Cairn India Limited - Ravva | Slime Hole Drilling

1
 
   
   
   
   
   
   
RAVVA Celebrating 16 Years of Technical
Excellence Innovating Development Slim Hole
Drilling
2
Ravva Innovating Development Slim Hole
Drilling The Slim Hole Drilling concept is
considered as one of the most cost effective
methods in oil gas field development. It
involves drilling smaller diameter holes and
using small diameter production casing
and tubing. It is an emerging technology expected
to make step-improvements in the efficiency and
economy of well construction. The narrow-gap
criterion says that if the drill string to hole
diameter ratio is greater than 0.8, it is
classified as slim hole. If one can extract
less rock, then the well costs should fall. Thus,
the use of small diameter well bore reduces the
overall cost of exploration, drilling and reserve
development. Slim hole drilling applications
are
Continuous coring Conventional rotary
drilling Downhole motor drilling Coiled-tubing
drilling.
? ? ? ?
The table here shows some of the wells drilled in
the Ravva exploration campaign, the hole sizes,
target depths, casing policy, production hole
sizes the year of completion for the respective
wells. As evident from the table, the initial
period of the campaign saw a three casing policy
being deployed to drill the wells. The data
received from these initial wells was then
processed to determine the right casing policy
for future wells. It can be seen in the table
that wells drilled in the later part of the
campaign have a two casing policy. The surface
hole was dropped and the intermediate 12 ¼ hole
extended from surface to intermediate depth and
the 8 ½ production hole was extended to cover
the longer hole section. The changes in the
casing policy with slim well design have resulted
in substantial cost savings. Slim Hole Drilling
advantages are
? ? ? ? ? ? ?
Considerably less usage of drilling
mud Smallercasing sizes (major cost reduction
factor) Can be used for High Angle
andHorizontal Wellbore from existing wells Could
be used for CO2 Flood Injection holes or
wells Reduced drilling time due to rapid drilling
The advantages of a smaller hole size and the
reduction in the volume and size of oil, wet
drilling cuttings helps in minimising the
environmental impact associated with the use of
oil base mud. The reduction in drilling fluid
volume and treatment cost, higher rates of
penetration, better hole cleaning
capabilities, savings in consumables (such as
bits, mud, cement, and diesel oil), less tangible
cost, and easier equipment mobilisation, produce
significant operational gains. By using slim hole
technology, a reduction of 40 to 70 in well
costs can be realised. Tubular costs alone are
reduced by 40 t50 with an overall reduction of
around 17 in the well cost. In addition to cost
savings, the operations can be performed safely
due to the smaller size of equipment involved. It
has also been seen that slim holes do not hinder
production. Although substantial savings are
realised by using the slim hole concept, there
are elevated risks as well. There is a lack of
good contingency in the event of unexpected hole
condition or geological condition requiring an
extra casing to be run prior to reaching the
total depth. Usually, very slow drilling ROPs
are associated with the erratic directional
responses, and can restrict the use of MWD
tools. Effective pre-planning between the
drilling, exploration, and the engineering team
to accurately define possible geological targets
and their potential drilling problems can
minimise these risks associated with drilling
slim holes and provide higher levels of success
in meeting planned objectives.