friction angle of the rock (right). Failure occurs under tension, compression or, most commonly, when the difference between the maximum and minimum principal stresses becomes large enough to produce excessive shear stress. The strength of a rock under downhole condi- tions depends on several factors. The most important are the cohesion, the internal friction angle, the maximum and minimum principal stresses and the pore pressure. Cohesion is strongly infl uenced by the degree of cementation of the rock. Well-cemented, consolidated sedi- mentary rocks tend to be stronger, while poorly cemented, unconsolidated rocks are weaker. Internal friction angle is influenced by the volume fraction of hard particles— typically quartz or feldspar grains— in the rock. Forma- tion grains in weak sandstone reservoirs become disaggregated, or loosened from the rock matrix, because of shear, tensile and volumetric failure.4 During production, shear failure caused by either drawdown or depletion can result in a catastrophic quantity of produced sand. Increas- ing the drawdown generates higher effective stresses around the well or perforation tunnel, and if these exceed the strength of the rock in this geometry, the rock will fail and sand may be produced. Increasing depletion can change the in-situ stresses in the Earth, which can also generate higher shear stresses around the bore- hole, possibly leading to sand production. Tensile failure occurs in weak sandstones primarily because of a high fl uid-fl ow rate, which is a function of drawdown. This type of failure is often sporadic, produces relatively low volumes of sand, is aggravated by rapid changes in well production rates, and often stabilizes with time. Volumetric failure, or pore collapse, is associ- ated with both drawdown and depletion and occurs in high-porosity, low-strength reservoirs. In weak but consolidated rocks, this phenomenon causes subsidence and has been studied exten- sively in North Sea chalk reservoirs.5 Not all sandstones yield disaggregated sand


grains under stress. Tests have shown that even weak sandstones— as determined by uniaxial- compression tests and confined triaxial tests— can have wide-ranging sand-production behaviors that are primarily related to rock type.6 Many events in a reservoir rock’s history can change its strength, eventually resulting in the onset of sand production. Additional stresses are placed on the rock matrix when drilling, complet- ing and stimulating a reservoir. Also, rock strength can be reduced by production events, such as acid stimulation, reservoir compaction or increases in water saturation.7


In weak and unconsolidated rocks, rock strength generally decreases with 12


σ 1


τ σ 3 σ n β


2β σ 3 σ n Principal effective stress


> Mohr-Coulomb failure criterion. The Mohr circle (red) represents the state of stress at any orientation in a material body, ranging from the smallest principal effective stress, σ 3, to the largest,


σ 1. If the Mohr circle intersects the failure condition (blue), the material will fail in shear. Mohr’ s circle also provides the normal stress (σ n) and shear stress (τ ) across the failure plane, and the failure angle β , measured from the direction perpendicular to maximum principal stress (inset).


increasing water saturation, with the largest decrease in strength occurring after only slight increases in water saturation from a dry state.8 Not all disaggregated sand grains are mobi- lized by produced fluids.9


They may stay in the


perforation, or in the well, and eventually cover the producing interval. The degree to which sand grains are mobilized depends on factors such as


fl uid viscosity and fl uid velocity, in complex and relatively poorly understood ways.10


When trying


to predict when and where sand production will occur, the failure of a rock and the resulting disaggregation of sand grains must be considered in conjunction with particle erosion and mobi- lization into the production stream. There are many ways to avoid or minimize sand production. In very weak, unconsolidated reservoirs, large-scale sand production may be inevitable, so downhole methods to exclude sand production or to consolidate the formation near the wellbore are practical. Sand-exclusion tech- niques include cased-hole gravel packs, high-rate water packs, frac packing, openhole gravel packs and stand-alone screens— such as slotted liners and expandable screens.


Consolidation techniques involve the injec- tion of resins to stabilize the rock while leaving enough of the original permeability intact to allow the production of reservoir fluids. These resins are sometimes used prior to hydraulic- fracturing techniques for sand control.


Choosing a method to reduce or eliminate sand production in moderately weak reservoirs is less straightforward. Underestimating the sand-production potential may lead to costly sanding problems in the future, while overesti- mating the sand-production potential may result in unwarranted and expensive installations of downhole hardware, or unnecessary reductions in production rate. Predicting the extent of sand production in moderately weak reservoirs is crucial to minimizing uncertainty when design- ing a completion. Furthermore, correctly predicting sand production can save operating companies millions of dollars per well. In many moderately weak reservoirs, screenless-completion methods provide an optimal solution.11


Techniques such as oriented


perforating, selective perforating, screenless hydraulic fracturing and consolidation treat- ments have reduced sand production, sometimes dramatically. There are also ways to manage sand production at the surface by using appropriate sand separators and through careful monitoring of erosion and accumulation. In such cases, the economics of cleanout and disposal of the sand must be taken into account in the fi nal choice of sand-management techniques. In conjunction with all sand-management methods— exclusion, screenless and surface— producing the well at an optimal rate can be essential to controlling sand production.


Oilfi eld Review σ 1


Shear stress (τ )


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