March 23 to May 25, 2010). X-axis expresses the date; Y-axis (left) expresses the number of aftershocks more than Ms 2.0 occurred daily; Y-axis (right) expresses the ratio of the minimum and mean value, the maximum and mean value per day.

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Fig. 6. Responding feature of fluid change with the aftershock occurred near WFSD-2. N2, Ar, CH4, CO2, H2, He, and O2 are showed as the concentration (v/v %), Ar/N2 and He/N2 are showed as the ratio of the concentration.

depth of aftershocks and distance from the epicenter to WFSD-2 hole. We selected the Ms 4.0 earthquake occurred at a quarter past six in the morning on April 27, which is 24.7 km. away from the on-site laboratory, the second nearest aftershock during this short period, and the Ms of that is greater than that of first one. Therefore, the concentration change of drilling mud gas was taken into consideration much more carefully during this short period around this day. The object of on-site laboratory is the drilling mud during circulating, which will only detect the circulating mud gas from deepen hole in the drilling process. The drilling mud is not usually circulating for 24 h every day, but only in the drilling process of about 2–3 h, thus it would limit the period of monitoring the drilling mud gas. Although the earthquake occurred frequently around the Longmenshan region, it is rare aftershocks occurred just in the drilling process and there are many influence factors during monitoring the drilling mud gas, both of which would increase the difficulty to monitor the fluid change caused by the aftershock, but fortunately the moment of this Ms 4.0 aftershock occurred on April 27 is just in the actual drilling process and the fluid change in the drilling mud gas is obviously different from that of the normal drilling process. During the period from April 24 to 30, shown in Table 2, WFSD-2 hole reached the drilling depth from 729.93 to 749.31 m, with a total length of about 20 m. In addition, the lithology of this section is mainly Triassic Xujiahe formation, which is consistent during this period and will difficultly affect the concentration changes of the drilling mud gas. But the drilling core at about 720 m-depth on the upper section is distributed with the thick fault breccia, cataclasite and fault gouge (Zhang et al., 2012), which is just a good channel for underground fluid. Fig. 6 shows the real time fluid analysis result, including the ratios of He/N2 and Ar/N2, which records the gas fluctuation before, during and after the Ms 4.0 earthquake. The period of drilling process started from ten past five to twenty five past six on the same day when this

aftershock occurred. Normally the gas concentration would increase and decrease smoothly during and after the drilling process. However, during this drilling process, the Ms 4.0 earthquake occurred and the gas concentration showed abnormal fluctuations. The concentrations of multi-components fluctuated about 15 min earlier before the moment the aftershock occurred and lasted for about 30 min. The normal trend of fluid change in this period is that the concentrations of N2, Ar, CH4, CO2, H2 and He decrease and that of O2 increases smoothly. However, for example, the concentration of CH4 was suddenly dropped from 9560*10−6 (v/v) to 5043*10−6 (v/v) during a short period of about 1.5 min and got back to 7306*10−6 (v/v), and that of O2 showed a sharp and increasing spike. Then the gas concentrations of N2, Ar, CH4, CO2, H2 and He slowly increased while that of O2 decreased but with relative large magnitude than those of normal. During and after the earthquake occurred, the concentrations of CH4, CO2, H2, and He and the ratios of He/N2 and Ar/N2 fluctuated with a decreasing trend, while that of O2 fluctuated with an increasing trend (Fig. 6). The abnormality might be related to the Ms 4.0 earthquake. The sharp decreasing in the relative contribution of various components and the sharp increasing of O2 in the drilling mud gas suggested that the abrupt reducing in permeability occurred in the access routes of the underground gas end member to the drilling mud. It might be caused by the changing stress during the earthquake. Through comparison of the fluid change and tectonic activity, especially the aftershock occurred during this period, we could see that the abnormal fluctuation of concentration in the drilling mud gas might be related some special event, which might mostly be affected by the tectonic activity from the underground. The on-site laboratory might record the different concentration changes in the drilling mud gas in before, coseismal and after this event. Which would indicate that the result of real time fluid analysis could not only respond to the lithological change of the drilling core (Li et al., 2010b, 2013; Luo et al., 2004a; Sun et al., 2005; Zhang et al., 2012), but also record the underground tectonic activity, such as earthquake. Especially, the abnormal change appeared much earlier than the aftershock occurred, which would provide a new efficient way to record, predict the earthquake and express the important technical role of real time fluid analysis. 4. Conclusions The WFSD project with on-site laboratories of real time fluid analysis provides a good opportunity to investigate the relationship between the fluid changes, lithology of the drilling core, tectonic activity and so on. Based on the comparison of different parameter, the following conclusions have been obtained. (1) The various on-site laboratories established in the WFSD project and some of the equipment devised by our group have been used well for a long time, which could meet the requirement of field analysis. Based on the position and height of the drilling platform and the diameter of the drilling mud pipeline, the reasonable reconstruction would ensure that the drilling mud gas could be extracted for determination without air pollution effectively. (2) As the response to great earthquake, the on-site laboratory with real time fluid analysis is one of the most rapid techniques to monitor underground fluid. Because the underground tectonic activity would occur more frequently after great earthquake than before, which make it more possibly to detect the abnormal concentration change of multi components in the drilling mud gas. In fact, a great deal of abnormal concentration change of components in the drilling mud gas have been captured, especially He and Rn, which could be affected difficultly by alteration. Furthermore, the frequency and the intensity of abnormal change in the drilling mud gas are both relatively high. All of which mentioned above might imply much information from the deep underground.

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(3) The fluid change has responded to the important section of the drilling core. The fault zones in the drilling cores of WFSD have the gouge width ranges from several millimeters to a few meters, which are rare and unexpected. Though the real time results of the drilling mud gas is shallower than the corresponding drilling depth in Fig. 4, the on-site laboratory has still timely responded to the PSZ in the WFSD-1 core, which appears that the concentrations of multi-components in the drilling mud gas are abnormal change simultaneously and correlate with the thickness of the fault gouge around the PSZ. (4) The on-site laboratory with real time fluid analysis might record the aftershock of earthquake. The period when the aftershock occurred relatively more frequently was selected to make investigation, and fortunately the moment of one Ms 4.0 aftershock occurred on April 27 was just in the actual drilling process. Through comparison of the relation between the abnormal concentration change of multi-components in the drilling mud gas and the aftershock happened during this period, we might conclude that some aftershocks have been recorded by the on-site laboratory with the obviously fluid change, which appeared that the concentrations of multi-components in the drilling mud gas fluctuated at about 15 minutes earlier before the aftershock occurred and lasted for about 30 minutes and changed their normal trend. The special change in the drilling mud gas earlier than the aftershock occurred might provide the new efficient way to record and predict the earthquake. Acknowledgments The paper was supported by Wenchuan Earthquake Fault Scientific Drilling Project of National Science and Technology Planning Project of China, funded by the Ministry of Science and Technology of China. The authors thank my colleagues Yuan Xin, Wang Xiaochun, Li Yingchun, Liu Jian, Sun Jianling and Yuan Jing for their hard and long-term work in on-site laboratories of WFSD despite the difficult condition. Thanks to the staff of the Engineering Department of WFSD Project for their help. Many thanks to the National Research Centre for Geoanalysis of China and the Department of Geological research of Project. Thanks to the chief geologist of WFSD project, Prof. Li, for his help and patience. We also thank two anonymous reviewers for their comments which greatly improved this manuscript. References Becken, M., Ritter, O., Park, S.K., Bedrosian, P.A., Weckmann, U., Weber, M., 2008. A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California. Geophys. J. Int. 173 (2), 718–732. Brodsky, E., Ma, K.F., Mori, J., Saffer, D.M., 2009. Rapid response fault drilling: past, present and future. Report of the ICDP/SCEC International Workshop of Rapid Response Fault Drilling. Tokyo, Japan, Nov. 17–19. Claesson, L., Skelton, A., Graham, C., Dietl, C., Mörth, M., Torssander, P., Kockum, I., 2004. Hydrogeochemical changes before and after a major earthquake. Geology 32, 641–644. Dong, S.W., Li, T.D., 2009. SinoProbe: the exploration of the deep interior beneath the Chinese continent. Acta Geol. Sin. 83 (7), 895–909 (in Chinese with English abstract). Dong, S.W., Zhang, Y.Q., Long, C.X., Wu, Z.H., An, M.J., Zhang, Y.S., Yang, N., Chen, Z.L., Lei, W.Z., Shi, W., Shi, J.S., 2008. Surface rupture investigation of the Wenchuan Ms 8.0 earthquake of May 12th, 2008, West Sichuan, and analysis of its occurrence setting. Acta Geosci. Sin. 29 (3), 392–396 (in Chinese with English abstract). Dong, S.W., Xu, Z.Q., Wu, Z.H., 2009. CAGS quick response to and geoscientific survey on May 12 Wenchuan Earthquake. Acta Geosci. Sin. 30 (1), 21–26 (in Chinese with English abstract). Erzinger, J., Wiersberg, T., Dahms, E., 2004. Real-time mud gas logging during drilling of the SAFOD Pilot Hole in Parkfield, CA. Geophys. Res. Lett. 31, L13S18. http:// dx.doi.org/10.1029/2003GL019395/pdf. Erzinger, J., Wiersberg, T., Zimmer, M., 2006. Real-time mud gas logging and sampling during drilling. Geofluids 6, 225–233. Fan, L.S., Jia, J., Wu, J.S., Zhan, Y.G., You, J.W., 2009. Overview on drilling operation of the WFSD-1 in Wenchuan Earthquake Fault Scientific Drilling Project. Explor. Eng. 36 (12), 5–8 (in Chinese with English abstract). Faulkner, D.R., Jackson, C.A.L., Lunn, R.J., Schlische, R.W., Shipton, Z.K., Wibberley, C.A.J., Withjack, M.O., 2010. A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones. J. Struct. Geol. 32 (11), 1557–1579.

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