Study Of Non-seismic Integrated Geophysical Technology And Experiment For Coal Bed Methane Enrichment Region

Seismic exploration method plays an important role in conventional gasexploration, but for coalbed methane (CBM) exploration, seismic exploration is stillin the experimental stage, and in seismic exploration areas with poor conditions,conventional seismic exploration method also met a lot of challenges. The cost oflarge areas of seismic exploration, especially the3-D seismic exploration is very high;it does not meet the need of the economic exploration and development of CBM.Non-seismic exploration techniques, with species diversity, and each has itsadvantages, including gravity, magnetic method, electic method and electromagneticmethod and so on, are all the important part of geophysical exploration technologysystem. With low cost, easy to form advantages of multi-method, many parameters,multi-scale, integrated exploration technicals, and carry out the non-seismicexploration technology experiment research for CBM exploration, to realize the CBMeconomical exploration and development has important significance.In order to adapt to the need of CBM economic exploration and evaluation, Withthe support of state major science and technology, the special projects of large oil andgas fields and CBM development project, the special research of CBM integratedgeophysical exploration technology was carried out. And in order to put forwardeffective method in the exploration of technology and their combination, realize theCBM enrichment region economic forecast and evaluation. This paper is intended tocarry out the non-seismic integrated geophysical exploration methods research forCBM enrichment region exploration. Hope to obtain more comprehensive andsystemic cognition, mainly aspects of research are as followed.First of all, the achievements of CBM exploration and development of geologicaltheory and practice exploration was summarized, and it provided a fairly goodgeological theory basis for the mechanism research by using the integrated geophysical exploration techniques for CBM exploration.According to the definition of CBM reservoir, summarized the CBM ofsignificant differences with conventional gas reservior, the generation, and the generalrule of CBM migration and enrichment, and the influence factors of CBM enrichment;Analyzed the regional geological structure, local geological structure, groundwaterdistribution and its dynamic conditions and the close relationship between the CBMenrichment; and the CBM enrichment and dilution geological structure model wassummarized.Secondly, according to CBM migration, the general rules and characteristics ofCBM seepage, and carried out the research of exploration mechanism and detectionmodel for the CBM exploration by using non-seismic methods.The CBM seepage characteristics and the physical or chemical change of theoverlying strata in CBM leakage process was detailed analyzed. And summarized thespecial secondary minerals forming processing in the overlying strata of CBMenrichment region, and the distribution law in the overlying strata; Combined with thecharacteristics of CBM reservoir, the model of electrochemical "chimney effect" ofconventional reservoir was revised, obtain the electrochemical detecting structuremodel for CBM exploration; According to the principle of the high-accuracy precisionground magnetic survey and Induced Polarization(IP) method, and the characteristicsof the secondary special mineral of CBM overlying strata, proposed by using thehigh-accuracy precision ground magnetic survey and IP method combining to detectethe special secondary minerals distribution to detecte the CBM enrichment regionindirectly, According to the electrochemical detection structure model, thecharacteristics of magnetic anomaly and IP anomaly may form above CBM reservoirwere analyzed in detail, and put forward some related issues of which need to payattention in the data processing and interpretation process.According to the influence factors of CBM enrichment, such as regional andlocal geological structures, the distribution of the groundwater, the dynamiccharacteristics of groundwater, and proposed by using Controlled Source Audiofrequency MagnetoTelloric (CSAMT) method to prospect the CBM favorableenrichment region, and proposed several typical geological models for CBMexploration; And put forward the CBM prospecting technical mode by detectinggroundwater, structure of CBM enrichment and dilution external conditions toprospect the CBM favorable enrichment region; Special points out that in thedetection of CBM by using CSAMT not only need pay attention to the reservoir structure factors, also need to pay attention to the groundwater distribution, it was thedifference of prospecting CBM reservoir and conventional gas reservoir by usingCSAMT method.Thirdly, according to the needs of data processing and explaining of CBMexploration, for the time domain IP and CSMT method to carry out the forwardmodeling and the data processing and interpretation method researchFor the time domain IP method, according to the characteristics of the complexgeoelectrical model and the need of2-D finite element forward modeling, proposedand implemented the complex calculation region unstructured triangular grid adsptivemesh method; For2-D point source geoelectrical problem, by using of finite elementmethod to carry out the2.5-D time domain IP forward modeling and inversion methodresearch with terrain; Use of potential swap principle and potential lineartransformation method to realize the fast forward modeling and inversion, Accordingto the CBM electrochemical exploration structure model which was proposed, andcarried out IP anomaly characteristics study, the theoretical model test results showthat the polarizability parameter was not effected on the topographic.Based on the1-D dopole source CSAMT, use of the electromagnetic fieldsuperposition principle to achive the bipolar source CSAMT forward modeling andinversion; and then compared the apparent resistivity curves of dipole source andbipolar source CSAMT. In order to improve the precision and efficiency of inversion,use of the constraint and optimization geoelectric parameters as the initial model, byusing the least-square method to realize CSAMT bipolar source inversion, andcompleted the line source two-dimensional finite element forward modeling studies, itlaid the foundation for two-dimensional inversion of CSAMT.Fourth, carry out the integrated geophysical exploration technique experiment forCBM, and summarized the method and technology application effect.Select the Heshun CBM exploration area to carry out the integrated geophysicalexploration technique experiment. And it proved that the weak positive and negativemagnetic anomalies "bright spot" concentration distribution area of the high-accuracyprecision ground magnetic survey was corresponded to the CBM favorable enrichmentregion; Large area of polarizability abnormal area and CBM favorable enrichmentregion has good corresponding relation, and for different geological conditions thepolarizability has the same performance; the apparent resistivity anomalies indifferent geological conditions have different performance; The "chimney effect" inthe area of the ground which produced the IP abnormal position was influenced by local open fracture and collapse columns; the results of polarization sounding wasbetter demonstrate the "chimney effect" in the stratum in the vertical distribution, andcombined with the vertical distribution of polarization anomaly, we can infer that therelationship between the plane anomaly of high polarization and the deep CBMreservoirs; The resistivity sounding technique can be better reflect the position anddistribution of the underground aquifer, however, the shallower low resistivity layerwas grater affect on its exploration depth and resolution. CSAMT method can give thedeep underground electrical structure and its distribution, combined with the electricalcharacteristics of each layer that obtained by drilling and well logging, can better inferthe distribution characteristics of underground stratas, The CSAMT inferred resultwas in good agreement with the known seismic profiles. Studies have shown that wecan accord the electrical distribution characteristics of CSAMT section to infer thedistribution of aquifer area, water-rich area, the fracture and fold, and then tospeculate the CBM relatively enrichment region, it also can provide the geophysicalreference for CBM well arrangement.Finally, according to the results of the integrated geophysical CBM explorationtechnique experiment, this paper presents the integrated geophysical explorationtechnique mode for CBM enrichment region exploration.The high-accuracy precision ground magnetic survey as the guide and accordingto the regional magnetic anomaly to get the roughly constructed partition of CBMexploration block; according to the concentrated distribution area of positive andnegative weak magnetic anomaly in local magnetic anomaly map to delineategenerally favorable CBM enrichment region; by using of CSAMT long profilemeasurements to control and verify the macro strata and the groundwater distributioncharacteristics, and divide the favorable segment of CBM enrichment region; Infavorable CBM enrichment region, by using3-D or fake3-D CSAMT survey, obtainthe planar distribution characteristics of stratus, and the local geological structuredistribution, so as to give favorable CBM enrichment area; In the favorableenrichment area, by using the IP survey to determine the high polarization anomalyarea, and with the polarization sounding, to correct the IP planar anomalies,combination the three methods of common anomalies to delineate the favorable CBMenrichment region; to realizate the multi-method, more parameters, multi-scale andperspectives, integrated geophysical detection for CBM enrichment region.Through the non-seismic integrated geophysical exploration experimentalresearch for CBM enrichment region, we considered that although the resolution of non-seismic methods can not be comparable with the seismic exploration method, butcan use its advantages of multi-method, many parameters, multi-scale from differentsides to relative the characteristics of CBM enrichment region, it can be also used asthe effectively complement and reference for seismic method, or to provide thefavorable target for seismic fine exploration. In addition, as the advantages of theexploration cost, may realize the CBM economical exploration and evaluation.