Well Log Data

Slide 1

• This unit covers well log data
• We will look at 5 common logs; each measures a specific parameter as a function of depth in the well
• Work through the interpretation of a set of well log curves
• And talk about correlating surfaces using a series of well logs

Slide 2

• This chart lists along the top some of the most common well data and along the left the property/use of the data
• The red circles indicate major applications
• The smaller green circles show secondary uses

Slide 3

• We will briefly look at these 5 classes of key logs

Slide 4

• One parameter that is important to know is the size (diameter) of the well bore as a function of depth
• Some logs need the tools to touch or be near the rock formations to get accurate measurements
• So if we know a certain portion of the well bore is much larger than average, we may need to correct or delete data from these zones
• The diameter of the drill bit is the primary controll on hole size
  • Changes in stress state
    • borehole breakout
    • induced fracturing
    • creep of salt
  • Chemical Reactions
    • swelling clays in shales
    • dissolution of salt
  • Drilling Process
    • spiraling of the borehole
    • bit marks
   

Slide 5

• Here is a diagram of a caliper log
• It measures the size of the borehole by using 2 or more "arms" that are pushed out hydraulically so that they touch the sides of the bore hole
• The hydraulic systems are calibrated to give us the hole size in inches or centimeters
• This information is used to:
  • Correct logs that are sensitive to hole size
  • Determine how much cement is needed when casing the well
  • To obtain some lithologic information, e.g., large diameter zones (washouts) indicate unconsolidated (loose) rocks
  • Determine stress fields from hole break-outs

Slide 6

• We will introduce two logs commonly used to determine lithology
• Gamma Ray log
  • a scintillation detector (similar to a Geiger counter)
  • It measures the natural radiation from a formation
  • Shales have a high level of natural radioactivity, hence the curve is far to the right
  • Sands have low levels of natural radioactivity, hence the curve is deflected towards the left
  • Analysts draw a "shale base line" (dotted red) that averages the high values
  • Where the curve is near this line, the interval is interpreted to be shale/clay
  • The further the curve is to the left of the baseline, the more likely it is sand
• SP (spontaneous potential)
  • measurements the potential difference between the voltage in the wellbore and an electrode on the surface
  • This log is displayed so, like the gamma ray log,
  • Deflections to the right = Shale
  • Deflections to the left = Sand

Slide 7

• This slide introduces two porosity logs
  • Density porosity (solid black line)
    • measure the bulk (average) density of the formation (rock & fluids)
  • Neutron porosity (dashed red line)
    • measures the hydrogen content
• For both logs,
  • Deflections to the left = more porous
  • Deflections to the right = less porous
• The way we interpret these logs is to draw them together (in the same track)
  • If the dashed red line is to the LEFT of the solid black line = Shale
  • If the dashed red line is to the RIGHT of the solid black line = Gas Sand
  • If the dashed red approximately overlies the solid black line = Wet Sand or Oil Sand

Slide 8

• The sonic log measures the time it takes for sound energy to travel a specific distance
• This measure is of interval transit time – often referred to as Delta Time of Dt
• The units are microseconds per foot (msec/ft)
• The inverse of Dt is the acoustic velocity – very important to the seismic people
• The tools has at least 1 transmitter and at least 2 detectors (receivers)
• We measure the time difference in receiving an acoustic pulse at each receiver
• As shown by the dashed white lines, the difference in travel paths is a small distance within the rock formation (yellow arrow)
• Thus Dt gives a measure of the transit time (and hence velocity) within the rock formation

Slide 9

• Another common log measures the electrical resistivity of the formation
• Tools are designed to investigate different distances into the rocks
  • Shallow = a few inches into the formation
  • Medium = about 2 feet into the formation
  • Deep = about 4 feet into the formation
• If the deep resistivity is high = either HCs or low porosity tight streaks
• If the deep resistivity is low = shale or wet sand
• If there is separation between the medium and deep measurements, it means
  • The formation fluid is different from the drilling fluid, and
  • The formation is permeable to the drilling fluid
• On the log that is shown,
  • deep = black; red = medium
  • The region boxed in red – the curves are separated, hence formation fluid different from drilling fluid
    • e.g., if drilling fluid = water, then interval does not have water in the pore space
  • The region boxed in green – the curves are NOT separated, hence formation fluid same as drilling fluid

Slide 10

• Here is a well log with 3 tracks
• Track 1 has the caliper and gamma ray measurements
• Track 2 has 3 resistivity logs – shallow = green, medium = red, deep = black
• Track 3 has 2 porosity logs – black = density, red = neutron

Slide 11

• First we can interpret lithology based on the gamma log
• Red dashed line = shale baseline
• Depth range subdivided into 3 litho-types
• High gamma, near baseline = shale (green)
• Low gamma = sandstone (yellow)
• Intermediate = silt (brown)

Slide 12

• Next we examine the 2 porosity logs
• Where the red curve is to the right of the black (cross-over), the sands contain gas in the pore space
• The top ~80% of the thick sand has gas

Slide 13

• Now we interpret the resistivity logs
• For the bottom of the thick sand and the deeper sand, the medium and deep resistivity have similar values
• That indicates that the drilling fluid is the same as the formation fluid
• If the drilling fluid was an oil-based mud, then we have oil zones
• If the drilling fluid was an water-based mud, then we have wet sands (pores filled with water)
• There is a lot more that a log analyst can do – this is basic stuff that any geoscientist should be able to do

Slide 14

• If we have more than 1 well, then we can work on correlating stratigraphy (rock layers) from one well to another
• Well log correlation is an important part of understanding both regional stratigraphy and field-scale stratigraphy
• We use log response patterns somewhat like fingerprints to make interpretations, for example, that the sand at 10,523 ft in well 1 correlates (is equivalent to) the sand at 12,010 ft in well 2
• To remove post-depositional tilting, people often datum (flatten) the logs from different wells on what is believed to be a time marker, e.g., a bentonite (volcanic) layer, a regional unconformity, or the top/base of a paleontologic zone (e.g., top of the Eocene)

• There are two main ‘philosophies’ used in well log correlation:
• Correlate based on lithologic units – Lithostratigraphy
• Correlate based on assume time lines – Chronostratigraphy

• Which is Better? A matter of heated debate!!

Slide 15

• Here we have 4 logs, either gamma ray or SP
• Several lithologies have been interpreted
• Green = coastal plain sandstones and mud
• Yellow = shallow marine sandstones (beach deposits and nearshore sands)
• Grey = shelf mudstones (offshore mud/clay)
• We would like to make some well-to-well correlations

Slide 16

• One option is to key in on the thicker nearshore sands and correlate their tops
• That is want has been done here
• The wells have also been datumed (shifted up/down) to align on the top of the thick sands
• This is called a lithostratigraphic correlation, since we are using lithologic type to say what correlates (is depositional time equivalent) with what
• When units are given formation and member names, we are usually dealing with lithostratigraphic correlation

Slide 17

• An alternative way to correlate is to define units in each well that were deposited at about the same geologic time
• These time lines may come from index fossils – first or last appearances
• Other units are easy to define as time correlative – e.g., a bentonite (volcanic) layer associated with a single volcanic eruption
• What can be done in many cases is look for unique log responses that can be tracked from well to well
• This is what you will be doing in the next exercise

Slide 18

• Does it matter if we correlate using a lithostratigraphic approach versus a chronostratigraphic style?
• In an exploration stage, it probably makes little difference
• You would probably want to drill the structure given either interpretation
• BUT it can impact details that are important in the development and production stage
• Differences in the 2 interpretations can lead to differences in:
• Estimates of HC reserves (volumes)
• Development plans, and
• How you might enhanced recovery – e.g., drill injection wells
• For example, consider the 2 deepest sands in well C
• In the upper interpretation, these 2 sands are totally isolated from the younger, thicker sands
• In the lower figure, these 2 sands are correlated with the thick sands in well A
• In the lower figure, we could inject water into the sands in the A well and it could enhance recovery from the 2 lowest sands in well C, whereas this is not true with the upper figure
• Our experience is that using a chronostratigraphic approach usually leads to better explanations of enhanced recovery efforts than does using the lithostratigraphic approach

Slide 19

• It is time for an exercise
• We have 5 wells that define a SW-NE transect
• Each well has an SP log (left track) that we can use to differentiate shale, silt and sand
• The right tract has a resistivity curve shown with 2 gain settings
• In the shale zones, the resistivity curve has a lot of ‘character’ – somewhat unique highs, lows, and transitions from highs to lows
• Several unique ‘patterns are given in well 5 – labeled A to H
• There is also a regional unconformity marked on each well log

Slide 20

• You are given 2 copies of the logs laid out as a transect
• You guessed it – one is for a lithostratigraphic correlation, the other is for a chronostratigraphic correlation

• See the READ ME file

Slide 21

• The uninterrupted log cross section

Slide 22

• Exercise ANSWER – part 1 – the lithostratigraphic correlation
• The regional unconformity is correlated in long red dashes
• Tops of sands are correlated in dashed orange lines
• This is a possible correlation – if you have done it slighly differently – that is OK

Slide 23

• Exercise ANSWER – part 2 – the chronostratigraphic correlation
• The logs were positioned such that the A marker surface is close to horizontal (our datum)
• Note how intervals from the A marker to the F marker are approximately constant thicknesses
• The lower part of well 5 and 4 thins dramatically We had a time of regional erosion and possibly tilting

Slide 24

• These are 5 logs from a totally different basin
• Three environments of deposition/rock types are color-coded
• The horizontal lines are what have been interpreted as parasequences
• The question is how to correlate between wells – to fill in the gaps

Slide 25

• Here is the interpretation as published by Van Wagoner et al. The lines represent parasequence boundaries – and are taken to be time correlative (time stratigraphic) surfaces