5自然电位测井

1、Self-Potential or SP LogsBy Zhang Qingguo5 Self-Potential or SP Logs5.1 GeneralitiesThe logThe SP log is a measurement of the natural potential differences or self-potentials between an electrode in the borehole and a reference electrode at the surface.The currents were actually called spontaneous p

2、otentials, by Conrad Schlumberger and H.G. Doll who discovered them.5 Self-Potential or SP Logs5.1 GeneralitiesPrincipal usesThe principal uses of the SP log: To calculate formation-water resistivity(Rw)；To indicate permeability；To estimate shale volume；To indicate facies；in some cases, for correlat

3、ion 5 Self-Potential or SP Logs5.2 Principles of measurementThree factors are necessary to provoke an SP current: a conductive fluid in the borehole; a porous and permeable bed surrounded by an impermeable formation; a difference in salinity slnt (or pressure) between the borehole fluid and the form

4、ation fluid. In oilfield wells, the two fluids concerned are the mud filtrate and (usually), formation water.扩散电位(Diffusion Potential)扩散吸附电位（Shale Potential）The Cl- ion is both smaller and more mobile than the larger, slower Na+ ion. Clay minerals which form shales, consist of layers with large nega

5、tive surface charge. Because of charge similarity, the negative chloride ions effectively cannot pass through the negatively charged shale layers, while the positive sodium ions pass easily. The shale acts as a selective barriermembrenThe actual spontaneous potential currents which are measured in t

6、he borehole are, for the most part, a result of the combination of the two electrochemical effects described above.1、diffusion potential (or liquid junction potential),17%2、shale potential,83%5.2 Principles of measurementLog presentation: units and scalesSP currents are measured in millivolts (110-3

7、 volts) and the scale is in + or - millivolts, negative deflections to the left, positive to the right . The log is usually run in track 1 with a gamma ray or caliper log.5 Self-Potential or SP Logs5.2 Principles of measurementUnwanted logging effectsAs indicated above, for the SP tool to work effec

8、tively, it must be connected to a surface earth. For onland wells this causes no problem and an iron probe can be pushed directly into the soil. Offshore, however, no such possibility exists. Without an effective earth, the SP will not be recorded. The SP from many offshore wells, especially from fl

9、oating rigs, is useless and mostly ignored. This is a pity. The SP is a cheap and useful log.5 Self-Potential or SP Logs5.3 Log characteristics Bed boundary definition and bed resolutionThe sharpness of a boundary will depend on the shape and extent of the SP current patterns. Generally when there i

10、s considerable difference between mud and formation resistivity, currents will be spread widely and the SP will deflect slowly: definition will be poor. The contrary also applies. When resistivities are similar, boundaries are sharper. In general, boundaries should not be drawn using the SP.5 Self-P

11、otential or SP Logs5.3 Log characteristics Bed boundary definition and bed resolutionFigure 5.9 Some conditions causing aberrant SP values when the SSP is not attained.SP bed resolution is also poor. For a full SP deflection(i.e. SSP or static SP, see Section 5.4) and proper bed resolution, as a rul

12、e of thumb一般来说, a bed should be thicker than 20 times the borehole diameter (Ellis, 1987). The exact minimum SP bed resolution will obviously depend on depth of invasion and salinity differences, in the same way as described above for bed boundary definition.5 Self-Potential or SP Logs5.3 Log charac

13、teristics Bed boundary definition and bed resolutionThe definition of the SP zero is made on thick shale intervals where the SP does not move: it is called the shale base line. All values are related to this line.The theoretical maximum deflection of the SP opposite permeable beds is called the stat

14、ic SP or SSP. It is the maximum possible SP opposite a permeable, water-bearing formation with no shale . It is only the log-derived SSP that can be used for the quantitative evaluation of Rw (the formation-water resistivity).5 Self-Potential or SP Logs5.4 Quantitative usesMethodologySP values for c

15、alculation - shale baseline and static SPthe shale base linethe sp zerothe static SP or SSPmaximum deflection of the SP5 Self-Potential or SP LogsFrequently the SP does not show its full deflection, for a number of reasons: the bed is not thick enough, there is shale in the formation, the invasion i

16、s very deep, there are adverse lithological effects (junction beds with highresistivity) or hydrocarbons are present .These conditions must be considered When taking SP values for calculation.Qualitatively, the greater the SP deflection, the greater the salinity contrast between the mud filtrate and

17、 the formation water. Generally these will be negative deflections, the formation waters being more saline than the mud filtrate. 5.4 Quantitative usesWater resistivity (Rw), quick look and calculationsDeflections to positive values however, occur with fresh formation waters, or at least those fresh

18、er than the mud filtrate . 5.4 Quantitative usesWater resistivity (Rw), quick look and calculations3、成像测井沉积相模式-辫状河成像测井沉积相典型模式分析 成像测井资料沉积学研究成像测井沉积模式辫状河由心滩和泛滥平原组成，发育细砾岩、含砾中细砂岩、中粗砂岩、粉砂质泥岩及泥岩等。垂向上由多个正韵律叠置而成，砂岩中发育平行层理、槽状交错层理、板状交错层理等。FMI模式图为亮斑模式与暗色块状模式的交替组合，动态图像上为一组不同角度的正弦曲线，上面发育砂砾岩的亮斑模式。PL19-3-8ST（1：200）

19、小冲刷面小冲刷面3、成像测井沉积相模式曲流河成像测井沉积相典型模式分析 成像测井资料沉积学研究成像测井沉积模式曲流河二元结构，砂体倾角逐渐变小呈蓝模式，指示古水流方向。5 Self-Potential or SP Logs5.4 Quantitative usesWater resistivity (Rw), quick look and calculations5 Self-Potential or SP Logs5.4 Quantitative usesWater resistivity (Rw), quick look and calculationsThe preceding meth

20、od allows an approximation of theresistivity of formation water.calcium and magnesium as well as sodium ions are pre-sent. The effects of calcium and magnesium are especially important at high resistivities in fresh waters. Corrections must be made to the formulae shown above.5 Self-Potential or SP

21、Logs5.4 Quantitative usesShale volume from SP (Pseudo-static SP)This simply assumes that the SP deflection between the shale base line (100% shale) and the static SP in a clean sand (0% shale) is proportional to the shale volume. This relationship is certainly true qualitatively, but quantitatively

22、there is no theoretical basis. The SP-derived Vsh is probably over-estimated.5 Self-Potential or SP Logs5.4 Quantitative usesShale volume from SP (Pseudo-static SP)5 Self-Potential or SP Logs5.5 Quanlitative usesPermeability recognitionIf there is even a slight deflection on the SP, the bed opposite

23、 the deflection is permeable. All deflections (with some rare exceptions) on the SP indicate, the priori, a permeable bed. 5 Self-Potential or SP Logs5.5 Quanlitative usesPermeability recognitionThe amount of deflection, however, does not indicate the amount of permeability: a very slightly permeabl

24、e bed will give the same value as a permeable bed (other values being equal). Naturally, the reverse is not true; not all permeable beds give an SP deflection, although these cases are rare .5 Self-Potential or SP Logs5.5 Quanlitative usesMineral identificationPyriteparat is possible that the SP rea

25、cts to excessively reduced and excessively oxidized beds (shales or sandstones) which are not in subsurface electrical equilibrium (Hallenburg, 1978).However, coals, which are extremely reduced, give alarge negative SP deflection or no deflection at all, although the reasons are obscure. shaleLignit

26、e褐煤Black shaleSand with disseminated Pyrite黄铁矿 Rhyolite ralat（流纹岩）Diabase dabes（辉绿岩）5 Self-Potential or SP Logs5.5 Quanlitative usesFaciesThe log shape, in sand-shale sequences, is related to shale abundance, the full SP occurring over clean intervals, a diminished SP over shaly zones. In so far as

27、shaliness is related to grain size, the SP is a good facies indicator. 5 Self-Potential or SP Logs5.5 Quanlitative usesFaciesThe SP has now been largely replaced by the gamma ray log for facies identification: the gamma ray log has more character and is more repeatable.5 Self-Potential or SP Logs5.5

28、 Quanlitative usesFaciesa well-marked channel sand; the coarse-grained base is clean while the finer-grained top is shaly. The SP is, therefore, following grain-size change .5 Self-Potential or SP Logs5.5 Quanlitative usesCorrelationPreviously, the SP log was one of those used for correlation but, for the reasons given above, has now been replaced especially by the gamma ray log. The SP is still useful for correlation, however, in areas of varied water salinities. The end.喀纳斯 Kanas Lake是中国新疆阿勒泰地区布尔津县北部一著名淡水湖，面积45.75平方公里，平均水深120米，最深处达到188.5米，蓄水量达53.8亿立方米。外形呈月牙，被推测为古冰川强烈运动