Experimental Research On The Mechanism Of Water And Oil Flowing Through The Water-sensitive And Low-permeability Reservoirs

The WN reservoir of Qinghai oil field is typical low permeability water-sensitive reservoir. Because of the specificity of the low permeability reservoir (low permeability, low porosity, complex pore types), often there is a big difference between its laws of fluid flow and medium to high permeability reservoirs’s. Therefore, based on the understanding for fundamental geological characteristics of WN reservoirs, through indoor core flow experiments and microscopic model of water flooding experiments, oil and water flow characteristics and influencing factors are given a systematic experimental study. Through research the following key understandings are obtained in this paper:(1) The reservoir’s main rock types are lithic arkose, feldspathic lithic sandstone and feldspathic sandstone. Rocks are mainly dominated by silt, followed by fine sand. Calcite is the main cement types of WN reservoir; type of rock cementation is porous cement-based; pore throat types include original intergranular pore, secondary intergranular pore, micro-pore and micro crack primarily; the throats which are the main contribution for seepage radius is about0.2μm～3.04μm, the throat radius is small. The WN reservoir porosity distribution range is from4.0%to33.5%,with an average9.7%, the permeability distribution is from0.1～81.8×10-3μm2, with an average2.54×10-3μm2,so the WN reservoir is a low porosity and low permeability reservoir.(2) The water-sensitive damage is the main damage factor of WN reservoir, illite/mixed layer and the chlorite/mixed layer,smectite are the major minerals which lead to reservoir water-sensitive;The reservoir water sensitivity damage rate is34.7%～90.7%, with an average64.3%,which is middle-high water sensitivity.(3) The WN reservoir’s single-phase water flow and single-phase oil flow are obvious non-linear flow characteristics of non-Darcy; the WN reservoir’s threshold pressure gradient of single-phase oil flow is in the range0.003408～0.033433MPa/cm, the threshold pressure gradient of single-phase water flow is in the range0.000779～0.028232MPa/cm. The threshold pressure gradient of single-phase oil and water decrease with the increase of permeability, in which there is a good exponential relationship. The threshold pressure gradient of single-phase oil is significantly greater than the threshold pressure gradient of single-phase water.(4) The WN reservoir’s the final efficiency of water flooding is in the range25.3%-68.1, with an average47.81%, the final efficiency of water flooding is relatively low; increasing injection pressure can improve efficiency of water flooding obviously, increase range is5%to10%.The injection water added into anti-swelling agent can effectively inhibit the expansion of clay minerals in the reservoir rock and improve water flooding efficiency.(5) The WN reservoir oil-water two-phase relative permeability curves take on typical oil-water relative permeability curves characteristic of hydrophilic low permeability reservoir, the aqueous permeability is low, the two-phase vadose zone is narrow.(6) Experimental studies with the micro model have shown that the residual oil in reservoir rocks mainly exist as in the four forms flow around,the edge corners and oil spots or oil film in pores of the small throat and large pores, in which the flow around residual oil is the main form of residual oil.The results of this research lay a foundation for better developing the WN low permeability reservoir of water sensitivity and have some guiding significance for developing similar low permeability reservoir of water sensitivity, too.