[Proj] Deepwater Horizon. Where is the bottom of the well?

[Noel Zinn]

A lot of science goes into wellbore surveying nowadays, and that applies to
assessing the quality of the survey, too.  Wells are surveyed while drilling
(MWD, measurement while drilling) with instruments in the bottom hole
assembly (BHA) that includes the drill bit.  Typically, those instruments
are magnetometers (magnetic azimuth) and accelerometers (inclination).
Wells are also surveyed when the BHA has "tripped out" with instruments
(typically gyros for true azimuth and accelerometers) that are lowered on a
wire line.  Distance along the wellbore trajectory (measured depth, MD) is
determined by drilling pipe tally or wire line length.  All these
instruments have biases that must be compensated and different random errors
that must be propagated from surface to TD (total depth).  Stochastic models
for MWD and gyro surveys have been published by the Industry Steering
Committee for Wellbore Survey Accuracy (ISCWSA), which is now the Wellbore
Survey Accuracy Technical Committee of the Society of Petroleum Engineers
(SPE).  BP provided early leadership of the ISCWSA.  For more information
follow this link => http://www.iscwsa.org/.  Given all this technology,
there are also geodetic and cartographic corrections that must be applied,
especially with deviated, extended-reach wells, which are common in deep
water.  

Regards,
Noel Zinn, Principal
Hydrometronics LLC, Consultancy and Technical Software
+1-832-539-1472, noel.zinn at hydrometronics.com
http://www.hydrometronics.com


-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Karney, Charles
Sent: Monday, June 14, 2010 1:09 PM
To: PROJ.4 and general Projections Discussions
Subject: [Proj] Deepwater Horizon. Where is the bottom of the well?

Here are some questions related to the Deepwater Horizon accident and
the drilling of relief wells for the readers of this list in the oil
industry.

(1) What is the method of determining the position of an instrument 3km
down a well?  Is it seismic, dead reckoning, or what?

(2) What is the typical absolute accuracy of the measured position,
horizontally and vertically (and, if possible, specifically with
reference to the leaking well)?  What is the *relative* accuracy?
(I.e., I can imagine that the absolute accuracy is poor because of the
unknown properties of the intervening rock.  However, it might be
possible to "find" the same position when drilling the relief well if
the same method is used.)

(3) Is the drill bit instrumented to return the position?  How?  Or does
a separate logging package need to be inserted into the well?  At what
frequency would this be done?

I can piece together some of the answers from wikipedia.  However, maybe
I can get more autoritative and more specific answers from this list.

	  --Charles

-- 
Charles Karney <ckarney at sarnoff.com>
Sarnoff Corporation, Princeton, NJ 08543-5300

Tel: +1 609 734 2312
Fax: +1 609 734 2662
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