Existing Nuclear Options Now Available As Thru-Tubing Tools

Table 1

Cased Hole Resistivity Logging

Crosswell Imaging

Figure 1

Slickline Innovations

More Information

The problem of targeting bypassed oil and gas in mature fields is one that faces many independent producers today. There are two paths to follow in finding a solution. One is to use the available data (production history, injection history, original open-hole logs, cores, etc.) to model the reservoir’s structure and producing behavior and indirectly predict the location of unproduced or unswept hydrocarbons. The other approach is to directly detect where the remaining oil or gas is within the reservoir. Many producers employ both.

The second approach has two fundamental challenges: how to gather data beyond the barrier imposed by steel casing, and how to accurately describe complex distributions of hydrocarbons at significant distances between wells. A number of new technologies either have been or are being developed to meet these challenges. Smaller, more compact nuclear logging tools now allow formation evaluation through both tubing and casing. A new resistivity log measures saturation data through casing. New techniques are being applied to image the interwell region beyond the wellbore. And improvements in downhole electronic components are making it possible to gather more data less expensively.

Existing Nuclear Options Now Available As Thru-Tubing Tools
Two closely related nuclear logging tools have provided through-casing measurements of saturation, porosity and lithology for decades: pulsed neutron capture (PNC) and inelastic carbon oxygen (CO) logs. In the past several years, a number of companies have made these tools compact enough to be run through tubing, widening their economic application.

Both of these tools irradiate the wellbore with pulses of high-energy neutrons that collide with the nuclei of the elements comprising the formation rocks and fluids. These collisions slow down the neutrons (a phenomenon called elastic scattering). Once the neutrons slow down to about 4900 mph they can be absorbed (captured) by these nuclei, creating a new isotope of the particular atom that does the capturing. The rate at which these neutrons are captured is important and can be measured. Chlorine atoms have a higher capture capability (capture cross-section) than hydrocarbons, so a formation saturated with salt water can be easily differentiated from one saturated with hydrocarbons or fresh water, but hydrocarbons and fresh water look very similar. In this case the CO measurement is used for hydrocarbon identification.

The CO measurement uses the inelastic scattering of the neutrons rather than the neutron capture for the measurement mechanism. The high-energy neutrons emitted from the source are slowed down by the inelastic scattering collisions with nuclei in the environment. Following a neutron-nucleus collision the nucleus emits gamma rays characteristic of the element. Measurements of these gamma rays provide a way to determine what sort of atoms are present in the formation and their relative abundance. CO logs measure the relative amounts of carbon and oxygen nuclei: high C/O ratios indicate oil while low C/O ratios indicate water. (Note: a much more thorough description of the basic nuclear processes at work in pulsed neutron logging can be found on the Baker Atlas web site at http://www.inteq.com/bakeratlas/resources/indepth5.2_RPM.htm). The CO log is unaffected by chlorine, so if the formation water is too fresh or if its salinity is unknown, it must be used to obtain a saturation measurement. Sometimes a combination of the two logs provides the best results.

The use of CO logs in particular has been somewhat limited by their relatively large size, relatively slow logging speed and sensitivity to the nature of the borehole fluid. Historically, these factors have made it necessary to kill a well and pull its tubing before logging, adding significantly to the expense. However, during the past five to ten years, logging companies have developed smaller-sized tools that combine the capabilities of both PN and CO logs in a single package that permits thru-tubing operation, eliminating the need to kill the well and pull tubing (Table 1).

Cased Hole Resistivity Logging
The case of resistivity measurement in cased holes has been one of a relatively straightforward theoretical solution waiting for the development of a measurement capability to support its application. Schlumberger’s new Cased Hole Formation Evaluation (CHFR) tool relies on this capability, the measurement of an electrical potential difference in the nanovolt range, to produce quality cased-hole resistivity logs.

When the CHFR tool injects current into the casing, most of it flows up and down the conductor, while a very small amount (in the milliampere range) leaks into the formation. Three voltage electrodes measure the potential difference created by this leaked current, which is proportional to the formation conductivity. The formation currents are sensed through the voltage drop they create in the casing segment. Since a typical formation is a billion times more resistive than casing, and the leaked current is only a few milliamperes, the measured signal is on the order of a few nanovolts. Relatively recent advances in electronics and electrical contact design have been the primary enablers of this nanovolt measurement capability.

Introduced earlier this summer, the CHFR tool was successfully validated against openhole resistivity tools in Europe, the Middle East and Alaska. Since its introduction in the US, it has been primarily used for contingency logging in wells where, for some reason, open-hole logs could not be acquired. However, according to Paul Beguin, CHFR Project Leader, "We expect the number of jobs, already over 40, to increase rapidly during the next several months. We have several reservoir monitoring projects currently beginning in California and the Permian Basin."

Because the electrical contact between the tool electrodes and the casing is so vital, measurements can be difficult in extremely corroded wells or when scale is present, possibly requiring a separate scale removal trip. Also, because the noise created by tool movement would be 10,000 times greater than the measured signal, the tool makes stationary measurements, which translates into a logging speed of 120 ft/hr. Longer stationary times can extend the range of measurable resistivities, which is about 1 to 100 ohm-m (with +/- 10% accuracy).

The cost of running the CHFR log could be comparable to the cost of a carbon-oxygen log, but would be a function of the particular situation. For instance, in a low porosity or fresh water environment a carbon-oxygen log would have to be run for multiple, slow passes, potentially making it more costly than the CHFR. Either of these tools would be more expensive than a conventional resistivity log.

The major limitations on the tool's applicability could be the 3-3/8" OD, making it necessary to be run in casing, and the resistivity range, says Beguin. "The maximum resistivity of 100 ohm-m sounds low compared to open-hole tools, but this tool allows producers to evaluate residual oil saturation and locate by-passed oil where conventional pulsed neutron tools can’t provide an answer."

As mentioned above, formation evaluation through casing can be accomplished with pulsed neutron technology. But nuclear tools work best in medium to high porosities, and the pulsed neutron capture measurement requires saline formation water. The CHFR tool works well in low porosity/low salinity situations, plus it has a relatively deep depth of investigation (between 7 and 32 feet). As with open-hole methods, saturation evaluation can be enhanced through a combination of resistivity and nuclear measurements.

Crosswell Imaging
While evaluating saturations at specific wellbore points within the reservoir is useful, in complex reservoirs the real challenge is characterizing the distribution of fluids across intervals and between wells. Research has been underway for some time to apply acoustic and electromagnetic measurements to image this interwell area. Commercial services are now available that offer interwell seismic imaging (e.g., Tomoseis Interwell Imaging). This technology has been shown to provide images of reservoir structure between wells, and also in some cases, flood front movement over time.

For example, Chevron has used Tomoseis crosswell seismic imaging to define the location and extent of fluid changes during alternating steam injection/oil production cycles in heavy oil wells in California. Figure 1 shows three crosswell seismic velocity images of the reservoir volume between two wells, with the injector/producer halfway between. The first image is at the beginning of an injection cycle, the second is at the peak of injection, and the third at the end of injection/production. The images identify the area to the right (downdip) as receiving the bulk of the injected fluid.

Another effort being carried out at Lawrence Livermore National Laboratory (LLNL), funded by the Department of Energy, has had some success in imaging saturation distributions between wells using electromagnetic (EM) induction rather than seismic signals. As with crosswell seismic imaging, at least two wells are needed, one for the signal source and at least one other for the receivers. All of these tools are designed for wells where the tubing has been removed.

The LLNL researchers have been successful in imaging saturations between wells with fiberglass casing, over a distance of about 90 feet, at a depth of about 1500 feet. These field tests were carried out in the Lost Hills Field in the San Joaquin Valley (with the collaboration of a producer, Seneca Resources Corporation, and a developer of electromagnetic imaging systems, Electromagnetic Instruments, Inc.). The problem these researchers are now wrestling with is how to make the technology work in steel cased wells. Current efforts are underway to attempt to use induction logs to normalize for the effects of the casing. “We are able to monitor fluid movement on the order of several feet in non-conductive casing,” says Barry Kirkendall, Project Manager at Lawrence Livermore, “Our current approach is to try to at least qualitatively subtract for the effects of steel casing with low-frequency EM induction. We will be doing this at a field site provided by Chevron in California. Their interest is in imaging the flood front of CO₂ injection.”

Slickline Innovations
While independent producers might like to apply these logging technologies, either the ones that are commercial now or those that might become so in the future, they are expensive. The costs of pulling tubing and running wireline equipment into multiple wells can be hard to justify in mature fields where even a successful characterization of the reservoir does not guarantee additional reserves. Even the thru-tubing services can be expensive in marginal operations.

But in another area, new technology is making it easier to apply older techniques in ways that might hold promise for locating bypassed oil in older fields. Two advancements: the ability to build smaller yet more efficient measurement tools and a rapid increase in electronic data storage capability, have combined to make it possible for some types of production logging and perforating to be performed using slickline equipment. While these tools do not measure fluid saturations, flowmeters and temperature logs can help identify what fluids are being produced from what intervals. Slickline perforating can be used to make the completion adjustments that might be needed to improve recovery, often at a lower cost than electric wireline. Micro-Smart Systems, Inc. has incorporated these technology advances into a slickline “memory logging and perforating” service.

A critical step in slickline memory logging is being able to correlate recorded measurements and depths. This is accomplished in this case by first establishing a downhole reference point (nipple) with a collar locator. As the well is logged with the downhole memory tools, a surface computer makes depth measurements that are precisely correlated in time with the measurements being taken downhole. After the tools are retrieved, the data are dumped to the computer, were the surface and downhole data are merged. The downhole reference point distance is then shifted to match an existing log or other form of correlation. The computer insures that all the recorded data remains in the proper perspective relative to the reference point.

According to Steve Arnold, Sr. Field Support Engineer with Micro-Smart, “A lot more data can be stored downhole than before, and the newer electronics use less power, making it possible to acquire more data with the same or even smaller batteries. Also, temperature is less of a factor than it used to be.” According to Arnold, Micro-Smart has completed close to 2000 successful runs over an eight year period with their perforating tools alone, continually making improvements to both the tools and the software for managing the data. “Offshore or in inland waters, where the cost of putting an electric wireline unit on a production platform or well caisson is a significant factor, we’ve been able to achieve a 40 to 60% cost reduction compared to using wireline,” says Arnold. “Onshore, where smaller electric wireline companies are very cost-competitive, the difference is not so great, but it’s still there.”

More Information
Independent producers can access additional information related to these technologies by contacting any of the following:

• Schlumberger’s web site has information on the CHFR tool at: http://www.connect.slb.com/.
• Baker Atlas’ web site has information on pulsed neutron logging at http://www.inteq.com/bakeratlas/.
• Halliburton’s web site has information on cased hole logging services at http://www.halliburton.com/.
• Tomoseis, a unit of Core Laboratories, has information on their crosswell seismic service at: http://www.tomoseis.com/.
• Steve Arnold with Micro-Smart Systems, Inc. can be reached at micro_smart@yahoo.com