Abstract:

The Chang-8 Unit of the Yanchang Formation is a major reservoir in Jiyuan area, Ordos Basin. However, the reservoir
shows strong heterogeneity and oil production varies  from well  to well dramatically. In  this paper, samples  from 4 dr  lling wells
were selected  to conduct  relative permeability sensitivity  test so  that  the percolation characteristics and  their  influence  factors
can be   studied during the reservoir water-flooding process. Twenty casting thin sections were observed carefully, and the image
analysis of grain size and  the analysis of pore-throat were carried out. Other  tests were also done  including capillary pressure
measurement, scanning electron microscope  (SEM), X-ray diffraction and sensitivity analysis of clay minerals  in  the sandstones,
and  the oil-water displacement experiment. The purpose of  the work  is  to analyze micro-pore structures of  the  reservoir, clay
minerals, and the impact of diagenesis on relative permeability curve. Results show that the permeability curve of the reservoir in
the study area can be further divided into two types: the up-concaved and linear percolation curves. The up-concaved curve of the
corresponding reservoir belongs to the distributary channel microfacies with a high content of skeleton minerals, coarse grain size,abundant primary  intergranular pores and  large-diameter dissolution pore and good connectivity between pores. Mineral grain
contacts mainly occur as point to point and point to line, and the throat is coarse and long. The porosity and permeability of the
reservoir are high, and  the  fluid percolation  is good. In addition, as  to reservoirs with micro-cracks, proper water—flooding rate
can  improve  the efficiency of oil displacement. Because of  the strong heterogeneity and high content of sensitive clay-minerals
of  the  reservoir, pressure should be  increased gradually  to  raise  the water-flooding pressure  to eventually solve  the problem of
clogging of pores by clay mineral. This research is important for understanding of reservoir percolation characteristics in the initial
stage of exploitation and is useful to enhance oil recovery

Key words:  percolation-curve characteristics, micro-pore , structure; , sensitive clay mineral, diagenesis, Chang-8 , sandstone
reservoir, Jiyuan area