Basic Gamma Ray Correlation &

Introduction to Geosteering

Tom Arnold, PetroSkills Instructor

Learning Objectives

Introduction to Correlation

Basic Understanding of Correlation

How Correlation Relates to Geosteering

Introduction to the Gamma Ray Log

Basic Gamma Ray Correlation

How Correlation is used in Geosteering

Define Correlation

…a relation existing between phenomena or things.

The Outcrop

What correlates in this image?

How is Correlation Done in Drilling?

Gamma Ray Log

Gamma Ray logs measure the naturalradioactivity present in rocks. Shale generally produce higher levels of gamma radiation than sandstones or limestone. This is because they contain more of the naturally radioactive minerals like potassium, thorium, and uranium. Therefore the Gamma Ray discriminates the shale from clean formations to estimate the relative shale proportions.

NOTE: In holes using salty drilling fluids, like oil based mud, the gamma ray log is very effective.

1. Average deflection value of the bed

2. Size of a bed3. Shape of the

deflection 4. Make note of high

and low spikes5. Depth of the bed6. Use other tracks in

the same manner

Correlation – Characteristics to Correlate On

Avg. Deflection

Size

of B

ed Shape of Deflection

DE

PT

H

Use other tracks

SPIKES

Correlation Using Gamma Ray Logs

In this example correlation is being done on the Eocene boundary. Notice the depth difference between Hole 1171D and Hole 1172D. It is around 100 feet down dip from the latter.

Notice also correlation being based on the spikeat the top of the formation.

Correlation – ExampleHere correlation is performed on the top of the J SAND

Similar Shape in ALL

Similar Spikes and Average Deflections

This is how log correlation is done!

Logs are correlated based on location of similar features.

J SAND

Gamma Ray Correlation

Difference - about 20 feet

Example

Higher by 20’

7960’7940’

What is Geosteering“Geosteering is the interactive, geological placement of a precise, high angle well path within a formation. Geological, as opposed to geometrical, placement is required because of uncertainties in the position of targets due to the unpredictability of structural and stratigraphic variations which can occur in any field. Real-time LWD data is compared to modeled log responses to guide the well path to optimum reservoir layers.” (MWD Vendor)

Geosteering Is About Geology

What would a gamma ray signature look like through a complex, folded structure such as this?

How would you steer a well under these conditions?

Geosteering Is About Geology

Introducing faulting along with folding really makes the task challenging.

Note: Numerous faults are visible in the above photo.

Well Bore

Target A

Target B

Being able to SEE the structure defined by the LWD data is critical in effective geosteering.

LWD

Geosteering Is About Geology

Introducing Steep Dip Adds to Complexity

Well Bore

LWD

Target

Unusual Structures Can Be Found Anywhere!

Understanding the geology is critical in providing accurate geosteering interpretations.

Target

Well Bore

Correlation in Geosteering

From correlation to cross-section, the location of the wellbore within the lateral can be defined.

Offset well- BLACK

Current well-COLOR

Wellbore

Target

Gamma Ray

The Terminology of

Horizontal Drilling

Quick terms for a directional well

Vertical –Y axis portion of the well

Horizontal/Lateral – X axis portion

of the well

KOP (kick off point) – depth at

which the well departs from

vertical.

Curve – section between the KOP

and landing point – usually curve

shaped

Landing Point – a point where the

well transitions into the

horizontal/lateral portion of the

well. This is usually determined by

depth or inclination.

Basic Terms

TVD-True Vertical Depth

RKB-Rotary Kelly BushingHL- Horizontal LengthTVDRT -True Vertical Depth for the Rotary TableRKB-Rotary Kelly BushingVS- Vertical Section

Predetermined point at which the well transitions from the curve to the lateral portion of the well

Signified either by a certain inclination or TVD

Factors that determine Landing point

Target formation

Formation dip

Possible reasons to adjust landing point (not limited to)

Adjusted KOP

Up hole (vertical) faulting, thinning or incorrect geoprog

BHA not achieving build needed

Landing

Geosteering Requirements

Geosteering requires that we are aware of where we are within our target zone and if a threat exist for us to exit our target zone. When we see that our drilling inclination is greater or lesser than our bed dip, we need to be able to calculate at which point we could exit our target zone if the drilling inclination is maintained and the bed dip is constant. One simple formula for calculating this is: we will move 1.74’ for every degree of separation we have per 100’ drilled.

Basic Geosteering Requirement

Remember: In Geosteering we are provided three things..•Survey Data•LWD Data•Offset Typelog

From this data we calculate all directional parameters and make correlations relative to the wellbore. If any of the input data is in error, EVERYTHING derived from it will be WRONG!

Accuracy Counts!!

Survey Data

Provided by the LWD contractor at the well site, the survey data and current gamma ray las file are sent to the operators distribution and used by the geosteerer as either Excel ,LAS or PDF files

The MD, inc and azimuth are input into the geosteering software. All other parameters are calculated.

Note: Initial TIE point must be input before any calculations are possible.

When Survey and LAS data Arrive, Import It NOW!

Helps in determining speculative correlations Identification of possible geologic hazards

1. Faults2. Synclines3. Anticlines4. Karsting events

Keeps you in tune with what the well is doing and on track with the data1. Verifying that the survey looks correct2. Verifying that the LAS tracts look correct3. Verifying that the data you are getting is

the most recent

Small Errors…BIG PROBELMS

Calculated survey data must be accurate. Even a few tenths can make a big difference in a 10,000 foot lateral!

Now

Later

Depth

Drilling inclination is the direction up or down of the bit.

Azimuth is the direction (N, E, W, or S) of the well

Range is from 0 - 359

Drilling Inclination & Azimuth

Drilling Inclination is different from dip!

Calculation of Bed DipGeosteering Bed DIP is only ‘apparent’ dip! There is insufficient information available to calculate true

bed dip.

Basic geosteering is understanding that when we have an offset between our drilling inclination and bed dip, we will see a change in our gamma pattern or response.

Advanced geosteering is understanding that changes in drilling azimuth can also affect our gamma response, although less dramatically than Inc. and bed dip

We must understand that while our beds may be dipping up or down, they are also dipping in a certain “direction” or azimuth.

Stratigraphic azimuth must be taken into consideration when gamma responses fall outside the realm of expected dips.

Geosteering Considerations

While measuring a formation with a gamma tool, we look for the amount of gamma radiation that the formation is emitting, potassium, thorium, and uranium.

Formations can either give a high gamma reading or a high amount of gamma ray energy or low gamma reading or low amount of gamma ray energy.

A broad classification is to say that formations with a higher clay content will have a high gamma reading and those with low clay content will have a low gamma reading

Gamma Ray Data

Gamma Interpolation

Before After

From the basic information defined above, the TVD, Northing, Easting and Vertical Section are calculated. LAS data from the well does not always provide this information.

In order to correlate within the horizontal, every foot of gamma data must have a inclination and azimuth interpolated for it.

Survey data is used to derive inclination and azimuth.

Offset TypeLog LAS

The offset typeloggamma ray LAS file contains the data that will be used during drilling to correlate the position of the project well.

The operator must provide the top of the target in this well!

Geosteering in the Curve

Though the majority of our time geosteering a well will be spent in the lateral, the curve or landing phase is the most critical (especially in areas where the operator doesn’t have a lot of control points)

Geosteering in the Curve Phase

Giving target top projections during the landing of a well is the primary function of geosteering in the curve. It lets the operator know if the planned land out depth needs to be changed. These changes are better done sooner rather than later especially, if the land out has to be moved up a significant amount. Landing high to target and then moving down into target eats up vertical section. Landing below target and moving up is preferred by some operators but is sometimes limited by the geology below the target zone.

Geosteering In The Curve

NOTE 1: This is also helpful in giving us a full picture of our target zone with our MWD tools measurements

NOTE 2: When giving target top projections we have to remember that the projection is at current VS and not at our land out VS. If dip is present those two depths will be different

Data will vary in value so normalization is the process of getting the offset data to match your real time data in value. You can normalize the data to lay on top of your offset or just a little below or above your offset data

Geosteering in the CurvePhase

Gamma Ray Normalization

Be sure all peaks and valleys are identified.Not NormalizedOver Normalized Correctly Normalized

As the data comes in, you will continue to match the last section to your offset. This is done by adjusting the “TVD” value. The TVD value usually represents the TVD top of your target area. This number will change periodically and is considered a prediction of Target Top.

Try to keep from giving dip predictions. Dip during the curve is a pitfall situation because of the curvature of the well bore and the constant azimuth change will result in an incorrect dip value.

TVD Adjustment During Curve Phase

Tie the data into the offset data by taking the last section of (gamma) data and finding the exact spot where it matches your offset data adjusting your TVD value (target top TVD) so they lay on top of each other

Do not pay attention to anything above your last section of matching data. In most cases ,it will not match our offset data exactly nor lay right on top of it. We have to remember that we will always have variation.

Correlation In The Curve Phase

•Keep track of where the formation tops come in during the curve using gamma data. Use Mud logger information if it is available. Compare the drilled tops to the estimated tops from the geoprog. This allows for the identification of certain trends:

1. Thinning of beds2. Thickening of beds3. Possible faults

•This serves as a secondary confirmation device when the possibility of land out changes are made.

•Try to maintain one full piece of data through the landing and not break it (data) up into sections. This could cause false faulting which is misleading.

•Create another section close to target or once land out is achieved

•Once land out has been achieved you will begin lateral correlations

Notes On The Curve

The Problem With TVD

Apparent

Downdip: Everything Appears Too Deep & Too Thick---------------------------

Updip: Everything Appears Too Shallow & Too Thin

The Solution: Relative Stratigraphic Depth

RSD is measured in feet above and below the top of the target as seen above. Everything is correct: thickness & depth.

Geosteering in the Lateral

In the lateral we will begin to use some functions of geosteering that we didn’t in the landing phase such as:

Dip calculationsTarget projectionsRepeat gamma correlations

Geosteering in the LateralIts important to be more than familiar with the target area and its characteristics

Some operators will give a “sweet spot” within a target area calleda “soft” target area or ‘centerline’.

Offset Bed Thicker Than the Target

The dip appears to turn opposite to the well path.

Wellbore

Vertical Offset

LWD

thicker

thinner

Vertical Offset

LWD

Wellbore

thicker

thinner

Offset Bed Thinner Than the Target

Dip appears to turn sub-parallel to well path.

When selecting your data for correlation,try to select as much as possible. Thiskeeps from over steering and gettinglost in the minute details. Too broad ofan area will lead to under steering andmiscorrelation as well.

Over Steering vs Under Steering

This is the most common problem to people new to geosteering!

The main principle of overlaying gamma still applies as it did in the curve but because now we are dealing with drilling inclination and formation dip, we have to either squeeze or expand the gamma to overlay it; this is labeled the dip function.

Gamma Characteristics During the Lateral

Compression will modify dip

In this example of lateral gamma, we have close to a 100’ of data that has been squeezed andstretched to match our offset gamma (in grey). This squeezing and stretching is actually affecting ourdip calculation. This dip calculation is based on:

1. Wellbore inclination2. Wellbore azimuth3. Correlation of the section

This dip adjustment is something that we don’t do in the landing phase but do constantly during the lateralphase. Getting the best possible match for the gamma as we drill our lateral is critical.

Note: changing dip affects correlation

Relating Correlation, Dip and Azimuth

Steer the Well in Segments, Incorporating as Much Data as

Possible

Small segments can lead to incorrect interpretations.

As we drill the lateral, we will continue to adjust the dip function to make our gamma “correlate” either to the offset gamma or to itself. If we follow the gamma in purple we see the progression of the well down the strat column and then back up again, continuously correlating to itself. This up/down progression will happen many times during the lateral. Because drilling inclination and dip are constantly changing, these correlations may have to be broken up into what we call correlation blocks.Correlation blocks allow us to better match our gamma to our offset or previous gamma with more efficiency.

Each different color indicates a specific strat block.

Creation of STRAT Blocks

Other Requirements

Initial Requirements: Well Plan•The well plan is how the well is expected to be drilled and is formed by the directional company with input from the operator

•Will be either in PDF, Excel, TXT or CSV format or all

•Can be imported into most steering software

•Is not to be considered a blueprint for the drilling of the well.

•Verify all information on well plan with the geoprog

Consist of 3 major partsVerticalCurve

Lateral

Initial Requirements: Well Plan

Pertinent information contained within the well plan

Grid coordinates of location

Kelley Bushing

VS azimuth

Well path data

(Verify that all this information matches the information that you have from the operator and information that you have put in the steering software)

When drilling, verify real time azimuth against plan azimuth

Real time inclination will most likely not match plan inclination

The Well Plan Is just a PLAN!!

There is always a level of geologic uncertainty, no matter how much control is present. Unexpected

dip changes happen regularly. In this case, had the plan been followed, little of the target would

have been encountered during the lateral.

Offset A

Offset B

Wellbore Plan

Well Plan

Geological Prognosis: GeoProg

The GeoProg will contain information about formation tops and thickness, top and base of the target, contact information , anticipated structural anomalies, drilling hazards or other information that will be critical for completion of the project.

Grid data or depth converted seismic data

Given in X, Y, Z formatX - Easting/WestingY - Northing/Southing Z - Subsea value of formation represented

•Must have X, Y and Z for wellhead location

•Not required but beneficial for “steering” wells

•Can be used and imported into some steering software

Grid Data

..provides assistance in determining correct correlations.

Learning Objectives

•Was correlation defined?

•Was a basic understanding of correlation provided?

•Was the relationship of correlation to geosteering defined?

•Was the Gamma Ray Log introduced?

•Was the basic process of Gamma Ray correlation provided?

•Was the basic use of correlation in Geosteering defined?

END