COPYRIGHTcloud1.activelearner.com/contentcloud/portals/hosted3/...Gamma Ray Logging Why This Module is Important The following GR logging topics are covered: • The basic physics
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Introduction
Gamma Ray and Spontaneous Potential Logging Core
Why This Module is Important
An understanding of petrophysics and petrophysical technologyat the core knowledge level is extremely important tounderstanding well and reservoir performance.
Petrophysical data are data obtained from the wellbore duringand after drilling.
In this module, a basic introduction to Gamma Ray andSpontaneous Potential (SP) Logging is provided.
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════
You will learn about Gamma Ray (GR) logging and how it provides early indications of the presence of potential reservoir rock.
GR data may be acquired either in open hole or cased hole.
Rock FormationRock Formation
Gamma Ray Logging
Why This Module is Important
The following GR logging topics are covered:• The basic physics of the Gamma Ray logging tool• Calibration standards used for GR logs and GR API Units• The three most common radioactive elements found in sedimentary
rock and applications of the Spectral GR tool• How the GR log is used to discriminate between reservoir and non-
reservoir rock • Recognize the typical GR log response in common formations
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════
Other options:• Coiled Tubing Unit • Use of a tractor to pull
the tool string down to desired depth
Logging While Drilling
Logging While Drilling (LWD) is the alternative to wireline logging.
LWD tools are built into drill collars and contain very sophisticated electronics designed to withstand extreme vibrations and high temperatures and pressures.
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════
Passive detectors respond to natural gamma radiation in the subsurface
Do not have a radioactive source
Harmless equipment
Rock FormationRock Formation
Net/Gross – Gamma Ray
To measure the natural gamma rays emitted from the formation, the Gamma Ray (GR) tool is lowered in the borehole.
The Gamma Ray tool consists of a detector and associated electronics to measure the gamma radiation originating in the volume of formation near the tool.
Gamma Ray Logging
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════
Correlation• Well to Well correlation • Depth matching between separate trips in the well• Positioning of open-hole sampling tools• Providing the depth control needed for cased hole perforation
– Note: gamma rays do pass through steel
General lithology indicator• Discriminate between reservoir & non-reservoir • Used to determine Net to Gross ratios
Qualitative “shaliness” evaluation of the reservoir rock• Gamma Ray data can be empirically calibrated to estimate percent
shale in a shaly sand interval
GR Response to Lithology
The Gamma Ray log is a good “first-pass” indicator of lithology
The Gamma Ray log records total abundance of the radioactive isotopes of Potassium (K), Thorium (Th) and Uranium (U)
K, Th and U are usually concentrated in shales and less in sandstones and carbonates (owing to differences in mineralogy) with some notable exceptions
Common GR readings, in API units*, are:• Limestones, anhydrites,
15-20 API• Dolomites and “clean”
(quartz-rich) sandstones, 20-30 API
• Shales, average 100 API, but can vary from 75 to 300 API
• Other lithologies: coal, salt (halite, NaCl) and gypsum usually low readings (~<20 API), volcanic ash and beds of potash salts (sylvite, KCl) give high readings (>75)
* 1 API unit = 1/200th of the response generated by a calculated standard that has 2x the average radioactivity of shale with 6ppm U, 12ppm Th and 2% K
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════
Del Rio: Typical shale. High potassium content associated with illite
Buda: Limestone. Very low radioactivity (<20 API)
Eagleford: Shale and source rock. High uranium content associated with high TOC (Total Organic Carbon)
Gamma Ray Summary
Relatively simple passive measurement of naturally occurring radioactivity
Primary sand/shale discriminator because shales are more radioactive than most reservoir rocks.
Primary well-to-well and logging run-to-logging run correlation tool
Vshale indicator via transform
Spectral version helps with fracture detection mineral identification, clay typing and infiltration of fluids carrying dissolved radioactive minerals (production scale)
Gamma ray, like all radioactive measurements, are statistical and log quality is affected by logging speed
High pressure, high temperature (HPHT) versions of tools available
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════
The SP log is recorded by placing a movable electrode in the borehole and measuring the difference between the electrical potential of this movable electrode and the electrical potential of a fixed surface electrode. The recorded curve is scaled in millivolts (abbreviated as mV).
SP log requires conductive (water-based) mud and cannot be recorded in oil base mud.
SP Presented in Track 1 of API grid
Key characteristics:• SP Scale has no zero• Calibrated for sensitivity• SP reference is shale
Useful for a number of things:• Field-wide correlation• Net/Gross estimation• Vshale estimation• Rw calculation (discussed later)
Number of factors can affect the response and interpretation• Hydrocarbon saturation• Salinity contrast between mud filtrate and formation water resistivity
“Last resort” method of shale volume estimation, and Rwcalculation
Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════