RELATIVE PERMEABILITIES AND TRANSPORT SYSTEM IN MIXED WETTED CARBONATE ROCKS

摘要

Two types of mixed wet reservoirs were observed.The first type was built by porous,permeable,organogenic carbonate rocks(bryozoan reefs)of the Main Dolomite,the second represents fractured – porous,low porosity carbonate sediments.Mixed wettability in the first type of reservoir was discovered during laboratory investigations of relative permeabilities and confirmed by Amott tests.Values of base permeabilities to water and to oil presented bimodal distribution.Additional tests and visualisation of displacement procedure shows mixed-wet character of the investigated rock.In a second case wettability phenomena in fractured reservoirs were investigated after field tests indicated very low values of permeability(ten times lower than calculated from logs and cores data).This situation led to performance of additional measurements and to reinterpretation of all data.The storage element of the investigated reservoir is microporous rock.Transport of reservoir fluids to boreholes by such rocks is not possible.Values of the threshold diameters are lower than 2 μm.Fracture systems were recognised using a fractal approach.It was found that three independent fracture systems existed in the field but only one system of fractures transports reservoir fluids.The fractures of this system are connected with organic and clayed interbeddings.Organic or clay-lined walls of fractures make the transport system neutral or weakly oil-wet in otherwise strongly water-wet reservoir rocks.These results were confirmed by relative permeability tests.