of "Gravity Gains Momentum"
A Reservoir Monitoring Case History
By: Johann van Popta, KSEPL, Rijswijk, The Netherlands
Stephen Adams, SIPM, The Hague, The Netherlands
Reprinted from Middle East Well Evaluation
A novel method of determining gas saturation has proved
successful in Oman's Natih Field where conventional methods
were giving anomalous results in difficult conditions.
Stephen Adams and Johann van Popta of Shell describe how, for
the first time in the Middle East, sensitive borehole gravity
measurements, interpreted as density logs, have been compared
with traditional density readings or previous borehole
gravity measurements to derive a quantitative estimate for
gas saturation changes.
Contributor: Richard Piggin, EDCON
Situated 250 km from Muscat in Northern Oman, the Natih
Field produces oil from a highly fractured limestone
reservoir (figure 1.1). Since production started the primary
method of recovery has relied on gas/oil gravity drainage
(GOGD). The physics behind this method is simple. Gas is
injected above the oil into a secondary gas cap, which
developed as the reservoir pressure declined. This controlled
injection prevents any further reduction in pressure and,
critically for the GOGD process, lowers the gas-oil contact
(GOC) in this dual permeability reservoir (figure 1.2).
The (lighter) gas flushes the (heavier) oil out of the
reservoir rock into the fracture system, lowering the gas-oil
contact. Some of the gas mixes with the oil held in the rock
matrix, encouraging flow into the fracture system where
gravity drainage then takes over. This drainage is a slow
process - a foot or two per year of vertical displacement.
Fig. 1.1: Location of the Natih Field.
Fig. 1.2: TRUE LIFE STORY: At
discovery, the Natih Field had no gas cap. Through
primary depletion the FGOC moved down and the FOWC moved
up. Oil was left above the FGOC and below the FOWC in the
matrix blocks. Finally, with gas injection, the oil rim
in the fracture system is pushed down. This exposes the
maximum possible amount of rock to the gravity drainage
process. Oil is produced at controlled rates from the oil
rim in the fracture system.
The effectiveness of the gravity drainage process depends
on a uniform and complete gas-oil contact being maintained in
the fracture system. To find out how well the process had
been working in the Natih Field, Petroleum Development Oman
(PDO) launched a gas saturation monitoring campaign four
years ago using conventional and pulsed neutron capture
measurements. Readings were taken through production tubings
and in workover wells. The estimated secondary gas
saturations were not only lower than predicted by material
balance calculations but were inconsistent - even within a
single well (figure 1.3). As the blocks of reservoir rock
between the fractures are relatively impermeable, it is
believed that the shallow-reading neutron devices could not
see beyond the mud filtrate trapped in the rock surrounding
the borehole, giving rise to pessimistic estimates of gas
flushing and variable results.
Because of this doubt over the validity of the gas
saturation results, an alternative method had to be found for
evaluating the reservoir's gas saturation.
Research by Shell in The Netherlands indicated that the
Borehole Gravimeter (BHGM) might be the answer. This is a
deep-reading tool which the research team believed would see
beyond the invaded zone and could be used to quantify gas
saturations (see box right). Extensive modelling studies were
conducted at Shell's research laboratories in Rijswijk, The
Netherlands. The study findings confirmed that bad hole
conditions, invasion of drilling fluid, poor cement bonds,
the presence of perforations and previously acidized
intervals would have a negligible effect on the gravity
Two factors stood out as being critical in achieving
accurate gas saturation measurements from the BHGM:
- A clear knowledge of the porosity around the well,
derived from open-hole logs.
- An accurate depth measurement for the tool at each
Consequently, the four wells selected for BHGM surveys in
the Natih Field were chosen not only for their structural
position above the fracture GOC and the completion status but
also for the quality of their open-hole porosity logs. Depth
control was achieved using a combination of wellhead, manual
and downhole measurements. A special odometer was mounted on
the wellhead and a casing collar locator log was calibrated
with a casing tally.
In addition, the length of cable run down the borehole
between measuring stations was checked manually using a steel
tape and a high-precision pressure gauge was included in the
tool string. This last gauge was added because it was assumed
the tool movement downhole could be estimated directly from
the difference in wellbore pressure - the borehole being
filled with brine, rather than weighted drilling fluid. Each
of the three depth measurements were examined in conjunction
with the gravity measurement and an optimum depth determined
for each BHGM measurement.
The provision of a stable borehole environment is
essential to ensure good quality gravity readings with such
sensitive equipment. In the depleted fractured carbonate
reservoirs of North Oman, heavy fluid losses are encountered
when a formation is open to the borehole.
Fig. 1.3: CONSTANT CONFUSION: Gas
saturation from Pulse Neutron logs gave inconsistent
results - even within individual wells.
EDCON's gravity meter (manufactured by LaCoste &
Romberg) is fundamentally a very sensitive spring balance in
which the weight of a hinged beam with a small mass on its
free end is balanced by the tension of a spring (figure 1.4).
As the gravitational acceleration - and hence the weight of
the mass - changes, the spring tension must be changed to
hold the beam in a stationary horizontal position. The spring
tension is calibrated in gravity units.
Essentially, the BHGM can be used to quantify hydrocarbon
saturations by reinterpreting the results to produce a
density log. To do this, the earth is modelled as a layered
cake with infinite horizontal slabs (figure 1.5).
The change in gravity between the top and bottom of each
slab is proportional to the slab's density and thickness.
The density of each slab is made up of the matrix (rock)
density, plus the fluid density. As oil is replaced by gas,
the fluid density decreases. This 'density' log from the BHGM
can then be compared with the original open-hole density
measurements. In the Natih Field, where there was no initial
gas cap, any differences between the two measurements can be
directly attributed to fluid movement and thus gas
Under normal conditions, measurements can be repeated to
within a standard deviation of about 3 µgal (three parts in
109 of the Earth's gravitational field).
Fig. 1.4: MAGIC MOMENTS: The sensor
within the BHGM is simply a spring balance. The tension
necessary to maintain the mass and beam in a horizontal
position is directly related to the gravitational
Fig. 1.5: Density of an infinite horizontal
Prior to running the surveys, three of the four wells in
the Natih Field were being worked over, thus all existing
perforations were closed with cement. This prevented fluid
movement distorting the results. In the remaining wells, the
surveys were carried out before perforating.
As a result of the co-operation agreement between
Schlumberger and EDCON, the surveys were conducted using
EDCON's Deep Density BHGM sonde in combination with
Schlumberger's gamma ray and high-precision pressure gauge
tools. Each survey took readings at the geological sub-unit
boundaries, producing an average gas saturation for each
layer. Typically, 15 to 20 gravity stations were selected in
each well from 20m below the GOC in the fracture system to
20m above the reservoir. Before each reading the tool had to
be left to stabilize for several minutes. Readings at some
stations were repeated in order to improve the precision of
gravity and depth measurements and to provide monitoring of
gravimeter drift. Overall, the logging time varied between 15
hours and 30 hours.
Using a BHGM to determine gas saturation is an innovative
use of the tool based on theoretical considerations and
experience in sandstone reservoirs in Texas, USA. To ensure
the validity of the results in complex fractured carbonate
reservoirs, a control test was planned in two new development
wells over a gas-bearing formation in the nearby Yibal Field
(figure 1.6). The estimates produced by the new tool were
compared with gas saturations calculated using resistivity
logs. Figure 1.7 shows the similarity of the results produced
by the two techniques.
Confidence in the tool is further increased by comparing
the BHGM-derived formation densities and the open-hole
density log for Natih-48 (figure 1.8). In this example, the
calibration intervals above the reservoir and below the
fracture GOC show good agreement. The discrepancies between
585m and 601m can be attributed to the gas-filled formation's
influence on the BHGM.
Fig. 1.6: Results from one of the
calibration surveys run in the Yibal gas reservoir. This
shows that the maximum and minimum gas saturations
derived from the BHGM straddle the estimates from the
resistivity log over the complete interval tested.
Fig. 1.7: Comparing the results from
the BHGM and resistivity logs for both the Yibal
calibration wells. The line of best fit indicates the
similarity of the results.
Gas saturations around the Natih Field have been estimated
based on BHGM measurements. When plotted with gas saturation
values derived from earlier pulsed neutron measurements the
BHGM estimates show consistently higher gas saturation. This
ties in with the neutron tool being affected by drilling
fluid trapped around the borehole (figure 1.9a and b).
Secondary gas saturations estimated for the Natih Field
using the BHGM are now similar to predictions from reservoir
simulation studies, according to Shell. Gas/oil gravity
drainage will continue to be the primary recovery mechanism
in the Natih Field, as will the use of the BHGM for gas
Fig. 1.8: A comparison between
BHGM-derived densities and the shale corrected open-hole
density log shows excellent correlation above the
reservoir and below the GOC.
Figs. 1.9(a) and (b): Comparison of
gas saturations predicted by the BHGM and PNL for two
different subunits show that an envelope created through
the BHGM results completely encompasses the PNL figures.
This is consistent with the latter figures being
influenced by drilling fluid trapped in the rock matrix
adjacent to the well.