Liqing Xu, Roger N. Anderson, Albert Boulanger, and Wei He
Lamont-Doherty Earth Observatory, Columbia University, Palisades,
New York 10964
More than half the oil and gas in every field in the world is
bypassed, even with the best of current technologies. However,
we are in the midst of a profound technological transition within
the industry that promises to significantly improve these recovery
efficiency percentages. Because we are experiencing it first-hand,
it is sometimes hard to discern the magnitude of the paradigm
shift. During the 1980's and on into the mid 1990's, the upstream
industry as a whole was testing, adapting, and finally accepting
3D seismic surveys as the cost-effective tool for both exploration
and enhanced recovery that geophysicists said it was all along.
Most 3D seismic acquisitions were over old fields, ironically,
and so from the very beginning, geologists were seeing changes
in oil/gas/water that had occurred since production began, usually
long before the first 3D survey was shot. These 3D surveys were,
in fact, the first 4D, or time-dependent seismic monitoring projects.
4D is 3D seismic imaging done many times over the same volume of the earth, and then integrated with other forms of time-depend information in a field. Examples of other 4D datasets include pressure histories, temperature monitoring from inside currently producing wells; cased-hole workover logs such as pulsed neutron, and reservoir simulations of the expected vs. actual behavior of the field.
The transition from 2D to 3D has resulted in improved recovery efficiencies that have increased from 25-30% of oil-in-place with 2D control, to 40-50% with 3D coverage. BP expects the industry's most ambitious 4D project in Foenhaven to improve those statistics to 65-75% (Petroleum Engineer International, January 1996).
The promise with 4D seismic monitoring is that we might now remotely
sense the real pattern of drainage of oil inside reservoirs beneath
the surface! Time-lapse images of producing reservoirs are acquired
through the repeated acquisition of 3D seismic surveys to track
oil's movement over time. The location and amount of bypassed
oil can be identified, so than new wells can be drilled to recover
much more than previously possible (Figure 1).
Figure 1. 4D Technologies are expected to improve recovery another 20% over 3D
We begin with the present state-of-the-art, 3D seismic imaging, where the static, fixed geometry's of reservoirs, faults and stratigraphy are described. We must know these first in order to properly interpret the meaning of 4D changes in drainage over time. Consider a 3D volume shot in the Gulf of Mexico in 1985 (Figure 2). Oil and gas reservoirs show up prominently as coherent "bright spots". Then the same area was re surveyed in 1988, and again in 1992. Assuming significant production between these snapshots, the reservoirs should have shrunk in volume over that time interval, as can be seen from Figure 2.
Figure 2. 4D is multiple 3D seismic surveys over the same fields.
The removal of large volumes of oil and gas from a reservoir produces velocity and density changes that can possibly be remotely detected from the surface. It is important to realize that the seismic response to changes in reservoir properties can vary from field to field. Depletion produces pressure drops that in the Pleistocene Gulf of Mexico (GOM) produce amplitude dimming, but in West Texas might produce amplitude brightening. Petrophysics and careful seismic interpretation can predict these changes. An acoustic response "wedge" can be generated by synthetic seismic modeling for each field. The impedance (velocity times density) changes of the rock matrix are held fixed, and the pressure drop from production is added to the density changes from changes in oil/gas/water mix to predict the acoustic response expected (Figure 3). Each reservoir in the field then defines a separate pathway along the surface of this wedge over time. That is what 4D seismic monitoring tries to detect in the real earth.
Figure 3. The "wedge" of expected seismic amplitude
responses to changes in fluid properties and pressure depletion
in a reservoir. This wedge takes a unique form for each field
in the world, and each reservoir describes a unique pathway along
the wedge surface during depletion.
Consider, for example, the Pleistocene GOM, water encroachment is predicted to produce a dim-out (blue) and formation of a gas cap produces a brightening (red) with time. Bypassed oil maintains its high amplitude (green), allowing the placement of new wells to drain missed pockets of oil in old fields.
This procedure is similar to that followed in the interpretation
of time-dependent well logs, of which Pulsed-Neutron cased-hole
logs are the most frequently used. A single seismic "bin",
or location of a vertical, stacked waveform, is like an acoustic
log from a vertical well (Figure 4). Suppose there are pulsed-neutron
logs from one well recorded in 1974, 1981, and 1990. After normalization
of the neutron fluxes, water saturations and capture cross-sections
(sigma) can be calculated for each. The drainage of the reservoir
should then show up in great detail. However, the information
is not extendible away from the wellbore. Here is where 4D seismic
comes in. The seismic waveforms from each bin around the well
can be examined for amplitude changes over time. They must first
be carefully normalized to each other, and not only amplitude,
but other attributes like phase, frequency, and particularly,
impedance, can be examined. Drainage should agree between the
two techniques at the well bin location. Then, seismic changes
from other bins away from well control can be examined.
Figure 4. 4D seismic compares bin-for-bin to 4D logs, such
as cased-hole pulsed-neurton logs.
We demonstrate the utility of 4D seismic monitoring in the Eugene Island 330 field of offshore Louisiana (Figure 5). 4 generations of 3-d seismic surveys have been acquired over the Eugene Island 330 field, first in 1985, then in 1988, 1992 and again in 1994. These datasets contain seismic snapshots of the field as it has been drained of more than 100 million barrels of oil equivalents. The 4D study was begun in 1992. At that time, a traditional, gravitational water drive was assumed to be sweeping the LF reservoir in the 4 corners area of Blocks 330/331/337/338. A 1992 snapshot shows considerable seismic amplitudes remaining downdip of the supposed oil/water contact. Up until 1992, primary wells draining the fault block were the A-11 and A-6 of Block 331, the B-2, B-5 and B-7 of Block 330, and the A-13 and 1-15 of Block 338.
Figure 5. Demonstration of the 4D seismic changes seen in Eugene Island 330 Field from 1992 to 1994. Red is increased amplitude, blue is dimmed amplitudes over time, and green are sustained, high amplitudes indicative of bypassed pay.
After the 3D seismic survey of 1992 was acquired, there were only
three wells active in the fault block: new wells B-5ST, B-6ST
in Block 330 and the A-12 in Block 338. A new 3D seismic survey
was acquired in 1994, and it clearly shows drainage, or dimmed
amplitudes, caused by production in the intervening years. About
2.4 million barrels of oil equivalents were produced from these
wells, and about 600 acres were dimmed. Assuming a 100 ft thick
pay sand with 30% porosity and a drop in Sw of 30%, that would
require that each ac-ft produce about 400 bbl/ac-ft, quite reasonable
for clean, Pleistocene sands in the GOM.
The bypassed oil remaining after 1994 will require the placement of yet another well into the fault block, and therein lies the power of 4D seismic monitoring. Each new well recovers an additional percentage of oil-in-place, increasing the ultimate recovery efficiency. In addition, the 4D seismic changes can be modeled within a reservoir simulator to risk the new well (Figure 6). Amplitudes drained by the new well are predicted to account for an additional 2 million barrels for a well placed horizontally along the 330/338 property line to the 4 corners. In another two years, that well will water out, an additional amplitudes are predicted to remain after that. Then a well projected more north-south across the property line will be required to recover an additional 2 million barrels, and so on into the future.
Figure 6. Model simulation of the flow paths and drainage
produced from a hypothetical well placed along the Block 330/338
property line in 1996. After two additional years of production,
a new 4D survey in 1998 is predicted to observed remaining amplitudes
tyhat would require another well orthogonal to this one to be
drilled in 1998. Each new well is projected to produce an added
2 million barrels of oil fro the same reservoir fault block, thus
increasing the ultimate recovery efficiency.
The recovery of oil and gas must become more efficient if we are
to supply the world with enough oil to support the increase in
standard-of-living required for a peaceful earth into the next
century. 4D seismic monitoring offers the hope of controlling
and optimizing oil drainage in real-time, and thus of recovering
more oil from old fields at a time when the world will surely
need these added supplies.