Page 213 - Well Logging and Formation Evaluation

Well Deviation, Surveying, and Geosteering 203

1. What is the relative dip between the borehole and formation?
2. If the direction of dip of the formation is the same as the borehole,
what is the absolute formation dip?
3. Suppose that it is known that the formation dip azimuth is at an angle
of 40 degrees to the borehole trajectory. What is now the true forma-
tion dip?

13.4 HORIZONTAL WELLS DRILLED ABOVE A CONTACT

Often there is a requirement to drill wells a fixed distance above a water
contact in order to optimize drainage. In an ideal reservoir, which is homo-
geneous, the contact would be at a fixed subsea depth, so in theory one
would only need to keep the well at a certain TVD. In practice, contact
depths may vary over a reservoir due to:

• Capillary effects. If the rock quality (particularly permeability) varies,
the oil/water contact (OWC) or gas/water contact (GWC) will vary,
while the free water level (FWL) remains constant.
• Depletion in the field. The contact may have moved due to aquifer influx
or injection during production.
• Depletion in neighboring fields. There may be observed an overall
tilting of the contact in a certain direction due to offtake in a neigh-
boring field affecting the aquifer.

The position of the contact will typically have been determined through
measurements made in nearby wells, and there may be some scatter in the
interpreted contact depths due to surveying errors. Typically, this uncer-
tainty will be on the order of 2–5m, although it may be greater if some
wells are particularly anomalous. Borehole TVD uncertainty as a result
of surveying errors, coupled with uncertainty in the true contact depth,
will lead to an overall uncertainty as to the distance between the well and
the contact. It may therefore be necessary for the petrophysicist to assess
the well’s proximity to the contact or FWL via real-time measurements
during drilling. The best way to do this is by using an established satura-
tion/height function, calibrated against core in earlier wells. Then the
water saturation calculated in the horizontal well while drilling may be
input to the model to back-calculate the height above the FWL. Once this
is known, the OWC may be estimated by observing the entry height on
the curve corresponding to the prevailing porosity and permeability.