CHARACTERIZATION OF POTENTIAL HYDRATE BEARING RESERVOIRS IN THE UGNU AND WEST SAK FORMATIONS OF ALASKA’S NORTH SLOPE

Richard F. Sigal¹, Chandra Rai¹, Carl H. Sondergeld¹, W.J. Ebanks, Jr.², William D. Zogg², and Robert L. Kleinberg^3
1 University of Oklahoma, Norman, Oklahoma
² PTS Labs
³ Schlumberger

During the winter operations seasons of 2003 and 2004, Anadarko Petroleum in cooperation with Maurer Technology and Noble Corporation and with the partial support of DOE drilled and cored a shallow well, Hot Ice 1, located at 30-T9N-R8E, Umiat Meridian, on the North Slope of Alaska. The well location is on an Anadarko lease that is located about five miles down dip from the established hydrate occurrences in the Cirque well and in wells in the Tarn Field. The primary goals of the operation were to recover and characterize hydrate-baring core and potential hydrate and shallow gas reservoir rocks, and to test several technologies designed to improve the economic viability of North Slope exploration. These technologies included a light weight continuous coring system, an Artic Platform, and an on site mobile core characterization lab. As part of the project Anadarko Petroleum worked with Rock Properties Resources to design and build the mobile lab that contained both standard core characterization capacity and equipment specially designed to measure properties of hydrate containing sediments. The lab has the capacity to make measurements on plugs at below freezing temperatures, under specified confining and pore pressures. This lab was operated on the drill site by a team from Corpro and PTS. All the measurements discussed in this paper were made in the portable lab on the drilling platform or in Deadhorse.

The Hot Ice 1 well was drilled from the surface to 2300 ft. There was almost 100% core recovery from the bottom of surface casing at 107 ft to TD at 2300 feet measured depth from the surface. Based on the best estimate of the bottom of the methane hydrate stability zone core was recovered over its complete range. Approximately 565 ft of good sandstone reservoir rock was recovered in the Ugnu formation and approximately 215 ft were recovered in the West Sak. There were gas shows in the bottom part of the Ugnu and throughout the West Sak. No hydrate baring zones were identified either in the recovered core or on the well logs.

The whole core was described by the well site geologists, Ebanks and Zogg, who produced a detailed lithology log and core description. Core photographs and a gamma ray log were also done for the whole length of the core. During the 2003 operations a CMR logging tool located in the mobile lab made a NMR measurement on a six inch section from each 40 inch length of whole core. The tool was donated for use by Schlumberger and operated by a Schlumberger team (Robert Kleinberg and Douglas Griffin). The CMR measurement provided an estimate of the amount of unfrozen brine in the “frozen rocks” from the permafrost section. The median value for the unfrozen porosity in the permafrost zone was .051. For the rocks in this section this converts to about 13% of the pore space being filled with unfrozen brine. All the whole core measurements were done in a lab module maintained at a temperature below the freezing point of water.

One inch plugs were cut from all the potential reservoir rock identified by the well site geologists. Thick sections had several plugs cut. At some stage all the plugs were cleaned and dried. After this porosity, permeability, and grain density measurements were done on them. The other procedures on the plugs followed two paths depending on if they came from the “permafrost zone” or from the deeper section. For samples from the permafrost zone velocity and resistivity measurements were made on recovered state samples at subfreezing temperatures. Some thermal conductivity measurements were also made. NMR measurements were made on a few frozen plugs and on thawed plugs, and some re-saturated samples. For samples taken from unfrozen formations, the complete suite of measurements was done after cleaning and drying.

With the exception of the deepest sands in the West Sak and some anomalous thin tight zones all the sands recovered were unconsolidated high porosity high permeability formations. At 800 psi the Ugnu sands had an average porosity of 39.3% and geometrical mean permeability of 3.7 Darcies. Their average grain density was 2.64 g/cc. The West Sak sands had an average porosity of 35.5%, geometrical mean permeability of 0.3 Darcies and average grain density of 2.69.

There were several one to two feet thick cemented sand zones recovered from the West Sak formation. These zones had only a few percent porosity and almost no measurable permeability. On a well log they appear as very resistive with a high velocity. In shallow sections of wells resistivity and velocity are usually the only logs available. Given the presence of gas in the Hot Ice 1 well, if only those logs along with a mud log had been available the tight sand zones would probably been identified as hydrates. Although this does not imply that mapped hydrate zones are just tight sands, it does add a note of caution to identifying hydrates from old well information.

The NMR measurements in the Ugnu made on plugs with a bench top NMR system present some interesting unanswered questions. There are significant differences between the thawed measurements and the re-saturated measurements made on the same samples. As of yet no completely satisfactory model to explain the differences has been found. The observed relaxation times for the samples are somewhat more rapid than usually observed for rocks with such high permeability. Finally the NMR measurements do not produce a good permeability estimator.

The NMR measurements on plugs from the West Sak samples are more what would be expected. This in part may be due to the fact that only measurements were made after re-saturation. For these samples a very good permeability estimator could be developed. The formula differed from the default formula by a factor of three so that the West Sak samples have a relaxation rate 1.7 times that of the samples used to develop the default formula. There was very little difference in the unfrozen NMR spectral types between the Ugnu and the West Sak.

The resistivity measurements on the Ugnu sands plugs were made at temperatures below the freezing point of pure water. The analysis of the data required making several reasonable assumptions. This analysis showed the percent of unfrozen material increases as the temperature approaches 0 degrees C. For the shallower parts of the well unfrozen saturations were roughly 6 to 10%. This is in agreement with the NMR measurements. The Ugnu electrical measurements also provided an estimate of the salinity of the brine before freezing. The estimated salinity had a median value of 7,100 PPM. The West Sak resistivity measurements were mostly made after cleaning, drying and re-saturation. The formation factor m had a small dependence on pressure. Its median value at 800 psi was 1.94. Electrical measurements on a few recovered state samples showed the brine in the West Sak also to be very low salinity.

Shear and compressional wave velocity measurements were made on the Ugnu samples at below freezing temperatures. The velocities of these samples should be very similar to samples containing hydrates when the hydrate containing samples have comparably high saturations of frozen material. At 800 psi the median compressional velocity was 3.86 km/sec. Median shear velocity was 2.13 km/sec. The West Sak samples were measured after cleaning drying and re-saturating. Since they were unconsolidated samples they had much lower velocity. The median compressional velocity at 800 psi confining pressure was 2.04 km/sec. Shear velocity was 1.08 km/sec.

In summary the methane hydrate stability zone below the Hot Ice 1 location has thick sections of sand stones that would make excellent reservoir rocks for either shallow gas or hydrates. The Ugnu contains more sand section than the West Sak formation and the Ugnu is slightly more porous and significantly more permeable.