Summary
A giant, fractured carbonate field in north Oman has both complex geology
and complex reservoir-drive mechanisms. The upper, densely fractured layers are
produced using the gas/oil gravity-drainage (GOGD) process, while the
less-fractured lower set of layers is subjected to waterflooding. The
production from the GOGD layers is through vertical and horizontal wells
completed in a thin fracture oil rim. Gas conformance control is a challenge in
many of these wells because the gas breakthrough occurs for a variety of
reasons: downward movement of fracture gas/oil contact (fracture-oil-rim
thinning), gas breakthrough via high-conductivity fractures (fracture gas
breakthrough), zonal-isolation failure at the wellbore (mechanical gas
breakthrough), and increasing gas saturation in the matrix (matrix gas
breakthrough). An integrated multidisciplinary team studied well and reservoir
performance and openhole (OH) and cased-hole logs to diagnose the source of
higher-than-expected gas/oil ratio (GOR) in several GOGD wells. The most
important logs in this work were memory-production-logging-tool (MPLT) surveys
used to identify the sources of gas production and formation-microimager (FMI)
logs used for fracture identification and characterization.
This paper illustrates the work carried out in horizontal openhole and
vertical cased-hole completions to shut off the undesirable gas flow
successfully. The horizontal wells identified with poor zonal isolation behind
the liner were treated with an innovative gel gas-shutoff procedure. The merits
of this procedure outweighed those of other proposed solutions: targeted
placement, a strong full-blocking gel to fill up channels behind the liner,
inert particles to control fluid loss of the full-blocking gel to small
fractures and the formation matrix, and displacement with an already-cured gel
that could be washed out of the wellbore.
Significant drops in the GORs of these wells resulted in sustained
oil-production increases. This is a step change in the ability to manage
detrimental gas production in this field and is expected to lead to further
opportunities for improved gas management and well performance in this field
and other fields where the GOGD recovery mechanism is used.
Introduction
The giant fractured carbonate field was discovered in 1964 and came on
stream 3 years later. The field has seven reservoir layers—A through G—and
multiple subunits within each layer. The upper layers A, C, D, and E1/E2 are
more intensely fractured than lower layers, the E3/E4, F, and G reservoirs.
Initial production from the reservoirs (1967 to 1970) was by natural depletion,
supported by gas injection in the A reservoir unit from 1968 onward. After this
initial period of gas injection, water injection was implemented in the A, C,
D, and E reservoirs (1970 through 984). Previously unknown fracture networks in
these layers resulted in rapid water breakthrough. This was followed by GOGD
development (1983 through 1998), which was successful in arresting the decline
in the oil production. Following a simulation study in 1996, it was decided to
implement a line-drive waterflood with horizontal wells in layers that were
considered sparsely-fractured. Because GOGD is not effective in sparsely
fractured reservoirs, waterflooding these layers was expected to increase
recovery substantially in those layers. Since 1997, field development and
operation have used this combination of GOGD and localized waterflood. A recent
review of production and petrophysical data showed that the fracture spacing
varies significantly both horizontally and vertically. This impacts the GOGD
efficiency and recovery factor, since GOGD is inefficient where fracture
spacing is larger, because the oil has to travel longer distances through the
matrix to the nearest fracture set to be recovered.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
23 October 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
30 April 2007
- Manuscript approved:
4 June 2007
- Version of record:
20 February 2008
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