Research On Developing Regularity Of Multiperiod Structural Fractures In Futai Carbonate Buried Hill

According to the structural analysis and combining previous research results,the evolution stages from Futai buried hills in lower Paleozoic can be divided into some stages,including the stable subsidence stage in Paleozoic;the forming of thrust-fold in late Mesozoic and the stretching rift depression stage during Cenozoic.The forming of thrust-fold in Yanshan epoch and the extensional rift in Himalayan period are two major tectonic deformation stages which Futai buried hill experienced.They are also the main formation period of buried hill fracture.Through core observation and well logging fracture identification,it showed that the rich amount of structural fracture developed in Futai buried hills.At the same time there were some solution pores,diagenetic shrinkage joint,sutures and other non-structural fracture.According to the strata occurrence and paleomagnetic method,core is directed and the fracture occurrence is determined.There were two strike directions of fracture: one was the NE-SW direction conjugate shear fractures which are dominant,another was nearly E –W.Fracture was given priority to with vertical cracks and high Angle fracture,low angle fracture and horizontal fracture development were less.Fracture mechanics properties were mainly shear joints,there was a small amount of extensional joint development.The core crack opening was mainly distributed in 0.6 ~ 3.0 mm while 0 ~ 0.4 mm took second place.Most of core is cut to wear and the extension of crack length can be long.The average line density of fracture in cored interval is between 2.19 ~ 11.75 A/ m.The non-structural fractures which mainly developed on the top of the buried hill are mainly affected by the lithology,fracture distribution,tectonic position and buried depth.The non-structural fractures can form a complex network by the connecting function of tectonic fractures and become channels of hydrocarbon migration and reservoir space.Based on the statistics of fracture filling in every layer in Futai buried hills coring wells,combining imaging logging interpretation results,the filling rates of fractures in every well were calculated by layers and the rule of fracture filling in the Ordovician and Cambrian Fengshan group was predicted.Think the size of the buried hill fracture filling rate is affected by the factors such as tectonic position,lithology.We considered the buried hill fracture filling rate is affected by tectonic position,lithology and other factors.The fracture filling rate in Chegu 202 fault block is the lowest,followed by Chegu 201 fault block and Chegu20 fault block.Influenced by lithology,upper Majiagou group and lower Majiagou group mainly filling calcite were mostly limestone and were highly filled generally.As a result,it is not favor to preservation for fractured reservoir.However,Yeli-Liangjiashan group and Fengshan group mainly filling anhydrite and calcite were by dolomite primarily and were lowly filled.Based on the calculating results of rock mechanics experiment and logging data,we established the relation between the elastic modulus and confining pressure of limestone and dolomite under different confining pressure,calculated the rock elasticity modulus under the condition of corresponding buried deep in fractures forming period and selected rock mechanics parameters by periods and horizons.Furthermore,the mechanics models of Badou group,upper Majiagou group,lower Majiagou group and Yeli-Liangjiashan group in Ordovician and Fengshan group in Cambrian.were established.The equivalent of ancient stress in the periods of fracture forming was determined by fitted inversion.We determined the direction of the present crustal stress according to the orientation of drilling induced fracture and bore-hole collapse and the size of present in-situ stress on the basis of the empirical formula and differential strain results.On this basis,numerical simulation in Yanshan period,Himalayan period and present in-situ stress field were carried out.The quantitative relationship of mutiperiodic stress field of fracture parameters was established based on rock mechanics and elastic mechanics theory.On this basis,using finite element methods,we carry on the numerical simulation of fracture linear density,the opening,porosity,permeability and other parameters in Futai buried hills.Simulation results shows that the fracture linear density of Futai buried hills is 3.5 ~ 3.5 A/ m and the present fracture opening is about 0.16 ~ 1.78 mm.Besides,the present fracture opening in Yanshan period is about 0.49 ~ 1.78 mm and is about 0.16 ~ 0.22 mm in Himalayan period.The present fracture porosity is between 0.14% ~ 0.14%.The present permeability in the east-west direction is between 2.84 ~ 19.1 mD and is between 0.1 ~ 18.6 mD in the south-north direction.The vertical permeability is between 2.95 ~ 15.28 mD.We calculated the quantitative parameters and predicted the distribution of effective fracture combing fracture filling with the results of numerical simulation on fracture parameters.Based on the size of fracture aperture,the fractures in Futai buried hill can be divided into three levels: large aperture fracture(aperture>1.0 mm),medium aperture fracture(aperture:0.5~1.0mm),small aperture fracture(aperture<0.5mm).According to the situation of fracture filling,effective fracture porosity,effective fracture density,development of non-structural fracture and distribution of fractures in different levels,the fractured reservoir performance in Futai buried hill can be divided into four types from high to low.