Formation Evaluation Petroleum Reservoir Eng. Dep.

/ Third Year

Mud logging is one of the first direct evaluation methods available

during the drilling of an exploration well.

Mud logging (Surface logging): is the process of studying the general

physical characteristics of rocks during the drilling operations to report
information about the drilling and well construction process.

During the drilling operations, the fluid is continuously circulated down

the inside of the drill pipe, through the bottom of bit and back up the annular
space (Fig-3).

 The drilling mud carries broken rock fragments (cuttings) to the

surface. Traces of oil or gas may be also brought up in the mud.

 The mud log may be of great use to the petrophysicist and geologist in
operational decision making and evaluation.

 The information obtained by the mud logger is presented in the form of

various logs such as the driller’s log, the cuttings log, or the show
evaluation log.

Drilling mud consists of a variety of clay and other materials (barite,

hematite) in a fresh or saline aqueous solution. We can classify borehole muds
into two groups, those that are conductive and non-conductive. The non-
conductive muds include air, gas, and oil-base fluids having infinite
resistivity. Current flow in conductive muds varies depending upon two
factors: the type of mud and the temperature.

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Gas trap

Mud Suction Shaker

pump pit pit Pit

Fig.3: Mud circulation system

The density of the mud is kept high enough so that hydrostatic pressure
in the mud column is always greater than formation pressure. This pressure
difference forces some of the drilling fluid to invade porous and permeable
formations. When formation pressure is greater than the mud hydrostatic
pressure the following may be seen:

 Large quantities of cavings (due to wellbore instability)

 Connection gases
 Trip gases
 Fluid incursions to the mud system
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1- Water-Based Mud (WBM)

2- Oil-Based Mud (OBM).
3- Aerated Mud.

drilling

1. Mud Properties: must be control and record the properties of mud

drilling such as density, pressure, viscosity, percentage of: solid,
chloride, alkalinity and PH.

2. Leakage (Loss) of drilling mud: can happen as a result of surface losses,

hole fill and down hole losses (thief zone)

3. Rate of Penetration: the changes in rate of penetration are very

important to the geologists because they indicate a change in lithology.

4. Weight on the bit and rotary table speed.

5. Mud system.

A mud log gives information recorded vs. depth, include:

1- Weight on bit and drill string rotation speed.
2- Detection of gas and oil in the mud.
3- Mud pump speed and Mud pump pressure.
4- Identification of the lithology and formation type being drilled.
5- Identification of porous/permeable zones (These information supports
wireline log data).
6- Detection of hydrogen sulfide (H2S).
7- Hydrocarbon staining on the cuttings.

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The mud logging unit is the information center on the rig site to serve both
exploration and drilling. It is located very close to the rig floor.

A number of cables extend from the unit to a number of sensors installed

at different locations on the drilling rig. These sensors are used to measure
many important variables or parameters of the rig operations (Fig.4).

Fig.4: Mud Logging Unit

The sensors that feel all parameters in mud logging operations are:
1- Stand Pipe Pressure sensor (SPP).
2- Casing Pressure sensor.
3- Rotary Speed.
4- Mud Flow out.
5-Mud temperature, Weight and Conductivity.
6-Hydrogen Sulfide H2S sensor.

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 Formation Evaluation Petroleum Reservoir Eng. Dep. / Third Year

To comprehend all of the information available, we need to understand

four important areas of mud logging:
 rate of penetration and lag,
 gas detection,
 formation evaluation and sample collection,
 hydrocarbon show evaluation

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One of the most important jobs of the mud logger is sample collection.
Most of the sample material is gathered at the shale shakers.
When the cuttings arrive at the shale shaker, they are covered in mud,
unsorted by size, and generally unidentifiable.

The mud logging unit will generally take a sample of the cuttings and
stored in company laboratories. These samples are two types: washed and
unwashed. Washed samples are examined under a microscope (fig.9) in the
mud logging unit to obtain on the following (Cuttings Sample Description:

1- (sandstone, limestone, dolomite, shale, anhydrite,…etc)

(type, texture: color, sorting, size, mineral contents).
3- (visual inspection of porosity type )

Fig.9: Microscopic examination of cutting

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 Formation Evaluation Petroleum Reservoir Eng. Dep. / Third Year

Hydrocarbon show detection and description is the key task of the

wellsite geologist and of the mud logging crew. However, despite the
importance of hydrocarbon detection, nearly all of the tests and indications of
a hydrocarbon show are weak, which depends on experience.

Show evaluation is the complete analysis of the hydrocarbon- bearing

formation with respect to lithology and type of hydrocarbon present.

A complete show evaluation identifies:

There are two types of shows are recognized: gas and oil.
1. A gas show: the mud logger may see a significant increase in gas
levels.
2. An oil show: is an increase in heavier-than-methane gas levels as
well as a physical indication of oil.

Evaluation of oil in the drill cuttings (and drilling fluid) begins with
inspection of those samples under the microscope and inspection in the
ultraviolet-light box. Tests and visual inspection should be performed on the
mud, unwashed and washed bulk cuttings, as well as individual grains.
Four tests are used to detect hydrocarbons, especially Oil Show: odor,
staining, fluorescence, and cut.

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 Formation Evaluation Petroleum Reservoir Eng. Dep. / Third Year

1- Odor: does not usually apply to cuttings, but it is a useful test on cores.
Although difficult to standardize, odor falls into one of four categories:
poor, slight, fair, or strong.
2- Oil staining, like odor, is more useful when applied to cores. Staining
is described in terms of both color and percentage of sample stained,
e.g:
 No visible oil stain
 Spotty oil stain
 Streaky oil stain
 Patchy oil stain
 Uniform oil stain

The color of the staining can be related to the oil's gravity, where lighter
colored stains are indicative of high gravity oils and darker stains are
indicative of lower gravity oils. A black asphaltic residue is indicative of
dead, residual oil lacking volatile components.

3- Fluorescence: Liquid hydrocarbons fluoresce under UV light, and the

amount, intensity, and color are the first and best indications of a show.
Intensity is subdivided into none, poor, fair, or strong.
4- Cut: defines the leaching of oil from a sample by a solvent.

Application of Show Evaluation:

Complete show evaluation can help us:
(1) Identify the presence of hydrocarbons.

(2) Make recommendations for coring and testing programs.

(3) Show evaluations correlated with offset wells and wireline logs can
assist in reservoir interpretation.

Dr. Maha M. Mahmood \ Mosul University -41- 2022-2023

 Formation Evaluation Petroleum Reservoir Eng. Dep. / Third Year

2- Cores and Core Analysis:

2-1 Introduction:

Coring is the removal of undamaged sample formation material from

a wellbore and brought to the surface for physical examination.

The objectives of Coring are to bring a sample of the formation and its
pore fluids to the surface in an unaltered state, to preserve the sample, and
then to transport it to a laboratory for analysis. The objective Core Analysis
is to reduce uncertainty in reservoir evaluation by providing data
representative of the reservoir at in situ conditions.

Core information includes Geological Evaluations as: lithology,

depositional environment, mineralology, formation age and geological
sequence, fracture analysis.
Engineering Evaluations as: Porosity Distribution, Permeability
Distribution, Permeability vs. Porosity Relationship, Hydraulic Flow Unit
Distribution, Formation Heterogeneity, Oil/Water Contacts, Reservoir Fluid
Saturations and Distribution, Wettability Relative Permeability, Capillary
Pressure.

2-2 Coring Methods:

Through the years, many types of equipment and many different

techniques have been devised for the taking of cores, some of which were
successful and have evolved into the equipment and techniques of today.
Generally, current methods of coring can be grouped into conventional
and sidewall coring.

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1- Conventional Coring:
A core bit is located on extreme end of the drill stem and a core
barrel is located immediately above the core bit for retaining the core
after the cut.

When the decision is reached to cut a conventional core (e.g., at a

drilling break, a planned depth reached), the drill pipe is removed
from the hole and dressed with a hollow core bit and a hollow barrel
equipped with a nonrotating inner barrel (Fig.10). Drilling fluid
circulates between the inner and outer barrels in order not to flush the
core.

Outer Barrel

Inner Barrel

Fig. 10: Conventional Coring

2- Sidewall Coring:
In sidewall coring, a sample is obtained from the wall of a
previously drilled open hole at chosen depths (Fig.11). The tool is
designed somewhat like a perforation tool, but instead of a bullet, a short
tube is driven into the side of the borehole and retrieved by means of a
cable attached to the body of the tool.

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 Formation Evaluation Petroleum Reservoir Eng. Dep. / Third Year

One advantage of this method is obvious: samples can be obtained

from zones that were not cored at the time of initial penetration. In
addition, with the aid of wireline logs, these samples can be taken at very
precise positions in the hole.

Fig.11: Sidewall Coring

2-3 Core Analysis Techniques:

There are three types of core analysis techniques. The technique used
depends all the coring method, the type of rock to be analyzed, and the
type of data to be obtained.

(1) Conventional or Plug Analysis:

The plug analysis method is used most frequently. In this method,

a small plug sample, which is easy to work within the laboratory, is
cut at selected intervals from the whole core (Fig.12). The data
obtained from the small plugs are then assumed to represent the
reservoir rock properties of the sampled interval.

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 Formation Evaluation Petroleum Reservoir Eng. Dep. / Third Year

Fig.12 Plug Samples

(2) Whole-Core Analysis:

The whole-core analysis method is used when the plug analysis
method becomes invalid because the heterogeneities such as
fractures or vugs. This method uses the whole core for rock property
measurement in as long a length as possible (Fig.13). The technique
requires larger equipment in the laboratory, and not all commercial
laboratories are equipped to perform this type of analysis.

Fig.13: Whole Core Samples

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(3) Sidewall Core Analysis:

Considering the process under which these cores are obtained and
the sample size of the core, the measured data will have limited value
in some areas in addition to the in some situations, this rock sample
is all that is available. It is, therefore, desirable to look at the relative
value of rock properties as determined from sidewall samples
(Fig.14) and those obtained from conventional cores.

These studies indicate in general that:

(1) Sample porosities in softer, looser sands are only slightly

higher than those of conventional cores.

(2) Sidewall sample permeability is decreased in higher

permeability formations.

(3) Water saturations from the sidewall cores are lower and oil
saturations slightly higher than conventional core data.

Fig. 14: sidewall Core Samples

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2-4 Types of Cores Analysis:

1- Routine Core Analysis (RCAL):

Routine core analysis attempts to give only the very basic
properties of unpreserved core. These are basic rock dimensions,
core porosity, grain density, gas permeability, and water saturation
(Fig.15).

2- Special Core Analysis (SCAL):

Special Core Analysis attempts to extend the data provided by
routine measurements to situations more representative of
reservoir conditions (Fig.15). SCAL data is used to support log
and well test data in gaining an understanding of individual well
and overall reservoir performance. However, SCAL
measurements are more expensive.

Fig.15: Routine and Special Core Analysis

Dr. Maha M. Mahmood \ Mosul University -00- 2022-2023