Depending on the location, frac-packing and openhole horizontal completions are the dominant completion strategies in the development of deepwater projects. Economics require that a minimum number of wells be drilled while maintaining effective reservoir drainage to maintain a high productivity index in the wells. Drilling long horizontal sections is an option, and openhole gravel-packing is a very effective way to achieve all these goals.

Offshore Brazil, more than 200 openhole horizontal gravel-packing operations have been

Figure 1. A completion tool assembly with a differential valve enabled gravel packing a long horizontal openhole section that would have been impossible to pack using conventional means. The tool was then converted to a stimulation tool to save 18 hours of rig time. (Images courtesy of BJ Services)
completed since 1998. Experience has been a good teacher. To better assess the improvements over time and look for additional efficiencies in proppant placement, time and other parameters, we recently studied data from more than 72 openhole horizontal gravel packing operations performed since 2001.

The data show clearly that gravel-packing rates have increased over time, also increasing packing process efficiency. Computer simulation and better understanding of downhole challenges have greatly affected job time over the years, but technology has also provided some leaps in efficiency. In particular, engineered single-trip completion systems and ultralightweight proppants have altered the status quo for gravel packs in even the most challenging conditions.

Improving tool efficiency
The typical well bore for openhole horizontal gravel packing completions in Campos Basin is 95¼8 in. production casing set at approximately 90° inclination with a horizontal openhole section of 81¼2 in. These wells also typically have a 40-ft (12-m) rat hole below the casing shoe, which in some fields with fracture gradients approaching or below 0.56 psi/ft must be eliminated or bypassed to prevent the alpha wave from stalling at the low pump rates required to successfully pack the entire horizontal length.

Tools for gravel packing have traditionally been simple devices, but recent tool developments have greatly increased operational efficiencies and job effectiveness for long horizontals. The ComPlete horizontal single-trip (HST) system, for example, converts from a gravel-packing service tool into a simulation tool in order to save valuable rig time. The system provides effective mechanical diversion of the stimulation fluid and isolates the horizontal section after stimulation to avoid formation damage due to fluid loss. Furthermore, to ensure effective bottomhole pressure control and manage carrying fluid leakoff in formations with low fracture gradient, a differential pressure valve tool can provide altered flow paths in the annulus between the screen and the washpipe.

For example, a well in the Campos Basin has an 81¼2 in. openhole section of 2,100 ft (640 m) at 88Þ inclination. The formation fracture gradient is 0.58 psi/ft and the reservoir pressure 2,140 psi. Engineers used proprietary gravel-packing software to simulate an operation in the well, including flow conditions, and develop a comprehensive gravel pack design.

After performing several circulation tests with the HST tool and a differential valve, the 1-ppa
Figure 2. Ultralightweight proppants have enabled successful gravel packs under extremely challenging conditions.
proppant slurry was pumped downhole at 7 bpm (Figure 1). The alpha wave lasted 136 minutes, packing the openhole section with 38,000 lb of proppant (67% annulus packed). The beta wave began at 153 minutes, delivering 19,095 lb of proppant (33% annulus packed). The differential valve opened at 161 minutes, when the surface treatment pressure suddenly increased and the return rate dropped off. Because of the valve’s successful operation, controlled screenout occurred at 223 minutes. Without the valve, the screenout could not have been reached.

Furthermore, rather than having to pull the tool out of the hole to stimulate the well, the stimulation was carried out with no need for an additional trip, saving a valuable 18 hours rig time.

Extreme conditions
Efficient gravel packing in a deepwater well completion can be challenging. These wells often have excessive fluid loss, variations in hole stability and hole geometry, and an extremely narrow pressure window between bottomhole pressure and fracture gradient. The narrow pressure window, in particular, can be a significant concern because high pump rates required for long-distance proppant transport may fracture the formation, causing fluid loss and a sand bridge during the alpha stage or an early screenout in the beta stage.

For example, one customer wanted to complete several wells whose openhole horizontal sections included four conditions:
• Extended-reach horizontals (more than 2,300 ft/700 m);
• Low formation fracture gradients;
• Washed-out zones; and
• Insufficient fluid returns to circulate conventional proppants into place.

The combination of unfavorable conditions suggested gravel packing the wells with conventional sand would be extremely risky and difficult, at best. First, washed out zones several hundred feet long would affect the dune bed, substantially raising the chances of stalling and premature screenout. Furthermore, reducing the pump rates to avoid
fracturing would very likely result in increased dune height and premature screenout. To avoid these problems, we proposed using LiteProp 125 ultralightweight proppant and the LitePack openhole gravel packing method.

Ultralightweight proppant
The pumping boundaries for openhole gravel packing are Qmin, the rate at which 85% dune ratio occurs, and Qmax, the formation fracturing pressure.

The highest bottomhole treating pressure arises as the beta wave develops, rapidly increasing the back pressure from fluid filtration through the screen/washpipe. This pressure will drop as the square of the pump rate, but reducing pump rates also increases the chance of premature screenout as solids settle out of the fluid.

Therefore, reducing Qmin is a critical element in planning a long, extended-reach openhole gravel packing operation. Increasing the viscosity of the fluid medium would increase proppant transport — but at the cost of increasing pump rates. A better solution is to reduce proppant density.

With specific gravity of just 1.25, ultralightweight proppant settles out of fluids much more slowly than do heavier conventional sands, drastically changing the alpha-beta modeling for dune height propagation. The slow settling effectively minimizes Qmin, greatly extending the length of the open hole that can be packed at a given pump rate.

Mixed with fluids, the lightweight proppant also produces lower hydrostatic head than denser conventional sands, and when mixed at 1 ppa, it produces an 8.8% solids volume. This results in more solids entering the open hole in less pump time (about half the time for the same volume of solids) compared with conventional and ceramic proppants.

Packing for a long trip
The initial application of the LitePack technology was in an injector well in the Campos Basin field. The 2,655-ft (800 m) openhole section included a significant challenge besides its length: a 500-ft, 12-in.-diameter (500 m, 30 cm diameter), shale-sloughed washout zone just past the last casing shoe.

Achieving a gravel pack with conventional sand would have required a pumping rate of more than 6.5 bbl/min to achieve a dune rate less than 85% of the sloughed zone. Instead, we designed the operation with ultralightweight proppant at 1 ppa so we could pump at a lower 4 bbl/min (0.64 m3/min) pump rate and achieve a 71% alpha dune height (Figure 2).

The alpha wave lasted 101 minutes, packing the openhole section from heel to toe with 15,200 lb (6,900 kg) of proppant (62% annulus filling). The beta wave began at 144 min of pumping time, delivering 9,400 lb (4,250 kg) of proppant (38% annulus filling) before controlled screenout occurred at 220 min. The packing time amounts to about half of the time required to pump an equivalent volume of ordinary sand — if it could even be done under the difficult well conditions.

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