Page 31 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:55 PM Page 7

Preliminary Work for the Well Design [ ]
1.1.2

ify. In reality, if you are given the maximum area to go for, you may aim
not at the center but at a place that gives you the most leeway for direc-
tional performance that does not go quite as planned.
For an offshore well (except for a platform), a seabed survey is
required to check for bathymetry, seabed obstructions, seabed com-
position, likely leg penetration (if applicable), and shallow gas indi-
cations. Generally, this would cover a 2 km x 2 km square, centered
on the proposed well location. Local currents should be checked (his-
torical data may be available in mature areas) both at surface and at
seabed level. Surface currents will affect rig positioning and marine
operations; seabed currents may cause scour. Apart from the seabed
survey, shallow seismic may be required to spot shallow gas anom-
alies and estimate leg penetration. In an area of soft seabed, the
drilling contractor may require a soil boring analysis to ensure that
the rig can be jacked up with minimal risk of punching through a
hard crust during preloading.
Sources of offset data. Now that the location and target depths/for-
mations are known, you can look for relevant offset wells. Except for a
rank wildcat well, quite a lot could be available from company sources.
This includes final well reports (which, if written properly, will be your
best source of information), daily drilling reports, etc. If people who
worked on the well are still with the company, make a note of it so you
can contact them later if queries arise.
Other data on offset wells may be available from sources outside
the company. For instance, if the mud records are missing or incom-
plete, ask the mud service company which was on the well if they still
have information such as daily reports from that well. Bit records are
often available from bit vendors. Wireline logs are usually archived for
at least ten years prior to disposal by the logging contractor. IADC and
geolograph reports can be more useful than daily drilling telexes
because they will often hold more detailed information and are usual-
ly more accurate than the daily report telex to the drilling office. The
drilling contractor may still have these somewhere.
Other outside data may include maps showing structures, surface
features (for planning access to the site, locating water sources, and
avoiding sensitive areas), and offset wells. In addition, government
records are an important source of information. In many if not most
areas worldwide, regulations demand that well information be filed

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