Formation damage can be defined as the
reduction in the original permeability of the reservoir rock close to
the wellbore. The potential of the near wellbore formation permeability
being reduced (damaged) exits from the minutes the drilling bits enters
the formation until the wells is abandoned.
From the time the drilling bit enters
the pay zone until the well is put on production, the zone is presented
to drilling fluids that could be detrimental to the future productivity
of the well. At the point when drilling through the zone, the nature of
the drilling liquid and the pressure differential are basic.
Generally oil and gas well reservoirs
are penetrated using water based drilling fluids. The presentation of
both the mud solids and polymers into the formation influences the
liquid saturation in the pore space. The response between the formation
fines and/or contradictory reservoir brine with moved mud fines and
filtrates brings about a decrease of the formation original
permeability. Oil wells that are completed in an open opening system,
even generally shallow attack close to the wellbore might considerably
hinder the stream in light of the fact that reservoir fluids must go
through the damaged zones before production.
The formation can be damaged in distinctive ways. Physically, the formation can be damaged by:
(a) The attacked mud solids obstructing the pore channels
(b) The narrowing of vessels because of adsorption of attacked polymers furthermore
(c) Water block, emulsion block and gas block.
Synthetically, the formation can be
damaged by the response between the filtrate and pore substance and/or
network materials. Swelling and scattering of muds and precipitation by
the response between mud filtrate and pore content and additionally
arrangement of salt and minerals from the network are the primary
Bacterially, the formation can be
damaged by the settlement of microscopic organisms and their hastened
items, obstructing the pore channels.
In this manner with a specific end goal
to augment the economic benefits of oil wells, it is important to know
the attributes of damage accounted during drilling and to create
strategies to minimize the degrees of damage brought on by drilling
Van Everdingen and Hurst presentation
the ideas of skin component to the petroleum business. They perceived
that for a given flow rate, the measured bottom hole flowing pressure
was not as much as that computed hypothetically. This demonstrated that
there was an extra pressure drop over that figured hypothetically and is
free of time. They ascribed this pressure drop to a little zone of
changed or decreased permeability around the wellbore and called this
"attacked/damaged/skin" zone. They suspected that attacked zone is
because of reservoir pollution by mud and stopping of some pore spaces
around the wellbore. Numerically, skin drop is exhibited by,
The concept of thin skin in the above
equation works very well in damaged wells. But because of mathematical
and physical difficulties when the well is stimulated i.e negative skin,
it has to be generalized.
Hawkins modified the above
equation by introducing the concept of thin skin. He defined skin factor
for a damaged zone of radius rs with permeability ks in a formation
with permeability k and wellbore radius rw 9see figure 1.2 & 2.2)
Theoretically, the skin factor for a
damaged well can vary from zero to and for simulated well, the skin can
vary from zero to a value as low as -6.
1.1 skin Damage & Formation Permeability
Skin damage is brought about by drilling
liquid attack, which decreases the permeability around the wellbore. A
high permeability reservoir exhibits a high attacked zone than a low
permeability reservoir. However the rate loss in permeability in a high
permeability zone is smaller than that in a low permeability zone. This
is on the grounds that those high permeability formations have extensive
pore throat sizes, which are not totally obstructed by solids in
Though pore throats or low permeability
formations are little, mud solids and filtrates presumably obstruct
those throats bringing about huge decrease in permeability. Comparison
1.2 demonstrates that change in permeability is more essential than
thickness of the attacked zone.
FIGURE 1.1: A Schematic Of A Well With Damaged Zone (Skin Damage)
FOR A GIVEN RATE, q
Pwf1 > pwf > pwf2
FIGURE 1.2: Effect Of Skin Factor On Well Flow Pressure.
1.2 OBJECTIVES/AIMS OF STUDY
The objective of this project is
- To recognize, diagnose the causes of formation damage.
- To analyze the effect of the damage to the wells.
- To evaluate the economic effects of formation damage.
- To make recommendation of formation damage removal base on evaluation and analysis of data.
1.3 SCOPE OF THE STUDY
The study is concerned with the causes,
effect on the wells and the economic effects of this damage on the wells
as well as solutions of formation damage in oil wells.
1.4 BRIEF HISTORY OF WELL X
This well was drilled as a deviated
appraisal well in 2003. It was also completed in 2004 as a TSM. It came
on stream in June 2004 and was rapidly beamed up to potential.
Production picked in January 2005 at 3702 BOPD on 44/46”. Interval
started cutting water in November 2006, thereafter, production started
to decline. Further beam-ups failed to increase production or arrest
decline. An abnormal decline in FTHP occurred in June 2007 following a
beam up to 52/64”. Since then THP has been fairly low. The production
fell to 1250 barrels per day. A stimulation program was carried out in
June 2008 when the production rate decreased to 975 BOPD.
This method adopted in writing this
research project is through abstracts obtained from journals, textbooks,
SPE technical papers and presentations. The research project also
involved the receiving of past project of similar subject matters and
obtaining relevant information.