Application of Seismic Anisotropy Caused by Fissures in Coal Seams to the Detection of Coal-bed Meth

Vo.74No.24ACTA GEOLOGICA SINICAJune 2000Application of Seismic Anisotropy Caused by Fissures in CoalSeams to the detection of coal-bed methane reservoirsLIU Mei, GoU Jingwei, yu Guangming and lin JiandongResearch institute of Coal Geophysical Exploration, Zhuozhou 072750, HebeiAbstract Coal-bed methane is accumulated in micro-fissures and cracks in coal seams. The coal seam is thesource terrace and reservoir bed of the coal-bed methane(Qian et al., 1996). Anisotropy of coal seams is caused bythe existence of fissures. Based on the theory of s wave splitting: an S wave will be divided into two s waves withnearly orthogonal polarization directions when passing through anisotropic media, i.e. the fast S wave with its direc-tion of propagation parallel to that of the fissure and slow S wave with the direction of propagation perpendicular tothat of the fiThis paper gives the results of laboratory research and field test on the S wave splitting caused by coal-seam fissures. The results show that it is feasible to detect fissures in coal seams by applying the converted s wave and finallygives the development zone and development direction of these fissureKey words: coal-bed methane, coal-seam fissure, anisotropy, splitting of s wave1 Introductionexerted to seek development areas of coal-seamfissures and cracks that are favourable for the a-Coal-bed methane is the natural gas accumulated incumulation of coal-bed methane( Qian et al., 199coal seams, is a kind of natural gas formed with and This implies that the conventional methods on geo-stockpiled in the coal seams(Dai, 1982). It is a kind physical exploration of natural gas is not suitableof clean energy that is easy to mine and transport for the prospecting of coal-bed methane reservoirwith high efficiency and low cost. It plays an mNew methods must be appliedportant role in improving the composition of energy,protecting environment and insuring safety of coalmines in China. Therefore, it has attracted a greatattention from the state and relevant industrial departments of China. China is the largest coal-producing country in the world and rich in coal-bedmethane reserves. The exploration of coal-bedmethane is being undertaken on a large scale (Qianetal,1996)However. coal-bed methane is different fromconventional natural gas that is accumulated freelyn strata. It exists in micro-fissures and cracksadsorption state. The coal seam is both the sourcebed and reservoir bed of coal-bed methane. Coalmethane is found, but the discovery of coal seamsdoes not mean that efficient coal-bed methane e-ig. 1. A sketch showing S-wave splitting as passingervoirs are found and further efforts should bethrough mediaYH中国煤化工CNMHGVol. 74 No. 2ACTA GEOLOGICA SINICAJune 2000Anisotropy is due to the existence of fissurescoal seams according to the theory of s wave split2 Seismic Wavefield Features of Coalting: an s wave shall be divided into two s wavesseam fissureswith nearly orthogonal polarization directions whenpassing through anisotropic media(Fig. 1)Since the state of coal-seam fissures is the mainave parallel to the direction of the fissure is calledcontrol factor for the accumulation, transportationthe fast S wave and that perpendicular to the direc- and development of coal-bed methane, interpretation the slow s wave (Ewen et al., 1996; Crampintion of development of the fissures(their strikes and1998). Thus, the research of seismic waveformdensities for example) based on the features offeatures of the coal-seam anisotropy caused by coal-seismic wave fields becomes a top task in the seisseam fissures is the basis for the detection of coalmic exploration of coal-bed methanegure 2 shows a physical model of coal-seammethane reservoirs by means of seismic exploration fissures made in the laboratory. At the beginning ofthe test, transmission probe and receiver of s waveS wave transmitterlocated on two sides of the model, perpendicularto the layering (fissure) direction of the modelThen the transmitter and receiver are rotated in thesame direction and at the same step( the beginninpoint is 0%), and one channel is recorded(onceevery 10. Fig. 3 shows the test record of the fissuremodel. It is shown from the figure that the velocityof the s wave is the highest, 1510 m/s(travel time49 us), when the propagation direction of the Swave is parallel to the fissure direction of the medium (the 9th and 27th recorded channels, 1.e. atpositions of 90 and 2700), called the fast S waveFig. 2. Physical model of coal-seam fissuresand that the velocity of wave is the slowest, 1370methodsn/s(travel time 54 us), when the turning directionwyl. dat二Fig. 3. Test records on the fissure model中国煤化工CNMHGApplication of Seismic Anisotropy to the Detection of Methane reLiu mei et aof the s wave is perpendicular to the fissure direcabsorbed energy by strata is just a half comparedtion of the medium(the first and 18th channels, i.ewith the pure S wave, the splitting is converted onlyat positions of o and 1800), called the slow S waveonce. and vertical and lateral resolutions are relaThe results of the above test show that the devel-tively highoping state and direction of fissures can be estimatFigure 4 shows the converted s wave(P-SV)ed based on the splitting of s waves when fissuressection of Line N and Fig. 5 the converted S waveare well developed in coal seamssection of Line N. Fig. 6 is the common-angle-bistack section arranged in a circle. The circle of 36003 Field testis divided into 48 bins and the common-angle-binstacking section is formed by stacking the data in aa field test was undertaken in some mining districtbin with nmo and dmo. the first channel is owith flat landform and an area of 5 km. The availpointing to the north(N), the second rotates clock-experimental area haswise by7.°, the third 15°," and the 12 th chansimple geological structure and a stable main mi-nel is 90, pointing to the east(E), 24th channelnable coal seam with buried depths about 700-1000180, to the south (S), 36th channel 270, to them. The area is rich in coal reserves and belongs towest(W), and the48 Sth channel is360°,i.e.0°,an area with high content of coal-bed methanepointing to the north (N)There is obvious impedancethe coalWseam and its overlying and u3005007009001100130015001700190derlying wall rocks, which1ea Etends to cause strong seismicreflected wavesWithin the area of 5 kmr2D1留朋(garea in longitudinal and lateral《directions and perpendicular toNHm小付付解的eameach other and six concentricobservations were designed你解数The coal seam is deeply buriedin this area and the pure s waveFig 4. Converted S wave(P-SV) section on Line Nhas no enough energy, thereforeint observation of p wave and800100012001400160018002000converted s wave is appliedCompared with the pure S wave,the converted s wave hasunique advantages: its in-duction mode is just the same asthat of the conventional longiand the s wavesource Is解省flected longitudinal and converted s waves can be obtainedy the same induction; (3theconverted s wave passes strataonly once (one way) and theFig. 5. Converted S wave(P-SV) section on Line n,中国煤化工CNMHGVol. 74 No. 2ACTA GEOLOGICA SINICAJune 20001111-1i;!i↓1111SV wave section and p-p wave section4 Conclusion:(;:1. Coal-bed methane is generally accumulatedin fissures and micro-fissures of coal seamsThe main task in the exploration of coal-bed计物r比methane is to seek coal-seam fissures andthe application of s wave splitting featuresPPD山时时时时上 is the basis for the exploration of coal-bed二·:D血t上⊥⊥上上上盖上业贮》》加》3shows that it is feasible to apply the convert-Fig. 6. Common-angle- bin section arranged in a circleed s wave in the practice of explorationCompared with Fig. 3, the common-angle-binManuscript received Jan. 2000section(Fig. 6) shows a very strong feature of Sedited by Zhang Yuxu and Liu Xinzhuwave splitting and anisotropy of strata caused by thefissure. Based on the features of the fast and slowReferenceswaves and the position of the converted point, it isCrampin, S, 1988. Polarization of s Wave: Applicationshown that the fissure is located around the thirdthree-component underground record to determination ofchannel (198.50), i.e. in a north-north-east (NNE)anisotropy. Collected works of theory on exploration of sdirectiwave, Beijing: Petroleum Industry Press, 101-118Jiuxing, 1982. Implication of coal-bed gas and its subdivi-Comparison of S-wave sections of Line N(Figon. Geological Review, 28(4): 370(in Chinese)4)and Line N(ig. 5)shows that To wave is com- Ewen, W, 1996. Elastic Wave in the Layered Medium, Transpletely closed at the intersection on the bottomlated by Liu Guangding, Beijing: Science Press, 112p(inboundary of the new stratum, suggesting that theChinese)converted s wave has the same propagation velocityQian Kai, Zhao Qingbo and Wang Zecheng, 1996. Theory onexploration and development of and experimental testingin directions perpendicular to each other withouttechnology of coalbed methane in the coal seam, Beijingpitting but the coal-seam reflected wave T7 at thePetroleum Industry Press, 50p(in Chinese)intersection is nearly 30 ms slower on section Nthan on Line n, which shows obviously that theabout the first authorsplitting of s wave occurs in the coal seam. TheLiu mei male born in 1942, graduated from thelonger propagation on section N, indicates the slowDepartment of Geology, Beijing Institute of MiningS wave, while the shorter one on Line n2 the fast s in 1967, majoring in coal geophysical explorationwave, which means that the fissure of the coal seamHe is at present Deputy Chief Engineer of the Re-exists approximately in the direction of N, basi-search Institute of Coal Geophysical Explorationally the same direction pointed by the circleProf. Liu has long been engaged in the research ofarranged section. The development area of fissures, coal geophysical exploration and relevant datai.e. the area favorable for the exploration of coalprocessingbed methane, can be determined by the neuronetwork analysis of the seismic attributes of the P中国煤化工CNMHG