Fluvial Facies Reservoir Architecture And Its Influence To Remaining Oil Distribution

Most of the oilfields developed in the early stage of China have entered the stage of high water cut or super high water cut because of the long-term water injection development.Although a large number of oil and gas resources have been produced in the middle and late stages of oilfield development,there are still a considerable number of remaining oil and gas resources that have not been recovered.These remaining underground oil reserves have great potential for increasing recoverable reserves and enhancing oil recovery.For old oilfields with high water cut and extra high water cut,the detailed study and characterizat ion of reservoir architecture has become an important means to tap the potential of remaining oil especially for fluvial facies oilfield.Fluvial facies sandbodies change rapidly in transverse direction.Complex reservoir spatial distribution and internal structure play an important role in controlling the formation and distribution of remaining oil.It is of great significance to make clear the characteristics of each configuration unit of channel sand body and quantitatively characterize it,to improve injection-production well pattern and to increase the success rate of infilling adjustment wells for tapping the remaining oil potential of old oilfields in high water cut stage.To reveal the distribution characteristics and complex connectivity of fluvial sandbodies,through detailed analysis of various kinds of data,the sedimentary model of Guantao Fromation in the study area was determined and a detailed characterization of the reservoir architecture was completed.On the basis of analyzing reservoir architecture characteristics,the distribution types of remaining oil in the study area are summarized.This research can provide guidance for further development and adjustment and established targeted injection and production mode in oilfield.Therefore,this study is a supplement to the existing fluvial facies reservoir geology,which is of positive significance to promote its development and has significant academic value.At the same time,the fine characterization of reservoir internal structure and the prediction of residual oil distribution can provide reliable ground for the subsequent development and adjustment of oilfields in high water cut stage and the enhancement of oil recovery.It has important application value and wide application prospect.Combining the well logging with seismic,chronostratigraphic framework was established in Kongdian oilfield.The Guantao Formation of Kongdian oilfield corresponds to a third-order sequence and can be subdivided into three fourth-order sequences,including LST,TST and HST,and the evolution model of fluvial sedimentary system in study area has also been established.The hierarchy of depositional units in fluvial system of Kongdian oilfield was established and a comprehensive analysis method was presented for fluvial facies reservoir architecture combining a variety of technologies such as interwell correlation method based on model fitting,Seismic Sedimentology,prediction based on geological knowledge data-base,analysis of development dynamic data analysis.Three architecture hierarchies were identified and described detailedly include stacking patterns and distribution characteristics of single channel sandbodies,distribution and scale of genetic units,occurrence and scale of the third-order boundary surface in different fluvial systems.The stacking pattern of the single channel sandbody include isolated sandbody lens,thick sandbody and the transition style between them.The meandering rivers developed the small scale channel sandbody with single strip-apparent and interlaced strip-apparent and large scale braided channels sandbody developed continuously in the plane.Genetic units are appeared as scattered sand-bars and crisscross channel filling deposits in channel sediments of braided and meandering channels.Relativity between width of channel and the width and the length of side-beach is relatively high in meandering channel and there is a positive relationship between the width and the length of the central-bar in braided channel.The development of interlayer in the point bar resultes in the phenomenon that the point bar is a "semi-continuous sandbody" with top connection but poor bottom connection.Interwell analysis indicates that the horizontal extension width of a single phase lateral muddy-layer ranges from 64 m to 81 m,and the angle of that is 4.3 ～ 5°.The central-bar can be divided into four parts: the head,the center,the tail and the wing.The internal structure varies in different positions and depositional periods,due to the different hydrodynamic conditions of each part during the deposition of the central-bar.The heterogeneity of different reservoir architecture units is analyzed from several aspects: channel contact relationship and connectivity,differences in reservoir physical properties and lithofacies composition of genetic units,differences in reservoir physical properties of accretionary bodies.Braided channel sandbodies are superimposed each other forming broadly distributed composite reservoirs.In the meandering channels,the channel sandbodies are relatively low in the strata,and the contact and connectivity of the channel are various.There are five types connection relations of sandbody corresponds to five types connection relations of meandering river channel units: disconnected isolated channel sandbodies,connected when channel sandstones contacted channel sandstones,disconnected when channel sandstones contacted channel mudstones,weakly connected when channel sandstones contacted levee siltstones and disconnected when channel mudstones contacted levee siltstones.Based on the analysis on the characteristics of porosity and permeability of the single genetic unit sandbody,the differences of the sedimentary environment and hydrodynamic condition of internal architecture units of fluvial reservoir lead to differences on the reservoir quality,which also leads to differences in porosity and permeability of each accretion sandbodies.The distribution law of remaining oil is analyzed and the remaining oil distribution pattern under the influence of reservoir architecture is summarized in the study area based on the analysis of materials such as reservoir architecture,heterogeneity,reservoir numerical simulation and development,etc.There are four types of remaining oil formed under the influence of the single channel:(1)Channel superimposed area has large original reserves,even though it has high recovery degree and high water cut,it is still the main distribution area of recoverable remaining oil,(2)The channel sand bodies with poor well pattern control or not drilled belong to the untapped area,which is a potential residual oil enrichment area,(3)the non/weak seepage interface formed by a single channel is sheltered to form the overlapping oil accumulation zone of the river channel,(4)Local one-way water injection caused by sand body pinch-out and fault-sand combination at single channel boundary controls the formation of local residual oil enrichment area.Remaining oil under the influence of forth-order architecture was enriched due to seepage differences of different genetic units for the infiltration channels will form in the infiltration unit,resulting in different degrees of spread between the units,and the seepage barrier between genetic units is also a factor because the injected water is blocked by the muddy unit.Remaining oil under the influence of third-order architecture was enriched due to relationship between sandbody internal interlayer and water injection channel.In meandering fluvial deposits,the tendency of lateral accretion layer has an important influence on waterflood development,controlling the distribution of remaining oil.The distribution of remaining oil in the central-bar is mainly affected by the development and preservation of muddy interlayer.