Drilling the U-formation (shale and sandstone) in Saudi Arabia requires high
drilling fluid density (±95 lbm/ft3) to mechanically stabilize the
shale and balance the reservoir pressure. Two fluids have been used to drill
this formation: KCl/BaSO4/CaCO3 and potassium
formate/CaCO3 drilling fluids. Barite is added in the first fluid
with CaCO3 to reduce the amount of solids needed to formulate the
drill-in fluid. However, BaSO4 is insoluble in acids and requires
chelating agents (DTPA) to remove it from the formation. Formate drill-in
fluids with low solids content can be used, but they are expensive and
corrosive at pH values less than 7–8. A third drilling fluid was developed
recently by Al-Yami et al. (2007) to overcome some of the problems associated
with these two systems.
The objectives of this study are to determine solids invasion and damage
characteristics for the three fluids. Coreflood tests were conducted using
reservoir cores at bottomhole conditions. The cores were then examined by
environmental-scanning-electron-microscope (ESEM) analysis to investigate
solids invasion, solids type, and location of damage.
The three drilling fluids examined did damage the core plugs either through
external filter cake, internal filter cake, or both. The ESEM images showed
that the least damage occurred when Mn3O4 water-based
drill-in fluid was used. The highest damage was observed when
barite/CaCO3 mud was examined. This was followed by the potassium
formate drill-in fluids. Kaolinite booklets were not attacked by the filtrate
of the three drill-in fluids examined. The damage that was noted with potassium
formate mud resulted from incompatibility of mud filtrate with the formation
brine where potassium chloride crystals were noted in the cores.
There are six scenarios in which drilling fluids can cause damage to the
formation (Bishop 1997):
- Fluid-to-fluid incompatibilities (e.g., emulsions generated between
invading oil-based-mud filtrate and formation water).
- Rock-to-fluid incompatibilities (e.g., contact of potentially swelling
smectite clay or deflocculatable kaolinite clay by nonequilibrium aqueous
fluids, such as fresh water, that have the potential to reduce near-wellbore
- Solids invasion (e.g., the invasion of weighting agents or drilling
cuttings into the formation).
- Phase trapping/blocking (e.g., the invasion and entrapment of water-based
fluids in the near-wellbore region of gas wells).
- Chemical adsorption/wettability alteration (e.g., changes in the
wettability and fluid-flow characteristics in the critical near-wellbore area
because of emulsifier adsorption).
- Biological activity (e.g., the introduction of bacteria into the formation
during drilling and the subsequent generation of slimes, which reduce
Moreover, in overbalanced drilling, solids invade the formation. The
invading particles, which are suspended in the drilling fluid, tend to plug
pore throats and cause formation damage. To minimize formation damage, properly
sized bridging material should be large enough not to invade the formation, and
it should form an effective filter cake to prevent solids and mud-filtrate
invasion (Bailey et al. 1999). Zain et al. (2001) developed a model to show
that solids particles with smaller diameter than pore-throat size will enter
the rock and reduce permeability. The depth of invasion by solids in drill-in
fluids will indicate the required flow-initiation pressure.
In the present study, three water-based drilling fluids were examined using
sandstone reservoir cores. Potassium chloride was added to the three fluids to
control clay swelling and fines migration. The objective of this work is to
examine solids invasion and damage characteristics that can occur with the
three drilling fluids. ESEM analysis was used extensively to determine the
nature and depth of the damage.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
13 August 2008
- Meeting paper published:
11 October 2008
- Revised manuscript received:
5 March 2009
- Manuscript approved:
20 March 2009
- Published online:
25 November 2009
- Version of record:
1 March 2010