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.
Poupon, A., Poupon, A., Schlumberger Technical Services Clavier, C., SPE-AIME, Schlumberger-Doll Research Center Dumanoir, J., Schlumberger-Doll Research Center Gaymard, R., Schlumberger Technical Services Misk, A., SPE-AIME, Schlumberger Well Services A primary feature of this interpretation method is that it uses all the logging data in a coherent manner. The formations are analyzed for clay, shale, quartz, water, hydrocarbon content, and changes in sand-grain mineralogy. Even where hole conditions are adverse, reliable results can be achieved through extensive crosschecking for likeness. Introduction During the past 7 years methods have been developed for interpretation of clean and shaly sands using combinations of sonic, density, and neutron logs, along with resistivity and auxiliary logs. Corrections for the presence of light hydrocarbons in the formation, which affect the log readings of the sonic, density, and neutron logs, were also developed. For these methods the shale parameter values are either assumed or deduced from the log readings in adjacent shale beds. This is satisfactory as long as the shales have uniform properties. Frequently this is not so, and the log properties. Frequently this is not so, and the log analyst is faced with determining the properties of the shales occurring in the shaly sands. Intuitively it would seem that sands and shales deposited in sequence during a continuous sedimentation cycle should possess logging properties related to their common geological background. The study of neutron and density logs made in sand and shale sequences has substantiated this. From this study a conceptual model of shales and shaly sands has evolved that is consistent with geological considerations as well as logging tool responses, and in turn has led to the interpretation method described here. The method makes maximum use of all the following logs: neutron, density, resistivity,* gamma ray, SP, microresistivity, sonic, and caliper. Some of these logs may be omitted (microresistivity, sonic, caliper, and either SP or gamma ray); however, doing so decreases the reliability of the results. Formation clay content is determined from several clay indicators and the formation is analyzed for shale, quartz, water, hydrocarbon content, and changes in sand-grain mineralogy. In favorable cases an estimate of hydrocarbon density is made. The associated computer program, SARABAND, solves the interpretation, crossverifies the input data and results and determines automatically many of the required parameters. The interpretation results of main interest are listed in special tabulations and displayed on a film especially coded for easy identification. The Sand and Shale Models Fig. 1 is a neutron-density frequency crossplot generated by a computer. The plotted values of andD, are the apparent porosities from neutron and density logs. Each of the one- or two-digit numbers on the plot represents the total number of readings, over a 390-ft interval in a sand-shale sequence, having the values of N and D, corresponding to the location of the number. The distribution shown on Fig. 1 is typical of sandshale sequences. JPT P. 867
Here is a method of interpreting well logs for formations having lithologies that are mixtures of silica, limestone, dolomite, anhydrite, and clay, or that are mixtures of any two specified minerals and clay. Analysis is made for clay content, matrix density, porosity, hydrocarbon saturation, and secondary-porosity index. Introduction During the past several years interpretation methods have been developed to obtain porosity and lithology from combinations of neutron, density, and sonic logs. One of these is the Dual Mineral Method which uses a crossplot of the neutron- and density-log data to arrive at values of porosity and apparent matrix density of the formation. The sonic log is used to indicate zones of secondary porosity and to help define the lithology. The Dual Mineral Method has been in use for a number of years. It gives good results in clean, liquid-saturated formations or clean, gas-bearing formations of known lithology. However, its use is restricted to these cases. This paper describes a more general method, based on the Dual Mineral technique, which has been developed for the interpretation of formations having lithologies that are mixtures of silica, limestone, dolomite, anhydrite, and clay, or that are mixtures of any two specified minerals plus clay. This new method takes account of both formation shaliness and hydrocarbon effects. In a preinterpretation procedure, log corrections are made, and certain parameters are evaluated. By study of crossplots of the log readings over intervals within lithologic units, the main mineral constituents of the rock matrix are identified and the corresponding properties are evaluated. properties are evaluated. Level-by-level computations are then made. Using the values of clay properties determined during preinterpretation, clay content is evaluated at each preinterpretation, clay content is evaluated at each level, and corrections are made for clay content and hydrocarbon effects. For the hydrocarbon correction, a value of hydrocarbon density is used, based on field information or crossplot study. Values of porosity, apparent matrix density, and water saturation are computed. Safeguards are included to minimize the possible effect of adverse borehole conditions on possible effect of adverse borehole conditions on some log readings. A moved-hydrocarbon index can also be calculated. The logs used for this method include a density log, a gamma ray-neutron log, a sonic log, and an induction log or a Laterolog, preferably with an SP curve. The addition of an Rxo log (Microlaterolog or Proximity log) and the value of the hydrocarbon Proximity log) and the value of the hydrocarbon density provides a reliable evaluation of hydrocarbon effect. The sonic log is used to determine lithology in the M-N plot, to evaluate secondary porosity, and to provide a limiting value of porosity in clay-free formations that are caved. The method can still be used without an Rxo log or sonic log, but the reliability of some of the results is diminished. The method is used in a computer program called CORIBAND. A preinterpretation pass is made in the computer, and depth plots and frequency crossplots are produced to determine parameters needed for the final computations. JPT P. 995
Included in the family of production logging devices are two types of flowmeters (a continuous and a packer type), high-resolution thermometers, gradiomanometers, production fluid samplers, densimeters, water cut meters, and through-tubing calipers. The instruments enable recording of: profiles of flow rate; fluid characteristics that enable determination of the fluid composition; hole sizes; and temperatures. In addition, casing collars are recorded for depth control. Fluid samples, suitable for PVT studies, can be obtained at any depth in the fluid column. Interpretations of these data, especially when used in conjunction with through-tubing gamma ray-neutron and cement bond logs, define most production problems. In new completions, production logging services are used both to ensure optimum ultimate recovery and to explain production problems brought to light by surface performance. In older wells, the logs aid in planning remedial work for declining producers. Monitoring of secondary recovery projects is another important application of production logging services. Introduction Experience indicates that surface fluid measurements are not adequate to describe the efficiency of the down-hole production system. In many wells that appeared to be producing properly, down-hole malfunctions were found that, if uncorrected, would appreciably reduce ultimate recovery. In some instances the malfunctions, if allowed to persist, would even preclude effective application of secondary recovery techniques. Because the operation of the down-hole system has a great effect on ultimate recovery, a more diagnostic means of measurement is required. Production logging tools are designed to operate downhole under dynamic producing conditions. Thus, this family of tools provides the information necessary to evaluate the performance of the well-completion scheme. Included in the family are instruments that locate fluid entry (or injection), measure flow rates, and enable determination of fluid composition from each zone of fluid entry. The tools are built to operate through 2-in. tubing, and to perform properly at pressures up to 10,000 psi and temperatures up to 300F. Pressure control equipment allows safe entry into producing wells with surface pressures as high as 6,000 psi. and into injection wells with pressures up to 7,200 psi. Production Logging Measurements Great progress in production logging has been made recently in the development of tools to work under dynamic conditions. Combinations of these tools (a flow rate meter, fluid identification devices, and some form of depth control) provide answers to the question, "How much of what fluid is coming from where?" Flow rate measurement is not new to the industry. However, downhole determination of fluid composition has always been a problem. The new devices enable identification of the various fluids flowing, and permit computation of the fraction of total flow that each represents. With the constituent fluids thus identified, down-hole flow rate measurements are much more meaningful. The problem of fluid identification is complicated by the variety of possible mixtures of oil, gas and water. and by the effects of fluid velocities on these mixtures. Both the fluid mixture and velocity must be considered in selecting appropriate logging devices. The first consideration must be measurement of parameters that vary sufficiently for reasonable resolution between the constituent fluids of a multi-phase flow. For example, the greatest benefits of a device that measures fluid densities are realized when defining gas entry into liquids. For oil-water mixtures the specific gravities are more nearly the same and resolution is difficult. JPT P. 137ˆ
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