It provides an insight to drilling fluids engineers how to protect the rok and retain its native productivity

1. FORMATION DAMAGE
1. Formation damage can be defined as a part of the drilling process which
impedes the ability of a hydrocarbon bearing rock to be able to flow to its
full natural potential.
2. Whether or not the rock is damaged, can be ascertained by virtue of skin
factor of the drilled rock, which if > 0, is an indication of damage.
3. Higher the value of skin factor, more is the extent of damage. Conversely if the
skin value is negative, it means that the process of drilling has triggered the rock
to flow more than its originally estimated recoverable reserve by primary
technique.
4. Zero value of the skin factor is an indication that the rock is neither damaged nor
triggered for higher flow by drilling process.
5. The first thought which immediately comes in most professionals’
mind while referring to formation damage is – The damage is due to
mud!!
6. This is a major reason of not being able to eliminate/minimize
formation damage to its full potential, as MID IS NOT THEONLY CAUSE!

2. FORMATION DAMAGE
 Having said that, it is also a fact that incorrectly designed mud and
improperly maintained properties of mud do cause fair amount of damage to
the producing rock
 It is essential to understand and acknowledge that the very process of drilling
is inherently damaging to the rock.
 Therefore until recently it was believed that it is not possible to drill the
reservoir and NOT DAMAGE IT!!
 This myth has been busted and wells have been drilled with negative skin
factor, an indicator that rock’s natural ability to flow has been enhanced by
drilling fluid related aspects!!
 Let us remember -
 DAMAGE MINIMIZATION ATTEMPTS MAKE SENSE ONLY FOR RESERVOIR SECTIONS.

3. FORMATION DAMAGE
 Feature conducive to damage, that are extremely difficult to control are
 Naturally damaging rock. These are the types of rock which have very high
permeability and porosity, leading to excess intake of fluid entering through
the inter-connected and large pore spaces
 Composition of connate water and wettability features of rock are often
capable of cause of damage, that cannot be completely overcome.
 Regression and/or abrupt ramp in pore-pressure can sometimes pose
challenge in well design and mud properties selection to the extent that
damage cannot be completely avoided
 Design optimization, sometimes calls for accepting minimum damage for the
increased overall success of the project and adopt technology which negates
the impact of drilling related damage. Filter-cake dissolving spacer
technology, fracturing the rock to deeper horizontal reach, are some features

4. FORMATION DAMAGE
 Drilling practices and drilling fluid design as well as management practices
often entangle with each other to a large extent when it comes to damage.
 Therefore, a holistic approach of good oil field practices, is more preferred
way of addressing the damage minimizing practices and the Dos and Don’ts
described below are hence not being segregated into distinct two categories
of drilling and drilling fluid.
 Do’s and Don’ts to minimize/eliminate formation damage are as below
 As far as possible, drill with just enough mud weight to keep reservoir content
in its native place during drilling and the well-bore mechanically stable. Geo-
mechanical modelling studies help achieve this.
 Do not drill at a rate faster than the overall ability of the drilling process to
keep the hole free of cuttings. ECD monitoring is a viable technique to
monitor this.

5. FORMATION DAMAGE
 Do not drill at a rate faster than the overall ability of the drilling process to keep
the hole free of cuttings. ECD monitoring is a viable technique to monitor this.
 Focus on mud design to help build a compact and impermeable filter cake around
open hole quickly to disallow filtrate invasion.
 If information is available on inter-granular pore space, then use the same wisely
to design a mud with particles in such a way that they do not invade the pore-
spaces and cause fluid migration along with filtrate.
 Mud weight more than optimum, will exert unnecessary extra pressure against the
wall of open hole, resulting into filtrate invasion which will displace the reservoir
content too far to activate the well during flowing back.
 Retain the native wettability of the rock by selecting the correct type of mud
system to disallow damage of rock as a result of incompatibility between reservoir
fluid and mud filtrate.

6. FORMATION DAMAGE
 If the reservoir bearing open hole is to be cemented, then the slurry design should
ensure that there is no mud / cement slurry lost during cement job. Software
models help in achieving this.
 The rheological profile of the mud should be conducive to hole flushing without
exerting unnecessary additional ECD. Ideally the LSRY ( 3RPM*2/6 RPM) should be
equal to the size of the open hole from which the fluid is expected to flow back.
 While designing well clean-up programme avoid use of too many fancy stuff
(surfactants /solvents/corrosion inhibitors, oxygen-scavanger) unless extremely
essential question the logic of inclusion by assuming that default pill programme
does not require any of these chemicals.
 Plain brine of correct weight should clean the fluffy part of filter cake and create
pin holes in compact filter cake to facilitate flow of oil/gas through them which
will then find their path out once the draw down is applied due to difference in
formation -pressure and applied pressure to activate the rock.

7. FORMATION DAMAGE
 Most important part of preventing formation damage is tom select a mud system after
conducting core flood tests on various formulations in a reputed core lab. Selection of
completion design also is an important aspect of preventing formation damage and
completion engineer should be an integral team member to finalize the plan.
 Monitor dynamic Fluid loss rather than static API F/L and use a PPT apparatus with ceramic
disc of size matching with average pore throat size of the reservoir rock. The PPT must be
kept below 2.5 cc and tests carried out at BHCT and not on predefined API temp., which are
good for static measurements.
 Formulate mud with fragile gels to negate the adverse impact of excess pressure on rock
while initiating circulation after considerable gap.
 While selecting brine weight always remember to apply temp. correction factor to arrive at a
realistic weight at bottom and know the corresponding weight at surface temp.
 The well bore clean up plan should be drawn with great caution to prevent excessive pressure
getting exerted on well bore while various density fluids entering the annulus.

8. • Whole/partial mud loss into the producing rock is one of the biggest threats to
formation damage and a fool-proof plan MUST be drawn to prevent loss of
circulation as far as possible
• While drilling through Limestone or fractured rock it is very difficult to prevent
losses and ideally such formations should be drilled with floating mud cap technique
or plain water (or brine of correct weight) to minimize damage. Such rocks are
invariably acidized subsequently and use of water / brine of correct weight helps in
ensuring that there is no incompatibility issue.
• Do not tempt to drill at a rate faster than the ability of system to flush out cuttings
no sooner they are generated. System efficiency can be improved by higher LSRV
readings with fragile gels and optimized YP.
• If situation warrants, flush the hole with low-hi vis pills without creating ay
pressure fluctuation in circulatory system. Monitor ECD very carefully and never
allow it to suddenly creep up. If required drilling can stop for a while and hole
circulated to flush the cuttings and bring ECD in realistic range.
FORMATION DAMAGE

9. • Do remember that with increase in depth there will be a nominal increase in ECD
and this will not adversely affect the hole, but any abrupt rise in ECD must be
addressed immediately and brought back in same range as prevailing before.
• Last, but not the least – ALWAYS keep experienced and knowledgeable personnel on
rig to protect the interest of the well.
RESERVOIR DRILL-in Fluid
For the last three to four decades there is an increased awareness level as well as
willingness to address formation damage elimination / minimization by thoughtfully
designing the mud system with focus on means of eliminating mud related damage
during drilling as well as during well flowing. With scientific approach, a system has
been devised which addresses
FORMATION DAMAGE

10. • Filtrate invasion minimization
• Quick formation of impermeable and thin, yet compact filter cake
• Thoughtful selection of mud products.
• Use of technical data to select suitable mud weight and not over emphasise only on
well safety even if it comes at a cost of damaging the well/rock.
• As far as possible, elimination of additives which have known potential to damage
the rock by chemical/physic-chemical mechanism.
• Knowing the type of rock and using the same as an input in selecting the
composition and the type of mud system.
• Selection of mud parameters which avoid/minimize formation damage.
• All these factors led to evolution of a type of mud which is intended to be used
specifically only for reservoir section of the well and such system has been
distinguished by the other types with the name of “ Reservoir Drill-in fluid” system.
FORMATION DAMAGE

11. • The primary function of this system is to not only facilitate smooth drilling, but
simultaneously attempt to protect the rock as much as realistically possible. One
such essential feature of this system is that it is designed with focus on selecting
the particles in mud of that size which does not intrude into the rock and make
conscious attempt to maintain the size and concentration in as much the desired
range as possible by continuous replenishment of sized particles in mud externally
and compensate for loss of particles due to crushing action of bit while continuous
circulation. At the same time, one has to select correct configuration and size of
shaker screens to dispel finer particles and keep inert solids in a range which does
not adverse the rheology of the system and yet build the much desired firm filter
cake ( internal as well as external).
• A lot of emphasise should be laid in correct use of solids control equipment to
retain essential sized particles and only selectively discard harmful colloidal sized
particles. This is being achieved by making arrangements of re-introducing the
correct sized particles into main stream from shaker discard.
FORMATION DAMAGE

12. Selection of various additives in RDF mud
• The focus, again is on eliminating the additives which are generously used in
drilling upper sections, like PAC, Asphalts etc to name few. Instead of them bio-
degradable starch as well as bio-degradable viscosifiers are used to build the
skeleton of mud which is then converted in a RDF by administering correct sized
particles of calcium carbonate and avoiding use of barite and instead achieve
desired weight either by high density brine or by adding higher dose of calcium
carbonate or a combination of both. The native wettability of the rock is
ascerta9ined and as far as possible simpler version of water-based mud is preferred
over other types, unless severe shale instability issues warrant otherwise.
• It is imperative to accept that the process of drilling will invariably damage the
rock and efforts are made to minimize the impact as much as possible. One can aim
for zero damage but that is not always achievable. The wisdom is to live with
minimum damage and yet drill the rock safely and timely.
FORMATION DAMAGE

13. When to use RDF:
The decision to use RDF must be taken with all factors being given due weightage and
selecting the system as well as components and properties which not only minimize
damage to the rock, but also facilitates safe and timely drilling. Wherever feasible,
core flood tests should be conducted in lab with samples of probable mud type and
system to find out which of them provide highest permeability regained figure. Out of
the top few such formulations the ease of drilling and completion design chosen for
the well have to be reviewed. It may not always be possible to select a system which
is zero damaging but the one which facilitates hassle free drilling and completion as
well as damages the rock to the least, is often the chosen one.
With advancement in drilling fluids technology now it is possible on many / all
occasions to use solids free design of RDF and yet get the benefit of drilling and
completion with minimum complication. Solids-free RDF in reservoir actually means
that whatever solids are there in system,, they are purposely retained to facilitate
damage minimization and other non-essential additives (Barite or other heavy density
additive which has potential to damage the rock).
FORMATION DAMAGE

14. CHANGING OVER TO RDF AND MAINTAINING RDF
• Once a consensus has been arrived by team to drill the reservoir with a
particular type of mud, it is imperative that the mud of previous system is
completely replaced with RDF correctly formulated, including but not
restricted to hole volume, surface volume. Before resuming reservoir
drilling it is a good practice to build at least twice the open hole volume of
the section planned for drilling to account for additional volume
requirement, failing which often there is a temptation to blend the
previous section mud. This can defeat the purpose and should be refrained
from. Monitor particle size of the active mud with the help of particle size
analyser to be made available on rig with qualified person to operate the
same periodically and help mud engineer to decide his tour dose of
essentially solids of correct size. This aspect is often overlooked and is a
cause of not being able to eliminate rock damage completely.
FORMATION DAMAGE

15. SUMMARY
• It is a bit unrealistic to conclude that use of RDF will eliminate
formation damage but certainly, with right planning and
correct execution the extent of damage can be brought with-
in manageable limits. The concept still needs enormous
support from the entire team for optimized results and
success/failure ca not be entirely executed to any single
individual.
• USE OF RDF SHOULD PROVIDE FLEXIBILITY TO ELONGATE THE LIFE OF
THE WELL AT SUSTAINABLE FLOW RATE. EARLY SPURT AND
UNSUSTAINABLE RESULTS REFLECT PARTIAL SUCCESS ONLY.
FORMATION DAMAGE

16. SOLIDS FREE NON-DAMAGING DRILLING AND COMPLETION FLUIDS
• Industry has now by and large realized the importance of solids-free
drilling as well as completion fluids for various advantages they offer to
drill and complete with.
• Without going in those details and focussing only on damage
minimization aspects of such fluids, these fluids offer
• Minimized fines migration
• Formation-friendly filtrate ( in some specific type of brine) to enhance
the flowability of the producing rock by virtue of chemical reaction with
connate water of the rock and converting it to a flow-inducing fluid
• Minimized solids in filter-cake, resulting into its easy lifting feature
• Complete avoidance of incompatibility between drilling and completion
fluid by using same base fluid in drilling and completion fluid ( brine)
FORMATION DAMAGE

17. As a result of the capability of some heavy weight brine/s to eliminate
use of weighing material completely, such brines offer huge advantage
in HTHP wells as they eliminate fines migration( and thus control
intractable formation damage), apart from offering host of other
drilling related advantages – reduced ECD, reducing flow-check time
(as they attain thermal equilibrium very quickly) etc.
They impart the flexibility of loading higher concentration of
proppants and yet transport the entire fluid to fractured zone by
providing excellent solubility in presence of VES( Visco-elastic
surfactants)
The challenge in such fluids is to keep the fluid-loss ALAP, because the
solids required to retain/disallow flow of water from mud due to
pressure application, are either absent or insufficient.
FORMATION DAMAGE

18. FORMATION DAMAGE

19. FORMATION DAMAGE

20. FORMATION DAMAGE

21. FORMATION DAMAGE