Extending MWD survey interval
27 October 2008
Since the start of the ongoing discussion on MWD survey accuracy,
another item has cropped up ..
In a series of development wells, an operator has increased the
interval between MWD surveys from once per stand to once per two
stands .. the proviso being that it is in the tangent section and
only after assembly behaviour has been established.
We are now looking at whether we can extend the interval further, at
least to once per 3 stands.
The current situation is that the directional drilling company has
no issue with this, but that the corporate survey policy of this oil
company will not allow it .. one the grounds of being outwith their
required relief well survey policy (absolute uncertainty < 30 m, at
2 standard deviations.
I'd be interested in your experiences (and, preferably, facts)
1. The policies you have encountered / used .. and what they are
2. How they apply to development drilling
3. Anything you did that allowed increased survey frequency
We have identified considerable savings from increasing the survey
interval and would like to learn what you are doing.
Relentless Pursuit of Perfection Ltd
Depending on the well deviation it is okay to survey once per two
stands or more as the number survey points for a vertical or low
inclination well, < or =20Â° will not affect the description of the
trajectory that much. For higher inclinations it is advisable to
survey per stand and for regions where the DLS exceed 3Â°/100ft,
perhaps survey at even shorter intervals. Our manual recommends at
least 2 surveys per stand (as amended). It was one survey per single.
Running a rotary steerable tool favours long interval, per stand or
two stand surveys as hole rigosity is lower. With a mud motor, I
will recommend at least a survey per stand.
Please look at the thread "MWD Survey Errors".
You will see a "rough guide" to what I am thinking about here.
I am in the process of developing a program that takes into account;
a) Intermediate virtual surveys - (real wellpath vs perfect curve).
b) Temperature effects in hole.
c) Bent sub effect.
d) Bouyancy and stretch of pipe in hole.
e) Actual path of pipe (as opposed to always along the axis).
So far i am about 80% to completion, but the results are amazing in
the following consequences;
a) Seems I have been able to get a well path drilled with MWD to
Gyro (+/-) accuracy (the gross errors were historically greater than
the systemic errors it seems).
b) By working out the "real" trajectory of a well it is much more
accurate to predict the Torque and drag figures of that well - ie
using included micro DLS instead of planned.
c) By knowing the TVD to MD relationship more accurately, the
placing of perforations and general completion accesories is much
By taking the ratio factor (ie the calc that converts from chord to
arc) the "curvature" of a hole can be more accurately defined. and
so by using this criteria in a "hole section of constant state" - I
have been apparently able to spot the failing of deflection tools by
subtle changes in their behaviour - much before the well has been
drilled further into the abyss and declared failed.
My G&G team seem "confident" they have 2 inch seismic resolution at
4000mTVD when landing horizontals.
Guess we need that extra survey after all!!
I would say undoubtably that increasing the survey interval reduces
accuracy. However, I think that the rules are very much well
specific. I don't know whether I agree that it increases the cone of
error as the tool is the same (and so by definition is its error
I am assuming the extended survey interval will apply to tangent
situations in non-anticollision sections of hole. There is also the
consideration that perhaps the section will be re-suveyed (eg - gyro
run after casing) and so the final inaccuracy will be quantifiable.
At the end of the day its the usual cost to benefit ratio. I am a
believer in accuracy for surveying - and have identified many causes
of error (both systemic, systematic and gross) that perhaps increase
the error beyond extending survey interval.
Directional Drilling Co-ordinator
Theorically, extending survey interval makes bigger diameter of
ellipses uncertainty. Ellipses's diameter also depends on hole
inclination, hole azimuth and tools error model. Oil companies
usually define maximum acceptable diameter of the ellipses (for
example : 100 ft at 9 5/8" casing point).
How much we can extend survey interval is unique on each wells.
Directional Drilling Co-ordinator