Effect Rule and Mechanism of Bedding Fractures on the Fluid Flow in Tight Reservoir with Extremely High Pressure

摘要

Yongjin oilfield in Junggar Basin is a tight oil formation with extremely high pressure(higher than 100MPa).This oilfield was not investigated optimistically in the past because of the super-low reservoir permeability and the considerable content of asphaltene and wax in the crude oil.However,the negative stereotype of such reservoir conditions with low-production was broken by a horizontal well drilled in Oct.2018,which had unexpected high-production constantly for more than half a year.The well-developed tectonic bedding fractures are believed as the key to the high production in this horizontal well.In this paper,we comprehensively investigated the rule and mechanism of bedding fractures in the high oil recovery of the tight reservoir by multiple theoretical and experimental methods.The tight rock samples with/without bedding fractures,which are used in the experiments,were drilled out from Yongjin oilfield.Based on nuclear magnetic resonance,the bedding fractures under different confining pressures(up to 30MPa)are characterized by the transverse relaxation time(T2)spectrum and magnetic resonance imaging.Besides,we designed a novel non-Darcy flow setup with the ultra-high-pressure holder and high-accuracy flow meter.With this setup,we observed the fluid-flow phenomena in the bedding fracture-matrix system under pressure of 100MPa and temperature of 135°C.We then introduced two low-velocity non-Darcy models for fitting the flow data in the fracture-matrix and matrix-only samples to reveal the flow mechanism in such reservoir conditions.By comparing the permeability,we distinguished the role of the bedding fractures in improving oil production.The magnetic resonance images illustrate that the bedding fractures run straightly through the rock.Meanwhile,the T2 spectrum indicates that the size of fracture decreased with the confining pressures.Besides,the results of non-Darcy flow experiments demonstrate that a non-Darcy flow curve has linear and nonlinear flow part.The nonlinear flow starts from the zero-pressure gradient.Under the given overlay pressure,the permeability of fracture-matrix decreases with the drop of pore pressure gradually,and the permeability free-falls demotion after reaching a closure pressure.The nonlinear flow region is widen in the closed status.During elevating the pore pressure,we tested the cracking pressure of the samples,which is 3-10MPa larger than the closure pressure.The results show that when the fracture is open,the permeability of fracture-matrix sample is very sensitive to pressure,and hundreds/thousands of times larger than the matrix-only ones.The permeability of closed fracture-matrix sample is still dozens of times larger.Although the permeability of the matrix-only sample changes with pore pressure,the permeability is reversely insensitive to the change of pressure.Both the non-Darcy flow characters in fracture-matrix and matrix-only systems provide the instructive function for the oilfield development management.Besides,maintaining the optimized flow pressure down hole to prevent the bedding fracture from closuring is a crucial advantage in Yongjin reservoir development.