Understanding reservoir rock characteristics and the forces that mobilize oil in unconventional reservoirs are critical in designing oil recovery schemes. Thus, we conducted laboratory experiments in three preserved Middle Bakken cores using centrifuge and nuclear magnetic resonance (NMR) instrument to understand oil recovery mechanisms in the Bakken. Specifically, we measured capillary pressure, pore size distribution, and oil and brine distributions. A series of oil and brine replacement experiments (drainage and imbibition) were conducted in the preserved cores using a high-speed centrifuge. T2 time distribution and one-dimensional saturation profile measurements were obtained using a 2-MHz nuclear magnetic resonance instrument before and after centrifuge experiments. Moreover, pore size distribution was determined from Mercury Intrusion Capillary Pressure (MICP) and Nitrogen Gas Adsorption experiments. We conducted scanning electron microscope (SEM) imaging on polished core slabs to determine pore shapes and mineralogy of pore walls using a field emission-scanning electron microscope (FE-SEM). Our measurements show that, contrary to the common notion, the preserved Middle Bakken cores are not oil-wet—but show mixed-wet characteristics. Water resides in smaller pores and oil resides in larger pores in all experiments. Using a low-salinity synthetic brine of 50,000-ppm to surround Bakken cores of much higher salinity, we produced up to 6.33 % (of pore volume) oil from two higher porosity (~8%) cores, and 10.72% (of pore volume) oil from one lower porosity (~2%) core in spontaneous imbibition experiment. Moreover, up to 6.62 % (pore volume) oil from the two higher porosity cores and 11.23% (of pore volume) oil from the lower porosity core was produced in forced imbibition experiment. These experiments indicate that chemical osmosis overrides the wettability effects in tight Middle Bakken cores. The new observations regarding osmosis have altered our classical notion of capillary imbibition in shale reservoirs.

Summary Understanding reservoir-rock characteristics and the forces that mobilize oil in unconventional reservoirs is critical in designing oil-recovery schemes. Thus, we conducted laboratory experiments for three preserved Middle Bakken cores using centrifuge and nuclear-magnetic-resonance (NMR) instruments to understand oil-recovery mechanisms in the Bakken. Specifically, we measured capillary pressure, pore-size distribution (PSD), and oil and brine saturations and distributions. A series of oil/brine-replacement experiments (drainage and imbibition) were conducted for the preserved cores using a high-speed centrifuge. T2 time distribution and 1D saturation-profile measurements were obtained using a 2-MHz NMR instrument before and after centrifuge experiments. Moreover, PSD was determined from mercury-intrusion capillary pressure (MICP) and nitrogen-gas-adsorption experiments. We conducted scanning-electron-microscope (SEM) imaging on polished cubical cores to determine pore shapes and mineralogy of pore walls using a field-emission SEM (FE-SEM). Our measurements show that these three preserved Middle Bakken cores show mixed-wet characteristics. Water resides in smaller pores and oil resides in larger pores in all experiments. Using a low-salinity synthetic brine of 50,000 ppm to surround Bakken cores of much-higher salinity, we produced up to 6.33% [of pore volume (PV)] oil from two higher-porosity (approximately 8%) cores, and 10.72% (of PV) oil from one lower-porosity (approximately 2%) core in a spontaneous-imbibition (SI) experiment. Up to 6.62% (of PV) oil from the two higher-porosity cores and 11.23% (of PV) oil from the lower-porosity core were produced in a forced-imbibition (FI) experiment as well. These experiments indicate that molecular diffusion/capillary osmosis overrides the wettability effects in low-permeability Middle Bakken cores. The new observations regarding molecular diffusion/capillary osmosis have altered our classical notion of capillary imbibition in low-permeability reservoirs.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCarbon/oxygen (C/O) tools have been designed for helping evaluate reservoirs under relatively simple conditions, such as cased wells with tubing removed and a single fluid present and reservoirs that have relatively high formation porosity. However, to reduce costs, tubing is often not pulled and wellbore conditioning is minimized. A new generation of small-diameter C/O tools capable of logging through-tubing has therefore been developed to deal with these circumstances. Logging through-tubing, despite its advantages, presents its own set of problems. Obtaining reliable measurements is often highly challenging because of different tubulars, various borehole fluids, and multiple contacts in a single well.A new high-resolution C/O system has recently been developed for use under such complex conditions, specifically in tubing of 2 7 / 8 -in. and larger diameters with various fluids in the tubing/casing/formation. The tool has been run successfully in borehole conditions with wells flowing and shut-in. Sometimes the conditions included multiple wellbore contacts of gas, oil, and water within casing and multiple contacts in a tubing/casing annulus. Despite the various and changing borehole environments encountered by this tool, advanced electronics and detector designs, careful characterization, and comprehensive software have resulted in high-quality measurements.This paper illustrates performance of the new C/O system under extremely inhospitable conditions in the Asia-Pacific area. Conditions include multiple contacts with water, oil, and gas in the borehole, as well as multiple contacts with water, oil, and gas in the tubing/casing annulus. Furthermore, reservoir porosity in the subject borehole was in the low range of 8 to 15%, where success with C/O logs has previously been minimal. Nevertheless, the tool obtained extremely useful data that provided an accurate assessment of the reservoir conditions, and thus allowed decisions to be made for secondary workovers and other reservoir product management remedies.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper discusses a new small-diameter, high performance pulsed-neutron spectrometry tool that has been introduced recently for reservoir monitoring applications. Basic measurement principles are explained, and important operating parameters are presented. Field examples from Western Venezuela and South Texas illustrate multiple applications of the new device and compare the performance of the new instrument with a larger pulsed-neutron spectrometry tool.The new 2-1/8-inch tool is designed for logging through 2-7/8-inch or larger tubing. The tool is slightly larger than traditional 1-11/16-inch through-tubing tools. This design accommodates larger detectors, thereby improving the performance of the carbon-oxygen (C/O) measurement. Two bismuth germanate detectors yield high gamma ray count rates with good spectral resolution.To optimize C/O measurements, one mode of tool operation interlaces 5ms of background measurements with 20ms of 10-kHz neutron pulses. The tool records 256-channel spectra from each detector:• during the neutron bursts (for analyzing inelastic events), • during the interpulse period (for analyzing capture events), and • during the background period (for analyzing activation and background events).The tool also provides a simultaneous neutron capture cross section (sigma) measurement. Carbon-oxygen and calcium-silicon (Ca/Si) response characteristics for saturation and lithology analysis are derived from laboratory measurements and are illustrated in the customary fan-chart format. In a second mode of operation, the tool optimizes the sigma measurement and provides high-quality neutron capture spectra for quantitative lithology determination.

Pulsed neutron capture logs have long provided a means of determining water saturation and estimating porosity of formations behind casing. Recent technological advances in pulsed neutron logging have resulted not only in improvements in saturation and porosity data, but also in the capability to provide information regarding water entry and flow, lithology, and improved differentiation of gas-filled zones from tight formations. These advances provide greater accuracy in reservoir evaluation and monitoring as well as improved completion and production diagnostics. Inelastic, capture, borehole, and background spectra are now utilized to enhance porosity estimates, identify water flow, and help determine lithology. Modular tool design allows the pulsed neutron tool to be combined with production logging tools for more reliable production diagnostics. Simple tool modifications permit quantitative measurements of water velocity and borehole oil holdup. The modular design also allows additional detectors to be placed in the toolstring for lithology identification and gravel pack evaluation. This paper discusses this new pulsed neutron capture technology and gives an overview of the expanded range of pulsed neutron applications. Field examples of waterflow detection, gravel pack evaluation, oil/gas discrimination, and conventional analysis are presented from offshore wells in the Gulf of Mexico and from onshore wells in Mexico and the U.S. P. 287