| This experimental research presents new phenomenological evidences that improve the understanding of the sand production process. The investigation studied formation, stability, behavior, and morphology of sand arches in non-consolidated two-phase-saturated sands. The experimental apparatus consisted of an X-ray CT scanner facility, an X-ray transparent high-pressure vessel, and a uniaxial strain cell. A method was developed for fabricating artificial samples, reproducing the physical properties of the natural non-consolidated formations. Samples were prepared, saturated and submitted to confinement, simulating wellbore conditions. Kerosene and brine were pumped through the sample, generating sand production. The effects of changes in flow rates and in wetting-phase saturation were tested and quantified.; The findings established that single-phase saturated sands submitted to fluid flow did not produce a stable arch. However, a minimum of 3% in wetting-phase saturation was enough to produce a stable arch. Rather than an empty cavity, a dilated, agglomerated, and plastic material filled the volume inside of the arched region. Changes in saturation showed strong influence on arch stability. Minimum amounts of sand were produced for wetting-phase saturation smaller than 20%. Episodic, evolving to continuous sands were produced for saturation higher than 20%, and massive amounts of liquefied sands were produced for saturation higher than 30%. Decrease in flow rate reduced sand production, but had no effect on arch size. Fines and small grains of sand were preferentially produced in the water-phase. Gravity was an important imbalance factor in cavity growth and arch instability.; The material inside the arch consisted of fractured blocks of dilated and agglomerated sands, which moved plastically toward the exit hole. The mechanism of arch growth was surmised as a balance between driving and resistive forces. Expansion of the dilated zone, fractures due to excess in tensile stresses at the grain level, and shear due to relative movement between agglomerated blocks determine the sequence of cavity instability and arch growth. |
