Sudent Handout Quotusing Well.v2

7/25/2019 Sudent Handout Quotusing Well.v2

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Well Log Exercise Borowski Eastern Kentucky University

Using Well Logs in the Petroleum Industry

Well logs are used in the petroleum industry to evaluate reservoir rock and determine the fluids

within subsurface strata. This exercise shows us how to determine both lithology and fluid type in rocks

of an actual oil field !handeleur "ound #lock $% &ield' "t. #ernard Parish' Louisiana. (oreover' we)ll

investigate the occurrence of oil within the reservoir. *ur sub+ect well logs will be ,inch logs that occur

within the oil field' highlighting the principle oilproducing unit' the ## "and' seen on the type log below.

The field was discovered in ,-% within coastal waters of Louisiana' which are controlled by the

states out to a distance of / miles. The map below shows the location of the field' situated below coastal

marshlands developed on a portion of the subsiding (ississippi delta. Water depths in these areas are

generally only tens of feet so that wells are drilled with +ackup rigs that are floated out over the drill

locations. !handeleur "ound #lock $% &ield has produced over 01 million barrels of oil 2((#*3 24ohn et

al.' $11/3.

The lithologies we will observe are unconsolidated sandstone' cemented sandstone 2usually with

calcium carbonate' !a!*/3' shale or mudstone' and gradational lithologies between these end members.

5 simplifying aspect of this exercise is that there is no limestone within the stratigraphic section revealed

by the borehole. Pore spaces in the rock either contain brine 2salty water3' oil' or various amounts of

calcareous cement that occlude pore space. The reservoir rock is Upper (iocene in age 2Textularia 6L7' a

foraminifera3 and were deposited within a deltaic system much like that see today in !handeleur "ound.

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Use the cross section and your notes to answer the questions below.

&I8"T "T9P: your cross section comes in $ parts' carefully tape these two halves together with tape

affixed to the #5!; "I<9 of the paper. Then get some yellow' green' and blue colored pencils.

9ach of these logs has been trimmed to take less space. !onse=uently' only a small portion of the totaldepth of each well is shown and only tracks , and $ appear on the cross section. Track , contains the "P

curve whereas Track $ contains resistivity curves. *ne of the logs' the 5tlantic ">L 0%0$ ?,@' still shows

the log scales for your reference.

Aote the vertical scale that separates the ?,0 and ?0 logs that marks the elevation of subsurface strata.

The sign of the scale is negative 23 because these elevations lie below sea level' the datum. 9ach of the well

logs 2excepting the ?,03 have been positioned according to their subsurface elevation' thus this is a

structural cross section. This fact will become pertinent later in the exercise.

,. 8efer to your notes: What is the "P curve and what does it measureB What are its unitsB

$. When the "P curve occurs at far right what is the lithologyB __________________________

When the "P curve occurs at far left what is the lithologyB __________________________

/. What is a shale baselineB <raw the shale baseline on the well log at far left 25tlantic ">L 0%0$ ?,03.

What do deflections of the "P curve to the left of the shale baselength meanB

8esistivity curves measure resistivity to an electrical current within the borehole wall rock' and thus give

proxy information about the type of fluid with the pore spaces of the rocks. In petroleumproducing

sedimentary sections' the fluids contained within the pore spaces of rocks are usually saltwater or brine'

i.e.' pore water with high concentrations of dissolved ions.

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0. Would brines have high or low resistivityB WhyB

%. What would be the resistivity of hydrocarbons C high or lowB WhyB

. If rocks contain brine' in which direction should resistivity curves move C left or rightB

__________________

If they contain petroleumB __________________

Look at the well log at far left 25tlantic ">L 0%0$ ?,03. The units in the depth track are in feet and note

that the well logs starts at about 0DD1 feet in depth.

@. What is the depth difference between successive horiEontal linesB ___________________________

This display is called a oneinch log because the vertical scale is , inch F ,11 feet. GHerify thisJ

Look in the depth interval between %,$/ to %$11 feet.

D. What is the dominant lithologyB Kive your reasoning.

Aote that the first $ resistivity curves in Track $ plot to the left the third curve to the right is the

amplified resistivity curve' which are the same data plotted on an amplified scale 21 to $ ohmmeters

instead of 1 to ,1 ohmmeters3. We will generally use the first $ curves.

-. What kind of fluid fills the pore spaces in the interval %,$/ to %$11B Kive your reasoning.

5gain refer to the well log at far left 25tlantic ">L 0%0$ ?,03. The arrow points to the ## "and' the main

reservoir in !handeleur "ound #lock $% &ield' whereas the sand unit ,1M feet above 2%1/%) measured

depth3 is the ## (arker. The marker stratum is somewhat sandy as indicated by the deflection of the "P

curve intermediate between that of pure shale or unconsolidated sand.

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The top of the ## sand and ## (arker is where the inflection in the "P curve occurs during the

transition from shale to sand 2or sand to shale3.

,1. <raw a line showing where each of these strata occur in all wells in the cross section' using the above

drawing as a guide. Label each stratum.

This process is called elog correlation and mimics lithostratigraphic correlation although we are

inferring lithology by proxy using well logs rather than referring to actual lithology as determined by

rock specimen.

Aow go to the next well in the cross section' the 5tlantic ">L 0%0$ ?0. Look at the top of the ## "and

2N%$%/ ft in depth3. Aote there is a depth interval of resistivity much higher than that in the ## (arker

above' or most of the ## "and below. The marker)s intermediate resistivity is due to partial cementationof pore space by calcium carbonate that increases resistivity of the unit by increasing the tortuosity of the

path of the electrical current.

,1. What fluid occurs below %$%/ ft in the ## "andB Kive your reasoning.

,,. 5t what depth does this depth Eone of high resistivity end in the ?0 wellB _______________________

,$. What fluid is contained in the ## "and below this depthB Kive your reasoning.

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,/. Oow thick is the column of this fluid in the ?0 wellB

<raw a line from bottom of this fluid on the well log' analogous to the tops of the ## (arker and ##

"and done earlier. <o this for all the wells with this anomalous resistivity' and colorin the high and low

resistivity Eones green and blue' respectively' so that your well logs look like this:

Aote that the upper portion of the ## "and is colored in yellow indicating sand color each well log of the

cross section in the same manner.

Aow' using a straight edge connect the top of the ## "and from well to well' not including the ?,0 well.

ou have +ust drawn the top of the ## "and reservoir across !handeleur "ound #lock $% &ield'

constructing a structural cross section complete with lithostratigraphic correlations as do professionalgeoscientists working in the petroleum industry. Looking closely' one can determine that the highest

elevation of the reservoir sand occurs in the 5tlantic ">L 0%0$ ?, well' the discovery well for the field.

Let)s turn our attention to the distribution of petroleum in the field.

Looking at the wells containing petroleum' what is the pattern of oil occurrence in all the wells C which

fluid is higher in elevationB

Why does this pattern occur in the ## "and reservoirB GThink of salad oil and vinegar 2N water3.J

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Let)s imagine a tub of water' place a clear bowl into it' and turn the bowl upsidedown. Aow let)s take a

big' fat syringe full of salad oil and in+ect that oil under the lip of our inverted bowl.

What happens to the oilB What is the relationship between the salad oil and waterB (ake a drawing

This experiment is analogous to the nature of the relation of oil and water within the pore spaces of

reservoir rocks. (any oil fields have a horiEontal oil>water 2*>W3 contact throughout their extent.

5 horiEontal contact means that the elevation of the *>W contact should be at the same elevation in every

well of the field where oil floats on water in the pore spaces of the reservoir rock.

Let)s investigate the elevation of the *>W contact in the ## "and. 9ach well in our cross section is

referenced to sea level by the ;elly bushing 2;#3 elevation' which is measured in feet above sea level. &orexample' the ;# elevation of the ?0 well is $%.1 feet above mean sea level. To calculate the elevation of the

oil water contact' subtract the ;# elevation from the measured depth elevation of the contact 2$% feet C

%$DD feet measured depth F %$/ feet subsea' or below sea level3. The illustration below shows the

rationale behind the calculation.

Calculating the subsea elevation: Subsea elevation = K elevation ! "easured depth.

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<o this calculation for each well showing your work at the bottom of each well log' and giving an example

of this calculation in the space below.

5re the *>W contact elevations across the field uniformB _______________________

9xamine in particular the 0%0$ ?0' ?/' and 0%0% ?0 wells.

Using your each other and your instructor as a resource and thinking stratigraphically about how sand

bodies are deposited and their lateral continuity or connectivity' please list some reasons why *>W

contacts are often not horiEontal.

These are reasons why sedimentary geologists are needed in the petroleum industry. 8eservoir sand units

are not uniform and their thickness and connectivity vary laterally over the expanse of a petroleum field.

8ealiEing that oil is distributed in many compartments within petroleum fields allows maximum recovery

of petroleum through additional drilling and enhanced oil recovery pro+ects.

89&989A!9"

4ohn' !.4.' #.L. 4ones' #.4. Oarder' 8. #ourgeois' (.#. (iller' $11/. &ield studies in the !handeleur

"ound area' offshore Louisiana 2state waters3. #ulf Coast $ssociation #eological Sciences

%#C$#S& Transactions' %/:/%,/$.

'nd exercise

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Sudent Handout Quotusing Well.v2

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