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Basic Relationships of Well Log Interpretation

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Basic Relationships of Well Log Interpretation CH01_v2.qxd 8/5/04 10:42 AM Page 1 Asquith, G., and D. Krygowski, 2004, Basic Relation- ships of Well Log Interpretation, in G. Asquith and D. Krygowski, Basic Well Log Analysis: AAPG Meth- 1 ods in Exploration 16, p. 1–20. Basic Relationships of Well Log Interpretation INTRODUCTION puter software, is intentional. It is only through expe- rience with such manual methods that the reader can This chapter provides a general introduction to well gain an appreciation for the effects of parameters on logging principles and methods that will be used the calculations, and gain a better understanding of the throughout the book. Succeeding chapters (2 through accuracy and precision of the techniques discussed 6) introduce the reader to specific log types. The text here. discusses how different log types measure various When the first edition of this book was published, properties in the wellbore and surrounding formations, virtually all well-logging data were acquired through what factors affect these measurements, where on a the use of wireline-conveyed tools; that is, logging standard log display a particular curve is recorded, and tools lowered in the borehole on a 7-conductor cable how interpreted information is obtained from the logs over which power, operating instructions, and data using both charts and mathematical formulas. Unlike were sent. Since the mid-1980s, a second formation- many other logging texts, the logging tools are evaluation technique, measurement while drilling grouped according to their primary interpretation tar- (MWD) or logging while drilling (LWD), has devel- get, rather than their underlying measurement physics. oped. In this method, the logging sensors are imbed- Spontaneous potential (SP) and gamma ray logs are ded in the thick-walled drill collars used at the bottom discussed first, as their primary use is correlation and of the drill string (near the bit), and measurement of their primary interpretive target is gross lithology (the formation properties is done continuously during the distinction between reservoir and nonreservoir). The drilling process (hence the name, MWD). Initially, porosity logs (i.e., sonic, density, and neutron logs) are MWD logging technology borrowed heavily from covered next, then the resistivity logs. Nuclear mag- wireline technology, with the goal being to produce netic-resonance logs, although they provide porosity LWD measurements comparable to wireline measure- (among other quantities of interest), are presented after ments. As LWD technology has progressed, sensor resistivity logs. This is due in part to their recent design and other features of LWD have been incorpo- arrival and to their relative absence in historical data rated back into wireline technology, for the improve- archives. ment of those measurements. The final four chapters again deal with interpreta- Unless specifically noted in the text, the interpreta- tion of the data, this time in detail with example prob- tion of borehole data is the same irrespective of the lems and their solutions. These chapters bring the source of the data, either wireline or LWD sensors and introductory material of Chapter 1 together with the measurement systems. The techniques shown here are specific measurement information and are intended to applicable to both data sources and can even be provide a coherent view of the interpretation process. extended to incorporate equivalent core measure- The reader is encouraged to work the examples to gain ments. familiarity with the interpretation techniques and to begin to understand the limitations on interpretation that are present due to the nature of subsurface infor- GENERAL mation. As logging tools and interpretive methods are The use of charts and simple calculations through- developing in accuracy and sophistication, they are out the text, rather than the use of petrophysical com- playing an expanded role in the geological decision- 1 CH01_v2.qxd 8/5/04 10:42 AM Page 2 2 ASQUITH AND KRYGOWSKI making process. Today, petrophysical log interpreta- some property in the rock surrounding the wellbore tion is one of the most useful and important tools (see Society of Professional Well Log Analysts, 1984). available to a petroleum geologist. For the reader unfamiliar with petrophysical logging, Besides their traditional use in exploration to corre- some confusion may develop over the use of the word late zones and to assist with structure and isopach log. In common usage, the word log may refer to a par- mapping, logs help define physical rock characteristics ticular curve, a suite or group of curves, the physical such as lithology, porosity, pore geometry, and perme- (paper) record of the measurements, a logging tool ability. Logging data are used to identify productive (sonde), or the process of logging. zones, to determine depth and thickness of zones, to Rock properties or characteristics that affect log- distinguish between oil, gas, or water in a reservoir, ging measurements are: porosity, lithology, mineralo- and to estimate hydrocarbon reserves. Also, geologic gy, permeability, and water saturation. Additionally, maps developed from log interpretation help with the resistivity of the rock is important because it is determining facies relationships and drilling locations. directly measured and is an essential part in the inter- Increasingly, the importance of petrophysics and well- pretation process. It is essential that the reader under- log analysis is becoming more evident as more atten- stand these properties and the concepts they represent tion is being devoted to the ongoing management of before proceeding with a study of log interpretation. reservoirs. The industry is realizing the importance of detailed petrophysical analyses, based on the details of the available data in monitoring, simulating, and Porosity enhancing reservoir performance to maximize the Porosity can be defined as the ratio of voids to the return on investment. total volume of rock. It is represented as a decimal Of the various types of logs, the ones used most fre- fraction or as a percentage and is usually represented quently in hydrocarbon exploration are called open- by the Greek letter phi, φ. hole logs. The name open hole is applied because volume of pores these logs are recorded in the uncased portion of the porosity,φ = 1.1 total volume of rock wellbore. All the different types of logs and their curves discussed in this text are of this type. The amount of internal space or voids in a given A geologist’s first exposure to log interpretation volume of rock is a measure of the amount of fluid a can be a frustrating experience. This is not only rock will hold. This is illustrated by Equation 1.1 and because of its lengthy and unfamiliar terminology, but is called the total porosity. The amount of void space also because knowledge of many parameters, con- that is interconnected, and thus able to transmit fluids, cepts, and measurements is needed before an under- is called effective porosity. Isolated pores and pore vol- standing of the logging process is possible. ume occupied by adsorbed water are excluded from a Perhaps the best way to begin a study of logging is definition of effective porosity but are included in the by introducing the reader to some of the basic con- definition of total porosity. cepts of well log analysis. Remember that a borehole represents a dynamic system; that fluid used in the Lithology and Mineralogy drilling of a well affects the rock surrounding the bore- hole and, therefore, log measurements. In addition, the In well-log analysis, the terms lithology and miner- rock surrounding the borehole has certain properties alogy are used with some ambiguity. Lithology is often that affect the movement of fluids into and out of it. used to describe the solid (matrix) portion of the rock, The two primary parameters determined from well generally in the context of a description of the primary log measurements are porosity and the fraction of pore mineralogy of the rock (e.g., a sandstone as a descrip- space filled with hydrocarbons (i.e., hydrocarbon satu- tion of a rock composed primarily of quartz grains, or ration). The parameters of log interpretation are deter- a limestone composed primarily of calcium carbon- mined directly or inferred indirectly and are measured ate). In the early days of log interpretation (with limit- by one of three general types of logs: ed measurements), this was usually a sufficient • electrical description. Probably the first instances of lithologic • nuclear effects on the logs were observed in shaly or clay-con- taining sandstones. With the advent of multiple poros- • acoustic or sonic logs ity measurements and the development of more The names refer to the sources used to obtain the detailed interpretive methods, it has become possible measurements. The different sources create records to estimate the primary solid constituents, normally as (logs), which contain one or more curves related to a mineral pair or triad. CH01_v2.qxd 8/5/04 10:42 AM Page 3 Basic Relationships of Well Log Interpretation 3 The literature has tended to follow the improved ferently, formation water takes up space both in pores understanding of the constitution of the solid part of and in the connecting passages between pores. As a the formations of interest, with most current literature consequence, it may block or otherwise reduce the referring to the determination of mineralogy instead of ability of other fluids to move through the rock. lithology. When one considers the physics of logging Relative permeability is the ratio between effective measurements, the ambiguity continues. Some meas- permeability of a fluid at partial saturation and the per- urements (primarily nuclear) are made as the result of meability at 100% saturation (absolute permeability). molecular-level interactions between the formation When relative permeability of a formation’s water is and the logging tool. These might be considered as zero, the formation produces water-free hydrocarbons being affected by the formation’s mineralogy.
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