Determination Of Water Saturation Model On Low Porosity And Low Permeability Reservoir And Its Application In SongNan Area

Low porosity and Low permeability reservoir is the main reservoir type of china's current lithostratigraphy hydrocarbon exploration and development,and it is the main source of China's future oil and gas reserve and output. The log evaluation of low porosity and low permeability reservoir is always one difficult point. The determination of saturation is a very important task on reservoir evaluation. However, the complexity of its pore structure make rock's conductivity and satuation deviate from Archie relationship, result in the rock becoming non-Archied rock. Usually, the property of low porosity and permeability leads to complicate log response, at the same time, it exacerbates the difficulty of saturation evaluation.For sandshale profile, the logging evaluation of reservoir in middle -high porosity has accumulated a set of very mature approach to explain,and it can really provides more reliable evaluation results. However, for the oil and gas logging evaluation of sandstone reservoir with low porosity, abundant shale, low permeability and low oil saturation,logging technology has shown a lack of capacity,and log interpretation mostly uses the methods and thinking in middle-high porosity and permeability reservoir. For explaining theory, Archie principle has been using during past several decades, but it is the product under uniform medium and ideal conditions, using it to solve the low porosity and low permeability reservoir, its accuracy can be imagined. So it is important to work out a water saturation model which can evaluate low porosity and low permeability more effectively.Basing on the study of a number of areas of low porosity and low permeability of the reservoir causes, rock characteristics, pore structure characteristics, physical characteristics, the characteristics of rock parameters and logging features,sumed up that the low porosity and low permeability reservoir is mainly due to material source, sedimentary environment, diagenesis and the role of tectonic stress. To a large extent, the content of plastic minerals of rock in source area determines the quality of properties. In depositional environment, the best lake water medium for reservoir is fresh water and brackish water. The properties of reservoir are transformed by digenesis, such as compaction, cementation, dissolution, metasomasis and cataclasis, of which compaction and cementation are the main reasons to destroy the primary pore. The impact of tectonic stress to properties has two sides, the positive aspect is that fissure and dissolution make properties improved, and the negative aspect is that compression stress makes rock denser. Usually, on low porosity and low permeability reservoir, debris content is relatively high ,fine-grained rock composition is much more and cementation occurred frequently. Pore structure is complex, and usually the primary pore, secondary pore and fissure is being simultaneous. Rock electrical parameter a of low porosity is usually higher than that of high porosity, and m in low porosity is usually lower than that of high porosity. In many low porosity and low permeability reservoirs, partition is needed basing on the relationship of F and porosity, therefore, a and m is selected respectively. Resistivity log curve is mostly effected by rock pore structure and shale. As a result, it lays the foundation for proposing satuation model which is suitable for low porosity and low permeability reservoir.In the paper, the Southern of SongLiao basin BaWu-SiWuJiaZi is study area. The reservoir of the area is low porosity and low permeability reservoir. The resistivity of oil layer changes in the scope of great value. Reservoir of the study area can't be evaluated correctly with Archie equation and identical rock electrical parameters. Through study the characteristics of petrophysical analysis data of 81 rock samples, sumed up two main reasons which affect low porosity and low permeability rock's resistivity value. First, the extra conductivity of shale itself. Second, because of fine rock particles, poor separating and high clay content. However, the increased clay content in the process will cause all reservoir rock particles structure to become fine, and pore structure also tends to be complicated. The result is equivalent to the average conductive cross-section becomes smaller and conductive length becomes longer, which causes resistivity of clay sandstone to be increased. Therefore, in the study area, extra conductive of shale itself and increased resistivity caused by the change of the pore structure are at least two coexist contrary factors.In the paper, the above two contrary factors of shale effect the rock resistivity are considered. Based on the analysis data of samples, satuation model which is suitable for low porosity and low permeability reservoir is proposed. Model displayed the three affecting factors of resistivity value. First, Archie equation. Second, the extra conductivity of shale itself. Third, the increased clay content in the process will cause pore structure tends to be complicated, the result is equivalent to the average conductive cross-section becomes smaller and conductive length becomes longer, which causes resistivity of clay sandstone to be increased.The proposed model can describe well the effects from the change of the pore structure and extra conductive of shale for resistivity. Thus the relative contribution of pore fluid value is highlighted and saturation can be calculated more accurately.In this paper, the log evaluation methods of low porosity and low permeability are established and the processing software is developed. Comparing the results of the model proposed in the paper and other general models, the result calculated by the model proposed in the paper mactched best with the Ro measured in laboratory, therefore, the model proposed in the paper can better reflect the conductivity of rock.The model is applied in the study area and 9 wells are handled actually, the interpretation results can reflect the test results and sealed coring results well. Therefore, the model proposed in the paper can calculate the water satuation of low porosity and low permeability reservoir well.