Identification And Prediction Of Dominant Seepage Channels In Water-flooded Sandstone Reservoirs

Penglai 19-3 Oilfield is located in the south-central part of Bohai Sea.The main reservoirs are fluvial sands in Guantao Formation and the lower part of Minghuazhen Formation of Tertiary.It is the second-largest integrated oilfield after Daqing Oilfield in China.Guantao Formation is dominated by the braided channel sedimentation and the lower part of Minghuazhen Formation is mainly meandering river sedimentation.With sandstone and mudstone interbedded,connectivity of the reservoir is good and pore is well developed.The reservoir is characterized by medium to high porosity and permeability.However,as a result of the difference in original formations and the effect of waterflooding,the oilfield is facing a series of production problems,such as the serious interlayer interference,water breakthrough along single layer,low waterflooding efficiency,unbalance of water injection profile and so forth.The contradiction between and within layers is more obvious than before and finally makes the thief zones formed in high-permeable zones.There are many disadvantages once thief zones are developed.For instance,inefficient or ineffective circulation of the injected water,low recovery efficiency,high water cut,which severely affects the waterflooding effect and economic benefit.Therefore,the accurate identification of thief zones and effective forecasting model are instructive to enhance oil recovery and for the waterflood profile control and water shutoff.Aiming at the problems in waterflooding,Guantao reservoir,the major pay zone in the study area,was mainly studied.Domestic and foreign literatures were widely investigated.On the basis of geological and development characteristics,a series of research work on thief zones was carried out,which includes the classification,identification,analysis of influencing factors,forecasting and so on.The study discussed in this paper not only offers technical support for treatment measures of the study area in future,but also provides reference for other high water cut oilfields.The main research results obtained are as follows:1.Based on the large amounts of previous research results,thief zone can be divided into two types according to the different causes and characteristics,primary thief zone and secondary thief zone.2.With full use of core testing data,tracer monitoring data,production performance data,production logging data,logging data and the like,the horizontal and vertical distribution of primary thief zone and secondary thief zone in flooding units were accurately identified respectively.Meanwhile,with the Grey Correlation Method,the correlation degree of water injected-fluid produced and water injected-water produced were calculated respectively,which ensures the results be even more accurate.3.Choose proper parameters to establish criterion to classify water-flooded level,which is one of the criteria to identify thief zones.4.Features of the layers where thief zone developed from were analyzed,therefore,the geological and the development factors were summarized respectively.5.On the basis of the geological research,the different sand body interconnectivity models were summarized and the connectivity factors were calculated respectively.6.According to the identification results,reasonable parameters were selected by analyzing the main controlling factors.Based on the geological features,the reasonable forecasting model and the forecasting criteria were established combined with the production performance data.7.The forecasting model in this paper was proved to have high practical value by the latest production and test data.