Study On Prediction Method Of Tectonically Deformed Coal And Coal Bed Fractures Using AVO And AVAz Technology

The structure and physical properties of the tectonically deformed coal are different from the undeformed coal,and its existence has a significant impact on the safe production of coal mines and the effective development of coalbed methane resources.In order to effectively predict the tectonic coal,we build a variety of coal rock models with different structure types based on the equivalent medium theory and coal structure characteristics,and then explore the AVO(Amplitude Variation with Offset)and AVAz(Amplitude Variation with Azimuth)response regularity of different forward models built with AVO and AVAz detection theory.After that,the correlations between coal properties and seismic responses are applied to the actual mining area,achieving the prediction of tectonic coal distribution and coal fracture characterization.The research results have guiding significance for coal mine production and disaster prevention.In the study on forwarding simulation of tectonic coal AVO response,we build a variety of coal rock models based on different elastic parameters data and calculate the AVO response with Zoeppritz equation.Thus,the P and G attributes of different AVO forward models are obtained by fitting the relationship between amplitude and incident angle.Finally,the variation trends corresponding to changes in coal properties on P-G crossplot is found and used for coal structure interpretation.The AVO forward modeling based on elastic parameters from different deformation degree tectonic coal demonstrates that P,G attributes increase appropriately linearly with the deformation degree,and P-G crossplot shows regular variation trend.Since the fracture density of coal rock is positively correlated with the deformation degree of tectonic coal and negatively correlated with confining pressure,we propose that the fracture density used as a quantitative index of tectonic coal deformation degree and to evaluate the deformation degree of tectonic coal indirectly.Therefore,we build forward models based on elastic parameters measured under different confining pressure to investigate the influence of coal fracture density on its AVO response.It is found that the P,G attributes increase appropriately linearly with fracture density,and P-G crossplot shows consistent variation trends of different coal samples.To further investigate the influence of porosity and pore fluid on AVO response of coal bed,coal rock models are built based on Gassmann's Equation to explore the variation characteristics of P,G attributes.It is found that the porosity has a similar effect with fracture density as studied above.In the study on forwarding simulation of coal bed anisotropy,we build fractured coal rock models of HTI(Transverse Isotropy with a Horizontal axis of symmetry)media based on Hudson equivalent medium theory and obtain the seismic phase velocity by solving Christoffel equation.The influence of fracture parameters on VVAz(Velocity Variation with Azimuth)characteristics is investigated with cross-plotting analysis.Reflectivity is calculated with plane wave reflection coefficients in anisotropic media-Schoenberg and Protazio algorithm,and the influence of fracture parameters on GVAz(Gradient Variation with Azimuth)characteristics is investigated.It is found that the anisotropy magnitude is positively correlated with fracture density and negatively corelated with the aspect ratio of fracture.To verify the correctness and validity of forward modelling conclusions,we take the No.3 coal bed in Fowa area of Xinjing Coal Mine as the case study object.For tectonic coal interpretation by AVO technology,firstly,coal structures identification and classification are taken on 11 well logs in study area according to the differentiation of log response.And then seismic pre-processing for AVO analysis is taken on 3D pre-stack gathers to eliminate interference and get high quality gathers.Finally,AVO attributes are inverted with two terms Aki-Richards appropriate equation,and the development and distribution of tectonic coal are predicted according to the interpretation method derived from forwarding simulations.For coal bed fracture characterization by AVAz technology,we firstly divide full-azimuth gather into six azimuthal bins with 30°bin size and take seismic data pre-processing on each azimuthal gather.Secondly,the fracture strike direction interpretation method with AVAz curves and fracture parameters inversion algorithm are determined.Finally,comparing the fracture orientation with regional tectonic stress field map and underground in-situ stress measurements,it is found that,within 3 km~2 mining area,fracture orientation deviate from the background trend because of the affection of fold structure and fault structure.In summary,we propose a feasible method of tectonic coal prediction and coal bed fracture characterization through the AVO,AVAz forward modelling and receive good result in case study.