Quantitative Prediction Of Multi-Phase Fractures In Tight Sandstone Reservoirs

As the main seepage channel and important reserving space in tight sandstones,the tectonic fractures generally show many unique characteristics,such as multi-period development,overlapping with each other,and distributed in shape of continuous net.Due to the multi-stage tectonic movements,the quantitative prediction of fractures has been a hotspot and difficulty in petroleum geology.Based on the similar outcrop investigation,core observation,CT scan and thin slice observation,this article takes the Keshen 2 tight gas field of the Kuqa Depression as an example to the identify the multi-phase fracture system and analyze the relationship between fractures.As a result,the formation period of fractures in Keshen 2 gas field was determined,the main control factors affecting fracture development were analyzed,and finally the geological models of multi-phase fracture systems were summarized.Based on identifying multi-phase fractures through core observation,rock thin section observation,similar outcrops observation,CT scanning,the interrelationship between fractures were analyzed and formation time of fracture was determined,and then we could analyze the factors of influencing the formation and the development scale of fracture,and classify the combination mode of the two-phase fractures.Through analysis of single fracture artificial rock triaxial compression test,found that with the increase of the major principal stress and the existing fracture Angle "?",the peak intensity decreases first and then increases,and connected curves of those peak intensity is a kind of "U" shaped curve;Combining with the study of finite element numerical simulation,the value range of "?" of different destructive forms of single fracture rock mass is determined.According to the analysis of triaxial compression test,the research of finite element numerical simulation,and the comparison of seven classic rock strength theory such as Coulomb-Mohr Principle,Griffith Principle,Byerlee Slip Principle,Wing Crack Principle etc,finally the Coulomb-Mohr Principle,Griffith Principle,Byerlee Slip Principle were determined to be a single fracture rock mass strength criteria;Combining with the related theory of rock mechanics,strain energy theory and the principle of conservation of fracture surface energy,the quantitative relational expression between the fracture density and stress field was established.Through studying literature and reaching the former research achievements,and combining with finite element simulation and stress analysis of six tectonic evolution models of Kuqa river profile,the main forming period of reservoir fractures in Keshen 2 gas field was determined;The main stress direction was obtained through the statistic and analysis of the fractures.And the principal stress of the construction period was calculated by indirect method.Based on the multi-level composite strength criterion and fracture mechanics model,the structural geological model of the Bashenjiqike Formation(K1bs)in Keshen 2 gas field was first established,and the mechanical parameters were assigned according to the experimental results of rock mechanics.The paleo-stress states of the early and late movements of the Himalayan since the Miocene were obtained from regional geological and tectonic mechanical analysis.Then,the paleo-tectonic stress field numerical simulation and fracture superposition calculations were performed to obtain the spatial distribution of the total fracture volume density of the two phases.Finally,a comparison between the statistical results by cores and the numerical simulation results was carried out to verify the the numerical simulation results were compared with the statistical results by cores,to verify the rationality of composite strength criteria for fractured rock mass.