Sand (Production)

Key Takeaways

• The sanding prediction problem requires determining the critical bottomhole flowing pressure (CBHFP) or critical flow rate below which the formation will not produce sand, using rock mechanical analysis of the near-wellbore stress state combined with the formation's unconfined compressive strength (UCS) and tensile strength: as the bottomhole flowing pressure is reduced during production (by increasing the production rate or reducing the wellhead backpressure), the effective stress in the formation rock near the wellbore increases (because fluid pressure is no longer supporting the grain contacts against the overburden and horizontal stress), and when the effective stress exceeds the formation's shear strength (as estimated by the Mohr-Coulomb failure criterion using the UCS and angle of internal friction measured from core triaxial tests), the rock fails and grains are detached from the matrix and transported into the wellbore by the flowing fluid; the depth of the critical stress zone around the wellbore (the "plastic zone") grows with increasing drawdown, and sanding initiates when the detachment rate exceeds the rate at which grains are redeposited in the pore structure; prediction requires measurement or estimation of UCS (typically from scratch hardness, log correlations, or laboratory triaxial testing of core), the in-situ stress state (overburden, horizontal stresses, pore pressure), and the completion geometry (perforation orientation, density, and phasing).
• Gravel packing is the most widely used sand control method for high-rate oil and gas wells in weakly consolidated sandstone formations, using a carefully sized gravel (typically 40/60 mesh to 16/30 mesh natural silica sand or ceramic proppant) placed between the formation face and a wire-wrapped screen or slotted liner to create a permeable filter medium that retains formation sand while allowing fluids to flow freely into the wellbore: the gravel size is selected based on the grain size distribution of the formation sand (from sieve analysis of core or cutting samples), with the optimal gravel size being approximately 5 to 6 times the median grain diameter of the formation sand (Saucier's rule), ensuring that the gravel pore throats are small enough to bridge the formation sand but large enough to maintain high permeability; open-hole gravel packs (packed around a screen in the open-hole completion interval) provide better contact with the formation and higher inflow performance than cased-hole gravel packs (packed through perforations), but require a stable wellbore during screen running and are not suitable for formations with severe wellbore instability; high-rate well gravel packs routinely achieve well productivities within 70 to 90 percent of the theoretical open-hole unstimulated productivity, while avoiding the near-total loss of productivity that sand influx and plugging cause in an unsandcontrolled completion.
• Frac packing combines hydraulic fracturing with gravel packing by pumping a fracturing treatment that creates a short, wide hydraulic fracture from the perforations into the formation, placing gravel (proppant) in both the fracture and the perforation tunnels, then over-flushing to pack the screen-wellbore annulus with gravel in a conventional gravel pack configuration: frac packing achieves both sand control (from the gravel pack around the screen) and stimulation (from the propped fracture that bypasses near-wellbore damage and connects to a larger drainage area) simultaneously, improving well productivity to 2 to 5 times the unstimulated open-hole rate in damaged or tight formations; the frac pack completion has become the standard completion method for high-rate Gulf of Mexico deepwater production wells, where the combination of near-wellbore damage from drilling, the high water-cut environment requiring large fluid throughputs, and the high sand production risk from young Miocene and Pliocene sands makes the combination of sand control and stimulation essential for economic well performance; frac pack completion design requires coordinating the fracturing fluid volume and rate, the proppant schedule, the screen and gravel pack design, and the completion tubular design in a single integrated operation.
• Sand management (accepting controlled sand production rather than preventing it) is the economic alternative to mechanical sand control in some low-rate or onshore wells where the cost of sand control completion ($1 to $10 million per well) is not justified by the well's production value: sand management programs define a maximum sand production rate (typically 0.1 to 1 percent of total fluid volume) and a maximum particle size (typically below 150 to 250 microns to minimize erosion of surface equipment) that can be tolerated in the production stream without causing unacceptable erosion or accumulation, and control sand production to within these limits by restricting the well's production rate below the critical sanding rate or by periodic wellbore cleanouts when sand accumulates; erosion-resistant chokes, high-chrome or tungsten carbide-lined separators, and sand cyclones (that remove sand from the produced fluid stream before it enters rotating equipment) are the standard surface equipment modifications for sand management programs; the risk management challenge is that the critical sanding rate can decrease over the producing life of the well as reservoir pressure declines and the effective stress around the wellbore increases, potentially requiring progressive rate reductions that reduce the economic life of the well below what a sand-controlled completion would achieve.
• Standalone screens (wire-wrapped, sintered metal, or mesh-type screens run without gravel packing) are used as a lower-cost sand control alternative in formations where the grain size distribution is coarse and uniform enough that the screen openings can be sized to retain the formation sand without gravel: standalone screens require a formation grain size sufficiently larger than the screen slot width (typically screen slot width of 150 to 250 microns for 200 to 350 micron median grain diameter sands) to prevent plugging of the screen slots by the formation sand, and are prone to erosion and plugging in fine-grained or poorly sorted formations where fine particles can enter the screen openings and accumulate inside, blocking flow; expandable sand screens (ESS), which are mesh screens on a corrugated base pipe that can be hydraulically expanded to conform closely to the borehole wall, provide improved performance in deviated and horizontal wells by maximizing contact between the screen and the formation, reducing the velocity of formation fluids through the screen openings and thereby reducing sand plugging and erosion rates compared to conventional non-expanded standalone screens.