Experimental Study On Unstable Water Injection In Heterogeneous Interior Reservoirs Of High Water-Cut Oilfield

With most domestic oilfields gradually get into the high water-cut stage,the development of remaining oil gradually changes from planes and inter layers mode to the integrated three-dimensional mode that includes planes,inter layers,and interior layers.Thus,enhancing the recovery of remaining oil from the interior layers becomes the key.Unstable water injection is one of the important methods to enhance oil recovery in high water-cut period.It is relatively mature in foreign countries,but it is known later in China.At present,there is a lack of sound research methods and necessary data support to deeply understand the displacement law of unstable water injection in heterogeneous reservoirs.According to the characteristics of the heterogeneous reservoirs in the interior layer,four groups of models were designed,and converted to unstable water injection to enhance oil recovery after conventional water flooding according to the actual development situation.The CT scan was used to analyze the characteristics of oil-water saturation during the displacement process.The pressure point was set to analyze the internal pressure change characteristics of the reservoir.The research deeply analyzes the change characteristics of oil-water saturation and the development effects during conventional water flooding and unstable water injection.The study also analyzed the pressure change characteristics of the heterogeneous interior layers during unstable water injection and its intrinsic relationship with the mechanism enhancing oil recovery.The oil-water saturation distribution characteristics of the conventional water injection process were obtained by CT scanning.The results show that with the increase of heterogeneity,especially when the thick oil layer itself has a higher permeability,it is easier to make a crossflow and breakthrough earlier,as a result,the development effect is rapidly deteriorated.In addition,with conventional water flooding inverted rhythm reservoirs behaved better than positive rhythm reservoirs.When the water cut reached 95%,change conventional water flooding into unstable water injection.the experiment used different injection pressures to carry out five rounds of secondary water injection for each model.The experiment used different injection pressures to perform five rounds of unstable water injection for each model.The oil-water saturation image obtained by CT scanning clearly shows the movement of the water-flooding front in the low-permeability layer,and the overall water saturation of the reservoir gradually increased.The experimental results show that the unstable water injection can effectively expand the swept range of water injection.It should be noted that under different permeability ratio and different reservoir rhythm,the remaining oil areas swept may be different.When the permeability ratio is small,mainly the low permeability area is affected.When the permeability and permeability ratio is high,due to the influence of gravity,for the positive rhythm,mainly the low-permeability area is swept,but for the inverted rhythm reservoirs,there is an obvious ripple effect on the top of the high-permeability area in addition to the low-permeability area.Comparing the recovery percent between the models,with greater injection pressure and stronger heterogeneity,it is more conducive to unstable water injection.Different from conventional water flooding,positive rhythm reservoirs behave better than inverted rhythm reservoirs in unstable water injection.Through the experiment,the pressure data of the heterogeneous reservoir during the unstable water injection are obtained,analyzed,and the pressure field patterns are established.The results show that the pressure in the high permeability zone changes faster than the low permeability zone,thus forming a pressure disturbance between the high and low permeability zones.Unstable water injection pushes the front of the water-oil displacement by the pressure disturbance,which expands the swept range of water injection.The remaining oil in the low-permeability area is brought into the high-permeability area under the pressure disturbance and is extracted through the high-permeability channel.Finally,it is pointed out that the pressure disturbance is mainly affected by the heterogeneity of the reservoir and the injection pressure.With stronger reservoir heterogeneity and higher injection pressure,the pressure disturbance would be greater,which will help to enhance the recovery by unstable water injection.The study provides theoretical reference and guidance for oil field when use unstable water injection to enhance oil recovery of interior oil layers,and provides methods and data support for experimental study.