A streamline approach for identification of the flowing and stagnant zones for five-spot well patterns in low permeability reservoirs

The mechanism of the fluid flow in low permeability reservoirs is different from that in middle-high permeability reservoirs because of the existence of the Threshold Pressure Gradient （TPG）. When the pressure gradient at some location is greater than the TPG, the fluid in porous media begins to flow. By applying the mirror image method and the principle of potential superposition, the steady-state pressure distribution and the stream function for infinite five-spot well patterns can be obtained for a low permeability reservoir with the TPG effect. Based on the streamlines distribution, the flowing and stagnant zones in five-spot well patterns can be clearly seen. By the definition of the effective startup coefficient （SUC）, the ratio of the flowing and stagnant zones can be calculated accurately. It is shown that the SUC for five-spot well patterns is not constant, but decreases with the increase of the di- mensionless TPG. By increasing the effective permeability of the formation （such as by the acid treatment and the hydraulic fracture）, in increasing the injection-production differential pressure or shortening the well space （such as by infilling well）, the SUC can be improved. The results of the sensitivity analysis show that a better choice for the SUC enhancement is to shorten the well spacing for small permeability reservoirs and to increase the pressure difference for large permeability reservoirs. This streamline approach can be used to determine the distribution of remaining oil and provide guidance for infilling well.

Displacement and development characteristics of fire flooding of vertical wells in old heavy oil areas

As the vertical-well fire flooding technology is industrially applied in the steam-injection old heavy oil areas of Xinjiang and Liaohe oilfields,its enhanced oil recovery potential is gradually clear.According to laboratory experiment,field test and reservoir engineering,the displacement characteristics of verticalwell fire flooding in the steam-injection old heavy oil areas are systematically investigated.Laboratory experiments and core data show that the vertical-well fire flooding has significantly high flooding ef-ficiency,no residual oil are remained in the firing front sweeping zone,and the lateral sweep efficiency and ultimate recovery can be achieved.The vertical-well fire flooding is a strategic replacement technology to enhance the recovery greatly.Development characteristics and advantages of areal and linear fire flooding are well investigated,and research results are applied in the design of industrial test plan of fire flooding in the Hongqian block,Xinjiang.The research shows that the linear well pattern has advantages of easy construction of ground facilities and management,less well of stage management,simple matching technology and easy achievement of purposeful control for fire front;the areal well pattern has advantages in reducing the air/oil ratio during the fire flooding period,increasing the total recovery rate of reservoirs and reducing the geological and reservoir management risks.To select well patterns of fire flooding,some factors such as geology,reservoir,fluid properties,oil price,and reservoir development degree should be mainly considered.In the Hongqian block,an improved linear well pattern with a combination of new wells and old wells is applied,this not only absorb experiences of linear well pattern pilot test,but also use advantages of areal well pattern.