AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Reservoir Geology of Mensa Field After 13 years of Production, Deepwater Gulf of Mexico (GOM)
(1) Shell Exploration & Production Company, New Orleans, LA.
Mensa, MC Block 731, is a deepwater GOM subsea gas field, producing from an upper Miocene; proximal lobate turbidite sheet sand associated with a faulted four-way closure. “I” sand production commenced in 1997 accumulating a total of 835 BCF from five wells. Field rates reached 300 MMCFGD, with current rates at 60 MMCFGD. Original reservoir pressure was 10,100 psi declining to 6,700 psi. A recent sixth well, targeted the faulted “attic” area and proved pressure communication with the main reservoir. The new well extends field life by several years and will improve field recovery to 1 TCF. Gas recovery is about 70% via a combination of gas expansion, moderate water drive and compaction. Reservoir structure is a nearly flat, covering some 5000 acres from 15,200 to 15,800 feet subsea. Net pay is up to 170 feet with average porosities of 30% and permeabilities up to 1 Darcy.
The depositional setting for the “I” gas sand appears to be a small lobate fan system associated with a turbidite basin entry-point. It can be further subdivided into two shingled lobate deposits referred to as the “Ia” and “Ib”. The “Ia” is the younger deposit and covers the northern portion of the field. It is probably completely filled with gas and is either shaled or eroded out before reaching the original gas water contact (OGWC). The older “Ib” covers the southern half of the field and has an OGWC. The lobes are high net to gross (> 90%) but contain very thin shale and other low permeability layers sometimes correlative between wells. The low permeability rock was not a barrier to gas flow as proved by a 2006 well test that penetrated swept (wet) “Ib” sand in a low structural reservoir position. In contrast, a combination of a post depositional channel downcut, shingled lobate geometries and the thin poor quality rock layers have proven to be a significant baffle to both higher pressure aquifer support and to water influx. Water took 12 years and 805 BCF cumulative to reach the first producing well.
There are three field pressure trends. The first trend is related to the original development wells, which for the most part, perforated the entire “Ia” sand and upper portion of “Ib” sand, indicating depletion drive. During second trend, pressures leveled off significantly due to the aquifer kicking in, with possible contribution from the unperforated “Ib” sand. The third pressure decline occurred in the last few years and indicates that the energy resource is finite.