Sand-Production Simulation in Heavy-Oil Reservoirs
- Liangwen Zhang (Porous Media Research Inst., U. of Waterloo) | Maurice B. Dusseault (Porous Media Research Inst., U. of Waterloo)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2004
- Document Type
- Journal Paper
- 399 - 407
- 2004. Society of Petroleum Engineers
- 4.3.4 Scale, 2.2.2 Perforating, 5.3.2 Multiphase Flow, 5.3.3 Particle Transportation, 3.2.4 Acidising, 1.2.2 Geomechanics, 2 Well Completion, 3 Production and Well Operations, 1.2.3 Rock properties, 5.6.9 Production Forecasting, 4.6 Natural Gas, 3.1.7 Progressing Cavity Pumps, 5.4.11 Cold Heavy Oil Production (CHOPS), 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.14 Casing and Cementing, 3.2.5 Produced Sand / Solids Management and Control, 2.4.3 Sand/Solids Control, 1.8 Formation Damage
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A new sand-production model is developed based on interparticle contact-force variations at the discrete micromechanical level, scaled up to the macroscopic level. Two mechanisms for sand production are expected in the field: dynamic detachment (plucking of particles) and equilibrium yield (shear dilation followed by liquefaction at <50% porosity). The model discussed in this paper describes the sand-production mechanism in the dynamic detachment process. Within the new model formulation, sand production arises because of either a large porosity gradient or a large pressure gradient. The 1D steady-state solution of the new model is also presented; it may be used for simple sensitivity analysis for pmeters such as field stress and pressure-depletion effect.
Production of sand during oil production is simultaneously a major concern1 and a benefit2,3 for both conventional and heavy-oil production operations. It is now well known that sand influx enhances production; more than 100,000 m3/d of heavy oil is produced in Canada using Cold Heavy-Oil Production with Sand (CHOPS), a primary production method involving encouragement of sand flow and driven by solution-gas exsolution and stresses. Yet sanding can also cause problems such as steel erosion (conventional oil only), impairment of surface-equipment operation, difficulty in well workovers, well cleanup issues, and additional costs for waste sand disposal. A sudden influx of a large amount of sand into a producing heavy-oil well can even damage progressing cavity pumps or plug production tubing. Downhole screens or liners may become eroded or blocked, especially in high-rate light-oil wells (heavy-oil production is screenless).
Sand-production problems can be experienced in various ways. In conventional oil wells, transient sand production, where the sand-production rate declines rapidly with time, is frequently experienced during the cleanup period after processes such as perforating or acidizing, after rapid bean-up of production, or after water break- through. It is assumed that sand production in these cases is merely the removal of weakened or sheared and dilated sand. However, there are different "types" of sand-production behavior.
Conventional-oil sand production has been classified on the basis of distinct reservoir evolution stages:4-7
- The early transient sand-production period, when some of the perforation-induced damage has been removed and the cavities may have a zone of reduced permeability around them, leading to a steep exit gradient.
- A stable production period with enlarged cavities, when the damaged zone and the permeability impairment have been removed.
- A general increase in sand influx after a water-cut increase, a process that is considered to be related to capillary forces and is a type of flow-rate-induced sand production.8-11
- An unstable sand-production period because of reservoir pressure depletion, considered to arise when large effective-stress increases cause straining, destroy cohesion, alter fabric, and induce almost-continuous sanding.
In the latter case, substantial strains (>0.002, or 0.2%) are induced by the effective-stress and shear-stress changes on the reservoir rock; this strain is sufficient to severely degrade the cohesion by breaking the small amounts of brittle grain-to-grain mineral cementation, thereby reducing the sand strength. Both the shearing resistance and the tensile strength will be reduced, and cohesionless sand has no resistance to tensile grain-plucking forces that arise because of hydrodynamic forces, even though these are small. Note that because of the removal of sand during the completion process and the creation of a cavity (the perforation channel), zones in the near-well formation sand are under a far lower effective confining stress than in the far field.
On free surfaces across which flow is taking place, the normal effective confining stress (s´n) will be near zero if the sand has strained and been weakened, making this material susceptible to entrainment in the production stream.
In the transient sand-production period, failed sand removal from cavities is a process considered to be limited by the volume of the damaged rock, rarely more than a cubic meter or two. In such situations, however, the failed sand within the perforation tunnel may act as a support to the surrounding intact sand skeleton, and if the failed sand is removed by flow, the stable cavity structure may be destabilized. This can lead to a single sand burst followed by stabilization, episodic sand bursts that take place at seemingly random intervals, or even continuous sand production if the unsupported perforation cavities are large enough to be unequivocally unstable under flow conditions. Fig. 1 is a sketch of the damaged rock region around a perforation tunnel; if the cavity is cleaned of supporting sand and grows, it can coalesce with adjoining damaged zones. Because cavity stability in geomaterials is scale-dependent (the larger the cavity, the less stable it becomes), this can lead to continued instability.
Mechanics of Sanding
In the case of sand production triggered by water influx, two factors appear to be relevant. First, water influx implies increasing water saturation; this leads to a reduction in capillary cohesion. During stable oil production, the sand around the well is close to its "connate," or irreducible, water content. There is a small but important contribution to the strength of the sand from capillary cohesion, on the order of several kPa, depending on the grain size (finer-grained sands have higher capillary cohesion). Although small, this force is of the same order of magnitude as the hydrodynamic forces and can resist erosive destabilization. When water influx starts, capillarity is destroyed as water saturation rises. At high water content, the curvature of the water menisci between grains is reduced, destroying the capillary cohesion (see Refs. 8 through 11).
Second, water influx and saturation changes also imply relative permeability changes in the near-wellbore region;12 this alters the pressure distribution and, therefore, the effective stresses, processes which lead to cohesion loss and sand failure. (We do not consider it likely that water influx leads to a geochemical cause for sanding, such as cementation dissolution or clay swelling, albeit exceptionally these may occur.)
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