Published: 06/14/2017
Published: 06/14/2017
An operator producing a California heavy oil reservoir by steamflooding wanted to periodically run cased hole logs to track changes in the steam front and oil saturation. The monitor well had been extensively cored and logged to establish a baseline when it was an 8.7-in-diameter borehole, prior to completion with 7-in 23-lbm/ft casing. However, the field's formation water is very fresh, so there would be no contrast in a conventionally logged capture cross section between oil and water in the reservoir. Because traditional cased hole tools do not have this differentiation capability, they could not be used for monitoring.
Pulsar multifunction spectroscopy service overcomes the limitations of conventional pulsed neutron logging tools by integrating a high-performance pulsed neutron generator with multiple advanced detectors in a single 1.72-in-diameter tool. The result is a complete petrophysical volumetric interpretation based on highly accurate elemental concentrations—including carbon as the basis for TOC—in addition to traditional sigma, porosity, and carbon/oxygen ratio measurements. Pulsar service's measurement technology can be operated in different pulsed neutron logging modes for monitoring consistency with previous conventionally obtained data.
For monitoring the steam front, the FNXS measurement introduced by Pulsar service now makes it possible for cased hole logging to differentiate gas-, air-, or steam-filled porosity from fluid-filled and tight formations.
The 8.7-in-diameter open hole had been cored and logged with an extensive suite of openhole logs including neutron density and Litho Scanner high-definition spectroscopy service.
Pulsar service was run in the monitor well to simultaneously acquire inelastic gas, sigma, and hydrocarbon index and dual inelastic and capture spectroscopy data. The TOC computed from spectroscopy and the resulting determination of oil saturation were confirmed by the openhole logs. As shown in the second track from the right, Pulsar service's dry-weight TOC (black) obtained at 50 ft/h compares favorably with TOC similarly obtained by the larger-diameter advanced spectroscopy tool during the initial openhole logging (magenta) at 450 ft/h. The oil saturation computed from the cased hole TOC is a good match to the core-measured saturation on the far-right track.
The initial openhole neutron density log shows steam- and air-filled sands above X,500 ft. Pulsar service's sigma, thermal neutron porosity (TPHI), and FNXS logged in cased hole all also show gas (steam or air) in the same interval. In this situation, where openhole porosity logs are available, they can be used to compute gas saturation in conjunction with any of these gas-responding measurements, usually with the deeper-reading sigma or TPHI.
However, if openhole logs are not available, the difference between the crossplotted FNXS and TPHI responses can be used to not only solve for the gas saturation but also the total porosity after gas correction. The overlay on the crossplot of FNXS and TPHI shows the expected response of various lithologies. The subhorizontal upper boundaries are where 100% waterfilled porosity plots, and the subvertical boundaries to the left represent where 100% gas-filled porosity plots. The responses of TPHI and FNXS significantly differ because TPHI is a hydrogen-dominated measurement, whereas FNXS is not.
Without the new FNXS measurement or openhole logs, solving both gas saturation and porosity from pulsed neutron logs is underdetermined. In openhole, an accurate formation porosity in gas-filled formations is usually computed from a combination of density and neutron porosities. In cased hole, FNXS plays a role similar to that of density because its response contrasts with the traditional neutron porosity–type response, which is dominated by hydrogen. As a result, the response for air- and steam-filled sands is in the gas region of the crossplot and the fluid-filled sands and siltstones plot along the 100% fluid line.
Challenge: Reliably track steamflooding in a heavy oil reservoir with fresh formation water that obscures the usual contrast of oil and water.
Solution: Run Pulsar multifunction spectroscopy service to measure elemental concentrations— including total organic carbon (TOC)—in addition to sigma, porosity, and the new fast neutron cross section (FNXS) measurement that differentiates gas-filled porosity for a complete, stand-alone cased hole interpretation from a single tool.
Results: Definitively quantified oil saturation and differentiated porosity filled with air or steam from fluid-filled zones to enable monitoring solely by single-tool, one-run logging.
