logging while drilling versus convencional wire line – comparison
TRANSCRIPT
1
“Logging While Drilling versus convencional Wire Line – Comparison”
Ana Rafaela Gonçalves Bastos
SUMMARY
While drilling a well for exploration of hydrocarbons, data is collected, physical, in the case of
sampling profiles and "electric ", magnetic, sonic, etc., logs are obtained by probes. Logs are
sophisticated geophysical measurements recordeddown hole. These measurements may be
from spontaneous phenomena, such as the natural radioactivity (gamma ray log), or
measurements of induced phenomena, such as the forming speed, or the speed of a sonic
wave through a certain formation (sonic log). Nowadays, there is a fairly extensive range of
equipment that performs logs as well as techniques for making. We can divide these techniques
into two main ones, these being the Wire Line (WL) and the Logging While Drilling (LWD). A big
difference between them is the timing at which the measurements are made, being in the case
of WL, these are performed post drilling and LWD, as its name indicates, performed
simultaneously with the drilling. Both have their advantages and limitations.
CHAPTER 1 - INTRODUCTION
" Oil (petroleum from Latin), Petrus = rock and oleum = oil, Greek πετρέλαιον [petrélaion], "rock
oil", the ancient Greek πέτρα [petra], stone + έλαιον [elaion] oil .. "
There are records of its use in the Middle East, 4000 ac, the Egyptians, the Persians and the
Mesopotamians used it in paving roads, heating, lighting, etc.The Chinese were already drilling
wells since 347 ac using bamboo canes. But the modern era of oil exploration begins in the
nineteenth century, in 1846 in Azerbaijan, although the U.S. only consider its start when drilling
a well in Pennsylvania , made by Edwin Laurentine Drake in 1859.
Given the knowledge that this would be "The" world resource, the need to make its exploration
as well as the ability to find it, more efficiently, was and it is a necessary, constant evolution of
techniques and equipment to achieve optimum efficiency.
The evaluation of geological formations is a parameter of utmost importance. This can be done
by direct methods, that is, with the collection of testimonies (colours) of probes and / or
sampling trough (cuttings) or by indirect methods, being these the collection of graphic records,
called logs or electric profiles, obtained from electronic equipment, denominated probes, which
go up and down the well.
2
The detection of properties from formations, as well as mechanical parameters related to
drilling, aims to detection and interpretation of possible reservoirs, in the first case, to ensure
that the drilling is as efficient as possible.
The Schlumberger brothers first developed the logs in France, in 1920.
The first log was an electric record.As already mentioned, the techniques for performing these
logs have evolved and diversified. Nowadays it is possible to log in different phases of drilling,
and particularly when drilling or interruption of it.
The number of features and parameters that are possible to measure these days also increased
in large quantity, we went from an electric log power to a set of logs performed in several ways,
depending on your goal, from which the following stand out:
However, whatever the technique used, the principle is always the same, consisting in bringing
down the well probes that perform the reading of certain mechanical and petro physical
parameters, being interconnected by a multiconductor cable, or any other telemetry mechanism,
transmitting signals to the surface. Being the data then processed and converted into graphs, in
which each represents a petro physical parameter related to the formation considered.
CHAPTER 2- WELL LOGS
2.1 - WIRE LINE WELL LOGS
A well log is a geophysical record done throughout a hole. Conrad Shlumberger and Henri Doll,
who gave them the name of Carottage Electrique (electrical coring), were the first to perform
these type of records or measurements. Nowadays the name Diagraphies Différées is applied
in order to distinguish the wire line log’s, which are made after the drilling. From the ones made
during the drilling, is called, the diagraphies Immediate. However the name Log, from the
English, is used worldwide.
These type of measurements can be spontaneous or induced phenomena, such as natural
radioactivity (gamma- ray) for the first and the transit time of the formation, in the case of a sonic
log, which is a phenomenon induced. The need to perform logs is due to the gap left by cuttings
cores and despite the information provided for example by a core being correct, whereas the
logs are accurate but may fail. However all this information when crossed, gives us the
possibility of knowing with certainty the geological formation that we are drilling. The logs using
the Wire line are performed after the removal of the drilling tools, unlike the MWD
(measurement while drilling) and LWD (logging while drilling) as they are made when drilling.
This type of Logs are made in open-hole, meaning that the coating has not been placed in the
hole, being the walls of the hole its own formation.
Effectively there are tools that record certain properties with the casing, coating the well, logs
cased-hole (referred further ahead in more detail).
Logs with wire line use highly sophisticated equipment and independent from the drilling tools.
In onshore, a truck is used containing all the hardware necessary for the collection of electrical
3
impulses that are transmitted through a conductor cable (wire line), which also serves to support
the tool itself into the well. In offshore, we have the same equipment, but in a small cabin near
the rig.
To perform (wire line logging), it is necessary to stabilize the hole and remove the drilling
equipment, as already mentioned. Then the probe is coupled to the wire line equipment, which
is placed in the hole at its maximum depth, recording when pulling the tool to the surface. The
cable is pulled by a motor existing in the truck at a speed between 300m/h and 1800m/h,
depending on the type of the log that is being run.
Due to the fact that these logs are made after drilling and the removal of equipment that
performed it, due to the instability of the hole and the invasion of fluids at its formation, there
was the need to create tools that performed various types of logging, thereby reducing these
factors. However there is the continuing need of some tools to perform any type of log 's, being
that each tool takes 4 to 5 hours to complete the reading, the combination of all will take about 2
or 3 days to make a complete hole reading (in case of deep wells).~
2.2 – TLC WELL LOGS
As in horizontal wells, it is impossible to descend tools in flexible cables, the industry has
adopted a technique called TLC (Tough Logging conditions), which basically consists in pushing
the tools to the end of the well using the tubing for the purpose of probing, or the drill pipe.
Electrical cables carrying electrical signals are attached to the pipe. Therefore, the operations
for this method are longer consuming and difficult to perform.
The data is however similar to those obtained with cable tools.
2.3 – TÉCNOLOGIES MWD and LWD
The MWD and LWD are the latest technologies used in the creation of wells for Oil & Gas,
these tools, unlike wire line, give us information while drilling, in other words, while drilling the
well, we are receiving information regarding the drilling of the hole and the formations traversed
by the same.
With the MWD, modelling while drilling, we can have speed information; direction, angle,
vibration, torque, etc. regarding the hole and the drill. They can also run logs such as gamma
ray. This equipment is located behind the front cutting, and all the information collected by the
same is conveyed by the drilling mud and stored at the surface in the internal memory of the
device itself.
For a more complete analysis of the well, LWD is used which means logging while drilling, or
simultaneously acquiring data with drilling. Unlike the methods by TLC and wire line, which are
based on post data acquisition drilling, the LWD tools are equipped with their own power
sources, and the measurements can be continuously acquired and recorded in memory as a
4
function of time or transmitted to the surface in real time through the pulse in the mud column.
In this method, there are no electrical cables between the tools and the processing unit. On the
surface, the impulses are decoded and translated into a similar chart to record data from wire
line or TLC.
The most frequently used tools in LWD are the ones that measure radioactivity (GR), resistivity,
density and porosity of the formations.
One of the great advantages of logs LWD compared to those obtained after drilling is that the
tool measures the petro physical properties of the geological formations still in its semi virgin
state, in other words, long before they are invaded by the mud used in drilling. For this reason,
the data obtained by these tools are not exactly identical to those obtained by wire line or TLC.
The other major advantages are related to the monitoring path (angle and azimuth) of the
horizontal wells, ease in decisions relating to the identification of areas of interest for collecting
the borehole, and pre warning about the approximation of high-pressure zones.
[The Universe of the oil industry - From research to refining, 2007]
CHAPTER 3- PARAMETERS OF A RESERVOIR
When you want to make the analysis of the formations traversed by a well to characterize the
type of reservoir with which we are dealing is necessary to obtain certain parameters, essential
for an evaluation. These parameters are porosity, permeability, fluid saturation.
3.1- POROSITY
Porosity is defined as the percentage of voids in a given volume of rock.
The porosity is characterized in a complete and effective manner, being the absolute, the total
volume of voids and the effective, the volume of voids that are interconnected, enabling the
circulation of fluids. In other words, is from the effective porosity, which the extraction of
hydrocarbons are made, when existing.The porosity may be further classified as primary and
secondary in view of the rock´s formation in question.
Formula 1: Porosity (φ)=
;
The porosity of the reservoir rock usually varies up to 30% of the total volume of the rock.
Generally only porosities above 10% are considered attractive from a commercial point of view.
3.2-PERMEABILITY
Is defined, as being the property of a body that lets fluids to diffuse through it. This is the most
difficult petro physical parameter to assess, being impossible to quantify directly. Using various
5
indicators, it is possible to make a qualitative and semi-quantitative parameter of the well using
a combination of logs. One way that allows the determination of the permeability of a rock is the
application of Darcy's Law. Being an empirical law, this property must be obtained from an
experimental way.
Darcy studied the flow of water through unconsolidated sands and obtained the following
formula:
Formula 2: Q= K
;
• Q - Flow, the quantity of fluid that passes through the sample per time unit [cm3/s];
• A - The cross-sectional area of the sample [cm2];
• μ - Fluid viscosity [centipoise];
•
- Pressure gradient along the flow line;
• K - Permeability [Darcy´s Law].
3.3-SATURATION OF FLUIDS
It is defined as the ability of a rock being occupied by a specific fluid, being the percentage of
the storage capacity of a rock.
The saturation (Sh) is given by hydrocarbon 1-Sw;
Sw is the water saturation, which is calculated as follows:
Formula 3: Saturação em água (Sw) =
;
In 1942, Gus Archie related the resistivity of a formation saturated in water (Ro), the water
resistivity (Rw) and the resistivity factor (F). Showing that this factor is related to the porosity of
the formation.
Expressing in the following equations:
Formula 4: Equação de Archie Sw= (
)1/n
;
• Sw- saturation in water, applied only to non-argillaceous formations;
• Rw - resistivity of the formation water, laboratory-measured or evaluated by analysis of logs;
• Φ - Porosity, measured in log 's acoustic or nuclear;
• Rt - resistivity, measured by log 's electrical or electromagnetic;
• m - cementing factor measured in the laboratory;
• 1/n - Exponent saturation, measured in the laboratory. This depends on the wettability of the
formation.
Formula 5: F=
;
Formula 6: F=
;
• a- Tortuosity factor, complexity measure flow between the pores;
• m - Cementing factor.
6
The higher the tortuosity, the larger the value of m, this varies between 1.3 and 3.0, being
dimensionless. The range is 0.6 to 1.0 and, depending on the type of rock (Asquith, 1982).
CHAPTER 4- CASE OF STUDY
4.1 – DESCRIPTION
In conducting a drilling, it resorted to various methods of acquiring data about the drilling itself
(MDW) and related with the formations drilled (LWD and WL). As already mentioned the
objective of this project is to discuss: why the use of two methods of acquisition and not just
one, interpreted the results of logs run by LDW and WL and the calculation of parameters such
as SW, PHIE and VSH. To achieve these goals we used the software IP (Interactive Petro
physics Schlumberger).
The Interactive Petro physics software uses the formulas described above as the basis for the
calculations of the parameters listed (being that the eq. Archie, formula 4, was selected
manually).
Despite being made about 10Run’s (racing’s log sets), we selected only some of the WL’s log in
order to do such a comparison, as we can see on table 1.
In carrying out the calculations using the IP, it was necessary to introduce some parameters, as
the software establishes most of them. The only parameter introduced was the geothermal
gradient, a, m and n, and also define the maxim and minim base line for gamma ray.
LWD TOOL LOG NAME DEPTH OF INVESTIGATION
ARCvision Gamma ray R2.3.RawLWD:Grds(Gapi) 6inch
ARCvision ResistivityR2.3RawLWD:P40Hds(oh
mm) 40inc
WL TOOL LOG NAME DEPTH OF INVESTIGATION
HNGS Gamma ray R1.1.2_WL:Gr(Gapi) 12inch
AIT Resistivity R1.1.2_WL:IDL(ohmm) 90inch
Caliper R1.1.2_WL:CALI(in)
Density R1.1.2_WL:RHOB(G/C3)
Neutron R1.1.2_WL:APLC_1(V/V)
Table 1: Case study logs [by Author]
7
4.2 – RESULTS
In the evaluation of a reservoir, it is necessary to determine certain parameters that will divide
the reservoir in productive zones areas and not so productive. These parameters are called cut-
offs. The cut-offs are the limiting values of certain parameters such as: effective porosity; water
saturation and volume of sand. After the determination of the cut-offs, it is possible to reach the
reservoir portion, which is considered to contribute to the production, to this ratio we, call the
Net/Gross ratio. After obtaining this Net/Gross, this is applied to the entire thickness of the
reservoir in order to get the Net pay. Producing areas of the reservoir, we call Net pay, to non-
productive, non-pay.
The following tables show the values used for cut-offs, as well as the values of the calculations
performed by the IP. We used only the data for the reservoir zone B, considering the most
interesting.
Figure 1: Reservoir parameters [ by Author]
8
In order to visualise the better the similarities of the results that we obtained, we plot the logs of
LWD and WL together. As we can see on fig.2 there are two well defined reservoirs, with really
great results of oil saturation.
The small difference between both methods can come from the difference of depth of
investigation the tools (Tab.1) or the result of fluid invasion during and after drilling. As we know
if the tool have a better depth of investigation, the log is more representative, in this case WL
have much more depth, but for other point of view the fluid invasion is bigger.
In this case we can conclude that if we only had used LWD as the source of log information, the
reservoir would also been well evaluated.
Reservoir Summary
Zn Zone Name Easting Northing Top Bottom Gross Net N/G Av Phi Av Sw Av Vcl Phi*H PhiSo*H
# Ari
X X X X X X 240.45 $$8.08 0.034 0.246 0.601 0.314 1.99 0.79
Pay Summary
Zn Zone Name Easting Northing Top Bottom Gross Net N/G Av Phi Av Sw Av Vcl Phi*H PhiSo*H
# Ari
X X X X X X 240.45 $$2.44 0.010 0.368 0.305 0.107 0.90 0.62
Cutoffs Used
Zn Zone Name Top Bottom Min. Phi Sw Vcl
# Height CALCS_WL:PHIE CALC_LWD:SW CALC_LWD:VCLGR
Reservoir
1 X X 0 >= 0.12 <= 0.5
Pay
1 X X 0 >= 0.12 <= 0.5 <= 0.5
Depth Units : m
$$ indicates missing or null data in the zone.
Reservoir Summary
Zn Zone Name Easting Northing Top Bottom Gross Net N/G Av Phi Av Sw Av Vcl Phi*H PhiSo*H
# Ari
X X X X X 240.45 $$9.14 0.038 0.242 0.583 0.322 2.21 0.92
Pay Summary
Zn Zone Name Easting Northing Top Bottom Gross Net N/G Av Phi Av Sw Av Vcl Phi*H PhiSo*H
# Ari
X X X X X 240.45 $$2.90 0.012 0.350 0.292 0.132 1.01 0.72
Cutoffs Used
Zn Zone Name Top Bottom Min. Phi Sw Vcl
# Height CALCS_WL:PHIE CALCS_WL:SW CALCS_WL:VCLGR
Reservoir
X X 0 >= 0.12 <= 0.5
Pay
X X 0 >= 0.12 <= 0.5 <= 0.5
Depth Units : m
$$ indicates missing or null data in the zone.
Table 2: Cut offs and results[by Author]
9
The main reason to justify the non-exclusive use of the LWD tool, as characterizing geological
formations, are the costs associated with each log, which is performed using the LWD. Although
WL engage a wide range of manoeuvres when it is intended to "run" logs in the well, it
continues to be less expensive than the LWD. Being the Logging While Drilling only used to
calculate the mechanical parameters of the survey, such as drilling speed, direction, etc. and
run logs as Caliper, Gamma ray and resistivity. Being its main use, to identify possible
reservoirs, which will then be analysed using the WL.
CHAPTER 5 – CONCLUSION
In the development of this work WL and LWD technologies were discussed for the purpose of
comparing the level of performance in evaluating a reservoir. As these were being described,
some questions were put up, such as " If there is a method that performs the
logging while drilling, why don´t we do all the logs using the LWD?" In addition "If we only use
the LWD, will the reservoir be misinterpreted?”.
The LWD technology has revolutionized logging; enabling, as already described, logs
performed simultaneously with the drilling. The expected would be that when this technology
appeared, the Wire Line would be replaced by it, as it came to resolve some of the
disadvantages of WL, such as the time elapsed between drilling and the completion of logs and
all the consequences from this factor (fluid invasion, time manoeuvring, etc.).
Figure 2: LWD and WL logs [by Author]
10
Actually, that is not what happened. These two methods are commonly used together during
drilling. In summary, the factors, which determine the use of each methods will clearly condition
the possible abandonment of the conventional method, are as follows:
- Availability of methods of data acquisition in LWD;
- Availability of the equipment;
- Costs associated with the use of both;
- Depth and location of wells;
- Tilt;
- Conditions of the well, etc..
There is a wide range of data procurement methods available, most of which has already been
described in this dissertation. It turns out that not all of these methods are available in LWD,
being coring and logging as we can observe in the figure 3.
When a well is drilled using the two methods described for data acquisition, the wire line
equipment does not need to be "present" at the perforation site, as it is required from time to
time, being therefore not necessary, in order to be used in nearby wells. Therefore, it is not a
unique equipment of a drilling. In the LWD case, since it is while drilling, this does not occur,
therefore the investment of the companies in this type of equipment is much higher, since a
greater number will be required (depending on the number of wells) and as in any industry the
goal is a maximum profit. For this reason, despite the LWD be a device with many advantages,
it is not used in all wells, for example in the north of Brazil (where PARTEX Oil & Gas operates),
where the wells only use the conventional, since in that case would be very expensive to use
due to movement or investment required, etc.
In the oil industry, costs can vary, this is, it is not only associated with the equipment or the
meters drilled, but the conditions and sites to which they have to operate.
The depth is a factor influencing the choice of the equipment used, therefore with greater
depths, the time of manoeuvres associated with the use of the conventional method is
Figure 3: Logging tools [Schlumberger]
11
proportional, and this is often infeasible. Another advantage not yet mentioned, from the LWD,
is the possibility of profiling wells with ravelling problems, this factor often prevents the operation
of the WL. The diverted holes are increasingly recurrent, conditioning the use of wire line, since
it operates only in vertical holes or with little tilt. No less important is the issue of the equipment
logistics, that is, the handling, the space it occupies, etc. The LWD presents a great
disadvantage to the WL, since it is a cable with a probe, its size is much greater than that of the
conventional wire line, this can be a problem and a barrier to its use. Therefore, we can reach
the following conclusions:
- The results obtained by both methods, in the case study, are quite similar;
- In the case study, we can conclude that using only the LWD was sufficient to identify the
reservoirs A and B and accurately determine the values of porosity, volume of sand and oil
saturation.
- The costs associated with each method are variable;
- The total replacement of the Wire Line for LWD will not probably happen in the near future.
12