COMPLETION TECHNOLOGY: Floater installs horizontal open-hole gravel packs, Level 6 multilaterals

Brazil's Campos Basin, home to 61 fields in water depths ranging from 100 meters to 2,000 meters, is one of the major deepwater developments in the world. When Petrobras wanted to implement extended horizontal gravel open-hole gravel packs and Level 5 multilaterals (ML) in its Campos Basin Marlim Field, the operator faced unique challenges. Such an installation would be the first to be implemented in deepwater from a floating drilling unit. To date, Baker Oil Tools has worked with Petrobras to successfully install 38 horizontal open-hole gravel packs and three Level 5 multilaterals in the Campos Basin.

The ML wells were notable because in cementing the main bore and lateral, pressure isolation had to be achieved at the junction with packers and isolation seals. In these Level 5 ML installations, full hydraulic and mechanical integrity of the junction is achieved through completion equipment designed to straddle the junction.

These straddles can tie into a dual packer above the junction and provide independent production to the surface. Alternatively, the two-straddle tubing strings can tie together above the junction, providing either comingled production or injection. This second option was used on Brazil's deepwater Level 5 multilaterals.

Open hole horizontal gravel packed wells are attractive in deepwater because they deliver the production and injection needs to efficiently deplete the reservoir. This type of completion provides extensive reservoir exposure. By utilizing extended open-hole horizontal completions with multilaterals, reservoirs can be effectively produced using fewer wells.

With recent advances in open-hole gravel packing, the length of laterals that have been successfully packed exceeds 6,900 ft. While stand-alone screens are often used for this purpose, it has become clear that gravel packing can extend well life and the economic viability of deepwater installations.

Floating rig operations

The main concern when completing horizontal wells from a dynamically positioned floating rig is the motion of the rig. To ensure correct depth control and tool position, a positive positioning tool is incorporated into the standard gravel pack system. The tool allows weight to be slacked off to 80,000 lb at each position with the motion compensator engaging, when necessary, to mitigate the rig's movement and keep the workstring stationary.

To facilitate tool movement and positioning of the crossover tool, a steel cable is attached to the riser and suspended above the rig floor. Circulating and reverse positions are marked on the drill pipe with reference to the stationary line index. With these positions marked on the drill pipe with reference to the index line instead of the rig floor, the crossover tool positions are located and maintained, providing an easier indication for the Baker tool hand and limiting risk to the operation.

For the Level 5 multilaterals, good control of bit weight is necessary for efficient window cutting. The Marlim Field multilateral operations used a long-stroke bumper sub immediately above the drill collars for weight control.

The collars provided weight on bit and the bumper sub absorbed rig motion. This milling assembly provided a smooth interface between the drilling and the completion operations, leading to an efficient casing exit procedure without requiring additional cleanout trips.

Maintaining hydrostatic pressure

In scenarios where the hydrostatic overbalance required to maintain open-hole stability is lost, or the formation is swabbed, there is opportunity to loose the drill-in fluid filter cake. When the drill-in fluid filter cake is removed from the formation, experience has shown that excessive fluid loss and hole instability make it difficult to achieve a successful gravel pack.

Historically, the most likely time for the hydrostatic over-balance pressure on the formation to be lost is during setting of the packer and while moving the gravel pack service tool. If there is any flow from the wellbore to the formation, the pressure can quickly equalize. Once the over-balance is reduced to less than about 100 psi, the risk of removing the filtercake off the borehole wall significantly increases.

Special design features incorporated into the latest open-hole gravel pack tools alleviate the potential for loosing the filter cake and having excessive fluid loss. The more recent open-hole gravel pack tools are designed to maintain hydrostatic over-balance on the formation during all phases of the gravel packing operation.

For the latest open-hole gravel packing tools, prior to setting the packer and moving to the test position, the annulus pressure is in communication with the formation face through bypass ports in the packer seal bore. To maintain hydrostatic over-balance, the annulus must be kept full until it is time to test the packer. During the packer testing process, the annulus is no longer in communication with the formation. However, communication is established on the workstring side. During the packer testing, it is critical that the workstring be kept full. This is a change from standard rig operations and must be communicated appropriately.

Swabbing the formation during tool movement can also lose hydrostatic pressure. An anti-swabbing tool, a single acting flapper valve device within the crossover tool, provides a fluid path down the inside diameter (ID) of the gravel pack system and a method of sustaining hydrostatic pressure on the formation.

Once actuated, the anti-swabbing valve closes and provides sandface isolation while reverse circulating for propant cleanup. This same valve is used in the frac pack tools system to facilitate pressure decline analysis for efficiency calculations. The valve is held open during the frac pack or gravel packing operation but can be closed at any time during the operation as necessary.

Hole cleaning procedures

Hole cleaning is a must for successful open-hole gravel packing. The hole is scoured by circulating a non-viscosified brine at a rate exceeding 300 ft/min. Several laboratory studies concerned with transporting solids from horizontal wellbores have substantiated the 300 ft/min. rate. Successfully displacing the open hole to a brine requires 300-500 psi over-balance pressure at all times. This over-balance helps to maintain hole stability and a durable filter cake. Poor well productivity may result if the solids-free pill is spotted without high-rate brine displacement and a thick filter cake is gravel packed in place.

To assist in removing solids from the hole, the hole cleaning operations in Brazil employed reverse circulation whenever possible. This helped to eliminate difficulties associated with low flow velocities in the riser section. A three-step process cleans the riser, casing, and the open hole in phases to compensate for the large volumes of fluid.

First, drill pipe was run to approximately 30 meters above the casing shoe and the pipe rams were closed. The riser was circulated clean with seawater down the upper choke and kill lines. This circulation should be at as high a rate as possible (using all available rig pumps) and continue until returns reach 30 neophelometric turbidity units (NTUs).

Following casing cleanup, a displacement pill (without casing cleaning chemicals) is spotted into the casing/workstring annulus, and then the workstring is run to within 2 meters of bottom. The annular blowout preventer (BOP) is closed and the open-hole is reverse or forward circulated depending on applied pressure limitations. After displacing the drill-in fluid and displacement pill from the open hole with filtered brine, the hole is circulated until returns achieve 30 NTUs.

There are similar hole cleaning issues associated with the window cutting operations of a multilateral well. Standard debris management procedures must be augmented to avoid cleanup problems in the deepwater riser.

Fluid loss pill formulation

If excessive fluid loss is an issue, it may become necessary to pump a fluid loss control pill prior to gravel packing. It is vital to guarantee that this pill can flow through the gravel pack screen without plugging. This formulation should be yard tested on full-scale screen samples whenever possible. Small-scale laboratory testing on screen coupons can provide misleading results concerning the ability of a given fluid loss pill to flow through the screen.

In addition, sufficient pill volume must also be pumped for this approach to be effective. It is equally important that any pills pumped after gravel packing bridge on the inside of the screen immediately. If too fine a particle size distribution is pumped after gravel packing, the fluid loss pill will penetrate the gravel pack. The likely result will be an expensive acid cleanup operation. Pills that are sized to bridge on the inside of the screen have been shown to effectively clean up with production.

Production/injection start-up

Following gravel packing, the final completion equipment is run. This is more complicated if the well is a multilateral. However, the one question that typically remains following a horizontal gravel pack is what needs to be done to remove the effect of the filter cake. In the producing wells gravel packed in Campos Basin, as well as in many other applications worldwide, the wells were simply brought onto production without acidizing. Since these wells are some of the best producers in the field, it appears that the remaining filter cake material effectively breaks up becoming "transparent" to production.

However, when injection wells are gravel packed, the cleanup is somewhat more involved. For the horizontal injector wells gravel packed to date in Marlim Field, the practice has been to produce the well for a short term test, with the purpose of relaxing and breaking up the filter cake.

There were expectations that this short production period would completely remove the filter cake, eliminating any injectivity impairment. However, this was not the case, and the resulting injectivity for some of the injectors was somewhat less than expected and required post acid treatment to achieve target injection rates.

A further advancement of the open-hole gravel pack system now enables conversion of the crossover tool to spot acid as necessary for filter cake cleanup without making any additional pipe trips. If an acid treatment is employed, it will be pumped immediately following placement of the gravel pack.

Level 5 multilateral systems that hydraulically isolate the junction area have contributed to the success of deepwater and extended reach wells. The new larger bore (13-3/8 in. by 9-5/8 in. by 9-5/8 in.) reformable Level 6 multilateral junction will reduce risk and the rig time required to implement a Level 5 ML. The larger junction will provide flexibility of junction depth placement and deliver the hole sizes necessary to achieve required production rates in deepwater.

Gravel packing multilateral wellbores also has become more feasible with the hydraulically isolated large-bore junctions. Constructed before drilling or completing either leg, this technology improves junction formation integrity and simplifies fluid selection, changeouts, and hole cleaning. Using this approach, each leg of the well can be drilled and completed using conventional single bore technology and equipment. Combining the latest proven technologies for multilateral wells and open hole gravel packing provides completion options to address the ever challenging effort to reduce risk and improve economics of deepwater developments.

Acknowledgement

This article was adapted from SPE/IADC 67750, presented at the SPE/IADC Drilling Conference in Amsterdam, The Netherlands, on 27 February-1 March 2001. Baker Hughes Incorporated and the Society of Petroleum Engineers granted permission to publish. Frank Radez, Baker Hughes Incorporated, provided adaptation and rewriting of materials published in SPE 67750.