1、 With electrical logging, parameters measured in situ by appropriate bottom hole instruments were continuously recorded at the surfaceMeasurements were performed only in the uncased portions of the boreholes It became a general practice, when a hole was drilled or at intervals during the drilling，to
2、 run an electrical survey for the purpose of quickly securing a complete record of the formations penetrated. This recording was of immediate value for the geologica1 correlation of the strata and also for the, detection and evaluation of possible productive horizons. The information obtained from t
3、he electrical log was sometimes supplemented by sidewall samples of the formation or by the types of investigations performed in boreholes deviation surveys, dipmeters surveys temperature surveys, etcSeveral different kinds of resistivity curves or logs, in addition to the SP curves, were recorded i
4、n the boreholes. These logs, obtained by using different resistivity measuring devices, are called conventional electrical logs（normal and lateral）, Laterolog, Microlog, etc. A typical electrical log recorded with the conventional devices is illustrated in Fig4The left track of the log contains the
5、SP curve，and the right tracks contain the resistivity curvesEach curves shows a sequence of peaks, deflections, and flat zones corresponding to the different formations traversed by the bore-hole. As will be seen in the following chapters, logs recorded with other types of resistivity measuring devi
6、ces have a similar general appearance, although the corresponding devices differ basically in their principles and in their performancesRESISTIVITY METHODS Resistivity methods are numerous, and it is convenient to classify them according to the corresponding types of subsurface measuring devices; ie
7、., macrodevices or microdevices Macrodevices The macrodevices investigate rather large volumes of material around the borehole from about 10 to 100 cu ft and moreTheir measurements are used for definition of formation for correlations，and for qualitative and quantitative analyses of reservoirs in te
8、rms of fluidsaturation and porosityThis group includes the conventional resistivity devices and the resistivity devices with focusing systems When Using the conventional electrical log（normal and lateral），the path of the current used for the measurement has no restriction other than the location of
9、the electrodes between which the current flowsAccordingly，the measurements are affected by the bed located at the level of the device, the mud column，and，if the bed is thin, the formations above and below itThe relative effect of the mud column and of the adjacent formations depends upon the physica
10、l configuration of the electrode system and the geometry and the respective resistivities of the media involved in the measurementsIn formations of moderate resistivities, such as sands and shales, this effect is not too important the conventional devices are generally suitable for a correct investi
11、gation except when the beds are very thinThe results are usually not so good in hard-rock countries where the resistivity contrasts between different beds and between the formations and the mud are high. The devices that have focusing systems, such as the Laterolog, force the current to flow within
12、a horizontal slice of formation of limited thickness. As a result, the effects of the mud column and of the formations above and below the limits of the horizontal slice are appreciably minimized and maybe negligible in many cases. The focusing devices are better for investigating thin beds; the Lat
13、erolog is chiefly appropriate to hard formations, particularly in high-salinity muds. MicrodeviesThese pad-type devise investigate a very small volume of material （a few cubic inches） behind and closed to the wall of the borehole, and the effect of the mud column on the measurement is practically el
14、iminated. The delineation of the beds is, accordingly, much more accurate and more detailed than with the macrodevices, even those using focusing systems. With the MicroLog system, the measurements are quite affected by the mudcakes, which makes it possible to detect permeable beds and define their
15、boundaries exactly.The Microlaterolog tool has a focusing system that minimizes the effect of the mudcake, or even eliminates it when the mudcakes are thin enough. Under these conditions the measurements give values close to the resistivities of the formations flushed by mud filtrate behind the wall
16、 of the hole. This provides an approach to determine the formation factor and, therefore, porosity.When the porosity of the formation is sufficiently high, the formation factor and the porosity can also be determined by using MicroLog cures and appropriate correction charts.FORMATION RESISTIVITIESTh
17、e resistivities of the formations arc important clues to their probable lithology and fluid contentEarth formations conduct electric current only by means of mineralized water they containThe minerals that constitute the solid parts of the strata are insulators when absolutely dryIn a similarmanner，
18、any pure oil or gas in the formations is electrically nonconductiveThe few exceptions to this are metallic sulfides, graphite, etc., which conduct electricity like metals Formations of lowest resistivity have increases in both amount of water and its mineralization. Other important factor in formati
19、on resistivity are the shape and the interconnection of the pore spaces occupied by the water. These depend primarily on the lithology of the formation.Unit of ResistivityLaterologs and electrical logs record the formation resistivity, and the unit is the ohm-meter（ohm-m）. When a uniform electrical
20、current is sent through a 1-m cube in a direction parallel to any edge, the resistance in ohms is equal to the resistivity of the substance. The same unit is also commonly designated ohm-meter squared per meter（ohm-m2/m）, which gives a good practical range of numerical values for the usual resistivi
21、ties encountered in the earth, from 1 ohm-m or less to several thousand.Relation of Water Resistivity to Salinity and TemperatureThe resistivity of a formation depends upon the resistivity of the water contained in its pores, which, in turn, is a function of the concentration of a formation water qu
22、ite often has to be deduced from a knowledge of its salt content. Also, most interpretations involve comparison of different resistivity measurements, such as the resistivities of the drilling mud and of the formation water. In making such determinations, the following laws of electrolytic conductan
23、ce should be kept in mind.（1）The conductivity of an electrolyte increases in proportion to the amount of chemicals in solution; the resistivity decreases with increasing salinity. Formation waters and drilling muds usually contain several chemicals in solution that differ in their ability to conduct
24、 electrical current. Since the predominant chemical is sodium chloride （NaCl）, the resistivity graph for NaCl solutions （Fig.5） can be used convert salinity into resistivity. If the water contains large amounts of other chemicals, the amounts of the various electrolytes present must be converted to
25、their equivalent weights of NaCl. The resulting equivalent concentration is expressed in parts per million or grains per gallon of NaCl.（2）The resistivity of an electrolyte decreases as its temperature increase. This is of great importance in electrical logging because the temperature in the earth i
26、ncrease with depth. It is frequently necessary to compare the resistivity of a drilling mud measured at surface temperature to that of a formation measured at a much higher temperature in a deep borehole. The two values of resistivity can be compared only after they have been converted to values tha
27、t would have been observed at a common temperature.This conversion can be made by using the chart in Fig.5. For example, if the drilling mud had a resistivity of 1 ohm-m at F, the chart gives its corrected resistivity as 0.5 ohm-m at F.It is common practice in wireline logging to measure the bottom
28、hole temperature （BHT） with a maximum thermometer contained in the body of the sonde. （The value read on the thermometer gives the maximum temperature of the mud, which id generally at the bottom of the hole.） In case a thermometer is not available, the BHT can approximated by using one of the two f
29、ollowing equationsThese were established empirically from the average of numerous measurements made in various geological areas.For coastal belts, such as Louisiana，Texas Gulf Coast, and California, （2-1）Where T = temperature in degrees FahrenheitAnd D = depth in feet.For the inland area, such as mi
30、d-continent, Rocky Mountains, west Texas, Mississippi, and Illinois, （2-2） Relation of Formation Resistivity To LithologyThe ability of an earth formation to conduct current is directly affected by the amount of water in the pores：i.e，by the porosity of the formationIf the voids inside the formation
31、 were parallel cylindrical channels, the resistivity of the formation would be inversely proportional to the porosityBecause of the relative sizes and shapes of the mineral grains and the presence of cementing materia1, an electrical current crossing the formation has to flow along multiple tortuous paths. The cross sections of these paths vary rapidly form comparative large values in the pores to very small values in the intervals linking the pores. The formati
