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ExxonMobil in Alaska: Nominated to spend Christmas at Prudhoe Bay Personal reflections from renowned Alaska geologist Gil Mull, who sat on the Prudhoe discovery well for ExxonMobil Gil Mull For Petroleum News
It was totally unexpected; it was mid-December 1967, not long before Christmas, and there I was, suddenly on an airline flight from Los Angeles to Fairbanks, where I transferred to a bush flight heading for the Prudhoe Bay State No. 1 drill site.
Although ARCO was the operator on the well, Exxon’s Humble Oil & Refining was a 50 percent partner in the well and wanted to have its own geologist there to observe operations and to assist the ARCO geologists with sample examination and evaluation of the stratigraphy encountered in the well.
The well was a rank wildcat, located 60 miles from the nearest well or outcrop control, so that prediction of the stratigraphy to be expected in the hole was based on seismic control and projections from what we had seen in our outcrop mapping during our summer field work.
I’m sure management had not originally planned to send me up as the Humble well site geologist, because I was a relatively inexperienced, recently hired junior geologist with less than six months with Humble. But, unexpectedly, my colleague Bill Schetter, who was the Humble well site geologist on the well, announced that he had accepted a college faculty position to teach geology, and suddenly the company needed someone to replace him.
I had had three years of field mapping experience with Richfield Oil (ARCO) in the Brooks Range and on the North Slope before I joined Humble and thus was familiar with the North Slope stratigraphy and the Prudhoe prospect. And, I also had well site experience as one of the ARCO well site geologists the previous winter on the Susie Unit No. 1 well — a dry hole in the foothills 60 miles south of Prudhoe Bay.
Thus, although my specialty was outcrop geologic mapping, I was nominated to spend Christmas on the North Slope for the second year in a row, to represent Humble and assist ARCO geologist Marv Mangus and his alternate Bill Pentilla.
Things were becoming interesting As the bush flight crossed the Brooks Range and out onto the North Slope in the mid-winter darkness, a single light in the distance became visible — the rig lights at the Prudhoe Bay well site — our destination. The airstrip was a snow and ice strip on the tundra, and in the mid-day twilight the plane taxied up to an unloading ramp right outside the camp and the drill rig.
The camp consisted of two parallel rows of ATCO trailers strung together end-to-end and roofed over with sheets of plywood, and was about three quarters buried by drifting snow. The drill rig stood about 100 yards away at the east end of the camp.
Only a short time before my arrival, the well had reached the top of the Sadlerochit Formation (also known as the Ivishak Formation) at a depth of 8,208 feet, and things were beginning to become interesting. In the nearest outcrops 60 miles to the southeast in the Brooks Range, the Sadlerochit is hard dense sandstone, but at Prudhoe the bit penetrated porous sandstone and conglomerate.
And, even more interesting — although there had been some oil and gas shows higher in the well, methane gas readings in the drilling mud abruptly went off-scale in the Sadlerochit — which was a really encouraging sign. Inasmuch as there was no way of predicting with any level of confidence how thick this interval might be, drilling progressed slowly, we cut several cores, and wire-line logs were run in order to get a better idea of the reservoir quality of the sandstone and conglomerate.
Communication limited to radio In the early stages of drilling at Prudhoe Bay, the only means of communication between the rig and the ARCO and Humble offices was by single side band HF radio — there were no telephones on this part of the North Slope and the nearest public telephone was at Barrow, 200 miles to the northwest.
The daily drilling reports and geological reports were transmitted to the ARCO office in Anchorage on an open radio frequency that anyone could listen in on. On a few occasions when the single sideband radio signals were out, a ham radio operator who had his ham set there in camp was sometimes able to contact someone on the ham network. In these cases, the daily drilling and geological reports were relayed to the ham radio operator on the other end, whoever and where ever he was, who then placed a collect phone call to the ARCO office in Anchorage to relay the reports.
Inasmuch as the radio link was often unreliable, company management gave the drilling and geological personnel on the rig a great amount of autonomy to proceed using their best judgment. This was a level of autonomy that is unheard of today in an era in which satellites enable continuous communication between remote rigs and the headquarters offices. But in 1967, the management folks in Anchorage and Los Angeles, where the Humble office was located, knew that if they did not receive a daily report from the rig, it was undoubtedly due to poor radio signals, and assumed that things were OK at the rig. They knew that if the rig personnel needed help or advice, they would be contacted somehow.
A thousand mile daily commute But after the well penetrated into the Sadlerochit Formation with its high gas readings in the drilling mud, it was obvious that things were getting more interesting by the day, and this sort of casual communication between rig and town came to a screeching halt.
Thus began a new daily routine. The first thing the geologists did in the morning was to update our sample logs and reports, and then picked up the daily drilling report from the tool pusher. Then one of us, usually me — leaving the ARCO geologist to monitor the drilling activity — hopped in the Interior Airways Beach Kingair that pilot Bob Jacobs was warming up.
Depending upon the weather, we flew to either Barrow or to Fairbanks to phone the reports in to the offices in Anchorage and Los Angeles. When we flew to Fairbanks, this was a daily commute of over a thousand miles to make two or three telephone calls, and I was usually back to the rig by early afternoon.
By Christmas day, the well had penetrated over 350 feet of predominantly sandstone and conglomerate, accompanied by continued high gas readings in the drilling mud, and oil shows in some of the lower core samples.
This was a phenomenal thickness of potential reservoir beds and the decision was made to run an open-hole drill stem test (DST) to determine the flow capability of the lower 180 feet of the Sadlerochit Formation.
The test tool was opened early in the morning of Dec. 27, 1967, with a result totally unlike anything I had ever previously experienced in a drill stem test, or DST. In the tests that I had witnessed in the past on other wells, all that happened when the tester was opened was a weak puff of air flowing from the drill pipe, which then died to nothing. In this test, there was an immediate roar of high-pressure gas flowing to the surface, which was diverted to a flow pipe and ignited to make a flare that was up to 30 feet long blowing into the teeth of a headwind.
The gas flow was estimated at 1.25 million cubic feet per day (1.25 MMCF/D) through a 1/8 inch choke at a pressure of over 3,000 psi; this continued all day, with a rumble that shook the rig and resembled the sound of a jet plane overhead. The pressure was so great that after the test tool was closed late in the afternoon, the flare burned most of the night as the high pressure in the drill pipe bled down.
Looks like gas discovery By the morning of Dec. 28, the gas pressure in the drill pipe was finally exhausted and at last the drill crew was able to begin to come out of the hole with the drill pipe and test tool.
But by that time, the bottom of the hole had begun to cave, and the 8,500 feet of drill string and DST tool could be moved only a few feet up and down. The tester and lower part of the drill string were stuck in the hole, and a fishing job was begun.
Although no wire-line logs were available for the lower part of the hole and the charts in the test tool could not be recovered, the test showed that the well had penetrated a high-pressure gas reservoir that was at least 385 feet thick, with no indication of either a gas-oil or gas-water contact.
It was beginning to appear that Prudhoe Bay might very well be a significant gas discovery. This was exciting, but oil, not gas, was the objective and the full significance of the discovery was going to have to await further drilling — and that was not going to occur until the fishing job was completed.
Clearly, there was going to be no need for geologists at the well site for some time, so I flew back to Anchorage and then on to the office in Los Angeles. The results of the DST were headline news in the Jan. 16 Anchorage Daily Times.
Side-track to oil After several days of unsuccessful attempts to free the stuck drill string and test tools, the decision was made to side-track the lower part of the original hole and drill around the stuck fish. This took a couple of weeks, and when drilling into new geology resumed in late January, Hank Repp, one of the Humble senior geologists, went back as the Humble well site geologist.
The base of the Sadlerochit sandstone and conglomerate interval was finally reached at 8,670 feet — an interval thickness of over 460 feet with about 300 feet net sandstone and conglomerate as potential reservoir beds. Even more significantly, the lower 40 feet of the sandstone was oil saturated, and no oil/water contact was encountered.
After wire-line logs were run, a string of casing was set through the Sadlerochit and drilling continued into the underlying Lisburne Formation, which was found to consist of hard limestone with interbedded brown, porous, oil saturated dolomite.
Another open-hole drill stem test in the top of the Lisburne recovered light oil that flowed intermittently with a high volume of gas. This test showed that the Lisburne was also an oil reservoir, but the flow of gas suggested that there was communication with the overlying Sadlerochit Formation, which was behind casing.
During the DST, some of the high-pressure gas from higher in the well was apparently bypassing the cemented casing and into the lower part of the hole, where it flowed with the oil from the Lisburne.
The level of excitement on the well was increasing. Although the rate of oil flow during the test could not be measured, the discovery of oil in the well was headline news in the Feb. 16 Anchorage Daily Times.
Mull, Pentilla back on well When drilling in the Lisburne resumed after that drill stem test, ARCO geologist Bill Pentilla and I were back on the well, which was then drilling in dense limestone with more beds of brown oil-stained dolomite.
By the end of the first week of March, we had drilled and cored over a thousand feet of Lisburne that contained a number of thin beds of oil-saturated dolomite. Another drill-stem test was run, to test a 320-foot interval in the lower part of the Lisburne. This test was a spectacular success.
About 20 minutes after the test tool was opened, the light flow of air from the drill pipe was followed by gas to the surface and then in about two hours oil began flowing to the surface.
Oil flowed for 7 hours at a measured rate of 1,152 barrels of oil per day (1,152 BOPD); this test confirmed beyond any question that Prudhoe Bay State No. 1 was a significant oil and gas discovery.
In addition to the oil saturated dolomite beds in the Lisburne, the Sadlerochit Formation was clearly an even better reservoir unit with as much as 300 feet of net sandstone and conglomerate in an interval about 460 feet thick.
And more importantly, there was no indication of an oil-water contact in either the Sadlerochit or Lisburne. The wire-line logs, core data, and drill stem test data indicated a gas column of about 420 feet in the Sadlerochit, and no way of knowing the height of the oil column.
Sag River confirmation well Evaluation of the drilling results to this point clearly indicated to ARCO and Humble management that additional evaluation was necessary. A second well was going to be needed to determine the lateral extent of the Sadlerochit reservoir beds and to find the oil-water contact to determine the height of the oil column. A drill rig that BP and Sinclair Oil had used to drill a dry hole near the Colville River west of Prudhoe Bay was brought along the coast by cat train over a winter road on the sea ice.
And, clearly, more detailed seismic data was needed.
Thus began a major mobilization of equipment unlike anything seen before in Alaska. In mid-March, while drilling continued at Prudhoe Bay No. 1, a massive airlift began and two Alaska Airlines C-130 Hercules cargo planes began flying around the clock from Fairbanks. The Prudhoe well site was a beehive of activity as about every two hours, night and day, another Hercules would taxi into the ramp just outside our sleeping trailer and offload another 40 tons of equipment. On some occasions, two Hercs were on the ramp at the same time.
The planes flew in thousands of feet of drill pipe and casing, thousands of sacks of drilling mud and cement, seismic equipment, seismic camps, trucks and construction equipment to build a second drill site (figure 10), and all of the supplies needed to support another large camp for the drilling of the second well. This location, named Sag River State No. 1, was to be near the banks of the Sagavanirktok River, seven miles southeast of the Prudhoe Bay drill site and, based on the available seismic data, was predicted to be three to four hundred feet structurally lower than Prudhoe Bay State No. 1.
By May, drilling at the Prudhoe Bay well had ended and the well was undergoing a very detailed testing program. Meanwhile, the Sag River drill site had been completed and drilling was progressing rapidly.
Hank Repp, Dean Morgridge, and I took turns as the Humble well site geologists, working with ARCO geologists Marv Mangus, Bill Pentilla, and Bob Anderson (no relation to R.O. Anderson).
In some ways, this well was even more interesting than the Prudhoe Bay discovery well. By early June, the top of the Sadlerochit was reached and was being evaluated by almost continuous coring. Most of the Sadlerochit was within the oil column, and some of the sandstones and conglomerates appeared to have even better reservoir quality than at Prudhoe Bay.
More than 500 feet thick Security was very tight, and only the geologists were supposed to see the rocks that were being extracted from the core barrels, but one 20-foot core was particularly memorable. Usually, a solid cylinder of rock came out of the core barrel and was laid out in trays to be examined in detail. But in this case, with the core barrel hanging vertically in the derrick, when the core bit was removed from the barrel, out poured a pile of unconsolidated sand, gravel, and oil — which flowed through openings in the derrick floor and into the rig cellar. The porosity and permeability of this interval was fantastic. The entire drill crew soon saw and knew exactly what we were finding.
The Sag River field confirmation well showed that the Sadlerochit reservoir interval was over 500 feet thick, with at least 300 feet of net reservoir-quality sandstone and conglomerate, and a 400-foot oil column below a gas cap that was also about 400 feet thick.
The drilling and test data from the Prudhoe Bay State No. 1 and Sag River State No. 1 wells, along with the seismic maps of the area were given to the consulting firm DeGolyer and MacNaughton for an independent evaluation of the significance of the discovery.
And on July 18, ARCO and Humble released the results of this independent evaluation, which estimated that Prudhoe Bay contained between 5 billion and 10 billion barrels of oil, which would make it the largest oil field in North America.
But by the time the announcement made the headlines, my field partner Howard Sonneman and I were back in the Brooks Range for another season pounding on rocks and making geologic maps.
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