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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.
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Much
is being written about using seismic methods as Core
and log data provide high vertical resolution, but sample only a small
volume of rock. On the other hand, surface seismic methods sample large
rock or Crosswell seismic profiling fills the gap between data types that provide high resolution (but small sample volume) and data types with lower resolution (but high sample volume).
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 resolution. 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. Resolution on the order of 10 feet (3 meters) is possible. Crosswell processing is similar to surface seismic processing in that it includes velocity estimation (“travel time tomography”) and reflection imaging. Reflection imaging usually provides more resolution than the velocity image (“tomogram”) and depends critically on the accuracy of the velocity model for good results. 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 Crosswell
profiling is a technology for 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 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 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
vertical scale of three meters has been attained in a pilot flood in a
carbonate 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 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 Crosswell
images fill a resolution gap between the more traditional 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. |