Openhole Gravel Pack
An openhole gravel pack (OHGP) is a sand control completion method in which a gravel-filled annulus is created between the borehole wall and a wire-wrapped or pre-packed screen in the uncased, exposed formation interval, providing a permeable gravel filter that prevents formation sand from entering the wellbore while maintaining the productivity advantage of the full-diameter openhole completion over a perforated casing completion; the openhole gravel pack is distinguished from the cased-hole gravel pack (CHGP) in that no casing is set across the producing formation — the well is drilled to the base of the reservoir, a short shoe track is cemented, and the production string is terminated above the reservoir, leaving the full reservoir interval exposed as an open borehole; a completion string consisting of the production packer, blank pipe, and the screen assembly is then run into the openhole section, and gravel is pumped down the work string and placed in the annulus between the screen and the borehole wall through a crossover tool that directs the gravel-laden slurry into the annulus; openhole gravel packs are the preferred sand control method in high-permeability unconsolidated sandstone reservoirs (particularly in deepwater and shelf environments in the Gulf of Mexico, offshore West Africa, and the North Sea) where the elimination of perforation damage and the maximum flow area of the openhole completion provide significant productivity advantages over cased-hole alternatives.
Key Takeaways
- The borehole quality in the openhole section is the most critical factor governing openhole gravel pack success, because an irregular, washed-out, or rugose borehole makes it difficult to place a uniform gravel pack with consistent coverage around the screen and leaves voids in the gravel pack that can become sand production pathways when the completion is put on production; borehole quality is maximized by using inhibitive drill-in fluids (calcium carbonate-based drill-in fluids that minimize fluid invasion and formation damage while preserving the borehole in near-gauge condition), maintaining a minimum equivalent circulating density (ECD) to prevent borehole washout from differential pressure erosion of weak formation, drilling at controlled weight on bit and rotary speed to minimize borehole deviation from gauge, and minimizing the time between drilling to total depth and running the completion string (to prevent borehole swelling from clay hydration or time-dependent creep that narrows the annular space available for gravel placement); post-drill caliper surveys (using multi-arm caliper or acoustic borehole imaging tools run in the drill-in fluid) provide the borehole geometry data used to calculate the gravel volume needed for a complete pack and to identify intervals where washouts may create gravel placement challenges.
- Alpha-beta wave gravel packing is the theoretical model that describes how gravel deposits in the openhole annulus during pumping: the alpha wave is the primary gravel dune that advances along the low side of the horizontal or deviated borehole from the toe toward the heel as the pumped gravel-laden slurry decelerates and deposits gravel; the beta wave is the secondary fill that rises upward from the settled alpha wave to fill the upper half of the annular space above the settled dune after the alpha wave reaches the screen's blank pipe section and the circulating path through the crossover tool is blocked; successful completion of a gravel pack requires both waves to propagate correctly — the alpha wave reaching the toe without bridging or premature dehydration (which would stop the pack before it reaches full coverage) and the beta wave filling the upper annulus completely without leaving voids; the alpha wave velocity and the slurry viscosity (controlled by carrier fluid viscosity and gravel concentration) must be matched to the wellbore geometry, pumping rate, and formation permeability to achieve complete pack coverage in a single pumping stage; design tools that model alpha-beta wave propagation (using the OHGP design software from Baker Hughes, Halliburton, and Schlumberger) help engineers specify the pumping rates, slurry concentrations, and carrier fluid properties that maximize the probability of complete pack placement.
- Screen selection for openhole gravel packs is more critical than for cased-hole gravel packs because the screen in an OHGP is the only backup sand exclusion mechanism if the gravel pack is incomplete or fails locally — there is no perforated casing providing additional structural support; the screen must be sized to retain the gravel while allowing reservoir fluid to flow through the screen-gravel interface, must be mechanically robust enough to withstand the annular loads imposed by formation compaction and gravel settling over the well's producing life, and must be resistant to corrosion by the reservoir fluids (H2S, CO2, chloride) at reservoir temperature; wire-wrapped screens (weld-free or welded wire on a perforated base pipe) and pre-packed screens (dual-layer screens with gravel packed between the inner and outer jackets during screen manufacturing) are the two most common designs for OHGP applications; screen gauges (the gap dimension between wire wraps or through the pre-pack gravel) must be smaller than the d10 of the gravel being packed (the diameter at which 10% of the gravel is finer) to prevent gravel from entering the production stream through the screen if the annular pack is disturbed.
- Frac-packing in openhole completions (also called openhole frac-and-pack or OHFP) combines the gravel placement of a conventional OHGP with a hydraulic fracturing step that fractures the formation immediately before or simultaneously with gravel placement, using the fracture as a high-conductivity pathway into the reservoir and using the gravel pack to prevent sand from the fracture faces from entering the wellbore; the OHFP technique was developed in the Gulf of Mexico deepwater turbidite reservoirs in the 1990s to address the productivity challenge of high-skin damage in openhole completions from the drill-in fluid invasion that occurs during the extended time between drilling and completing in deepwater wells; by fracturing the formation, the frac-pack bypasses the invaded zone and connects the screen to undamaged formation at distances of 5-25 feet from the borehole wall, achieving skin factors of negative 2 to negative 5 compared to the zero to positive 5 skin typical of conventional openhole gravel packs; the combination of sand exclusion from the gravel pack and bypassed damage from the fracture makes OHFP the highest-productivity sand control method available for unconsolidated sandstone reservoirs, explaining its dominance in ultra-deepwater completions where every percentage of productivity improvement significantly affects the economics of the billion-dollar development program.
- Formation damage during openhole gravel pack placement (specifically, the deposition of filter cake from the drill-in fluid during the period between drilling and completion, and the invasion of gravel pack carrier fluid filtrate into the formation during the packing operation) is managed through the design of the drill-in fluid and the cleanup procedure after gravel placement: calcium carbonate-based drill-in fluids form an external filter cake on the borehole wall that is designed to be easily removed by acid treatment or by the returning formation fluids during initial cleanup flowback; the gravel pack carrier fluid must be compatible with both the formation and the drill-in fluid (incompatible fluids can cause sludge or emulsions that block the gravel pack or damage the formation); post-gravel-pack cleanup (using an initial flow period that draws formation fluids through the gravel pack and disperses the internal filter cake damage) is designed to restore near-wellbore permeability before sustained production begins; in high-permeability deepwater reservoirs where wellbore productivity is the key economic driver, detailed formation damage testing (using core flood tests that simulate the drill-in fluid and gravel pack fluid system under reservoir conditions) is standard practice before specifying the fluid system for a new development campaign.
Fast Facts
The deepwater Gulf of Mexico introduced the most technically demanding openhole gravel pack applications, with screen-gravel pack assemblies of 3,000-5,000 feet placed in horizontal or near-horizontal openhole sections at water depths exceeding 5,000 feet and at reservoir depths of 15,000-20,000 feet. The combination of extreme reservoir depth, very high temperatures (150-200°C), very high pressures (15,000-20,000 psi), and long horizontal openhole sections pushed the limits of screen design, carrier fluid formulation, and placement modeling. The development of premium sand control completions for deepwater has driven significant advances in screen manufacturing technology, real-time gravel placement monitoring (using downhole sensors to track alpha wave propagation), and formation damage prediction that have benefited openhole gravel pack design globally.
What Is an Openhole Gravel Pack?
An openhole gravel pack is the preferred completion for high-permeability unconsolidated sands when the full productivity of the formation must be accessed without perforation damage or casing flow restrictions. Instead of casing across the reservoir and shooting perforations, the borehole is left open and a screen is run into the exposed formation interval; gravel is then pumped into the annulus between the screen and the formation wall. The gravel acts as a filter, holding the formation sand in place while allowing reservoir fluids to flow through into the screen and up to surface. The result is a completion that accesses the full reservoir thickness without the skin damage of perforations or the reduced flow area of cased completions. In deepwater turbidite sands — often the highest-productivity reservoirs in the world — the openhole gravel pack is the standard completion method, and the precision of its execution directly determines whether a billion-dollar well delivers the production rates that justify the investment.
Synonyms and Related Terminology
An openhole gravel pack is also abbreviated OHGP and is sometimes called a barefoot gravel pack or an uncased gravel pack. Related terms include cased-hole gravel pack (CHGP, the alternative sand control completion method in which casing is set and cemented across the reservoir, perforations are shot through the casing, and gravel is pumped through the perforations to fill the perforation tunnels and the casing-screen annulus, used where wellbore integrity requires casing across the reservoir or where the openhole quality is inadequate for OHGP placement), wire-wrapped screen (the most common screen type for openhole gravel packs, consisting of a spirally wound wire wrapped around longitudinal support rods on a perforated base pipe, with the wire spacing controlling the slot size that retains the gravel while allowing fluid passage), alpha wave (the primary gravel dune that advances along the low side of the borehole during OHGP placement, representing the first phase of the two-phase alpha-beta wave gravel deposition process that fills the annular space from bottom to top), frac-pack (the combined hydraulic fracturing and gravel packing completion that fractures the formation before or simultaneously with gravel placement, bypassing near-wellbore damage and achieving negative skin factors in high-permeability sands where conventional OHGP productivity is limited by drill-in fluid invasion damage), and drill-in fluid (the specially formulated completion fluid used to drill the openhole reservoir section in openhole gravel pack completions, designed to minimize formation damage through controlled filtration properties and to form an easily removable filter cake on the borehole wall).