Study Of Reservoir Change Law After Waterflood In Glutenite Reservoir And Its Application

Baobei Block is a typical low-porosity and low-permeability glutenite reservoir,which is more heterogeneous than conventional sandstone reservoir.After more than 20 years of waterflood development,the comprehensive water cut of the block has reached over 80%,and the recovery is only 17.68%,and the production rate is only 0.08%.The analysises show that the main reason for the difficulty of the block exploitation is the changes of reservoir characteristics caused by long-term waterflood,which affect the fluid seepage in the reservoir.In order to improve the development status of the block,the reservoir characteristics and their changing rules,seepage characteristics and their changing rules after long-term waterflood in the block are studied by combining the indoor core experiments and reservoir engineering methods.Based on the changes of reservoir and seepage characteristics,the development policies of low-permeability glutenite reservoir after long-term waterflooding is studied.The following achievements and understandings have been achieved:(1)The long-term waterflood makes the clay mineral content decrease,and then enlarges the diameter of the pore throat and increases permeability,and finally causes the threshold pressure gradient to decrease.The decrease of threshold pressure gradient is conducive to the establishment of an effective interwell displacement pressure system.At the same time,the clay minerals migrate out of the pore throat and block in the pore throat,resulting in the enhancement of the reservoir heterogeneity.The increase of vertical reservoir heterogeneity may lead to fingering of injected water.(2)The decrease of clay mineral contents make the water sensitivity and velocity sensitivity of reservoir become weak water sensitivity and weak velocity sensitivity.In the current process of waterflooding and oil recovery,the reservoir damage caused by velocity sensitivity and water sensitivity is relatively small.The permeability change can weaken the stress sensitivity of reservior,but it is still medium to strong.By decreasing the effective stress,the damage degree of the stress sensitivity to the reservoir can be reduced.(3)The method of threshold pressure gradient is used to optimize the well spacing.It is found that the well spacing in the block is large.The well spacing in oil groups I and II should be reduced to 153 m,and that in oil group III should be reduced to 167 m.At the same time,the upper limit of the well injection pressure is determined to be 51.84 mpa by empirical formula method and field data method,but it is 31.5Mpa at present,so there is still room for the improvement of the well injection pressure.(4)The reasonable development policies are put forward: to increase water injection pressure and to infill well pattern.Increasing water injection pressure can increase production pressure difference between the wells and reduce stress sensitivity damage of reservoir.