Packing lite for the long haul
  from: Offshore Engineer
  by: Russell McCulley
  Friday, September 05, 2008

More and more operators are discovering the appeal of ultra-lightweight gravel for completions in challenging environments. OE’s Russell McCulley examines completion technology one service company has used with success in the Gulf of Mexico, including a horizontal open-hole gravel pack in a well at high risk of formation fracture.

Using ultra-lightweight proppant to gravel pack extended-reach horizontal open holes, some as long as 800m, has become a fairly routine, if challenging, practice in deepwater fields offshore Brazil. But the practice is rare in the Gulf of Mexico, where cased hole gravel packing and frac packing with conventional proppant is more common. But a pair of horizontal open-hole completions in the Gulf of Mexico’s shallow waters point to what may be a growing trend in challenging fields there and around the world.

Arena Offshore, a privately held oil and gas exploration company in Texas, together with completions experts from BJ Services teamed up last year on two completions offshore Louisiana and Texas. Arena, which specializes in what drilling manager Kalil Ackal describes as ‘overlooked resources in old-life fields,’ had opted to drill horizontal open holes in fields where economics couldn’t justify multiple penetrations. At less than 500ft, both open sections of the wells were relatively short. But the diameter of the first hole to be completed – a mere 4 3/4in – presented a real challenge.

Ultra-lightweight proppant (ULWP), rather than conventional sand, proved to be the solution. While BJ had experience with lightweight gravel packing in challenging environments – particularly offshore Brazil, where many fields also contain unconsolidated sands and extended-reach open-hole horizontal wells are common – the job marked the first time the company’s Houston completions group had used an ultra-lightweight proppant to complete open-hole horizontal wells in the Gulf of Mexico.

ULWP is not necessarily the first choice for gravel packing horizontal wells. But under certain unfavorable conditions, such as extended reach horizontals and well conditions that create circulating pressures that exceed frac gradients, ULWP can provide more complete packing of the open hole. Such was the case with the Arena Offshore wells: in the first, at Grand Isle block 34, Arena had drilled a sidetrack in shallow water to 5389ft MD and 2692ft TVD. The open-hole section of 418ft had been drilled with an ID of 4 3/4in, and an eight-guage pre-packed screen with an OD of 3.68in and ID of 2.441in was selected.

The estimated bottomhole pressure was 1180psi, with an estimated fracture pressure of 1762psi. The hydrostatic head of the slurry was 1368psi, which left less than 400psi to gain during pumping; greater pressure would risk fracturing the formation, an undesirable result in most horizontals. ‘The estimated friction pressure in the wash pipe alone was 449psi at two barrels per minute,’ says Jason Steckler, regional technical representative for BJ Completion Services. ‘To keep us under 270psi of friction pressure in the wash pipe, the expected pump rate for the job was 1.5bpm.’

Tight space, and the accompanying small equipment, meant that the gravelbearing fluids could not be pumped at a high enough rate to carry conventional proppant and be safely returned through the system’s wash pipe; the standard flow rate for conventional proppants, Steckler notes, is a minimum 4bpm. ‘Ideally, we need a minimum of 80% returns for a successful horizontal gravel pack. If you start losing the returns to the formation, the result could be premature screen-out and incomplete packing,’ he says. Due to the well’s 90° hole deviation, Steckler’s team chose the company’s HST Lite system for the completion, which allowed them to hold wellbore hydrostatics on the formation at all times prior to the installation of the gravel pack to help prevent sloughing. The system uses a flow path down the casing screen annulus and back up through a wash pipe, which in this case was 1266ft stretch of 1 1/2in, 4.19lb pipe with an ID of 1.462in. At 1.5bpm, the friction pressure reached 326psi and the system achieved full returns. ‘That’s the crucial number here, 1.5bpm,’ he says. ‘That would not have worked with conventional proppants.We would not have been able to get it off the boat.’ The Grand Isle operation pumped 3800lbs of proppant, with about 3140lbs going around the screen and about 640lbs used to fill the well’s section of blank pipe.

ULWPs offer a good alternative when conventional proppant is not a viable option, Steckler says. ‘The greatest benefit in the Gulf of Mexico is when you have long intervals and/or small holes like this one. In this well, they would have had to go with a screen-only completion’ had the lightweight gravel pack not been an option.

Ultra-lightweight gravel packing has, over the past few years, become a more refined practice. Conventional proppant, because of its high density, can lead to increased dune height and premature screen-out if the transport rate is reduced; the lighter material allows operators to pump proppant at lower rates with low-viscosity fluids, such as completion brines, and achieve the proper alpha-wave and beta-wave needed to successfully pack the entire horizontal open hole. Early efforts to develop a lightweight alternative to conventional proppants often ended in failure because the materials were not strong enough to maintain permeability at even relatively low closure stresses.

Researchers hit pay dirt with walnut hulls, the basis of BJ’s ULW-1.25 proppant, which the company markets under the names LiteProp 125. The finely ground hulls are impregnated with resin, coated and chemically modified; the size and somewhat irregular shape of the material has proven more than sufficient in most hydraulic fracturing applications and in gravel pack applications, and its low specific gravity allows near-neutral buoyancy and ease of transport and placement. The proppant offers closure resistance of up to 5000psi at bottomhole temperatures of up to 225°F, and is less than half the density of Ottawa sand, a common material used in gravel packs. The company also offers a spherical ceramic ULW-1.75 proppant, LiteProp 175, that can handle bottomhole temperatures of up to 275°F and may be used in oil and gas reservoirs with closure stress up to 8000psi.

The well at Grand Isle had been drilled out of the zone, so Arena ended up using less screen during the completion than had been anticipated; for the subsequent completion, at High Island block A-515, the company opted to go with the remainder of the eight-gauge screen as a cost-saving measure, even though the open hole, at 6 3/4in diameter, was considerably larger than the first. Some of the same conditions encountered with the first well were present at High Island A-515, including a narrow window between the hydrostatic head of the slurry and the estimated bottomhole fracture pressure of 1131psi. The completions team determined that ULW 1.25 should again be used to pack the 500ft open section. Pump rates reached 1.5bpm with full returns.

‘It was a textbook case, where you take all these different factors into consideration,’ says Arena’s Ackal. During installation of the lightweight gravel pack, he says, ‘we saw a real good, clean alpha-wave and beta-wave. And the well is performing as best as we could have expected.’ The company had predicted that it would recover about 8mmcf/d with the horizontal open hole. ‘Once we got the well completed and flowtested with the compressor in place, the well was performing better than we expected,’ eventually reaching 10mmcf/d, he says.

As offshore exploration and production continues to move into more challenging environments, Steckler says, ultralightweight proppants will increasingly play a role in completions. ‘Any well where conditions dictate that we need to pump at a slow rate, as well as these small-hole conditions,’ he says. ‘But it’s not as common in the Gulf of Mexico as in other parts of the world.’ OE

Wellbore model

Ultra-lightweight proppants have been the focus of tests conducted with BJ Services’ life-size wellbore model, which the company acquired in February 2008.

The modular system, housed at the company’s Technology Center in Tomball, Texas, allows engineers to observe the behavior of gravel pack proppants in 6ft sections of 8in diameter clear acrylic pipe that simulate actual hole size; the pipes are encased by a larger, 10in diameter clear pipe that simulates leakoff. The model accommodates screens or blanks up to 6 1/2in OD.

Designed primarily to model horizontal gravel packs, the system gives engineers the ability to evaluate the efficiency of gravel packing within a number of variable parameters, including open hole geometry, completion configuration, washpipe size, pump rate, and proppant and fluid densities, says BJ senior applied engineer in sand control Leslie Kennedy. ‘The critical transport parameters that describe this behavior are then used to validate [the] BJ PowerSand computational model, which can in turn provide accurate job design and simulation,’ he says.

The company is working with a group of ‘major operators,’ Kennedy says, to develop ‘a wellbore model testing matrix that can see the expansion of the ULWP technology worldwide, specifically for development of deepwater fields using horizontal well technology.’ OE

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