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BOREHOLE GRAVITY RESPONSE TO COTTON VALLEY REEF STRUCTURES

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.