Basic Well Log Analysis for Geologists George B. Asquith, Charles R. Gibson. Basic Well Log Analysis is a general introduction to common openhole logging measurements, both wire line and . Gibson ebook PDF download. Basic Well Log. echecs16.info - Ebook download as PDF File .pdf), Text File .txt) or read book online. Basic Well Log Analysis for Geologist - Free ebook download as PDF File .pdf) or read book online for free. A good manual for geologist.
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DOWNLOAD PDF Basic Well Log Analysis for Geologists (AAPG Methods in Exploration 3) Basic Growth Analysis: Plant Growth Analysis for Beginners. As logging tools and interpretive methods are developing in accuracy and sophistication, they are playing an expanded role in the geological. PDF | On Jan 1, , Fadhil Sadooni and others published Basic Well Log Analysis for Geologists Arabic.
And, all saturation and is a function of porosity, type of fluid i. Because both the rock and hydrocarbons act as insulators Now that the reader is introduced to some of the basic but saltwater is conductive, resistivity measurements made concepts of well log interpretation, our discussion can be by logging tools can be used to detect hydrocarbons and continued in more detail about the factors which affect estimate the porosity of a reservoir. Because during the logging measurements. A well's borehole and the rock surrounding it side of the borehole and form mudcake Rmc; Fig. Fluid are contaminated by the drilling mud, which affects logging that filters into the formation during invasion is called mud measurements. Figure 1 is a schematic illustration of a filtrate Rmf; Fig. The resistivity values for drilling mud, porous and permeable formation which is penetrated by a mudcake, and mud filtrate are recorded on a log's header borehole filled with drilling mud.
Quick-look Methods. Bulk Volume Water. Saturation Crossplots. Permeability From Logs.
Shaly Sand Analysis. Neutron-Density Lithology Plot. Neutron-Sonic Lithology Plot. Density-Sonic Lithology Plot. M-N Lithology Plot. Rock Typing and Facies Mapping. Electrical Borehole Images. Acoustic Borehole Images. Downhole Video Images.
Emerging Techologies: Other Borehole Images. Borehole Image Interpretation. We all agreed that a revision of Basic Well Log Analysis for Geologists was in order, to capture the technological advancements in well logging that had been made since the books publication. George suggested that I start the revisions, to provide a different perspective on his original efforts.
Our collaboration began in that way, with the revisions as a starting place for a continuing dialog which resulted in this edition. My sincere thanks and appreciation go to George for his confidence in my abilities, his willingness to put all of his work on the table, and for his efforts as the managing partner in this endeavor. Our thanks to Bob Cluff who critically reviewed the original book at the beginning of this project. His comments were taken to heart.
The review efforts of Rick Erickson and Gary Stewart are to be commended. Not only did they review the text, but they also attacked the case study data in great detail, comparing log displays with printed log values and final results. Many charts and figures used in the text were provided by Baker Atlas, Schlumberger Oilfield Services, and Halliburton. Our thanks for their willingness to share their information with this project. The log displays from the original book were scanned by Neuralog and provided for the project.
Neuralog software converted those images to digital data for display and interpretive processing. The raw data were stored, processed, and displayed using software from Landmark Graphics a Halliburton Company. The PetroWorks and OpenWorks products were used for this purpose.
The log plots and crossplots in the text were produced using PetroWorks software. Our thanks to both companies for providing the means to efficiently convert this work from the paper realm to the digital realm. And finally a very special thank you to my wife, Monica Krygowski, who has supported me in an effort that took much longer than originally anticipated.
Her comments, positive outlook, and encouragement are an integral part of this publication. Daniel A. Krygowski Austin, Texas, U. He received his B. His 25 years of petroleum industry experience include work as research geologist, Atlantic-Richfield Co. His industry projects have included the determination of the reservoir architecture and remaining gas reserves in the Hugoton and West Panhandle fields and exploration and reservoir characterization of selected reservoirs from the Gulf Coast onshore and offshore , Permian, Alberta, San Juan, Williston, Arkoma, Cooper Australia , Neiva Colombia , Maracaibo Venezuela , and Anadarko basins.
He has authored publications including 5 books in the fields of petrophysics, computer geology, and carbonate and clastic sedimentation and petrology.
Asquiths research interests include the documentation and quantitative mapping of relationships between petrophysical responses and depositional and diagenetic lithofacies, the petrophysics of carbonate and shaly-sand reservoirs, and the application of computers to petrophysical analysis. As a Domain Expert in the research and development organization, he is focused on the usability, user interface, and petrophysical technology content of PetroWorks and other software products.
He received a B. Previous to his employment at Landmark, he held a number of technical and management positions in petrophysics and software development at Cities Service Company now Occidental and Atlantic Richfield Company now BP.
He received his M. His thesis work involved stratigraphic studies in the Permian reef complex of the Guadalupe Mountains, New Mexico. From through he worked as an exploration and research geologist for Conoco in Denver, Colorado; Lafayette, Louisiana; and Ponca City, Oklahoma. In , he received his Ph. From to , he worked in reservoir characterization at Marathons Petroleum Technology Center in Littleton, Colorado. In , he toured the U. At CSM, he teaches beginning and advanced log analysis, carbonate geology, field seminars, and integrated exploration courses.
His specialties include carbonate sedimentology and diagenesis, fractured reservoirs, formation evaluation, borehole-imaging logs, and horizontal drilling. His research interests include carbonate diagenesis, clay mineralogy, and their implications in well log analysis. Each discipline that encounters and uses well log data does so from its own perspective. In doing so, each discipline sometimes uses the data without a full understanding of how the measurements are made.
That incomplete understanding can encompass the processing of the actual measurements into the raw data provided by the data logging companies and to the interpretation methods that convert that data into usable information about the subsurface. It is this incomplete understanding of well log data that commonly produces conflicting interpretations from different sources, when the goal should be a single cohesive model of the subsurface that can be consistently applied by all disciplines.
It does not claim to provide all information about well logs from all perspectives. Like the original publication, it remains focused on the interpretation of basic, or common openhole logging measurements. It also remains focused on the traditional interpretive goals of formation porosity, fluid saturation, and lithology.
The impetus for this revised text was a perception that an update was needed to address the technologies that had been introduced in the two decades since the original publication. We have endeavored to do so, from inclusion of the photoelectric effect Pe or PEF curve of the newest-generation density tools, to chapters specifically addressing nuclear magnetic resonance NMR logging by Steven Henderson and borehole imaging by Neil Hurley. Accompanying this book is a CD, which you will find attached to the inside back cover.
The CD contains 10 data-based files so that readers of this book will be able to practice the techniques described in the book. The authors hope that this introductory text will lead the readers to seek other sources on well logs and well log interpretation, which will lead to a deeper and broader understanding of formation evaluation.
George Asquiths Preface to the original publication reproduced in this edition still rings true; an understanding of the data and the discipline still comes primarily from the hands-on application of the information and methods shown here, and in other sources.
If you have read this far, take the time to read that Preface as well. There are many resources for petrophysical data. We hesitate to list specific sources here, especially online sources as websites can appear, change, and disappear quickly. Study of the properties of rocks by petrophysical techniques using electric, nuclear, and acoustical sources is as important to a geologist as the study of rock properties by more conventional means using optical, x-ray, and chemical methods.
Nevertheless, despite the importance of petrophysics, it is frequently underutilized by many geologists who are either intimidated by logging terminology and mathematics, or who accept the premise that an indepth knowledge of logging is only marginally useful to their science because, they feel, it more properly belongs in the province of the log analyst or engineer. The enormous importance of logging dictates that as geologists, we put aside old notions and apply ourselves diligently to learning log interpretation.
The rewards are obvious; in fact, no less than achieving an understanding of the ancient record hangs in the balance. And, it is likely that the success or failure of an exploration program may hinge on a geologists logging expertise. In the interest of conciseness, and so that logs used most often in petroleum exploration are thoroughly discussed, the text is restricted to open hole logs. I hope that the reader initiates his or her own study of other log types which are beyond the scope of this book.
Unfortunately, learning about open hole logging requires more of the reader than a light skimming of the texts material.
The plain truth is that a great deal of hard work, including memorizing log terminology, awaits the serious student; and even then, a facility with logs develops only after plenty of real-life experience. The intent here is simply to provide a foundation of knowledge which can be built upon later. Consequently, many exceptions to rules are left to more advanced books. It is quite possible that some colleagues will raise objections about the lack of time devoted to tool theory; they may also comment on the paucity of qualifying statements in the text.
These objections are understood and indeed there may be disagreements about what constitutes over-simplification. In defense of brevity, it should be pointed out that the surfeit of information available on petrophysics often discourages all but the most ardent beginner.
Certainly, many of the difficult decisions which had to be faced in preparing the manuscript dealt with selecting information judged indispensable at an elementary level. For those who are interested in expanding their knowledged of logs, his book will be a great help. Finally, a last word a substantial effort was expended to ensure that a minimum number of errors would appear in the text. However, given the nature of the subject and the almost infinite possibility for mistakes, there may be slip-ups, regardless; hopefully they will not be too serious.
George B. Succeeding chapters 2 through 6 introduce the reader to specific log types. The text discusses how different log types measure various properties in the wellbore and surrounding formations, what factors affect these measurements, where on a standard log display a particular curve is recorded, and how interpreted information is obtained from the logs using both charts and mathematical formulas. Unlike many other logging texts, the logging tools are grouped according to their primary interpretation target, rather than their underlying measurement physics.
Spontaneous potential SP and gamma ray logs are discussed first, as their primary use is correlation and their primary interpretive target is gross lithology the distinction between reservoir and nonreservoir. In general, an equal volume of mud Sxo - water saturation flushed zone filtrate can invade low porosity and high porosity rocks if Some of the more important symbols shown in Figure 1 are: the drilling muds have equal amounts of solid particles.
The Hole Diameter dh A well's borehole size is described solid particles in the drilling muds coalesce and form an by the outside diameter of the drill bit. But, the diameter of impermeable mudcake. The mudcake then acts as a barrier the borehole may be larger or smaller than the bit diameter to further invasion. The size of the borehole is measured by a caliper be affected. General invasion diameters are: log. The excess of borehole Flushed Zone Rxo The flushed zone extends only a pressure over formation pressure prevents blow-outs.
If invasion is deep or moderate, most often the hydrostatic pressure in the mud column is always greater flushed zone is completely cleared of its formation water than formation pressure.
This pressure difference forces Rw by mud filtrate Rmf. When oil is present in the some of the drilling fluid to invade porous and permeable flushed zone, you can determine the degree of flushing by formations. As invasion occurs, many of the solid particles mud filtrate from the difference between water saturations in the flushed Sxo zone and the uninvaded Sw zone Fig.
As the mud filtrate invades the saturated with formation water R w , oil, or gas. Even in hydrocarbon-bearing reservoirs, there is always a Next, formation water is pushed out in front of the mud layer of formation water on grain surfaces.
Water saturation filtrate forming an annular circular ring at the edge of the S w ; Fig. The annulus effect is detected by a reservoir evaluation because, by using water saturation higher resistivity reading on a deep induction log than by data, a geologist can determine a reservoir's hydrocarbon one on a medium induction log. The formula for calculating hydrocarbon Log resistivity profiles illustrate the resistivity values of saturation is: the invaded and uninvaded zones in the formation being investigated.
These profiles vary, depending on the relative resistivity values of Rw and Rmf. At irreducible water saturation, water will not move, and the relative permeability to water equals zero. Rcsistivitv-is the rock property on which the entire science of logging first developed.
Resistance is the inherent property of all materials, regardless of their shape and size, to resist the flow of an electric current. Different materials have different abilities to resist the flow of electricity. Resistivity is the measurement of resistance; the reciprocal of resistivity is conductivity. In log interpretation, hydrocarbons, the rock, and freshwater all act as insulators and are, therefore, non-conductive and highly resistive to electric flow.
Saltwater, however. J low resistivity. The unit of measure used for the conductor. Resistivity is a basic measurement of a reservoir's fluid saturation and is a function of porosity, type of fluid i. Because both the rock and hydrocarbons act as insulators but saltwater is conductive, resistivity measurements made by logging tools can be used to detect hydrocarbons and estimate the porosity of a reservoir.
Because during the drilling of a well fluids move into porous and permeable formations surrounding a borehole, resistivity measurements recorded at different depths into a formation. Conrad Schlumberger in began the first experiments which led, eventually, to the development of modern day petrophysical logs. The first electric log was run September 5, by H. Doll in Alsace-Lorraine , France. In , G. Archie's experiments showed that the resistivity of a water-filled formation Rn , filled with water having a resistivity of R; can be related by means of a formation resistivity factor F:.
Archie's experiments also revealed that formation factors can be related to porosity by the following formula:. The higher the value for tortuosity the higher the m value. Water saturation SW is determined from the water filled resistivity R,, and the formation resistivity Rt by the following relationship:. This is the formula which is most commonly referred to as the Archie equation for water saturation Sw. And, all present methods of interpretation involving resistivity curves are derived from this equation.
Now that the reader is introduced to some of the basic concepts of well log interpretation, our discussion can be continued in more detail about the factors which affect logging measurements. Figure I is a schematic illustration of a porous and permeable formation which is penetrated by a borehole filled with drilling mud. Hole Diameter dh -Awell 's borehole size is described by the outside diameter of the drill bit. But, the diameter of the borehole: The size of the borehole is measured by a caliper log.
Drilling Mud Rm --l day, most wells are drilled with rotarv bits and lise special mud as a circulating fluid. The mud helps remove cuttings from the well bore, lubricate and cool the drill bit, and maintain an excess of borehole pressure over formation pressure. The excess of borehole pressure over formation pressure prevents blow-outs. The density of the mud is kept high enough so that hydrostatic pressure in the mud column is always greater than formation pressure.
This pressure difference forces some of the drilling fluid to invade porous and permeable formations. As invasion occurs, many of the solid particles. Fluid that filters into the formation during invasion is called mud [iltratc Rrnr: The resistivity values for drilling mud, mudcuke , and mud filtrate are recorded on a log's header Fig.
Invaded Zone-The zone which is invaded by mudtiltratc is called the invaded zone. It consists otailushcd Z. OtlC R,o and a transition or annulus Rj zone. The flushed zone. Rw occurs close to the borehole Fig. The transition or annulus Ri zone, where a formation's fluids and mud filtrate arc mixed, occurs between the flushed R,o zone and the uninvaded Rt zone.
The uninvaded ;: The depth of mud filtrate invasion into the invaded zone is referred to as the diameter of invasion d, and dl: The diameter of in vasion is measured in inches or expressed as a ratio: The amount of invasion which takes place is dependent upon the permeability of the mudcakc and not upon the porosity of the rock.
The solid particles in the drilling muds coalesce and form an impermeable mudcakc. The mudcake then ads as a barrier to further invasion. Because an equal volume of fluid can be invaded before an impermeable rnudcake barrier forms.
This occurs because low porosity rocks have Jess storage capacity or pore volume to fill with the invading fluid, and, as a result, pores throughout a greater volume of rock will be affected. General invasion diameters are:. Flushed Zone R,o - The flushed zone extends only a few inches from the well bore and is part of the invaded zone If invasion is deep or moderate, most often the flushed zone is completely cleared of its formation water Rw by mud filtrate R",r.
When oil is present in the flushed zone, you can determine the degree of flushing by mud filtrate from the difference between water saturations in the flushed S,,, zone and the uninvaded Sw zone Fig.
Uninvaded Zone Rt - The uninvadcd zone is located beyond the invaded zone Fig. Pores in the uninvaded. Even in hydrocarbon-hearing reservoirs, there is always a layer of Iormat ion water on grain surfaces. Water saturation Sw: The formula for calculating hydrocarbon saturation is:. Invasion and resistivity profiles are diagrammatic.
They illustrate the horizontal distributions of the invaded and uninvudcd zones and their corresponding relative resistivities. There are three commonly recognized invasion profiles: I step, 2 transition, and 3 annulus. These three invasion profiles are illustrated in Figure 3. The step profile has a cylindrical geometry with an invasion diameter equal to d..
Shallow reading, resistivity logging tools read the resistivity of the invaded zone Rj , while deeper reading, resistivity logging tools read true resistivity of the uuinvadcd zone R1. The transition profile also has a cylindrical geometry with two invasion diameters: It is probably a more realistic model for true borehole conditions than the step profile. Three resistivity devices are needed to measure a transitional profile: By using.
Two modern resistivity devices which use these three resistivity curves are: An annulus profile is only sometimes recorded on a log because it rapidly dissipates in a well. The annulus profile is detected only by an induction log run soon after a well is drilled.
As the mud filtrate invades the hydrocarbon-hearing zone, hydrocarbons move out first. The annulus effect is detected by a higher resistivity reading on a deep induction log than by one on a medium induction log. Log resistivity profiles illustrate the resistivity values of the invaded and uninvuded zones in the formation being investigated.
They are of particular interest because, by using them, a geologist can quickly scan a log and look for potential zones of interest such as hydrocarbon zones. Because of their importance, resistivity profiles for both water-bearing and hydrocarbon-bearing zones are discussed here.
These profiles vary. A freshwater mud i. A saltwater mud i. Figures 6a and 6b illustrate the resistivity curves for wet zones invaded with both freshwater and saltwater muds.
A hydrocarbon zone invaded with freshwater mud results in a resistivity profile where the shallow Rxo , medium R;. In some instances, the deep resistivity will be higher than the medium resistivity When this happens. A hydrocarbon zone invaded with saltwater mud results in a resistivity profile where the shallow Rxo ' medium R,. Figures 7a and 7h illustrate the resistivity curves for hydrocarbon zones invaded with both freshwater and saltwater muds.
Table I is a list of the different methods for calculating forrnation t'aCl lf, ami illustrates how lithology affects the formation factor.
Icnipcnnurc olFormarioll-Formation temperature Tf is also important in log analysis because the resistivities of the drilling mud Rrnl. The temperature of a formation is determined by knowing: I formation depth; 2 bottom hole temperature BHT: Modified after Asquith,19RO. The formation temperature is also calculated Asquith, ILJ80 by using the linear regression equation:.
After a formation's temperature is determined either by chart Fig. X or by calc ul arion. Figure 9 is it chart that is used for correcting fluid resistivities to formation temperature. This chart is closely approx irnutcd by the Arps Iorrn ula:. Using a formation temperature of and assuming an R; of 0.
Resistivity values of the drilling mud Rm , mud filtrate Rmtl. The resistivity of a formation's water Rw is obtained by analysis of water samples from a drill stem test, a water producing well. Formation water resistivity Rw is also determined from the spontaneous potential log discussed in Chapter H or can be calculated in water zones i. These formulas are discussed in detail in subsequent chapters. The four most fundamental rock properties used in petrophysical logging are 1 porosity; 2 permeability: Where a porous and permeable formation is penetrated by the drill bit, the drilling mud invade, the formation as mud filtrate Rmf.
The invasion of the porous and permeable formation by mud filtrate creates invasion zones RXD and R, and an uninvaded zone R. Shallow, medium, and deep reading resistivity logging tools provide information about the invaded and uninvadcd zones and about the depth of invasion.
The lithology of a formation must be known because: The four fluids that affect logging measurements are: The resistivities of the drilling mud ROl , mudcake ROlJ, mud filtrate Rmf and formation water Rw all vary with changes in temperature. Figure 1. The borehole environment and symbols used in log interpretation. This schematic diagram illustrates an idealized version of what happens when fluids from the borehole invade the surrounding rock. Dotted lines indicate the cylindrical nature of the invasion.
Reproduction of a typical log heading. Information on the header about the resistivity values for drilling mud Rill and mud filtrate Rn1f are especially useful in log interpretation and are used in calculations. Figure 3. Typical invasion profiles for three idealized versions of fluid distributions in the vicinity of the borehole. As mud filtrate Rmf moves into a porous and permeable formation, it can invade the formation several different ways. Various fluid distributions are represented by the step, transition, or annulus profiles.
Step Profile-Mud filtrate is distributed with a cylindrical shape around the borehole and creates an invaded LOne. The cylindrically shaped invaded zone is characterized by its abrupt contact with the uninvadcd zone. The diameter of the cylinder is represented as dj. In the in vaded zone, pores are filled with mud filtrate ROlf: The resistivity of the invaded zone is Rxo.
Transition Profile-This is the most realistic model of true borehole conditions. Here again in vasion is cylindrical. In the flushed part Rxo of the invaded zone. In the transition part of the invaded zone, pores are filled with mud filtrate ROll. Beyond the outer boundary of the invaded lone dj on diagram , pores are filled with either formation water, or if present hydrocarbons.
In this diagram, hydrocarbons arc not present, so resistivity of the uninvaded zone is low. The resistivity of the invaded zone" flushed part is R"" and the resistivity of the transition part is Ri. Resistivity of the uninvaded zone is R, if hydrocarbon-bearing or R, if water-bearing.
Annulus Profile-This reflects a temporary fluid distribution, and is a condition which should disappear with time if the logging operation is delayed. The annulus profile represents a fluid distribution which occurs between the invaded zone and the uninvaded zone and denotes the presence of hydrocarbo ns.
In the flushed part Rxo of the invaded zone, pores are filled with both mud filtrate Rlllli and residual hydrocarbons RH. Thus the resistivity reads high. Pores beyond the flushed part of the invaded zone R, arc filled with a mixture of mud filtrate Rmf , formation water Rw " and residual hydrocarbons RH. Beyond the outer boundary of the invaded zone is the annulus zone where pores are filled with residual hydrocarbons RH and formation water Rw ' When an annulus profile is present.
The abrupt resistivity drop is due 10 the high concentration of formation water Rw in the annulus zone. Formation water has been pushed ahead by the invading mud filtrate into the annulus zone. This causes a temporary absence of hydrocarbons which, in their turn, have been pushed ahead of formation water. Beyond the annulus is the uninvaded zone where pores are filled with formation water Rw and hydrocarbons.
Remember that true resitivity of a formation can be measured in the uninvaded zone because of its virgin nature. True resistivity Rt will be higher than the wet resistivity R,, because hydrocarbons have a higher resistivity than saltwater. Figure 4. Horizontal section through a permeable water-bearing formation and the concomitant resistivity profiles which occur when there is invasion by either freshwater- or saltwater-based drilling muds sec Fig.
These examples are shown because freshwater muds and saltwater muds are used in different geographic regions. The geologist needs to be aware that a difference exists.
OR find out which mud is used in your area. The type of mud used affects the log package selected. Freshwater Muds-The resistivity of the mud filtrate Rmt is greater than the resistivity of the formation water Rw because of the varying salt content remember. A general rule when freshwater muds arc used is: Away from the borehole.
With a water-bearing formation, the resistivity of the uninvaded zone will be low because the pores arc filled with formation water Rw ' In the un invaded zone. To summarize: RXl, R, R,. Figure 5.
Horizontal section through a permeable hydrocarbon-bearing formation and the concomitant resistivity profiles which occur when there is invasion by either freshwater- or saltwater-based drilling muds see Fig. Freshwater Muds-s-Because the resistivity of both the mud filtrate Rmf and residual hydrocarbons RH is much greater than formation water Rw ' the resistivity of the flushed zone Rxo is comparatively high remember that the flushed zone has mud filtrate and some residual hydrocarbons.
Beyond its flushed part Rxo ' the invaded zone Rj has a mixture of mud filtrate Rmf. In some cases. The presence of hydrocarbons in the uninvaded zone causes higher resistivity than if the zone had only formation water Rw. Away from the borehole as more hydrocarbons mix with mud filtrate in the invaded zone, the resistivity of the invaded zone Rj begins to increase.
Resistivity of the uninvaded zone Rt is much greater than if the formation was at I OO,! Resistivity of the uninvaded zone is greater than the resistivity of the invaded R zone. Ignore the left side of the log on the opposite page, and compare the three curves on the right side of the log tracks 2 and l. Resistivity values are higher as distance increases from the left side of the log.
This is a measure of the un invadcd zone. In water-hearing zones in this case from 5, to 5, ft , the curve will read a low resistivity because the resistivity of the formation water Rw is less than the resistivity of the mud filtrate Rlllf '.
In a water-bearing formation, the curve will read intermediate resistivity because of the mixture offormation water Rw and mud filtrate Rmf.
In a water-bearing zone, the curve will read high resistivity because freshwater mud filtrate ROlf has a high resistivity. Figure 6B. See Figure 4 for review. Ignore the left side of the log on the opposite page. Log Curve SFL-Microspherically Focused Log" resistivity curves measure the resistivity of the flushed zone R,o ' In water-bearing zones the curve will record low resistivity because saltwater mud filtrate has low resistivity. Figure 7 A. Exampk of Dual Induction Focused log curves through a hydrocarbon-bearing zone.
Sec Figure 5 for review. Resistivity values arc higher as distance increases from the left side of the log. This is a measure of the uninvadcd zone. In hydrocarbon-bearing zones in this case from 8. In a hydrocarbon-bearing zone. This resistivity is normally equal to or slightly more than the deep induction curve lLD.
III a hydrocarbon-bearing zone. The SFl. J' pictured here records a greater resistivity than either the deep IlO or medium lLM induction curves.
Figure 7B. The reason for the increase in resistivities deeper into the formation is because of the increasing hydrocarbon saturation. See Figure 5 for review.
In hydrocarbon-bearing zones in this case from 9. O O ft TO line. This intersection defines the temperature gradient. In the United States as an example 8Ct is used commonly as the mean surface temperature in the Southern States. Figure 9. Because resistivity varies with changes in temperature.
Use the chart on the opposite page. Follow the diagonal line constant salinity to where it intersects a temperature value of 16 OF point B on the chart. From point B. This chapter and succeeding chapters III through V introduce the reader to spec-ific log types such as SP, resistivity, porosity, and gamma ray logs.
The text discusses how different log types measure various properties in the well bore ami surrounding formations, what factors affect these measurements, where a particular curve is recorded, and how data arc obtained from the log using both charts and mathcmatical formulas;.
The spontaneous potential SP log was one of the earliest electric logs used in the petroleum industry, and has continued to playa significant role in well log. By far the largest number ofwellx today have this type of log included in their log suites. Primarily the spontaneous potential log is used to identify impermeable zouc s such as shale. However, as will be discussed later.
The spontaneous potential log is a record of direct current DC voltage differences between the naturally occurring potential of a moveable electrode in the well bore, and the potential of a fixed electrode located at the surface Doll. It is measured in millivolts. Electric currents arising primarily from electrochemical factors within the borehole create the SP lug response. These electrochemical factors arc brought about by differences in salinities between mud filtrate Rll1r and formation water resistivity R,, within permeable beds.
The SP log is recorded on the left hand track of the log in track I and is used to: I detect permeable beds, 2 detect boundaries of permeable beds. The concept of static spontaneous potential SSP is important because 5SP represents the maximum SP that a thick, shale-free, porous and permeable formation can have t r a given ratio between RnjRw.
SSP is determined by formula or chart and is a necessary clement for determining accurate values of Rw and volume of shale. The SP value that is measured in the borehole is influenced by bed thickness, bed resistivity, invasion, borehole diameter, shale content.
Bed thickllcss-As a formation thins i. However, the SP curve can be corrected by chart for the effects of bed thick ness. As a general rule whenever the SP curve is narrow and pointed in shape, the SP should be corrected for bed thickness. Borehole and il1l'asioll--Hilchic R indicates that the effects of borehole diameter and invasion on the SI' log arc very small and, in general. In water-bearing zones the amount of SP reduction is proportional to the amount of shale in the formation.
In hydrocarbon-bearing zones the amount of SP reduction is greater than the volume of shale and is called "hydrocarbon suppression" Hiichie , 19 8. The SP response of shales is relatively constant and follows a straight line cal leu a shale baseline.
SPcurve deflections are measured from this shale baseline. Permeable bed b. The magnitude of SP deflection is due to the difference in rcsistiv ity between mud filtrate Rmf and formation water Rw and not to the amount of permeability.
In this example, the SP curve is used ttl find a value for R; by the following procedure: After you determine the formation temperature, you correct the resistivities obtained from the log heading of the mud filtrate Rmt and drilling mud Rm to formation temperature see Chapter I.
ThL' data necessary to usc this chart arc: I bed thickness. Once the value of SSP is determined. Equivalent rcsistivitv Rw. The value of Rllc' IS then corrected to R". It is important to remember that normal ly the SP curve has less deflection in hydrocarbon-bearing zones: Table J. Instead of charts. IOA-SP deflection with different resistivities of mud filtrate Rrnf and formation water Rw ' Where resistivity of the mud filtrate lRmf is equal to the resistivity of the formation water Rw there is no deflection.
Where ROlf is greater than Rw. Where Rrnf greatly exceeds Rw. Where Rmfis less than Rw. This is called positive deflection. All other deflections are less. PSP pseudo-static spontaneous potential is the SP response if shale is present. Note at bottom of diagram: A formula for the theoretical calculated value of SSP is gi veri. MOT 0.!
Figure II. Determination offorrnation water resistivity Rw from an SP log. This example is an exercise involving the charts on Figures 12 through It is measured here as two 20mv divisions from the shale baseline. The deflection is negative. Determine Tr-Use Figure 8 to determine the temperature of the formation Tf. It measures two units at a scale of20 mv per di vision from the shale baseline. The deflection is negati vc , so your answer is also - 40 rnv negative. Bed thickness read from SP log Fig.
II equals 8 ft. Correction factor from Fig. The term short normal describes a log used to measure the shallow formation resistivity, or the rcxi st i vitv of the invaded zone R. Figure Drop vertically from this intersection and read the SP correction factor on the scale across the bottom in this example, a value of 1. I Follow the value horizontally across until it intersects the sloping formation temperature line l3 OF; imagine one between the lines for and temperature lines.
Drop vertically from this intersection and read the ratio value on the bottom scale in this example, the ratio value is 5.
Rwc is calculated by dividing Rmtcorrected to formation temperature Tr by the ratio Rm! Drop vertically from the intersection and read a value for R; on the scale at the bottom in this case O.
Therefore, to determine Rw from SP it is best, whenever possible, to usc the SP curve opposite known water-bearing zones. The SP Ing can be used to calculate the volume ofshale in a permeable zone by the following formula:. The volume of shale in a sand can be used in the evaluation of shaly sand reservoirs Chapter VI and as a mapping parameter for both sandstone and carbonate facies analysis Chapter VII.
The spontaneous potential log SP can be used to: The variations in the SP are the result of an electric potential that is present between the well bore and the formation as a result of differences in salinities between Rmf and Rw'. The SP response in shales is relatively constant and its continuity of amplitude is referred to as the shale baseline.