Research On Oil/water Distribution Character Of Karst Carbonate Reservoir In 6th Block Of Ordovician TAHE Oil Field

The Ordovician reservoir of Tahe Oli Feild is complex fissured and vuggy karstic carbonate reservoir which experienced multiphase tectonic disruption and fossil weathering. The reservoir's main storage space is emposieu and It's reservoir quality is controlled by limestone karst.The carbonate matrix has almost no storage sense.It only can be the blockage of the available storage space.The distribution of the storage space is random and controlled by fractures, stratigraphic distribution, palaeotopography, palaeogeomorphology, paleoclimate, burial history and so on.The shape of emposieu is irregular and it's distribution is heterogeneous The reservoir located under the depth of 5300m and experienced multiphase migratory and filling processes.The distribution of fluid in the reservoir is very complicated.Although the proven reserves of TaHe Oilfeild is huge it's development status is very poor and the recovery ratio under the present production practice is low too because of big amounts of wells' high annual decline rates caused by high water cut.The descent of deliverability which are caused by water breakthrough and rising of water cut. is the main reason of the reservoir's poor development status. The key methods to improve development status and maintain productivity of the reservoir are figuring out the origin of the water, Oil/water distribution character and taking proper steps to control the rising of water cut.It has great theoretical and actual meaning for the efficient development of the field that taken 6th block of Ordovician TaHe oil field as an example to inquire it's oil/water distribution character.The research took karst geology, well logging interpretation theory, seismic reservoir prediction theory and petroleum reservoir engineering as guides.On the basis of the research of the controlling factor of karst reservoir the characteristics of the reservoir are inquired.At the same time the reservoir's storage space and types are classified It is believed that the valid storage space are classified into three categories which are pores, fractures,and vugs.The large scale vugs are the main storage space.The reservoir of lower ordovician in the region of interest are classified into three categories which are I class vuggy reservoir, II class fissured and vuggy reservoir,III class fractured reservoir.The I class vuggy reservoir is the best reservoir.The characteristics of the vuggy reservoir are inquired from it's vertical and plane distribution,filling characteristics,size,and so on.It is believed that the vuggy reservoir is well developed in the region of interest.The distribution of vugs are bound with the developing degree of fractures and faults and the plane distribution of vugs are mostly located in fracture belt and tectonized region.Whenever the middle-late Caledonian or the early Hercynian vugs distribute in layers apparently when the palaeogeomorphology of the region is recovered.The vugs in the region vertically distributed in the range of 100m from the upper surface of yingshan formation and the probability to drill vugs decrease with the depth of the well.The ancient karst landform of 6th block of Ordovician TaHe oil field distributed in NNE direction.According to the karstic feature of the reservoir the region of interest was divided into 5 fracture-cavity systems.The II and IV fracture-cavity system which lying in the high position of the ancient karst landform are developed.and are the main oil producing area. The I,III and V fracture-cavity system which lying in the low position of the ancient karst landform are not developed.and their productivity are low.The 5 fracture-cavity systems were divided into 30 different types of development blocks according to formation pressure, fluid property, multiwell interference, breakthrough and rising feature of water cut of the wells in the region.There are 9 multiwell development blocks and 21 single—well development blocks in the region.The reservoir of 2 typical single—well development blocks and 1 multiwell development block's were classified on the basis of well logging, miscellaneous geologic investigations,well test, production performance of the wells in the region. And the plane distribution of the typical development blocks' were inquired according to the seismic data.The breakthrough , water cut rising feature and history of the wells of different production performances in the typical development blocks were discussed. On the basis of the research above mentioned the oil/water distribution modes of the typical development blocks were founded according to geologic feature, seismic reflection features of the reservoir and the water analysis.It is believed that there are 3 types of water body in the reservoir which are residual water body in the constant volume cavity of Ordovician limestone,water body in the Ordovician limestone reservoir which is interconnected to water body in deep dolomite reservoir,and water body in the dolomite reservoir of Penglaiba Formation.The single—well development blocks are not all constant-volume reservoirs.There is a type of reservoir which is interconnected to water body in deep dolomite reservoir.The difference between it and the multiwell development blocks is reserve scale.The development state of the fissured and vuggy karstic carbonate reservoir determined that the possibility that there are large scale bottom water in the ordovician limestone system is very low.So the 6 Oil/water distribution modes that the predecessors proposed are actually one mode—the constant-volume reservoir in production performance.The original oil/water interfaces of the typical development blocks were calculated preliminarily on the basis of in situ data statistics and log interpretation of the wells.And the feature of the current water-oil interface of the typical multiwell development block's was researched on the basis of production logging data.The deepest original oil/water interface of the typical development block is under 5762m of S66-TK604 block while the shallowest interface is 5566m of TK618 block. The difference value between them is 196m.It means that there is no unified oil/water interface in 6 block of Tahe OH FieldAt present the depth of the top interfaces of effluent positon of wells in the typical blocks basically tallies with the depth of the well section where vugs developed.It means that the effluent position is mainly locate in the well section where vugs developed and is not closely related with the height of structure.The water free oil production period is not closely related with the depth of the well section where vugs developed. The water breakthrough time depends on the oil production intension and connectivity degree between oil and water.When the development blocks were put on production the hights of water channeling of different wells are not the same because of different connectivity degree between oil and water, different position where vugs developed.So there is no unified oil/water interface in development block. It's oil/water interface doesn't uplifte as a whole like in sandstone reservoirs also.Different development techniques and methods to maintain productivity and control rising of water cut are proposed according to oil/water distribution and reservoir characteristics of different types of development blocks.It is believed in the paper that the characteristics of the vuggy reservoir and the types of water body codetermined the dynamic changes of reservoir development. Formulating reasonable exploitation policy of fissured and vuggy karstic carbonate reservoir in TaHe oil field bases on the understanding of oil/water distribution and reservoir characteristics of different types of development blocks.