FREQUENTLY ASKED QUESTIONS

Q. How many Crosswell surveys has Z-Seis run?
A. Crosswell Imaging technology has been successfully used for more than 10 years in such applications as locating Bypassed Oil, monitoring EOR activities, and directing both SAGD and Unconventional Gas.  Z-Seis and its predecessor companies have performed over 400 surveys in locations around the globe.

Q. How far apart can wells be spaced?
A. This depends on several factors, but most surveys are between wells no more than ½ mile apart. (160 acre spacing), and typically in the 1,000 – 2,000 ft. (300m – 600m) range. Well completions, formation types and well noise levels can influence the spacing limit in each case.
Z-Seis’ technology is constantly under development with the goal of expanding the operating range.

Q. Can you operate in deviated and horizontal wells?
A. Yes.  Z-Seis routinely work in deviated and directional wells.  Since Crosswell Imaging acquisition is very similar to wireline logging operations, any well configuration that can be effectively logged is a potential candidate for use in Crosswell.  Z-Seis has even operated in horizontal wells using coiled tubing conveyance.  Our advanced processing techniques process the data in a fully 3D format, assuring our clients of reliable and accurate imaging.

Q. Do the surveys work in openhole, cased hole or production tubing?
A. Yes. Most surveys conducted by Z-Seis are from cased well to cased well. This is because most surveys are run in mature fields. The technology operates also in open hole wells, and in fact, the absence of casing improves the signal strength.  In cased holes it is always preferable to run surveys without production tubing in the wells, because the tubing (or multiple casing strings) can degrade the signal. However, our receiver tool will fit inside most production tubing, and we can discuss through-tubing operations if applicable.

Q. How do the results compare to surface seismic and VSPs?
A. Crosswell Imaging surveys focus specifically on the reservoir (or other zone of interest), and provide highly detailed images of the reservoir. Typically, the vertical resolution of Crosswell Imaging is 10 to 100 times better than surface seismic. Crosswell surveys are referenced in depth directly to the wells, and the results can be easily correlated and integrated with logs and cores. Surface seismic surveys have uncertainty in time-depth conversion, and also in lateral positioning in dipping areas. VSP surveys are referenced in depth to the wells, but have vertical resolution typically two times better than surface seismic.

Q. Who does the data processing of Crosswell surveys?
A. Z-Seis provides full service capability: survey design, data acquisition and data processing. Although the principles are the same as those used for processing conventional surface seismic, conventional seismic processing software does not handle the geometries and bandwidth of Crosswell data efficiently.

Q. How long does a Crosswell image survey take?
A. The time requirement to acquire a Crosswell image varies considerably depending on height of zone, distance between wells and sonic properties of the reservoir.  Acquisition times vary from several hours to several days.  In general the taller the zone and the further apart the wells, the longer the acquisition time will take.  As part of the planning process, a qualified Z-Seis representative will discuss each profile with our clients and carefully plan out the timelines involved not only for acquisition, but also the processing of the data.

Q. How do the wells need to be prepared for a Crosswell survey?
A. Crosswell surveys can be run in open hole or cased wells. The source and receiver tools are designed to be operated like standard wireline logging tools. The source tool is 3 1/2” in diameter and the receiver tool is 1 11/16” diameter. Both source and receiver wells should have liquid (oil or water) over the interval to be surveyed: gas does not couple the seismic energy well. The liquid level should be stable during the survey because flow often generates noise that can degrade the signal.

Q. How does time-lapse monitoring work?
A. Time-lapse monitoring is done to observe changes over time of the distribution of fluids in the reservoir. Normally the changes of interest are those associated with production or injection. The survey design and planning are slightly different for time-lapse studies than for static characterization objectives. The principle is to take multiple surveys (i.e. at least two) at different times, and be able to interpret the differences between the respective datasets. The first survey is termed a “baseline” survey and is best done before production or injection starts. The basic premise is that the reservoir rocks will not change over time, but the fluids will move. The response of seismic measurements (of any kind, including Crosswell) to fluid changes in the reservoir is the subject of rock physics, and the response varies from reservoir to reservoir, and is normally best determined by core analysis. Normally the changes from oil to water phase causes a few percentage change in the seismic velocity. These types of changes are often hard to resolve with surface seismic, but are much more easily resolved with Crosswell. This is because it is easy to exactly repeat the geometry of a Crosswell survey (assuming the wells don’t move!) – we can put the same source and receivers into the same wells at the same depths. As a result, Crosswell can often resolve changes as small as 1-2% in velocity and these can be interpreted in terms of subtle fluid movements in the reservoir. Crosswell has the added advantage of high resolution, so changes of fluid over intervals as thin as 5ft to 10ft (1m to 3m) can be imaged. Again the comparison with surface seismic is significant in that surface measurements typically can resolve changes of 5% or larger over 100ft (30m) or thicker intervals.

Q. Can surveys be operated offshore or internationally?
A. Surveys can be conducted either on land or offshore. If the offshore wells are on different platforms, Z-Seis uses a microwave or infrared link to communicate between the source and receiver equipment (for land operations we use a fiber-optic cable link). Z-Seis operates wireline trucks (for US land operations) and wireline skids (for offshore and international operations).  In addition, Z-Seis routinely operates surveys outside of the U.S.  Data processing is normally done in the U.S. at our Houston, Texas data processing center.

Q. Where did the technology come from?
A. The basic acquisition and imaging technology was originally developed at Stanford University in the early 1990’s.  With the support of Chevron, Shell and the Gas Research Institute, TomoSeis Corporation continued the technology development to the level of a practical oilfield service. Z-Seis’ has built on the TomoSeis Technology to broaden the envelope of Crosswell applicability.