formation Characteristics of a well being drilled .
Objective: To establish the existence of producible hydrocarbon reservoirs (oil & gas).
Type of information obtained from formation
Evaluation in Petroleum Geology :
· Presence of Hydrocarbon
· Pore pressure analysis
· Caliper for hole size
· Core and fluid samples
· Cuttings
· Reservoir characteristics
· Formation strength
Methods to obtain information :
· Wireline logging : GR , Laterolog , Litho-density tool ,Neutron tool,
· Logging while drilling
· Coring
· Mud logging
· Well testing
Basic logging Tools measurements:
1. Wireline Logging tools :
Electrical Logs: measure the electrical properties of the |
| Example of a Gamma Ray Log |
Gamma Ray Logs: measure the natural radioactivity of the
formation. It reflects the shale content of
the formation.
GR
Logs are used as:
- Correlation tool
- Shale indicator
- Shoulder bed delineation
- Differentiate reservoir rocks from non-reservoir rocks.
Spontaneous -Potential Log: measures the potential
the borehole and a grounded electrode at surface.
The SP is a passive measurement of very small electrical
voltages resulting from electrical currents in
the borehole caused by the differences in the salinities
(resistivity) of the formation connate water
(Rw) and the drilling mud filtrate (Rmf) , and by the presence
of ion selective shale beds .
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The tool can be run : |
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Open hole |
centered |
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Cased hole |
eccentered |
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In a borehole fluid of : |
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Water or water –based mud |
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Logging speed : the logging speed is constrained by other
measurements in the toolstring |
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Comments : Usually run with induction logs and old electric logs ,
the SP can also be run with laterologs , sonic, micrologs , dipmeters , and
sidewall cores .There usually is no separate SP tool . |
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| Spontaneous Polarization |
Density Logs LITHO-DENSITY TOOL (LDT): measure electron density of the formation which is
related to formation density. The density value is used to determine the porosity of the formation LDT
measurement are made by emitting medium energy gamma-rays into the formation and measuring the
number, and the energy of the gamma-rays returning to the tool.
Neutron logs: measure hydrogen index of the formation. This log is used primarily for delineation of
porous formations and determination of porosity.
A combination of neutron log with one or more other porosity logs provides more accurate porosity
values and lithology identification.
Laterolog Tools: require current to be passed through the borehole and into the formation in order to
measure fluid resistivity. It
consists of resistivity and conductivity tool.
- RESISTIVITY TOOL : Send current into the formation and measures the ease of electrical
flow through the formation.
- CONDUCTIVITY TOOL: Induces an electric current into the formation and measures how
large the formation is Rt is measured by both tools and is used to calculate hydrocarbon reserves.
Sonic Logs: measure the elastic or (sound) wave properties of the formation. It records interval transit
time (dt) in us/ft. It is the time taken for a sound pulse emitted from a transmitter to transverse 1 ft of
formation.The interval transit time for a given formation depends on its lithology and porosity. When
the lithology is known, sonic log is a useful porosity tool
Caliper Logs: measure the size or geometry of the hole.
2. LOGGING WHILE DRILLING (LWD):
LOGGING WHILE DRILLING (LWD) is a method used to perform formation evaluation similar to
electric wireline logging .However , the difference between the tow is that the the log results from
LWD are obtained Real –Time while drilling the hole section .
3. CORING :
Is a process used to recover cylindrical rock sample from the wellbore by using specific coring tools .- Advantage of coring - more accurate and reliable data of porosity and permeability.
- Disadvantage - additional cost required for the extra BHA trip to cut the core, late information in place
du to the core need to be analyzed and studied in laboratory.
- Coring is performed between drilling operations once the formation for which a core is required has
been identified on the mud log or LWD log , the drilling assembly is pulled out of hole .
- A special assembly is run on drill pipe comprising of a core bit and core barrel.
- The core is cut free and prevented from falling out of the barrel by a core catcher while being brought
up to surface.
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Sample of Core |
3.
MUD LOGGING :
Is the recording of information derived from examination and analysis of formation cuttings
circulated out of the hole for Oil shows and Gas reading through mud circulation?
Cuttings are taken across the full width of the shakers to be representative of the interval drilled.
Cutting logs shall be known at all times. Lithological analysis of the cuttings normally is performed
on the washed samples indicating fluorescence after treatment with solvents to indicate presence of hydrocarbons.
4. WELL TESTING :
The main objectives of running a well test are to make an indirect determination of the reservoir
physical characteristics and to get physical measurement of well performance and representative fluid
samples .
• It is performed after the presence of hydrocarbon is confirmed from the logs.
• Well test results are very important for development plan.
By performing a well test, we gather information on:
1- PERMEABILITY
2- SKIN
3- RESERVOIR MODEL
4- FLOW EFFICIENCY
5- FLOW RATE
6- GOR
7- FLUID COMPOSITION
8 - PVT SAMPLES
