Details of a novel approach to drilling and completing open-hole horizontal wells with a fully compatible synthetic- and oil-based fluid system using shunt tube technology have been revealed.

MI Drilling Fluids' proposed reversible drilling fluid (RDF) is a synthetic/oil-external emulsion that is reversible through an exposure to a fluid of pH less than 7.
A surfactant package included in the RDF water wets the bridging and weighting agents (CaCo3) upon reversal of the emulsion. MI said the synthetic/oil-external emulsion developed for gravel packing typically contains a 50% to 75% by volume aqueous phase as the internal phase and is completely solids-free. The internal phase can be brine or a pH-reducer, as well as a fit-for-purpose dissolver for the bridging agent.
In areas requiring sand control, gravel packing of open-hole horizontal wells is increasingly becoming standard practice, particularly in deep water, where reliability is of utmost importance due to the high cost of intervention. Often, the productive zone in these wells is drilled with a water-based RDF because synthetic/oil-based (S/OB) RDFs are thought to be more damaging or their damage removal is considered more difficult.
Furthermore, the lack of S/OB gravel-pack carrier fluids to provide well control without productivity impairment (due to formation or completion damage) often necessitates the use of water-based RDFs for the reservoir section.
A compromise has been to drill the top-hole section with S/OB mud, particularly in shaly zones, mainly due to the ability of S/OB fluids to provide greater shale inhibition, and then switch to a water-based RDF for the horizontal reservoir section.
MI believes this fluid swap complicates fluid management on the rig site, increases the risk of fluid incompatibility and increases overall completion costs.
An alternative method recently practiced offshore West Africa and the Gulf of Mexico has been to keep drilling with the S/OB-RDF and subsequently displace the S/OB-RDF to solids-free, water-based fluids in preparation for a water-based gravel-pack carrier fluid.
According to MI, this technique saved one operator 30% in direct completion costs and involved a switch to a water-based RDF for the horizontal section, displacements to solids-free water-based fluids, gravel packing with a water-based fluid and performing a post-gravel-pack cleanup treatment with coiled tubing.
Although this technique has been successful, an additional step forward would be to include breakers for the filter cake in the gravel-pack carrier fluid. This, MI said, would help achieve uniform cake removal and provide more uniform hydrocarbon influx. Another improvement would be to develop S/OB gravel-pack carrier fluids and avoid all potential incompatibility issues by eliminating displacements to water-based fluids, preferably by incorporating a cleanup mechanism into the gravel-packing process.
Chemistry
The reversible emulsion S/OB-RDF is made with a synthetic or oil as the external phase and brine as the internal phase, much like a conventional invert emulsion fluid.
Other components include an organophilic clay (barite), lime and an emulsifier and surfactant package. The brine internal phase is modified through the surfactant package to enhance reversibility (the ability to change from oil-external to water-external.)
The particular emulsifier and surfactant package in the S/OB-RDF offers a stable invert emulsion in alkaline conditions (pH greater than 7) while drilling, providing the rate of penetration and inhibition observed with conventional S/OB-RDFs.
However, when the emulsifier and surfactant package is exposed to pH conditions below 7, the emulsion reverses - making the base oil or synthetic the internal phase and brine the external phase. The reversible S/OB-RDF typically is formulated with oil (synthetic)-water ratios from 50:50 to 80:20. Higher brine content in the emulsion can provide similar rheological properties, with reduced clay concentration, minimizing the solids content in the RDF and potential damage to completion or reservoir. The lime content is kept at sufficiently high levels to maintain alkalinity while drilling.
S/OB gravel-pack carrier fluid
Potential fluid management issues on the rig can be simplified through the use of an S/OB gravel-pack carrier fluid in wells drilled with S/OB-RDFs.
The main role of such a carrier fluid is to provide enough density to balance the formation pressure without using any solids. This can be attained through the internal phase volume fraction or the internal phase type and density if, for example, an oil-external emulsion system is used.
Although emulsions with an internal aqueous phase volume fraction exceeding 90% can be formulated, these emulsions become highly shear-sensitive at concentrations greater than about 70% internal phase, getting increasingly more viscous with shear rates. Based on these results, an upper limit of 70% was selected for the internal phase volume fraction, and the brine type and density were varied.
The chemistry of the reversible S/OB-RDF described allows the RDF to be converted into a water-external emulsion upon contact with a fluid of pH less than 7 (acid).
By incorporating a pH reducing agent in the internal brine phase of the gravel-packing fluid, the carbonate bridging and weighting agents in the RDF can be water-wet, making the carbonate easily accessible to a dissolver. Such an integrated gravel carrier and cake cleanup fluid would combine an otherwise two-stage process (gravel packing followed by a cleanup treatment) into a single step, saving rig time as well as ensuring contact of the dissolver with the filter cake.
The pH modifier and CaCO3 dissolver can be a chelating agent solution (CAS), organic acid or mineral acid. The brine used in the internal phase may determine the choice of dissolver. For example, CAS is not compatible with divalent brines. Similarly, some organic acids may form precipitates with calcium brines (for example, calcium acetate). While hydrochloric acid can be used in all brines, corrosion of shale shaker screens could be costly, particularly at high temperatures and sustained exposure.
Water-based carrier fluid
As an alternative approach, the gravel-pack carrier can be aqueous-based with a cleanup system in contrast to field practices conducted without integrated cleanup chemistry, when the reservoir section is drilled with a S/OB-RDF.
Although the field results indicate significant improvement in productivity without integrated cleanup chemistry, compared to wells drilled with water-based RDFs and gravel-packed with the same carrier fluid, a further improvement would be to include cleanup chemicals in the gravel-pack fluid to minimize drawdown and optimize inflow profiles. In the field applications, a VES fluid was used with the shunt technology for gravel packing long horizontal intervals. These fluids provide higher retained permeabilities than polymeric fluids and do not require breakers because they break via exposure to produced liquid hydrocarbons and dilution with produced brine. Their low friction pressures make these fluids particularly suitable for shunt packing long horizontal intervals.
Developments
A solids-free S/OB gravel-pack carrier fluid has been developed. It is compatible with S/OB-RDFs and well suited for use in conjunction with a reversible S/OB drilling fluid because cleanup chemicals can be incorporated into the aqueous internal phase.
Laboratory data relevant to simultaneous gravel packing and cleanup in wells drilled with an S/OB-RDF have been presented.
The results can be summarized as:
• incorporated cleanup chemicals into the gravel-pack fluids increase retained permeability and decrease FIP;
• breakthrough times with gravel-pack carrier and cleanup fluids depend strongly on drill solids concentration, temperature and gravel size;
• the solids-free S/OB gravel-pack system can be formulated up to 15.4ppg with rheological profiles suitable for shunt packing at temperatures higher than 250°F (121°C).
• the solids-free S/OB gravel-pack system can simplify fluid management on rig site and decrease process times for gravel-pack completions; and
• combining this novel S/OB gravel-pack fluid containing cleanup chemicals with a reversible S/OB-RDF in conjunction with shunt technology provides means for gravel packing challenging open-hole completions, minimizing the risk of intervention or remediation, which can be costly in subsea and deepwater environments.
This S/OB carrier fluid also can be used without cleanup chemicals, providing a compatible system with conventional S/OB-RDFs.

Data Source: SPE Paper 64399. C Price-Smith, Schlumberger; M. Parlar, Schlumberger/M-I; S. Kelkar and M. Brady, Schlumberger; B. Hoxa, M-I; R.J. Tibbles, Schlumberger; T. Green and B. Foxenberg, M-I.