Borehole Gravity (BHGM) surveys have
been run in a number of wells in the current reef play at the
top of the Haynesville Limestone in Leon and Robertson
counties in East Texas. The surveys were run in wells which
did not intersect targeted porosity. In some of these wells,
the BHGM data has indicated potential production away from
the well and sidetracks have been successfully completed.
The drill targets are reefal mounds rising several hundred
feet into the Bossier shale above the top of the regional
Haynesville Limestone. Porosity occurs in the reefal mounds
and below the level of the top Haynesville. Porosity
distribution in reefs tends to be very irregularly
distributed. Production is gas.
A Borehole Gravity Meter is used to measure gravity at a
sequence of depths through the zone of interest. The density
of the rock surrounding the well is calculated from the
differences in measured gravity. In perfectly flat uniform
geology, the BHGM densities will match the normal gamma gamma
density log data. When the geology away from the well varies,
the gravity field and the calculated BHGM densities are
perturbed and no longer match the gamma gamma density. By
comparing the BHGM and gamma gamma densities, we can
determine changes in geology up to several hundred feet from
the well.
A BHGM log is normally run when the reef section drilled
is tight, or if there is a suspicion that the drill missed
the reef buildup. The BHGM responses can often determine if a
sidetrack is likely to intersect production.
The following figures illustrate the BHGM responses for
several possible well locations with respect to a reef.
Case 1.: Well on flank of a tight reef.
The well is shown about 50 feet to the left of the reef.
The red BHGM density trace follows the blue gamma gamma
density within the Haynesville (dark blue). Adjacent to the
reef in the Bossier Shale (pink), the BHGM density is higher
than the gamma gamma density because of the proximity of the
reef which has a higher density than the shale.
Case 2.: Well on flank of porous reef.
With the addition of 20% gas filled porosity (yellow) to
the reef, the BHGM response adjacent to the reef in the shale
becomes less than the gamma gamma density. Depending on the
amount of porosity in the reef the BHGM response will grade
between Case 1 and Case 2.
Case 3.: Well inside tight reef.
In the case of a BHGM survey in a well penetrating a tight
reef, the BHGM response is lower than the gamma gamma density
response below, within and above the reef. Within the reef
the BHGM density is lowered by the flanking lower density
shale. This response to the shale continues below and above
the reef.
Case 4.: Well inside reef with adjacent porosity.
The presence of gas filled 10% porosity adjacent to the
well inside the reef is shown by a corresponding lowering of
the BHGM density.
