True Formation Resistivity Research Articles

A Review of Current Techniques for Determination of Water Saturation From Logs

Abstract The basic saturation and log response equations are reviewed. It is concluded that conventional saturation calculations account for lithology and rock-type changes, but that they are susceptible to uncertainties in water resistivity (Rw), true resistivity, porosity and cementation factor (m). It is shown that resistivity-apparent porosity plots are useful in wells with minimum petrophysical data. Knowledge of Rw, m, the slope of the sonic log-porosity relation or the slope and intercept of the neutron log-porosity relation are not necessary, provided they are constant. Advantages and limitations are illustrated with examples. It is concluded that Rwa -depth plots are useful where Rw is unknown and lithology varies, provided m is known for all lithologies involved. Ros -So relations may be useful for determining water saturation when only a few porous intervals of constant rock type are present, and when either Rw or formation factor (but not both) are unknown. Finally, an example is reviewed to illustrate that, often, no one of the above techniques by itself may be diagnostic, and also to emphasize the need to utilize all available data. Introduction The determination of fluid saturations is still one of the prime functions of the petrophysical engineer. Although this problem has been continuously faced in day-to-day evaluation work since the advent of petrophysics, it still presents technically challenging problems. It is realized that hydrocarbon saturation is the quantity of real interest. However, with few exceptions, the problem resolves into determination of water saturation as defined by the following relationships: ....................(1) ...................(2) ....................(3) where = the fractional part of the pore volume filled with water of resistivity Rw = resistivity index = saturation exponent = true formation resistivity = formation resistivity factor* = fractional porosity = cementation exponent. Historically, the approach to this problem has been to determine resistivity index 1 from borehole measurements, and from 1 to calculate Sw using either an assumed value for n or one established from laboratory experiments. A discussion of the validity of laboratory determined values of n is beyond the scope of this paper. It will be assumed that the appropriate value for n is known, and this paper will discuss recent experience with the following techniques for determining 1:conventional saturation calculations,Ra vs plots,Rwa plots, andSo vs Ros relations. Conventional Saturation Calculations The time-honored process for making water saturation calculations involves the following steps:porosity is obtained from a core or a porosity log (sonic, neutron or density log);formation factor is calculated from Eq. 3 using an estimated m or one obtained from laboratory measurements or from resistivity measurements in 100 per cent water-bearing intervals;1 is calculated from Eq. 2 using a true resistivity Rt obtained from an appropriate resistivity device F, as calculated from Eq. 3 and an estimated Rw or one obtained from a water recovery in a nearby zone or another well, or one calculated from the SP log; andSw is calculated from Eq. 1 using the 1 calculated from Eq. 2 and n. This technique has the advantages of being well established and, therefore, relatively easily discussed with management and other log analysts. JPT P. 1425ˆ

A Method of Analyzing Electrical Logs Recorded on a Logarithmic Scale

Abstract A minimum of three resistivity measurements are necessary to estimate the influence of invasion and thereby assure an adequate determination of Rt. This requirement is fulfilled by the Dual Induction-Laterolog, the first modern logging tool to combine three focused resistivity devices. The three resistivity curves are recorded simultaneously on a logarithmic resistivity scale. Proportional dividers can be used to obtain ratios and products of the resistivity measurements by treating the logarithmic scale in a manner similar to the principle of the slide rule. This leads to a very quick, easy and mechanical method of determining Di, Rt, Rxo. and estimates of Sw. These values are obtained as plots directly on the log, thus providing a permanent record that is an integral part of the log itself. The method significantly reduces interpretation time by eliminating the necessities of making calculations and reading scales. Also, the accuracy of the log is more nearly maintained, particularly since interpolation is mechanical rather than interpretive. Thin-bed corrections and borehole-enlargement corrections are also greatly simplified. It is believed that the use of the proportional divider techniques with logarithmic resistivity scales will significantly improve the otherwise time-consuming task of log analysis. Introduction To determine true formation resistivity has always been a fundamental problem in quantitative log analysis. Modern induction logs have simplified this problem for cases of moderate invasion so that a value adequately close to R is often recorded directly. However, the problem remains that one cannot be sure that the recorded resistivity is adequately close to Rt, and too often the analyst fails to recognize when it is not. The Dual Induction-Laterolog is an important step forward in obtaining adequate values of Rt for invaded formations. This logging sonde includes three focused resistivity devices:a Laterolog (LLs) which has a comparatively shallow depth of investigation (resistivity designated RLLs);an induction log (ILm) which has a moderate depth of investigation (resistivity designated RILm); andan induction log (ILd) which has a depth of investigation about twice as large as ILm (resistivity designated RILd). Resistivities from these three devices are recorded simultaneously on a logarithmic scale. In effect, the Dual Induction-Laterolog provides three "equations", each containing the three variables Di, Rxo and Rt. This assumes a simple frontal invasion pattern; that is, a permeable formation with a homogeneous invaded zone of resistivity Rxo extending to a diameter Di, beyond which lies a homogeneous zone of true formation resistivity Rt. A simultaneous solution of the three "equations" to obtain Di, Rxo and R is shown as Fig 1. A fullscale reproduction of Fig. 1 is shown in the Appendix. The purpose of the paper is to introduce a simple, quick and accurate method of mechanically determining Di, Rxo, and Rt, and of estimating water saturation. With this method, the time-consuming necessities of mathematical manipulation and the reading and interpolation of numerical values are essentially eliminated. A systematic procedure of log analysis develops from this method. The proposed method utilizes the logarithmic resistivity scale from which ratios of resistivity values can be extracted in a manner similar to the principle of the slide rule. That is, the distance between two resistivity curves represents the logarithm of the ratio of the two resistivities, inasmuch as the logarithm of a ratio is expressed by subtracting the logarithm of the denominator from the logarithm of the numerator. This distance can be extracted with a pair of proportional dividers. Similarly, the proportional dividers can be used to enter this ratio into the logarithmic scale of an interpretation chart. With this idea as a beginning, the technique can be expanded into a systematic method of determining Di, Rxo, Rt and Rxo/Rt. Continuous plots of Rt and Rxo/Rt can be quickly made on the log. Estimates of water saturation Sw can be made directly from the Rxo/Rt plot and, in this manner, possible oil-bearing zones can be isolated. With the Rt plot and a porosity log, the value of Sw can be verified. JPT P. 844^

Improved Interpretation of Formation Productivity by Combined Use of Core Analysis and Electric Log Data: ABSTRACT

ABSTRACT Reservoir properties, such as permeability, porosity, residual fluid saturations, and capillary pressure characteristics are obtained by direct measurement of core samples. Electric logging devices provide information for computing approximate values for the formation factor or porosity, formation water resistivity, and formation water saturation. In the past, these two formation evaluation tools have been used more or less independently of each other. This paper presents a method of combining core analysis and electric log information to obtain more reliable interpretations of fluid productivity. The porosity and the immobile interstitial water saturation derived from core analysis may be used to compute a value which is called productive resistivity, or (Rp). Productive resistivity is defined to be the resistivity of a hydrocarbon-productive formation with an interstitial water saturation equal to or less than the immobile value. The deep investigation electric logging devices, such as the induction log, approach in measurement the true resistivity or (Rt), from which the formation water saturation can be calculated. If the true formation resistivity measured by the electric or induction log is less than the calculated productive resistivity, it may be concluded that or induction log is less than the calculated productive resistivity, it may be concluded that the actual water saturation is greater than the immobile interstitial water, and the formation interval may be either partially or wholly water-productive. On the other hand, if the measured resistivity equals or exceeds the productive resistivity calculated from core analysis, it may be concluded that the actual formation water saturation is no greater than the immobile interstitial water and the sand is probably wholly hydrocarbon-productive. A similar calculation may be made using a higher water saturation value taken from the transitional zone to determine a value called the minimum productive resistivity, or Rmp.

The Use of Differential Sonic- Resistivity Plots to Find Movable Oil in Permian Formations

Abstract In recent years, the plotting of sonic transit time vs the resistivity recorded by a deep-investigation device has become one of the most popular methods of interpreting electrical logs run in wells drilled with salt mud. This paper reviews the reasons why this method is valuable. It also discusses pitfalls in the method, which may be overlooked because of its very convenience. A possible source of error emphasized is the common assumption that all zones analyzed are affected in the same degree by mud-filtrate invasion. The use of a second "sonic-resistivity plot", this time with a shallow-investigation resistivity device, is presented as a way of avoiding entrapment by these pitfalls. The difference between the water saturations found from this second plot and from the standard plot can be used, according to rules-of-thumb to be stated, t9 estimate D, for the purpose of correcting the Laterolog reading for invasion. A sonic-resistivity plot with the corrected Laterolog readings will give better values of saturation, and the differences between these and the corresponding saturation values from the second plot are indicative of oil mobility. Several examples of Permian formations of West Texas, combined into a composite log, are computed by both the standard sonic-resistivity plot and by the more comprehensive "differential sonic-resistivity plots". The results of these computations are compared with the production of the formations to show how the additional information can improve the log interpretation. Introduction In attempting to decide where and whether his well will produce oil or gas, the oil operator needs to know several things, the most important of which is the water saturation Sw. Since Sw cannot be measured directly in situ, it is usually inferred from measurements of the porosity f and the true formation resistivity Rt. If the formation water resistivity Rw is known, it is possible to determine Sw from these indirect measurements by the use of Archie's fundamental equation.1 Equation 1 F usually is computed from the relationship F = f2, and Archie's saturation equation, therefore, may be written. Equation 2 This equation may be readily solved arithmetically, but a more adroit method consists of a graphical solution in which the readings from a porosity log are plotted vs the corresponding resistivity readings from a deep-investigation device.3 The possibility of such a graphical solution becomes apparent by rewriting Eq. 2 in the form Equation 3 According to Eq. 3, a plot of f in abscissa vs 1/Rt½ in ordinate will be a straight line, for constant values of Rw and Sw. Shortly after the introduction of the Sonic log, Wyllie, et al, proposed the time-average relationship.3 Equation 4 which can also be written as Equation 5 where ?t = the sonic transit time, Vm = the sonic velocity in the matrix material, and Vf = the sonic velocity in the formation fluid. This equation states that sonic transit time in excess of 1/Vm is proportional to porosity. Combining Eqs. 3 and 5, Equation 6 When values of ?t are plotted in abscissa vs values of 1/Rt½ in ordinate, Eq. 6 yields a straight line provided the term Sw·Vm·Vf/Rw½ (Vm-Vt) is held constant.

New Developments in Induction and Sonic Logging

Abstract In the combination induction-electrical log used at present in the field, the induction logging tool is appropriate for the investigation of moderately invaded formations. A new induction sonde with a radius and investigation about twice as large has been developed recently for the case of deep invasion. It has very nearly the same vertical resolution as the present sonde so that thin beds are defined as accurately as before. The characteristics of the new tool are described, the corresponding interpretation charts are given and field examples are discussed. The design of the sonic logging tool has been modified to improve the calibration and the reliability. The fact that porosity can be accurately recorded by means of the sonic log has prompted new interpretation. procedures for saturation estimation, wherein the data concerning the various permeable beds in a given well are correlated. One approach consists of plotting transit time vs true resistivity, with an appropriate scale. With this approach, saturations can be estimated conveniently even in cases where formation water resistivity is not well known. In another approach, a comparison is made of the values of the formation waters computed from the resistivity and sonic logs. Using the concept of continuity, this procedure makes possible a quick determination of zones of saturation in shaly sands and/or in case of appreciable variations of formation salinities with depth. It has been found that the comparison of porosity from the sonic log with the apparent porosity computed from a short-investigation resistivity log may reveal, in many cases, the presence of residual oil and thus detect potentially productive formations; this procedure is valuable when the true formation resistivity and the resistivity of the formation water are in doubt. Introduction During the past year, the efficiency of log interpretation has been vastly improved. The improvements have largely resulted from the introduction of a deep-investigation induction device and from the application of new interpretation techniques that utilize sonic vs resistivity readings. Since the new interpretation techniques depend, in part, upon good values of true formation resistivity, the new induction log will be discussed under Part I. The sonic interpretation techniques will be studied under Part II.

An Improved Interpretation Method for Salt Mud Logging

Abstract The increased usage of the FoRxo, Guard and gamma combination of logs in salty muds has led to the development of interpretation techniques which are fast and simple. This is possible because:The effects of bed thickness on both FoRxo and Guard logs are minimal and in moot cases may be neglected.With salt muds, true formation resistivity, Rt, is linearly and directly proportional to recorded Guard log resistivities over a wide range of conditions.Likewise, invaded zone resistivity, Rxo is approximately equal to recorded FoRxo resistivities. This direct reading of these two vital parameters makes possible a fast but accurate visual location of all zones of interest. The same characteristics of the logs also make possible simplified interpretation curves to calculate both porosity and wager saturation. Inasmuch as shaly sands are best measured without the disturbing influence of fresh waters, their interpretation is greatly enhanced by this system of logs. For this technique to be used, the following conditions must be fulfilled:The zones of interest must be invaded by mud filtrate.The ratio of resistivities of mud filtrate to connate water must be known and the ratio must not exceed 10.Mud filtrate resistivity must be known.Mud cake thickness must be small. Field examples are given which exhibit the inherent features as well as the disadvantages of the proposed techniques. Simplified charts for interpretative procedures are also presented. In conclusion, the FoRxo, Guard and gamma logs provide a detailed log of the lithology from which porosity and water saturation may be calculated in detail by a simple interpretation method when certain conditions exist. The logging system also provides an improved method of interpreting shaly sands when both formation water and mud filtrate are highly conductive.

New Log Interpretation Techniques for Gulf Coast: ABSTRACT

The fact that porosity can be measured accurately with the sonic log has prompted new procedures for estimating saturation, wherein data concerning the various permeable beds in a given well are compared. 1. In one approach a comparison is made of the values of the formation water resistivity computed from the resistivity log and from the sonic log. Actually, apparent formation water resistivities are calculated in assuming that all sands are wet. With the concept of continuity, this procedure makes possible a quick determination of zones of saturation in shaly sand and in cases where there are appreciable variations in formation water salinity with depth. 2. It has been found also that comparison of the apparent formation factor obtained from the sonic log with that computed from a short investigation resistivity log may reveal in many cases the presence of residual oil or gas and thus detect potentially productive formations. This procedure is valuable when true formation resistivity and the resistivity of the formation water are in doubt. Although these two procedures permit only a qualitative interpretation of the log, they have the advantage of speed and simplicity. The quantitative interpretation that remains to be done can be performed very quickly since all the non-productive formations have been eliminated by the foregoing procedures. The first procedure is best adapted to formations of high porosity and in fresh mud. The second procedure works best in formations of low porosity and with little contrast between the mud and formation water resistivities. Examples illustrate the application of these two techniques in the Miocene, Frio, and Wilcox sections of the Gulf Coast. End_of_Article - Last_Page 2517------------

ELECTRIC ANALOGUE OF RESISTIVITY LOGGING

The electric analogue device described below was designed to determine the relationship between the true resistivity of a ground formation and the apparent resistivities obtained by various electrode arrangements. This analogue is a network model which simulates properties of the formations and other pertinent variables encountered in actual logging.

Resistivity Logging in Thin Beds

Published in Petroleum Transactions, AIME, Volume 201, 1954, pages 155–161. Abstract Conventional resistivity logs consisting of a short normal, a long normal, and one or more long lateral curves do not give data that allow a complete quantitative interpretation in beds thinner than 20 ft. Reservoir rocks usually exhibit zones of continuous homogeneity of quite limited thickness where the long lateral curves become useless because of adjacent bed effects and boundary phenomena. If the beds are 12 ft or thicker, the short and long normals may be used for qualitative interpretation, which can be streamlined by the application of simplified departure curves. For beds of a thickness less than twice the long normal spacing, this procedure breaks down. The combination of the limestone curve, the laterolog or guard electrode log, and the microlaterolog permit quantitative interpretation for beds that are at least 10 ft thick, provided the mud resistivity and the hole diameter are known with sufficient accuracy. For beds thinner than 10 ft, combinations of the microlaterolog with short spaced laterologs and pseudo laterologs appear to be promising. Interpretation of these curves again requires the application of simplified departure curves. Resolution of various possible combinations was analyzed using departure curve data calculated on the Whirlwind I computer at the Massachusetts Institute of Technology. A field example is shown using the microlaterolog-microlog combination, and the combination of a 6-in. modified laterolog plus a 6-in. pseudo laterolog. Introduction For the purpose of quantitative interpretation of resistivity logs in porous formations, we want to obtain two essential quantities from the logs, namely, the true resistivity of the undisturbed formation, and the resistivity of the part of the formation invaded by mud filtrate. The apparent resistivities of all conventional logging devices are functions of these two parameters and are also influenced by a third unknown parameter, the diameter of the invaded zone.

A Contribution to Electrical Log Interpretation in Shaly Sands

Abstract Simple qualitative methods are explained for identifying those shaly sands in a well that are most likely to contain oil. A need for more precise measurement of the variables that enter shaly sand analysis is indicated. Field examples are given to illustrate the methods. A theoretical discussion of the quantitative interpretation of shaly sands is given as a basis for discussion and as a guide for the future. While not generally capable of practical application at the present time because of the lack of sufficient accuracy of the electric log data, these methods may become more feasible in the future as the result of the improved logging methods now being introduced. Introduction Experience has shown that for porous formations containing only a negligible amount of clayey material, reliable information on the fluid saturation and porosity of the reservoir rocks can usually be derived from the electrical logs. The interpretation is based on empirical formulae relating the true resistivity of a porous formation to its lithologic character, to the resistivity of the interstitial water, and to the proportions of water and hydrocarbons in the pores. If the resistivity of the interstitial water is not known, its approximate value can be derived from the SP curve. The porosity can usually be determined to a good approximation from a MicroLog, or a MicroLaterolog. When the reservoir rocks contain an appreciable percentage of clayey material, an additional factor is introduced into the analysis. In a clean formation, the matrix is an electrical insulator, so that the ability of the formation to conduct current is due only to the conductivity of the electrolytes in the pores; in a shaly formation, the shale constitutes a part of the rock matrix able to conduct current, and influences the resistivity of the formation.

The Guard Electrode Logging System

Abstract The guard electrode system measures the resistivity of formations by employing a thin disk of current which is caused to flow perpendicular to the bore hole. The control of this current disk is obtained in a brute force manner through the use of relatively long equipotential electrodes above and below the measuring electrode. The log obtained from the system is free of "lag," "plateau," "shadow," "reflection" and other distortions evident on conventional logs. The proportionate contributions of the hole size and invaded zone resistivity to the apparent resistivity reading are reduced when the mud and filtrate resistivities are lower. Thus, the guard system favors the use of conductive muds. The mathematical development of the theory is given in an appendix. Equations are derived to evaluate the effects of mud resistivity, hole size, invaded zone resistivity and depth, and the true formation resistivity. Sample logs from several provinces are reproduced and compared with conventional electric logs to illustrate the much greater detailed lithology that can be presented by the guard electrode system. Introduction With increased ability of those in the petroleum industry to analyze formation characteristics from electrical resistivity data, there has become apparent the need for more precise measures of that parameter. The measurement of the resistivity of a homogeneous and isotropic material can be made simply when the body of the material is infinite in all dimensions and the measuring electrodes can be placed within the body. In such a case, the current leaving an electrode follows a spherical distribution and the mathematical expression for the potential level at any distance from that electrode can readily be derived. If it is considered that the active electrodes are within a bore hole filled with mud of a resistivity different from that of the formation, the current does not flow in a spherical pattern near the current source or sink, and the calculations of potential levels become more difficult. The additional consideration of a section of formation invaded by mud filtrates, and thus having resistivity different from either that of the mud, the adjacent formations or of the undisturbed formation, makes the mathematical problem extremely complex. Should the vertical dimensions of the formations be considered as finite, the problem becomes so complex as to preclude reasonable solution. Yet, even these considerations have not described adequately the conditions encountered in a large proportion of the wells to be studied.