Click to view this article in PDF format.
CROSSWELL SEISMIC PROFILING: PRINCIPle TO APPLICATIONS*
By
Jerry M. Harris1 and Robert T. Langan2
Search and Discovery Article #40030 (2001)
1Stanford University, Palo Alto, CA.
2Chevron Petroleum Technology.
*Adapted for online presentation from the article by the same authors, entitled “Crosswell Seismic Fills the Gap” in Geophysical Corner, AAPG Explorer, January, 1997. Appreciation is expressed to the authors, along with Spyros Lazaratos of TomoSeis Inc. and Mark Van Schaack of Stanford University, and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
|
Much
is being written about using seismic methods as reservoir management and
monitoring tools. However, when we try to apply these methods, there are
always issues of Core
and log data provide high Crosswell
seismic profiling fills the gap between data types that provide high
Crosswell seismic profiling is conducted between wells with the source and receivers placed inside the wellbore, as illustrated in Figure 2. The receiver arrays are held fixed in one well while the source is slowly drawn upwards in the other well and is “fired” at preset intervals. After one source “run,” the receivers are relocated and the source run is repeated. The typical spacing between adjacent source points ranges from 2.5 feet (0.8 meter) to 20 feet (six meters). Receiver separation is usually similar. It is possible for these systems to acquire 20,000 or more traces in a single, 24-hour day. A
complete survey can be as small as a few thousand traces or as large as
several hundred thousand traces. Such factors as the well separation, the
thickness and structure of the imaging target, and the frequency content
of the received signal dictate the necessary size of a survey. The
distance between the source and receivers, which is on the order of the
well spacing, is considerably less than the propagation distances
associated with surface seismic methods. The short propagation distances,
combined with avoidance of weathered zones, allow the use of frequencies
at least an order of magnitude higher than used with surface seismic
methods, resulting in a proportionate increase in spatial Crosswell
surveys currently employ a frequency band between 20 Hz and 2000 Hz,
depending on the type of source used, the distance between wells and the
attenuation characteristics of the zone under investigation. In
Figure 3 comparison is made of a
crosswell velocity image and reflection image with modern surface seismic
data, a sonic log, and core data. All of these data were collected in a
carbonate reservoir in the Permian Basin of West Texas. Crosswell methods
are not a replacement for 3-D surface seismic technology in areas where
the frequency content is similar and where surface accessibility is not a
problem. It is 2-D by nature and the insufficiencies of 2-D versus 3-D
seismic data are well documented in the literature. However, by requiring
multiple profiles, a 3-D perspective can be achieved. One should view
crosswell profiling as being complementary to both surface seismic methods
and logging methods (as illustrated in Figure
1), and it is best targeted at locations where the enhanced Crosswell profiling is a technology for reservoir delineation, development, characterization, and monitoring, but not exploration. Monitoring changes in reservoir conditions (e.g. saturation or pressure) is easier than absolute imaging of reservoir properties (e.g., porosity), but monitoring requires multiple visits to the same site in order to obtain time-lapse images. In the United States, a majority of the crosswell activity has been in the San Joaquin Valley of California and the Permian Basin of West Texas, but there has been recent work in the Mid-Continent and the Gulf Coast as well. In the San Joaquin Valley, the primary interest has been managing the heat budget of thermal recovery processes. The well separations are usually small, the reservoirs shallow and the thermal recovery processes create large velocity changes that make it easy to monitor the progress of thermal fronts. The images used for monitoring are predominantly time-lapse tomograms, although reflection imaging has been used as well. The main difficulty with using crosswell profiling in this environment is that the sedimentary rocks are commonly quite attenuating, which can restrict the useful upper frequency range, and a powerful source may be required to propagate energy between wells. A second problem is that some wells will not hold water for a sufficient period of time, which prevents the use of fluid-coupled sources and receivers. In
West Texas, the reservoirs are dominantly carbonates with favorable
attenuation characteristics. As a result, frequencies as great as 2,000 Hz
can propagate over hundreds or thousands of feet between wells. The high
degree of Although
there are a variety of applications for which crosswell profiling is
technically feasible, for some of them the technique is currently too
expensive to implement on a routine, operational basis. For example,
successful imaging of CO2-saturation and -pressure effects on a
One
of the first applications where it is thought that crosswell profiling is
likely to find wide operational acceptance is in providing an accurate
“roadmap” for directional wells. It is becoming an increasingly common
practice to optimize recovery in a reservoir by targeting specific units
for a directional well. Directional wells are relatively expensive, and in
areas where the structure or stratigraphy between wells is not easily
predicted using traditional data types, crosswell methods may be the only
way to obtain the high The acquisition systems currently available commercially are based on two different source technologies: · A small airgun that is impulsive and relatively widebanded. · Piezoelectric elements that are swept in frequency in a manner similar to surface vibrators. Both sources are frequently used with hydrophone receiver arrays. The airgun system has been used successfully in clastic rocks in Kansas at a well separation exceeding 2,000 feet (600 meters), while the piezoelectric system has been used in carbonates at a well separation of 1,800 feet (550 meters). Greater well separations are possible and are slated for future projects. An axial hydraulic vibrator is currently under development by a cooperative Research and Development Agreement (CRADA) between the U.S. National Laboratories and numerous industry partners and was scheduled to be commercially available by the time this article was published in the AAPG Explorer. Because of its relatively high power, we expect it to be applicable to large well separations and to other acquisition geometries, such as that found in a 3-D reverse VSP or in a single-well mode (where the source and receiver are in the same well). Crosswell
images fill a For some applications, crosswell technology is currently moving from being a purely research activity to being an operational technique. Among the current barriers it faces in gaining a wider acceptance are the cost of data acquisition, the potential disruption to normal field operations and insufficient experience using technology in a variety of environments. The cost of data acquisition is dropping quickly, however, due to hardware improvements and the expanding experience base. It is expected that future improvements in data processing will reduce the disruption in field operations by carefully scheduling the survey during normal maintenance activities or before tubing is placed in a new well. Recent advances in multi-level receiver systems that can operate through production tubing and can be used simultaneously in multiple wells will permit more rapid data acquisition, reduce field disruption, and reduce costs. More and more case studies will expand the routine acceptability of crosswell profiling. |