Prediction Of Formation Water Resistivity Of Chang 4+5 And Chang 6 Oil-bearing Formations In Zhidan Oil Region

Typical low porosity and low permeability lithologic reservoirs developed by the Triassic Yanchang Formation in the Ordos Basin.At present,the main difficulties encountered in exploration and development are strong heterogeneity of reservoirs,complex and changeable formation water systems,and difficult identification of oil and water layers.Due to the dual effects of the formation water system complexity and infiltration water,the Triassic Yanchang Formation is characterized by a complex source of formation water and varying resistivity of formation water.The resistivity of formation water is an important parameter for the calculation of fluid saturation and the evaluation of reservoir oil and gas.It is necessary to accurately determine the source of formation water and determine the resistivity of formation water.The paper carried out in-depth investigates on the properties of formation water in the study area?including formation water ion composition,formation water salinity,and formation water ion characteristic coefficient?by using reservoir geology,experimental tests,logging curves,and oil production tests.The research has realized the discrimination of oilfield water and infiltration water.Furthermore,under the constraints of reservoir geological,the water resistivity of the formation water was respectively calculated using the water analysis method,the Archie formula method and the adjacent mudstone information method.The preferential selection is suitable for he study area the calculation method of formation water resistivity,and the following main understandings and achievements are obtained:The results of water chemistry analysis of formation water sampling indicate that the Triassic Yanchang Formation in Zhidan Oil Region of the Ordos Basin is dominated by CaCl₂ type water,and there is also a small amount of Na₂SO₄ type water.The formation water salinity is mainly affected by Na⁺+K⁺and Cl^-.The formation water salinity in the Chang 4+5 and Chang 6 oil-bearing formations is between 23701 mg/L and 169017mg/L,mainly in 40035 mg/L and 90017 mg/L,with an average of 70305.46 mg/L.The formation water salinity is overall high.The analysis of formation water ion characteristic coefficient show that the sealability of the reservoir in Zhidan Oil Region is overall good,but due to the influence of infiltration water formation water salinity value is low in the local area.As a result,the formation water system is complicate and changeable in Zhidan Oil Region.The formation water resistivity of water samples in Chang 4+5 oil-bearing formation in Zhidan Oil Region is in the range of 0.03?·m to 0.20?·m,mainly in 0.04?·m to0.10?·m,with an average of 0.065?·m.The formation water resistivity of water samples in Chang 6 oil-bearing formation is between 0.013?·m and 0.14?·m,mainly in 0.04?·m⁰.10?·m,with an average of 0.049?·m.The formation water resistivity calculated by the Archie formula method is closest to the formation water resistivity estimated by the formation water analysis data method.