The Liuhua 11-1 Field,
located 130 miles southeast of Hong Kong under 1,000 feet of water
in the Pearl River Mouth Basin (figure 1),
was discovered in 1987 and is currently being developed by the consortium
of BP, China National Offshore Oil and Kerr-McGee.
The reservoir
zone at 3,850 feet subsea is producing 16-22 API degree oil through
25 long-radius horizontal wells.
The Liuhua
Field is bounded by high water-flow faults and karst features that
affect the production of bottom water within the heavy oil reservoir.
Three-D seismic data reveal details of the reservoir heterogeneity
in spectacular images of gas chimneys associated with both linear
and circular karst features.
An ultra
high-resolution 3-D seismic survey over Liuhua was acquired in July
1997. With peak frequencies over 200Hz, the seismic data have allowed
for temporal and spatial resolution on the order of 14 feet. Faults,
fractures and karst features in the reservoir were analyzed on this
dataset using coherence technology.
Complex
attribute analyses added a greater understanding of rock matrix
continuity, which was initially thought to provide a tight, competent
seal to underlying aquifers.
The focus
of this article is on carbonate solution collapse and the associated
development of gas chimneys.
Liuhua
Geology
The Liuhua
reef carbonates are projected to have in-place reserves of 1.2 billion
barrels.
After the
initial production in 1996 peaked at 65,000 BOPD but declined rapidly,
it became clear that the reservoir lithology was more petrophysically
heterogeneous than originally thought, and that a 3-D seismic dataset
was needed for a reservoir characterization.
A structure
map of the top of the reef (figure 2)
shows bounding faults on the north and south sides of the Liuhua
reservoir. The southern fault system is associated with several
circular karst-collapse structures clustered south of the production
platform.
Figure
3 is an enlargement of this area from a coherence image showing
the internal detail of these features and a modern analog in Belize.
Oilfield
Karst and Gas Chimneys
The gas
chimneys associated with karst leaching are caused by the CO2,
H2S and methane byproducts of the bacterial degradation
of the oil. The actual karst-collapse results from carbonic acid
dissolution associated with the generation of the CO2.
In the
Liuhua reservoir the major faults provide channels for significant
vertical movement of water at the edges of the reservoir. Several
poor quality wells have been drilled into or near these fault zones.
At the same time, the ongoing karst solution collapse, which appears
to have been active for almost 15 million years, also creates vertical
zones for water encroachment both outside of and within the productive
area of the reservoir.
Figure
4 is a seismic reflection strength section showing the chaotic
reflectivity associated with the vertical deformation and gas chimney
over the large collapse feature at the reservoir's southern edge.
This feature spans about 5,000 feet of vertical section and is rooted
at the base of the carbonate platform in a sandstone aquifer that
crops out on the seafloor.
Geochemical
and mechanical effects caused by dissolution microfracturing and
stratigraphic brecciation of the brittle carbonate matrix ultimately
create pathways for the upward movement of water into the horizontal
well bores. Tight rock appears to become more permeable, while porous
reservoir rock becomes less porous and permeable as a result of
these combined processes.
Because
of the preferential permeability of water relative to oil in a heavy
oil reservoir, the tighter rock now produces water almost to the
exclusion of oil.
Figure
5 is a north-south section showing the vertical dim amplitude
zones, gas sag and collapse adjacent to the bounding faults. Both
groups of bounding faults are adjacent to partially collapsed gas
time-sag zones within the reservoir. This subtle low velocity sag
(about four meters) is linear rather than circular, and is thought
to represent incipient carbonate dissolution.
Figure
6 is an east-west reflection strength section within the chimney
zone, parallel to the south edge of the reservoir in figure
5. Again, the amplitude anomaly that extends to the sea floor
in the chimney collapse zones within and above the reservoir is
due to gas, suspected microfracturing and some carbonate porosity
changes.
This same
zone is connected to the large off-structure sinkhole complex shown
in figures 2 and 3,
and was modeled as a major source of water influx responsible for
poor production in the western field area.
Conclusions
Much of
the prior geoscience understanding of the Liuhua reservoir was revised
as a result of this work, including:
- Oilfield Karst is
now thought to be a significant factor affecting the hydraulics
of the Liuhua reservoir.
- The relationship
of the karst features and gas chimneys with an abnormally large
upward movement of water explains the high water cuts in many
of the wells.
The field
fluid movement was modeled successfully in a reservoir simulation
guided by seismic attribute analyses of the fault, fracture, gas
chimney and partial dissolution zones.
The resulting
production history matching of the fluid flow around the horizontal
well bores confirmed the reservoir's complex character.