Applied The Finite Difference Method In SP Log

When a single electrode is moving along the borehole wall,we can record the natural potential. After a long period of test, People find that this phenomenon is not an accident, so people call it natural potential. Natural potential logging can be used to judge the lithology, determine the formation water resistivity, divide the points permeablestratum, etc.Natural potential logging measurement'results are often qualitative, More and more people begin to pay close attention to quantitative calculation of natural potential logging.In this article, we use finite difference method simulate the natural potential.Through discretization of partial differential equations, we get a large-scale linear algebra equations. This paper focuses on the discretization of two-dimensional poisson equations and with the three layers of classical model of strata we finally use MATLAB language deal with the linear equations.The natural potential curve has something to do with the natural emf and natural current. Natural emf and natural electric currentare can be affected by some external factors, such as thickness of strata, mud resistivity, rock resistivity, diameter etc. This paper mainly simulate the effect of these factors to natural potential curve and the shape of natural potential curve and the amplitude of the features are summarized. Through the analysis of the test, we konw that natural potential curve is symmetrical in the middle strata and it does not change with the environmental impact. At the interface, natural potential curveis are largely abnormal. And natural potential curve and performance are gentle far from the interface. Taking the mudstone as the baseline ,When C w > Cmf, natural potential will show negative abnormal; When C w < Cmf, natural potential is abnormal in sandstone layer; When C w = Cmf, no emf,no natural potential abnormity. Along with the increase of temperature, amplitude of natural potential curve will increase. The relation between the growth of the temperature and the changes of natural potential amplitude is similar to linear relationship. When the rock water is rich in salt and the mud salinity is lighter, the natural potential value of sandstone layer is negative. When the sandstone layer water is poor in salt, the natural potential of sandstone layer is positive; When the local layer is thick, natural potential amplitude is close to static natural potential. The turning point of natural potential curve is at the formation boundary. When no mud intrusing, natural potential curve is symmetrical about the midpoint. In the interface, the amplitude variation of the curve is intense and with the increase of the depth of invasion, natural potential amplitude is lower and lower. When local layer containing water flooded layer, natural potential curve will change greatly. When the other external conditions are same, we inject low salinity water into the formation rich in original water,the amplitude of natural potential curve will increase and with the degree of water flooded increasing, it increase.When we inject high salinity formation water into the formation low in orginal water, amplitude of natural potential curve will decrease and with the increase of water flooding, it is lower and lower. Water flooded side'baseline will swept when the top or the bottom of the formation is water flooded. When the injected water'salinity is lower than primary reservoir water'salinity, baseline will swept to the negative direction; When the injected water'salinity is lower than primary reservoir water'salinity, baseline will swept to the positive direction. With the degree of water flooding increasing, the extent of the baseline offset is greater.