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Meeting the challenges at Oooguruk Pioneer has been using a variety of drilling techniques to deal with geologic issues in its successful Beaufort Sea oil field Alan Bailey Petroleum News
With oil resources exceeding initial expectations, the Oooguruk oil field in the shallow waters of the Beaufort Sea off Alaska’s North Slope has proved successful for Pioneer Natural Resources, the field’s operator. But developing the field has involved solving some tricky drilling problems associated with the field’s geology, Craig Knutson, Pioneer geological advisor, told the Alaska Geological Society April 19.
Three oil pools Oooguruk, drilled and produced from an artificial island about six miles offshore, has three distinct oil pools. The main pool is in a reservoir sand called the Nuiqsut, upper Jurassic in age and broadly similar in age and geologic setting to the oil reservoir of the Alpine field in the Colville River delta a few miles to the south. A younger and shallower reservoir, referred to as the Kuparuk C, is early Cretaceous in age and is equivalent to one of the producing horizons in the Kuparuk River field. The third and shallowest oil pool is in the Torok formation, also of Cretaceous age, a little younger than the reservoir of the Tarn field on the mainland and a little older than the reservoir of the Nanuq satellite field at Alpine, Knutson said.
Pioneer originally planned to produce oil just from the Nuiqsut and the Kuparuk C. But after drilling wells through the Torok and realizing that this formation also contains a substantial oil resource, the company is now conducting a Torok pilot project to test the oil production characteristics of this rock unit. That pilot has involved the drilling of three lateral wells from the Oooguruk island. In addition, this winter Pioneer drilled an onshore well, the Nuna 1, to appraise the southwest extension of the play.
Drilling challenges The drilling challenges at Oooguruk start in what is referred to as the intermediate section of a well, the section above the reservoir rocks. At Oooguruk this intermediate section consists primarily of shale. The upper shales contain zones with abundant fine-grained volcanic material which, tending to readily absorb water, can clog up the well, Knutson said. The deeper of the shale units, on the other hand, is under high stress in the subsurface: This shale wants to expand into the wellbore, thus disrupting the drilling operation.
Dealing with the shallower of these shale layers requires an oil-based drilling mud, to avoid hydration of the rock. Stabilizing the lower shale, on the other hand, requires a relatively high mud weight. And the more a well is deviated from the vertical, the more acute the problem becomes. In fact, the shale-related problems do not appear when drilling a vertical well, Knutson said.
Drilling solutions The drillers at Oooguruk have overcome the problems in the intermediate section by inserting a well casing string additional to the original well design, switching to oil-based mud, and using a technique called managed pressure drilling. Managed pressure drilling involves a surface choke and pump system that allows the instant adjustment of downhole pressures, enabling quick and accurate balancing of mud weight and pore pressure Knutson said.
Essentially, the drillers drive the well to below the hydrating shales using a light, oil-based mud. They then run a casing string down to below that shale. They then drill through the stressed shales using a heavy water-based mud, before installing the casing string to below that deeper shale. The additional casing seals off and stabilizes the walls of the well.
The Torok Once through the intermediate section, the drilling can proceed down through the first of the reservoir rocks, the Torok, where Pioneer has been conducting its recent production test program.
The Torok at Oooguruk consists of 200 to 250 feet of a type of rock called a turbidite, composed of interlayered sand and mud formed from the periodic flow of sand down the side of a marine basin. Individual sand layers in the Oooguruk Torok are very thin — typically just one to six inches thick, Knutson said. The thinness of the sands makes it impossible to calculate porosity, permeability or oil saturation from the well logs, he said.
However, the sands are quite porous, with a porosity of 15 to 20 percent, and are also reasonably permeable. Core from the Kalubik 2 well suggests that the sands are at least locally filled with oil so that, with a Torok section around 200 feet thick, there appears to be quite a large amount of oil in place in the reservoir. Oil has not been produced on the North Slope from Torok as thinly bedded as this so it was important to try a pilot program to determine whether the formation can be produced economically.
The production from a three-well Torok pilot is appearing quite good so far and Pioneer now plans to convert one of the wells into a water injector, to test the effectiveness of waterflood as a technique to flush the Torok oil into production wells, Knutson said.
Kuparuk C The Kuparuk C reservoir, the middle of the currently producing reservoirs at Oooguruk, generally shows up distinctly in seismic data. The reservoir consists of a permeable marine sand with grains which grade from very coarse at the base to fine at the top. The sand contains minerals called glauconite and siderite which affect the results of the well logging and have an impact on oil production from the reservoir, Knutson said.
Typically using rotary steerable drilling technology but also sometimes using a drill bit driven by a mud motor, Pioneer has drilled lateral wells 2,000 to 5,000 feet in length through and along a Kuparuk C section that ranges anywhere from 10 to 40 feet in thickness. The geologists use downhole tools called azimuthal resistivity logging tools to monitor the location of the drill bit within the reservoir rock, and hence to steer the well through the reservoir, Knutson said.
Pioneer is using a waterflood technique for production from the Kuparuk C, and currently has three water injection wells and three production wells.
Fractures Although the reservoir rock has reasonable porosity and permeability, the overall performance of the reservoir in delivering oil to the wells indicates that fractures in the rock dominate the flow characteristics of fluids through the rock, Knutson said. And, as a result of the fracturing, at one point injected water prematurely flowed directly to one of the production wells, preventing oil production through that well and causing Pioneer to have to shut the well in, he said.
However, Pioneer generally sees initial production rates of around 2,500 barrels of oil per day from the Kuparuk C, with expected ultimate production of about 2 million barrels per well. With production to date of around 6 million barrels, the company estimates an ultimate recovery of 10 million to 15 million barrels of oil from the reservoir, Knutson said.
The Nuiqsut Pioneer anticipates an ultimate production of 60 million to 80 million barrels of oil from the Nuiqsut, the main oil reservoir for the Oooguruk field, Knutson said. And a re-interpretation of the seismic and well data for the Nuiqsut after the field went into production indicated that the reservoir was more extensive than Pioneer had originally thought, he said.
The Nuiqsut consists of relatively clean but fine-grained sand with quite low permeability, he said. The rock is laminated, with wavy layers of clay, so that fluids tend to flow through the rock much less easily in a vertical direction than horizontally.
The fine grained nature of the reservoir rock bears similarities to the reservoir rock of similar age in the Alpine field. But unlike Alpine, which contains relatively light, low viscosity oil from the Kingak source rock, the Nuiqsut reservoir at Oooguruk is charged with a heaver, more viscous oil from a different source rock, the Shublik. And the Nuiqsut oil at Oooguruk becomes more viscous the deeper it lies in the reservoir.
Just to add to the challenges, the seismic data for the Nuiqsut is difficult to interpret because it is not a strong seismic event, while thinning of the subsurface permafrost in the offshore affects the velocity of the seismic sound waves, thus creating depth uncertainties, Knutson said.
Thin, hard layers of siderite, which were too thin to resolve in early vertical exploration wells, can cause havoc with horizontal drilling by deflecting the drill bit away from its intended direction. To avoid this problem Pioneer has adopted a rule that a well must approach the face of a siderite layer at an angle of at least four degrees, Knutson said.
And the orientation of subsurface stresses within the Nuiqsut reservoir tends to favor hydraulic fracturing in one particular direction, leading to a preference to orient wells in a northwest-southeast direction, a directional preference also seen in the Alpine field.
Evolving techniques With a plan to ultimately drill around 15 production wells and 15 water injection wells in the Nuiqsut, and with the wells having lateral lengths in the range of 6,000 to 8,000 feet, Pioneer has tried an evolving series of well completion techniques to overcome the low vertical permeability of the Nuiqsut reservoir.
The initial well design, used back in 2008, involved steering the drill bit to create a lateral well that undulated up and down, like a wave, through the reservoir. The wave-like geometry of the well enabled the well at various points along its length to access different layers within the horizontally layered rock.
This well configuration proved really difficult to drill and did not contain a liner to support the well bore after completion. Production from the well declined after a few months and the well appears to have collapsed.
The next well design involved some smoothing out of the undulations in the well bore and then the use of hydraulic fracturing to stimulate vertical communication and production. One well of this type proved very successful, but two subsequent wells of the same design were less effective. And because of the undulations in the well bore, well remediation of the troublesome wells proved impossible, Knutson said.
In 2010 Pioneer tried using dual lateral wells in the Nuiqsut. The first of these laterals worked very well, but the second proved less encouraging, Knutson said.
Massive frac Pioneer is now trying another approach, using a smoother horizontal well and a 12-stage, 2 million pound hydraulic fracture. The company has recently conducted the three largest hydraulic fractures ever executed in Alaska, two in the Torok and one in the Nuiqsut, Knutson said. The company has yet to flow back the Nuiqsut well but the two fracs that were conducted as part of the Torok test program were very encouraging, he said.
The Nuiqsut development has space for about 17 more wells, with Pioneer anticipating the use of a variety of completion strategies, Knutson said.
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