Sand Production
1. We may end up with ‘sanding’ (flow of sand-grains or particles into the wellbore along with hydrocarbons – leading to damages such as erosion on surface facilities and wellbore shutdown due to blockage of flow lines) - either - immediately following the well completion; or, sometimes, at a later stage, during the production: Which is quite more common and why?
2. How easy would it remain in reality - in order to segregate the fraction of sand production resulting from
(a) shear and tensile failure resulting from ‘changes in stresses’ in the wellbore - associated with the ‘opening of wellbore’ in the formation (mechanical instability); and
(b) ‘erosion’ resulting from the damage of pore-structure by fluid hydrodynamics (hydrodynamic instability)?
Feasible to distinguish the quantum of sand production associated with various ‘operational production strategies’ - from that resulting from (a) & (b) above?
3. Feasible to capture the commencement of sanding @ laboratory-scale, where stresses reach the limit of rock (core) strength – associated with the enhancement of ‘effective stresses’ resulting from ‘reduction in pore pressure’?
How precisely will we be able to capture the development of ‘plastic zone’; and its associated ‘extent of plastic strain’ @ laboratory-scale?
4. While applying an elastoplastic model with strain softening, the first part, where the stress following an elastic path until it reaches the peak point - can be easily captured. However, after reaching the peak point, how easy would it remain to capture the ‘variations in stress and rock properties’ with the ‘evolution of the plastic strain’ ‘until the residual state is reached’?
How exactly a damaged zone followed by a plastic zone gets developed around the boreholes; and can we directly correlate the above problem with the ‘linear elastic zone’ (by estimating elastic radial & tangential stresses along with the radial displacement around an opening); ‘non-linear softening zone’; and the ‘linear residual zone’ (by assuming that the stresses and radial displacement distribution follow the plastic behavior in both softening and residual zones – beyond the initial failure point); while analyzing the failure criterion along with the residual criterion?
Can we extend the analysis when the rock gets subjected to non-uniform horizontal principal stress - in the absence of plain strain condition?
5. How easy would it remain to delineate the ‘critical drawdown pressure’ in order to avoid entering the plastic zone development, where the reservoir within the whole drainage area follows elastic behavior in the absence of any sand production (by retaining cohesion with the least possible plastic strain)?
OR
Need to focus critically on the ‘extent of fully damaged (collapse) zone’ rather than focusing on ‘plastic development’?
6. What is the major drawback associated with the prediction of onset of plasticity using Mohr-Coulomb criteria?
M-C model is extensively used by experts, who know its limitation. It is also being abused by half-knowledge analysts, who don't understand its limitations. Typically, once plastic state is reached, there is no control or limit for plastic strain in this model. In other new wording, there is no bounding surface or failure surface, to limit the strains. Thus, the model is very good for initial approximation, for small strain problems only, where, strain changes from elastic to plastic zone, with limited non-linearity.
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