Evaluation And Prediction Of Structural Fractures Of Ultra-deep Sandstone Reservoirs Of Block Well Keshen 8 In Kuqa Depression

Ultra-deep tight sandstone reservoir usually presents low porosity and low permeability,the development of fracture system can effectively improve the capacity of reservoir flow.In this paper,the buried depth of K1 bs reservoir is more than 6000 m,which is an ultra-deep layer of tight sandstone reservoir,and fracture system has become a key geological factor for high yield of natural gas wells in this area.The complex structure,the limited drilling coring and the poor quality of seismic data of the study area lead to unclear understanding of the characteristics of fracture development.Based on field outcrop observation,lots of core observation,imaging logging and core sheet analysis,and numerical simulation,a comprehensive study was conducted on the K1 bs reservoir fractures of Block Well Keshen 8 in Kuqa Depression.The basic characteristics of fractures are obtained by field outcrop investigation,core crack observation and imaging logging data.It is concluded that the study area K1 bs mainly develops NWW-SEE and NNW-SSE tectonic fractures,which are mainly tensional fracture and high angle-straight fracture.Fracture opening is mainly distributed between 0.1-0.8mm,which is mainly non-filling to semi –filling.In the plane,the fracture strike of eastern structural high part and western anticline is nearly south-north,and of other areas is nearly east-west.The inclination of the fractures gradually decreases from the structural high part to the two limbs.The fracture linear density of high part of anticline is lower than that of the two limbs,and of western high part is lower than of eastern high part.In the longitudinal direction,with the increase of depth,the fracture density increases,and the fracture opening decreases.The development of tectonic fractures in K1 bs of Keshen 8 Block is mainly controlled by tectonic action,strata thickness and burial depth.It is concluded that the strata thickness is negatively correlated with the fracture linear density,and in the thin rock layer,fractures are developed.For the ultra-deep reservoir,with the depth increasing,the fracture density,opening degree and porosity have a decreasing trend.There are three epochs of structural fractures in the study area: In the end of Yanshan,with a short nearly south-north compression,it formed a small amount of NWW-SEE and high degree–straight fractures,of which filling degree is high.In the middle of Himalayan,with the slow nearly south-north tectonic compression,it formed NNE-SSW and high-angle fractures,of which filling degree was low.In the late of Himalayan,with the rapid nearly south-north compression,it formed a large number of NNW-SSE straight fractures,of which filling degree was low,with non-filling to little–filling.Using ANSYS finite element numerical simulation method to predict the fractures in the study area,it is concluded that the development of fractures has a certain correlation with the structural location,and the fracture density in the high part of anticline is lower,from which to two limbs and the low parts of anticline it gradually increased.The fractures near the fault zone are significantly higher than those of other regions.Fracture permeability and opening in structural high part presents high value.On the whole,although the density of fractures in structural high part is low,fracture permeability and opening are very high,and the fractures are effective,of which overall development is still higher than other areas.