CH-4 Log Correlation Techniques 22 48-1-10.PDF

7/28/2019 CH-4 Log Correlation Techniques 22 48-1-10.PDF

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Electric Log Correiation Directionally Drilled Wells 81

ELECTRIC LOG CORRELATION - DIRECTIONALLY DRILLED WELLS

In this portion of the chapter we discuss fundamental concepts and techniques for correlating directionally

drilled wells. Additional complexities in correlation arise when working with logs from wells deviated

from the vertical. We also look at the correlation of vertical wells with directionally drilled wells (oftenreferred to as deviated wells).

What is a directionally drilled well? We discussed earlier that a vertical well is one drilled 90 deg to

the horizontal reference, usually sea level. A directionally chilled well can be defined as a well drilled at

an angle less than 90 deg to the horizontal reference, as shown in Fig. 4-15. Some general directional well

terminology was discussed in Chapter 3. These terms are again illustrated in Fig. 4-15 for ease of

reference. Other terminology discussed earlier in this chapter for vertical wells is also applicable to

deviated wells.

Most wells drilled in an offshore environment and many wells onshore are drilled directionally. The

most common well is a simple ramp well (Fig. 4-1521), sometimes called an shape hole These wells are

deviated to a certain angle, which is usually held constant to TD of the well. Many wells are drilled withan 8" shape design. With an S shape hole, the well builds to one angle, ma intains this angle to a

designated depth, and then the angle is lowered again, often going back to vertical (Fig. 4-15b). Today we

see a large number of horizontal wells, which are shaped by continually building the angle until the

desired near-horizontal orientation is reached (Fig. 415c).

Log Correlation Plan

Just as with vertical wells, there must be some system to log correlation of directionally drilled wells. Due

to the nature of deviated wells, a good correlation plan is critical to accurate correlations. For this log

correlation plan, we once again use the structure map on the SOOO-ft Sand on a normally faulted

anticlinal structure (Fig. 4-16). The correlation plan outlined here is intended to make correlation

systematic, provide a logical method for correlating directionally drilled wells with other directionallydrilled wells or with vertical wells, and reduce correlation problems.

Step 1. Construct a correlation type log. Refer to the section on correlation type logs for the complete

definition of a type log. Do not use a deviated well in the construction of a type log because a log from a

directionally drilled well does not represent the true vertical stratigraphic section. Wells farthest off

structure serve as good type log candidates.

Step 2. Correlate all the vertical wells before correlating the deviated wells, since the vertical wells are

usually easier to correlate. For the vertical wells, use the same plan outlined in Fig. 4-7.

Step 3. Once the vertical wells have been correlated, begin correlating the deviated wells. To begin

directional well log correlation, first organize the wells according to their direction of deviation withrespect to structural strike. Devlated wells are classified into one of three groups: (1) wells drilled down-

dip, (2} wells drilled along strike, and (3) wells drilled rip-dip. Step 4. Begin correlation of these three

groups with the wells drilled generally down-dip. First correlate the wells with the least amount of

deviation, and where possible, correlate in closed loops with each well log comelated with a minimum of

two other wells. The wells with the least amount of deviation will have a log section thickness closer to

that seen in a vertical well than other wells drilled down-dip. Looking at the wells drilled from Platform B

in Fig. 4-16, the first directional wells correlated are those represented by a billiard ball type cor


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Figure 4-15 Diagrammatic cross sections illustrating (a) a simple ramp or "L" shape well; (D) a more

complicated "S" shape weli

; (c) a horizonial well. [(a) and (b) published by permission of Tenneco Oil

Company, (0) pubfished by permission of J. Brewton]


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Electric Log Correlation Directionally Drilled Wells 83

relation sequence number 1. There are two wells drilled with a minimum down-dip deviation (Wells N0. B-5 and

These Wells can be correlated to each other and then with the straight hole, Well No. B-l, drilled as a vertical well

from the platform.

Step 5. Continue correlating wells with increased deviation in the down-dip direction. For this example,

these are Wells N0. B-2 and B3, indicated by correlation sequence number 2. These two highly deviated

wells can be correlated with each other and then with Wells No. and 8-6. Also, the vertical Well No. 3

may be used to correlate B-2 and B-3, since it is an off-structure well exhibiting a thick stratigraphic

section.

Step 6. When all wells classified as being deviated down-dip are correlated, the next group to correlate

are those wells deviated along structural strike. From Platform B, Wells No. and B-9 fall into this

category. These wells can be correlated to each other and then with straight hole B-l to close the loop.

When correlating wells drilled along strike, the effect of bed dip is removed from the representative

thickness of the directionally drilled wells. This can often simplify correlation.

Step 7. Finally, correlate the wells deviated up-dip. Those Wells drilled closest to the crest of the structure

usually are complicated by stratigraphic thinning, faulting, and unconformities. The correlation of

these wells can be most difficult; therefore, they are normallyr correlated last when all other correlation

information is available and you can recognize the best correlation markers. Wells No. B-4 and B-8

drilled from the B Platform fall into this category. They are labeled as correlation sequence number 4.

Wells drilled in an up-dip direction can have variable log section thickness due to the geometric

relationship between a wellbore and stmctural dip. A log section from a well drilled in an up-dip direction

can be thicker, thinnet, or equal to the thickness of a log section from a nearby vertical well drilled

through the same stratigraphic section. This potential complexity can add to the difficulty of correlating

wells drilled in an up-dip direction. Because of these complexities, we recommend that these wells be

correlated last, after significant knowledge is gained from other correlation work. Step 8. Generally, it is

best to correlate wells located nearest each other, especially in areas where significant changes instratigraphic thickness are probable. Wells nearest each other and approximately in the same structural

position usually are expected to have the most comparable interval thicknesses.

Step 9. After correlating the wells from one platform, begin correlation of wells on any additional

platforms in the area. In Fig. 4-16. the A Platform wells in the northwest portion of the field should be

correlated next. It is not necessary, however, to isolate correlation to a single platform. Often, wells from

one platform are drilled in a direction toward another platform. If wells from separate platforms are in

close proximity to one another, they should be correlated to each other. Notice that correlation sequence

number 5 illustrates the correlation of 13-4 with A-5, and B-8 with A-4. Wells N0. A-1 and are straight

holes drilled from separate platforms, but since they are located in a similar structural position, they can

also be correlated to each other.The primary focus of this correlation plan is to provide a logical method for correlating all vertical

and deviated wells in an area of study. The plan outlined is by no means the only one that can be used.

The important point is to have a plan. Without one, log correlation becomes a random process, often

resulting in some type of correlation problem or in miscorrelations.


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Chap. 4 f Log Correiation Techniques

Figure 4-16 An example of a log correlation plan for direclionally drilled wells. The plan shows the hierarchy of the log

correlation sequence and illustrates how to correlale deviaied wells in closed loops.

Correlation of Vertical and Dlrectionally Drilled Wells

In this section, we discuss general procedures for correlating vertical wells with directionally drilled

wells. Directional wells have a measured log thickness (MLT) that can be less than, greater than, or equal

to the log thickness in a vertical well drilled through the same stratigraphic section. These different MLTs

result in additional complexities that must be considered when undertaking correlation work using well

logs from both vertical and deviated wells.

Now we look at the correlation of a vertical well with a deviated well. Figure 4- 17 shows a portion

of an electric log from vertical Well No. A-1 and the electric log from directionally drilled Well No. A-2.

The wells are in close proximity to each other. The detailed electric log correlation


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Electric Log Correlation - Direclionally Drilled Wells 85

(sand and shale sections) for both wells indicates that they have penetrated the same stratigraphic section.

Although both wells have a high degree of correlation (see SRM 1 through SRM 4), the stratigraphic

section in Well No. A-2 is much thicker than the same section seen in Well No. A-l. The log section in

WelINo.A-1 from SRM 1 to SRM 4 is 490 ft thick. The same section in Well No. A-2 is 735 ft. Earlier in

the chapter, in the discussion on vertical wells, we mentioned that a short section in one well with respectto another might be the result of stratigraphic changes or a fault. If the short section is isolated to one

particular location, the short section is most likely the result of a fault rather than variations in

stratigraphy. Conversely, if the short section is uniformly distributed over a series of intervals, the short

section is probably due to stratigraphic variations rather than a fault.

Based on correlation criteria, the thinner section in Well No. A-] appears to be the result of

stratigraphic thinning rather than a fault. In this example, however, we introduce another possible

explanation for the shortening. Since Well No. A-2 is directionally drilled, the thickness seen in the well

with respect to Well No. A-l may be completely the result of the wellbore deviation. Figure 4-18 shows

vertical Well No. A-1 and deviated Well No. A-Z in its true orientation with respect to the vertical. Well

No. A-Z is drilled due west at a deviation angle of 48 deg (48 deg from the vertical). The correlationmarkers in each well show that the strata are horizontal and the thick section seen in Well No. A-2 is

solely the result of wellbore deviation. We have now

AMPLIFIED SHORT NORMAL

Figure 4-1? Portion of an electric log from a vertical well (A-l) and a directionally drilled well (A-2). The electric log

sections show detailed correlations.


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Chap. 4 1 Log Correlation Technique:

introduced another complexity in correlation that must be considered when both vertical ant deviated

wells are present in the area of study.

Here are several procedures that can be used to help correlate a vertical well with a directionally

drilled well.

1. Mark the angle of deviation for the directional well on the log at least every 1000 ft. Thi: provides a

reminder that the well is deviated and indicates the angle of deviation at 1000ft intervals on the actual log.

'

2. To compare interval thicknesses, slide the vertical well log as you correlate from markei to marker.

This allows you to compensate during correlation for the expanded or reducet section in the directional

well as a result of its deviation.

3. Calculate a thickness ratio for certain correlation intervals of interest to help evaluate whether any short

section is the result of faulting. stratigraphic thinning, or just WCllbOl'f deviation (Fig. 4-18).

4. If a copy machine with a reduction mode is available, calculate the correction factor required to convert

the deviated (stretched) log section to a vertical log section, and then reduce the log by the appropriate

reduction factor. Use the reduced log for correlation.

S. In areas of horizontal beds or low reliefi the MD leg from a deviated well can be corrected for wellborc

deviation and converted into a TVD (true vertical depth) log to use for correlation.

6. In areas with bed clips greater than 5 to 10 deg, if dip data are available from a diprneter log or

previously constructed structure maps, these data can be used to convert the deviated log to a TVT (true

vertical thickness) log. A TVT log is one in which the measured thickness has been corrected for wellbore

deviation and bed dip to the thickness represented in a vertical well. In areas of dip, a TV!) log provides

little aid, if any, in correlation and can actually cause correlation problems (see section on MLT. TVDT.

TVT, and TST).

Estimating the Missing Section for Nbrmai Faults

Earlier in this chapter, we discussed the procedure for estimating the depth and missing section for a fault

in a vertical well by correlation with another vertical well. Now we present the method for estimating the

depth and missing section for a fault when deviated wells are considered. First, we look at the situation

involving an area with horizontal beds.

Horizontal Beds. We begin with a fault in the deviated Well No. A-2 (Fig. 4-l9a). By correlation with

Well No. A~1, this well cuts a fault near the 10,000~ft Sand level. To determine the depth and missing

section, We correlate the logs in the same manner as previously outlined in this chapter. First, correlate

down the logs starting with SRM 1. We can say that correlation is lost at points A in both wells. Mark this

location on the two logs. Next, find a correlation point below this section on the logs, such as SRM 4, and

correlate up the logs. We now lose correlation in the wells at points 8. By detailed correlation of the shale

markers and sands, we have determined that Well No. A-2 is faulted, and the section in Well No. A-l that

is strati graphically equivalent to the missing section in Well No. A-2 is highlighted in Fig. 4-19a.

The faulted out or missing section in Well N0. A2 is equal to 150 ft by correlation with Well N0. A-

l. Notice that the base of the 10,000-ft Sand is faulted out of Well No. A-2. This information is annotated


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on the log along with the amount of missing section, the depth of the fault, and the well(s) used to

correlate the fault.


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THICKNESS RATIO :

Figure 4-18 Vertical Well No. A-1 and deviated Well No. A-2 (shown in its true orientation with respect to vertical).

The correlation markers show that the thicker section in wet! No. A-2 is a direct result of its deviation from the vertical.

Electric Log Correlation Direclionally Drilled Wells


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SHORT NORMAL

AMPLIFIED SHORT NORMAL

TOP OF 10000 SAND

MISSING SECTION 150'

TOP OF 10,000' snug

10,000' SAND

BASE OF SAND FIO

1 ,000' SAND

INTERVAL FAULTED

10,300' SAND

10,300' SAND

DEVATED WELL vEnTlcAL WELL

(a)

Figure 4-19a Detailed correlation oi a deviated well with a verlical well to locale the de

pth and the missing sec-.

tion for a fault in the deviated well. The base of the 10.000-ft Sand is faulted out.

Chap. 4 f Log Correlation Techniques


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"mi-1 AMPLIFIED SHORT

2.x NORMAL

TOP OF 10,300' SAND

(b)

Figure 4-19b The simplified stratigraphic section through Wells No. A-1 and A-2 illustrates that the missing section inWell No. A-2 is equivalent to the vertical section highlighted in Well No. A-1. No thickness correction tactor is required

in this example.

Electric Log Correlation Directionally Drilled Wells


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Chap. 4 I Log Correlation Technique

The missing section in directional Well N0. is determined by correlation with Well NI which is a vertical

well. In a vertical well, the log thickness and vertical thickness are ll'. same. Since missing section is

expressed as the verticair thickness of the stratigraphic intent: faulted out of a well, the vertical thickness

of the missing section in Well No. A:2 is 150 ft. Th 150 ft represents the missing section for the fault.

This information will be used in future fau and structure mapping.

Figure 4-19!) is a simplified stratigraphic section showing Wells No. A-1 and A-2 positione. in their

true orientation with respect to the vertical. Well No. A-2, which is deviated at 48 de from the vertical, is

pulled apart at the fault to show the restoration of the faulted-out section. Thi cross section clearly

illustrates that the missing section in Well No. A-2 is equal to the 150 ft c vertical section highlighted in

Well N0. A-l.

Now consider a fault in vertical Well No. A-l correlated with deviated Well No. A-2 (Fig 4-20a).

Well No. A-] has a fault near the base of the 10,000-ft Sand. Detailed correlation, a shown in the figure,

identifies a 225-ft section in deviated Well N0. A-2 that is faulted out of We] No. A-l. The faulted-out

section is highlighted in the figure. Since the missing section for th fault is determined as the TVT of the

stratigraphic interval faulted out of the well, the estimate 0 225 ft of missing section based on the deviated

log thickness must be corrected to express thl missing section in terms of TVT.

Figure 4-20!) is a stratigraphic section showing Wells No. A-1 and A-2 positioned in thei: true

orientation relative to vertical. The log section of Well No. A-l is pulled apart at the fault tr Show the

restoration of the faulted out section. Since we are working in an area with horizonta beds, the correction

of the measured log thickness in Well No. A-2 to TVT is determined by tht simple trigonometric solution

of a right triangle. The insert in the center of the figure shows that the TVT of the missing section is

equivalent to the vertical side of a right triangle whose hypotenuse is equal to the log thickness of the

missing section in deviated Well N0. A2.

Where

TVT (431 TVT True vertical thickness

MLT = Measured log thickness in deviated well

: Angle of wellbore deviation from vertical

Therefore,

TVT (225 ft) (cos 48)

= (225 ft) (0.669)

TVT : 151 ft

The actual (corrected) missing section for the fault in Well A-l determined by correlation withdeviated Well N0. A-2 is 150 ft.

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CH-4 Log Correlation Techniques 22 48-1-10.PDF