The Prudhoe Bay oil field at the fulcrum of Alaska’s North Slope oil industry is so large that, even after more than 30 years of operation, development activity at the field continues apace, squeezing as much product as possible from the field’s 254 square miles of underground reservoir.
According the most recent Prudhoe Bay progress report submitted by field operator BP to the Alaska Department of Natural Resources, 2009 saw more than 3,000 jobs involving work on wells at the field, with 300 of those jobs adding to oil production volumes through techniques such as perforating wells and stimulating the oil reservoir.
62 new wellsBP drilled 62 new wells in the field in 2009, the report says. Those wells primarily involved sidetracking, a technique involving the drilling of a new well bore out from the side of an existing, poorly performing well, the report says. The sidetracking done in 2009 involved both conventional rotary drilling in which a jointed drill pipe rotates a drill bit, and coiled tubing drilling, in which a continuous length of narrow-bore steel tubing is fed into the well, with the drilling mud circulating through the well driving a mud motor that turns the bit at the down-hole end of the tubing.
These sidetrack wells often run at relatively low angles out from the original well bore, with precision drilling techniques enabling the drillers to thread a well through the relatively small pockets of oil left, as the oil pool in the field becomes increasingly fragmented with age.
“As Prudhoe Bay has matured, drilling targets continue to become smaller and more complex, with increased drilling and reservoir risk,” the report says.
BP anticipates Prudhoe Bay drilling activity in 2010 to be similar to that in 2009, with 14 to 24 rotary drilling penetrations and 32 to 42 coiled tubing penetrations, the report says.
Slowing the declineHowever, the work program at Prudhoe Bay is slowing rather than reversing the inevitable long-term decline in oil production. The average daily oil production of 252,000 barrels in 2009 will likely decline to somewhere in the range of 206,000 to 247,000 barrels in 2010, the report says (at its peak, the field delivered about 1.5 million barrels per day of oil).
And a pipeline replacement project is continuing the inspection and upgrading of the field’s aging pipeline infrastructure, with some further pipeline replacements in the offing. In 2009 BP used devices called smart pigs to check the condition of nine three-phase pipelines, nine produced water injection lines, three natural gas lines and six oil lines, the report says.
Field reservoirsThe main reservoir in the Prudhoe Bay field is the 450-foot thick Permo-Triassic Ivishak formation, with the much thinner Sag River formation forming a second reservoir above the Ivishak. The rock formations have been bent into a broad arch, with a discontinuity called the lower Cretaceous unconformity cutting out the crest of the arch. Oil and gas in the reservoir rock became trapped against the unconformity to form the oil field.
The detailed geology within the reservoir is complex, with layers of impervious rocks known as shales interstratified with sandstones and conglomerates and inhibiting the free flow of hydrocarbons through parts of the field.
BP maintains a computer model of the oil field, to simulate the effect of different field development approaches and hence determine how best to maximize oil recovery. The company has continued to refine and update this model, the progress report says.
“The geologic model has been improved, incorporating new well data and a refined description of the Sag River formation,” the report says.
Pressure managementHowever, reservoir management continues to focus on managing the reservoir pressure and optimizing the placement of injected water to support oil offtake across the field.
When Prudhoe Bay initially went into production a huge natural gas cap sat on top of an equally huge “rim” of oil. Pressure in the gas cap forced oil up through production wells. Oil would flow down the gently sloping reservoir rock layers toward production wells, thus giving rise to the term “gravity drainage area” for the section of the reservoir where this phenomenon occurred.
And as field production has progressed, gas produced from the field has been pumped back into the gas cap to maintain reservoir pressure.
In peripheral areas of the reservoir, too distant from the gas cap for the gas pressure to effectively drive oil production, massive volumes of water have been injected underground to flush oil from the reservoir rock, in a technique referred to as waterflood. And in 2002, to stem a pressure decline in underground gas pressure, BP also started pumping water into the gas cap.
Tertiary recoveryIn recent years so-called tertiary enhanced oil recovery techniques have come into play, to extract yet more oil from the reservoir. An important technique used at Prudhoe Bay has been the sweeping of a solvent known as miscible injectant through the reservoir, to act like laundry detergent, clearing residual oil from the rock pores. Miscible injectant consists of a mixture of natural gas and some of the natural gas liquids that drop out from natural gas when the produced gas is chilled.
A state-of-the-art technique known as miscible injectant sidetrack, or MIST, involves releasing bubbles of miscible injectant from a specially shaped sidetrack well around a production well.
And the alternating use of injected water and gas (referred to as “water alternating gas,” or WAG) has also been a commonly used technique for flushing out oil.
In the eastern waterflood area five MIST injectors have been added to WAG injectors, to tease out some of the more elusive oil. Drilling in this part of the field has targeted both the Ivishak and the Sag River reservoirs, the report says.
The western part of the waterflood area has been subject to a similar enhanced oil production program and saw nine new wells drilled in 2009. Those new wells included, in the Sag River formation, a multilateral well in which several separate well bores were drilled out from the same central well. BP acquired some 3-D seismic data in early 2009 for this part of the field and this data is providing improved subsurface imagery for identifying new drilling targets, the report says.
Sidetrack wells are also continuing to find new oil targets in the gravity drainage part of the field, the report says.
However, in an area of the field where the gas cap has expanded, gravity drainage has become less efficient, so that water injection and miscible injectant are being used to increase oil recovery.
New technologiesBP is also evaluating some new technologies that might further increase oil recovery, the report says. Initial tests of a trademarked Bright Water technology have shown promise — the technology involves the use of temperature sensitive polymers in injected water to help control the route of the water flow through the reservoir. LoSal — another trademarked technique under evaluation — uses low salinity water to improve oil displacement during water flood.
And BP has been working with Halliburton to test a new tool for the downhole separation and re-injection of gas, with field testing of the tool expected within the next two years, the report says.