|
North Slope gas options
USGS team uses downhole well pressure data to assess potential gas resources
Alan Bailey for Petroleum News
With recent moves towards the potential development of a natural gas pipeline from Alaska's North Slope to the Cook Inlet, scientists in the U.S. Geological Survey have been conducting research into where the best areas in northern Alaska may be for gas exploration and development. The research has involved using historic exploration well data to determine where downhole well pressures are particularly high. A prime reason for the high downhole pressures is likely to be the presence of natural gas at pressures amenable to commercial gas production: The formation of gas in underground porous rocks, sealed in by impermeable rocks, tends to increase the hydrostatic pressure within the rock, the team conducting the research told Petroleum News in a recent interview.
Dave Houseknecht, senior research geologist emeritus, said that USGS has been trying to better understand and constrain the natural gas resources in Arctic Alaska, including offshore regions, especially given that in the past, given the lack of a market for gas from the region, gas resources discovered during exploration drilling have tended to be discounted.
"What we have set out as an objective is to better understand natural gas resources throughout Arctic Alaska," Houseknecht said.
Houseknecht commented that an over-pressured gas accumulation can facilitate the production of gas, particularly from low porosity or low permeability reservoirs.
An indicator of natural gas William Rouse, the geologist working on the well overpressure project, commented on the value of overpressure as an indicator of the presence of natural gas.
He said that overpressure occurs when the pressure in a porous rock is higher than what would be the normal pressure resulting from the weight exerted by shallower rocks. In the past over pressures have been studied in the context of well safety and well control, to prevent well kickbacks that can injure workers or damage rigs, he said.
Essentially, a dense, fine grained rock deposited on top of a porous rock tends to increase the pore pressure in the porous rock. Then, if oil in the porous rock cracks into natural gas, the gas has nowhere to escape and the pressure rises further, Rouse said.
It is possible to directly measure the downhole pore pressure during wireline formation tests when drilling a well, Rouse said. However, companies do not always run that type of test. Instead, they tend to use the weight of the mud that is pumped into the well to prevent a well blowout as a proxy for the pore pressures in the rock formations that are being penetrated. The drillers do not want to risk a blowout by under balancing the mud load but also do not want to lose mud into the rock formation by overbalancing. As a consequence, the drillers try to use mud weights that exert pressures on or slightly above the formation pressures, Rouse said. Hence, it is possible to use the records of mud weights as proxies for the downhole formation pressures in wells, he said.
The use of well data Rebecca Smith, the physical scientist working on the project, said that the well data required for the study was publicly available from the Alaska Oil and Gas Conservation Commission. She said that it was possible to obtain the required data by going through well history files, including the mud records. The research only used data from exploration wells, rather than production wells. The researchers used data from 649 wells, 240 of which had pore pressure test data while 515 wells had mud weight data. A plot of nearly 1,500 data points where wells had both pore pressure and mud weight data demonstrated that the formation pressures inferred from the mud weights tended, as expected, to be slightly higher than the measured formation pressures.
Rouse said that by using appropriate values for what is considered to be a normal subsurface pressure gradient for Arctic Alaska, it was possible to identify downhole situations where there was overpressure. However, given some known factors that can give rise to incorrect apparent high pressures, the scientists decided to use a relatively high mud pressure gradient of 0.65 pounds per square inch per foot of depth as a cutoff value for identifying significant overpressure situations.
The distribution of over pressures The outcome was the development of a map showing levels of maximum mud pressure gradients in wells north of the Brooks Range, including offshore wells, and identifying well locations where there were significantly high overpressures. Smith commented that the resulting map shows much overpressure in the area of the Point Thomson condensate field on the Beaufort Sea coast to the east of Prudhoe Bay, and under the outer continental shelf near Point Thomson.
The depths and ages of the rock formations exhibiting especially high over pressures vary across the region, tending to be older in the more northerly part of the region, near and to west of the Prudhoe Bay area.
Houseknecht commented that a fold and thrust belt associated with the northern edge of the Brooks Range runs west to east from the Chukchi Sea under the foothills of the Brooks Range, before swinging north to pass under the Beaufort Sea near Point Thomson. The overpressure wells in the Point Thomson area are in the region of the thrust belt, as are a number of overpressure wells in the foothills. There are some anomalies to this overall pressure trend, such as the Inigok well in the western NPR-A to the north of the foothills, and the Tunalik 1 well in the western NPR-A, close to the Chukchi Sea coast -- both of these wells exhibit significantly high over pressures in rocks at depth. Some wells with relatively high over pressures are located fairly near the Dalton Highway and Trans-Alaska Pipeline. And there are some offshore wells with high over pressures to the east of Point Thomson.
The researchers have extrapolated the likely profiles of over pressured zones between wells, using seismic profiles of the subsurface, to trace zones in the subsurface where overpressure is likely to exist.
Undiscovered oil and gas assessments William Craddock, USGS supervisory research geologist, commented that for many years USGS has used a formal methodology for assessing the potential for undiscovered oil and gas in regions such as northern Alaska. The overpressure data determined from this new project will now form an input into determining what are termed "assessment units" in these oil and gas assessments. The overpressure information can provide insights into where there may be areas of gas accumulations and how productive gas wells may be, Craddock said.
|
