Formation Evaluation Part 2

Density Log Applications
• Porosity estimations
• Mechanical properties (in combination with sonic tool)
• Acoustic properties (in combination with the sonic tool)
• Shale compaction and pressures
• Natural fractures
• Gas identification (in combination with the neutron tool)
• Lithology (in combination with other tools)
Density Log and Radioactivity
-active log-emits GR from chem source, cesium 137, and reads scatter radiation
-cesium 137: .662 MeV GR energy
• Gamma rays emitted from radioactive source
• Gamma rays collide with electrons in formation, losing energy
• Detectors measure intensity of backscattered gamma rays
- Low energy GR relate to - Lithology
- High energy GR relate to - Density
* more dense rock, the less amount that returns to the source; related to density
Gamma Ray Interactions with Matter
• Pair Production; not of significanceto density tool operation since source strength is .662 MeV and it requires 1.02 MeV
• Compton Scattering:
-Medium to High Energy GR's
- scattered by electrons in formation
- each interaction loses energy
- more electrons => more scattering
- Related to elect. & rock den.
• Photoelectric Adsorption:
-Low Energy GR's
- absorbed by atoms
- more electrons => more absorption
- Indicates the atomic number - lithology
-second section because needs lower energy
Detector Count Rates and formation density and atomic number
-lower counts means more electrons which means higher density; graph in density section (2nd part) lower curve has higher density
-lower counts means higher absorbed means higher atomic number; on graph in the first section, lower curve has higher atomic number
bulk density and density logs
Bulk density, depends on:
• Lithology
• Formation porosity
• Density and saturation of fluids in pores
density porosity=pma-pb/(pma-pf)
Corrections for density log
- often needs correction; caliper usually run with it in same run
-density correction curve is measurement quality
-due to breakout in wellbore
-iregularities in mudcake
-poor pad contact
- if correction > .2 g/cc, bulk density curve is invalid
Coals and density logs
- coal has low density so stands out in a coal clastic sequence
-found interbedded with sandstones
- density depends on thermal maturity; higher thermal maturity and depth, density increases
-coals have issues with impurities so often use cut off of 1.75 g/cc
Shale compaction with Depth; density logs
- increase depth, increase density
-over pressure decreases density; lower rain contact, supporting over burden with fluids so porosity increases and density decreases
- on log with bulk density; increases and then in over pressure zone, suddenly decreases
-often appears in shales as much as several hundred feet above high pressure perm sands
density logs
• Very reliable tool
• Open or cased hole
• Used to determine
- bulk density
- Porosity
- lithology
• Shallow depth of investigation (10-15 cm); 6 to 8 in; limitation
- track 3
-RHOC: density
-DPHI: porosity
- scaled in different directions
-lower density and higher porostiy in region: gas
-lower density and higher proosity in sandstone
Gas Effect density logs
-low density, high density porsity
-low neutron porsity
-no acoustic anomaly
Factors Affecting Density Log Response
1. Shales and clays
• May cause porosity reading to be too high or too low
• Vsh and density of sh can be obtained from log readings in shale zones
2. Hydrocarbons
• In oil zones, phc = po which can be measured from fluid samples
• In gas zones, phc = pg which can be measured or
calculated using gas properties
• Gas will cause anomalously low density, and thus,
high density porosity
environmental effects on density logs
-borehole size
-mudcake effects
-mud filtrate invasion
-quality of the borehole
limitations of density logs
1. Lithology
• Matrix (pma) must be known for porosity computation
• Shales vsh and psh must be known to compute porosity accurately
• No need for compaction correction
• Shale effects very evident
2. Fluid Type
• Residual Oil causes pb to read slightly lower and computed porosity to read slightly higher due to the lower density of oil compared with water
• Residual Gas causes pb to read lower and computed porosity to read higher due to the very low density of gas compared with water
why is it imp to estimate lithology
-reliable porosity assessment
-reliable assessment of fluid saturations
reliable rock typing
detect zones for perforation and completion jobs
well logs that are sensitive to lithology
GR, PEF, Density, Neutron porosity, Acoustic, ECS (does a lot of the work for you; combines other logs and calc for you)
Common porosity crossplots
• Neutron-density
• Sonic-neutron
• Sonic-density
• MID plots
• NGS (Natural Gamma Ray Spectrometry)
• All have complicating effects
- Shaliness
- Hydrocarbons (gas)
- Fractures
Neutron Density Crossplots
• Most frequently used
• Developed for clean, liquid-sat form
• Boreholes filled with water or water based muds
2 measurement porosity Crossplots
• Two measurements determine two unknowns
- Formations with one lithology
• Lithology
• Porosity
- Formations of two known constituents
• Can determine a more accurate value of porosity
• Can determine the percentage of each mineral
- Complex lithologies
• Can determine a more accurate value of porosity
• Cannot determine percentage mineral makeup
Dual mineral Model
-porosity, Vm1, Vm2 are 3 unknowns and 3 eq required
Neutron Density Crossplot
-if calcite and freshwater, it will plot on calcite line
neutron density calc from pb so doesn't make a diff if there or not
- on the plot connect line of equal porosity
-go straight down to find density
- SS is higher than calcite because reads too high
• Density log may be displayed as porosity
- Density-Neutron overlay for water- filled lithology
- Curve order as with g/cc scaling
• Shale/Gas effects
- Across litho lines: Lithology most affected; as you cross from sandstone to limestone to dolomite
- Along porosity lines: Porosity least affected (along lines)
• Response lines change with tool type
shale effect on Neutron Density Crossplot
-shales move to the lower right b/c higher neutron density b/c reading bound water
-gases move upper left b/c gases read too low density (density is from low to high going from top to bottom)
- fractures cant be read of the logs; has no effect b/c both logs respond to total porosity; sonic neutron log can
sonic neutron and sonic density
-gives total porosity; cross the two to get fuggy or fracture porosity
-sonic log doesn't see fractures or vugs not total porosity
sonic neutron crossplots
• Developed for clean, liquid saturated formations; like neutron density
• Boreholes filled with water or waterbase muds
-shale is the NE region of the map
-Fractures are in the south
-gas in the NW region
-sonic on y moves from low to high
-neutron: low to high
sonic density
• Poor lithology and porosity
• Multiple lines
• Useful for Evaporites and Vsh
- not a lot of sep on curves which is hard to use
shales are distinctive; higher density and higher travel time
- to find Vsh locate 100% shale and clean formation and grid to give Vsh; lines lof 20%, etc.
MID Plot
Matrix Identification Plots: 2 Logs
1. Determine Apparent Matrix Density from neutron density plot
pb is where the neutron porosity value is
pma is if go linearly down; when porosity value is 0
2. Determine Apparent matrix transit time from sonic neutron plot
delta t is where nuetron porosity is
delta t ma is where porosity is 0
3. determine lithology using pmaa andtmaa plot
Elemental Capture Spectroscopy Sonde
- use AmBe neutron source to measure relative elemental yields based on neutron induced capture GR spectroscopy
lots of uncertainty b/c based on just 1 region
seismic data
- primarily acoustic data
-rock and fluid data vs. 2 way travel time; needs to be adjusted for depth
-acoustic waves; rock and fluid data vs. depth
- sonic is interface between logging and seismic data
applications of sonic logs
• Determine lithology (needs crossplots) and porosity(affects speed waves travel) more porous rocks are the slower travel time b/c acoustic waves depend on grain to grain contact; more space, means less grain to grain contact
• Determine rock mechanical properties, such as Poisson's ratio; imp for hydraulic fracturing
• Determine Rwa
• Detect gas; lower molecules/vol; slows down travel time
• Detect natural fractures and permeability
• Evaluate overpressure in basin; lowers grain to grain contact
• Combined with density logs to produce seismic traces (synthetic seismograms)
• Evaluate cement bond
Acoustic (Sonic) Log Tools
- transducers oscillate to produce sound; recievers capture the sound
-try to run it centered with a 3 arm caliper to get hole diameter which will cause issues in respose
- early tools had 2 recievers; modern tools have an upper and lower transmitter and 4 recievers
* helps to compensate for borehole breakout or for tools that aren't centered; can avg responses in both directions and wipe out problems
-modern tools can read 6 to 18 in into formation; reads beyond damaged zone
usually has delta t compression and delta t shear
• Travels thru mud & rock
• Velocity depends on
- Lithology
- Porosity/Pore fluid(s)
• Fastest mode
- mud 5,200 ft/sec (190 usec/ft)
- rock 18,000-25,000 ft/sec (55 - 40 usec/ft)
- much faster in rock than in liquid
• Weakest mode
- Fracture insensitive
-compressional wave; particles oscillate in the direction the wave propagates
-travel the fastest; first to arrive at the reciever
-good for fracture density and orinetnation
• Travel thru rock only
• Velocity (Vs) depends on
- Lithology
- Shear modulus
• Slower mode
- 11,000 -14,000 ft/sec (90 -70 usec/ft)
• Stronger mode
- Fracture sensitive
- Shale sensitive
- 2nd to arrive back at reciever; travel time is greater
-fluids have no resistance to shear so shear waves won't travel through fluids; good for detecting gas
-travels 90 degrees to direction of travel
guided waves
- can be propagated in a finite medium; borehole
1. Rayleigh waves: occur at the mud formation interface; slightly less speed than shear waves
2. Stoneley waves: travel in mud by interaction between the mud and the formation ; very slow
-reason why need receivers at the bottom
-velocity is lower in mud compressional wave velocity
-from them can find fractures and assessment of perm
arrival times
1. P waves; smallest amplitude
2. shear waves
3. rayleigh waves
4. mud waves
5. stonely waves; largest amplitude
Factors Affecting Sonic Log Response
• Unconsolidated formations
• Naturally fractured formations
• Hydrocarbons (especially gas)
• Rugose salt sections
Causes of Bad Sonic Logs
• Road noise
• Cycle skipping
Road Noise
• Caused by tool movement along the borehole, generating a high frequency noise component that is superimposed onto the normal acoustic signal
-tool or caliper is dragging on borehole wall causing noise that can interfere with reading
• Far sonic detectors are more affected by road noise than near detectors because of the reduced signal amplitude with increased travel time
• Attenuation (decreased amplitude) of the compressional acoustic wave is the major cause of poor sonic logs; occurs over distance of travel
• Attenuation results in the signal at the receiver crossing the threshold amplitude later than for a stronger signal.
- can cause some signals to not be reduced; causes cycle skipping
- in fractures, stoneley and shear waves are attenuated
Cycle skipping
• Low sonic transmitter strength may result in less than optimal receiver signal amplitudes
• Under extreme conditions this will result in cycle-skipping
- will miss a peak; amplitude is too low to trigger a response in reciever
- next wave comes in and is interpretted as that missing wave that passed underneath threshold
- get very low veloctiy interpretation
-happens if there are washouts, presence of gas in mud (very low velocity and miss a cycle), threshold level is set to low
- abrupt skikes on sonic log
Sonic as a Porosity Tool
-can be run in both open and closed hole; open is for mech prop and formation evaluation; closed is for cement quality and casing integrity
Sonic affected by:
1. Lithology
2. Porosity
1. Fluids
2.Compaction/ consolidation
-also affected by borehole conditions
-young formations usually undercompacted; gulf coast
Estimating Porosity from Well Logs
-Openhole logging tools are the most common method of determining porosity:
• Less expensive than coring and may be less risk of sticking the tool in the hole
*Coring may not be practical in unconsolidated formations or in formations with high secondary porosity such as vugs or natural fractures.
-If porosity measurements are very important, both coring and logging programs may be conducted so the log-based porosity calculations can be used to calibrated using the core-based porosity measurements.
triple combo
1. something to determine lithology; passive logs; SP, GR, Caliper
2. resisitivity to evaluate fluids
3. porosity: density, neutron, or sonic
sonic velocities of common fluids and matricies
- increasing velocity, decreasing travel time, increasing density: quartz, limestone, dolomite
- shales have a very large range for both time and velocity: due to compaction:
*as compact clay to shale have a colume reduction as much as 50%; squeezes out water and increases grain to grain contact
*compact SS: only about 10% reduction in vol
* if shale is <100 usec/ft: considered compacted; if greater than undercompacted
- acoustic waves great for identiying coals; low velocity
-decreasing velocity, increasing time: water, oil, time
* in gas zones: shear waves travelling only in solid parts, so will show up; P waves show lots of small fluctuations
* in shale, shear time is small
wyllie time averge
-uses delta t to find porosity of formation
porosity=t log- t ma/t fl-t ma
-consolidated, compacted formations
correction for sonic porosity; wta
-wyllie's time avg needs compaction correction factor for:
1. shale
2. if undercompacted or uncompacted; if undercompacted; C is 1
Rock Properties affecting acoustic waves
- Reservoir Rocks depend on location:
*west TX and Saudi Arabia: carbonates
*Gulf Coast: sandstones
-most abundant rock type is shales (type of sed rock); 75% of surface rocks
-Carbonates are known for having vuggy porosity that aren't well connected
-want to know not just the type of porosity, but how connected pores are
1. primary
2. seconadary
Types of porosity
• Developed at deposition
• Typified by
1. Intergranular pores of clastics or carbonates
2. Intercrystalline and fenestral pores of carbonates
• Usually more uniform than secondary porosity
• Developed after the sediments were deposited
• More complex and usually less predictable than primaryporosity
• Typified by
- Dissolution pores of clastics or carbonates
- Cementation (clays) (clays have micropores so not good at storing HC)
- Fractures (perm of fractures much better than perm of matrix; so like them unless leakage)
*Diageneis: all physical and chemical changes in the rock after original sed were deposits
• Size and shape of grains; grain shape affects porosity, not grain size; both affect perm
• Sorting; porsoity and perm
• Rock - fluid interactions
- Dissolution
- Cementation
• Fractures
• Stress; not a huge affect on porosity; big effect on perm because decreases size of pore throats
• Formation damage
1. Framework: carries the load
2. Matrix: small grains between framework; if they are there, they are reducing the sorting and decreasing the perm; reduces storage capacity
3. Cement: chemically precipitated when water flows through pores
4. Pores
-1 to 3 are considered Matrix by engineers
- if secondary porosity, there will be 2nd stage cement, fractures that form from tectonics
Clastic depositional systems
- primary control on porosity and perm is depsitonal environ and associated dep facies
-grain sizes depend on energy of transporting mech (water, wind, and ice)
*higher energy, light weight grains will be trans and heavy grains will be depsoited; makes a diff in res quality
-fluivial channel filled sed facies have best res quality
-strike: parallel to shore
-dip: perpindicular to strike
Depositional systems determine:
• Where reservoir and source rock occur;
• Geometry and orientation of reservoirs; and
• Initial reservoir quality.
Channel Fill Reservoir
-channel fill: higher energy
*levee deposited when river overflows banks; deposits heavier grains to channel and heavier ones farther away
*floodplain: mud; peat which makes coal
-Lagoon: bioturbation
*clams, shrimp, etc. are stirring up sediments that were once laminated
*negative impact on res quality
*flood plain: higher capillary pressure; lower perm; greater irreducible water saturation
relationship between porosity and permebiliaty
- in most sandstones relationship between the 2
-carbonates not nearly same relationships can'tpredict it as well b/c much more soluble and reactive than SS
-Determine porosity and perm from core and create a plot and then to the same from log; need them to match
-no common log to measure k, so use one of the porosity logs to measure or estimate k
Log patterns and grains
-Upward coarsening: sand moves over silt, silt moves over clay
-GR or SP Log patterns reflect grain size and mineral composition, which in turn, are related to depositional environment and sediment transport energy
• Particle sphericity and angularity
• Packing and Sorting (variable grain sizes)
• Texture
• Cementing materials (water moving through has ions that will precipitate ions as mineral; trap original water as bubble in the cement)
• Overburden stress (compaction) (grains rotate)
• bioturbation (lifeforms stirring up sed; messing with original pores)
• Vugs, dissolution, and fractures
-logs telling you that there's porosity, but not telling you how interconnected they are
-fractures gen increase res quality
-cubic packing; Porosity = 48%; not found in nature
-rhombus packing; after compaction cubic turns to this;
Porosity = 27 %
1. Primary:
Intergranular-Interstitial Void Space Between Framework Grains
2. Secondary (diagenetic b/c results from changes of pores):
-Dissolution: Small Pores Mainly Between Detrital Framework Grains or Cement
-Micropores (clays have lots of pores and lots of bound water; not related to effective porosity): Partial or Complete Dissolution of Framework Grains or Cement Small Pores Mainly Between Detrital or Authigenic Grains (Can Also Occur Within Grains
-Fractures: Breakage Due to Earth Stresses
Mech of compaction
1. platy grains-shales; flaten out when compacted; shales are fisal splits on planes
2.Non-platy Grains: sandstones
3. Ductile Grain Deformation; structural clays