BP to do gravity surveys at Prudhoe in conjunction with gas cap water flood

Kristen Nelson

Prudhoe Bay operator BP Exploration (Alaska) Inc. has told the state that it will be doing gravity surveys offshore and onshore at Prudhoe Bay this winter and each winter for several years to come to help monitor water movement from the gas cap water injection project.

This winter’s survey will provide baseline data before the water flood begins in the fall. Repeat gravity surveys will measure gravity change with time.

The survey includes global positioning system and high-resolution gravity readings at precise locations along survey lines in townships 10-12 north and ranges 14-15 east, Umiat Meridian.

BP told the state that small-scale gravity surveys have been conducted in the greater Prudhoe Bay unit to provide information on water movement in the Sadlerochit formation gas cap. This survey will be on a larger scale with 350 to 400 stations in winter 2001-2002 on sea ice in Prudhoe Bay and on frozen lakes and tundra mainly south of the bay. Test gravity surveys in the past several years have verified equipment and established baseline information, the company said.

The gravity survey will provide BP with the general water flood shape and accurately determine the average flood front. The accuracy of the survey will be checked with water mass balance.

BP said water injection creates a positive gravity anomaly in the gas cap due to higher density water replacing gas.