SOLUTIONS FOR CYLINDRICAL CAVITY IN SATURATED THERMOPOROELASTIC MEDIUM

Based on the thermodynamics of irreversible processes, the mass conservation equation and heat energy balance equation are established. The governing equations of thermal consolidation for homogeneous isotropic materials are presented, accounting for the coupling effects of the temperature, stress and displacement fields. The case of a saturated medium with a long cylindrical cavity subjected to a variable thermal loading and a variable hydrostatic pressure （or a variable radial water flux） with time is considered. The analytical solutions are derived in the Laplace transform space. Then, the time domain solutions are obtained by a numerical inversion scheme. The results of a typical example indicate that thermodynamically coupled effects have considerable influences on thermal responses.

A cost-effective chemo-thermo-poroelastic wellbore stability model for mud weight design during drilling through shale formations

Drilling through chemically-active shale formations is of special importance due to time-dependent drilling fluideshale interactions.The physical models presented so far include sophisticated input parameters,requiring advanced experimental facilities,which are costly and in most cases unavailable.In this paper,sufficiently-accurate,yet highly practical,models are presented containing parameters easilyderived from well-known data sources.For ion diffusivity coefficient,the chemical potential was formulated based on the functionality of water activity to solute concentration for common solute species in field.The reflection coefficient and solute diffusion coefficient within shale membrane were predicted and compared with experimental measurements.For thermally-induced fluid flow,a model was utilized to predict thermo-osmosis coefficient based on the energy of hydrogen-bond that attained a reasonably-accurate estimation from petrophysical data,e.g.porosity,specific surface area(SSA),and cation exchange capacity(CEC).The coupled chemo-thermo-poroelastic governing equations were developed and solved using an implicit finite difference scheme.Mogi-Coulomb failure criterion was adopted for mud weight required to avoid compressive shear failure and a tensile cut-off failure index for mud weight required to prevent tensile fracturing.Results showed a close agreement between the suggested model and experimental data from pressure transmission tests.Results from a numerical example for a vertical wellbore indicated that failure in shale formations was time-dependent and a failure at wellbore wall after 85 min of mudeshale interactions was predicted.It was concluded that instability might not firstly occur at wellbore wall as most of the conventional elastic models predict;perhaps it occurs at other points inside the formation.The effect of the temperature gradient between wellbore and formation on limits of mud window confirmed that the upper limit was more sensitive to the temperature gradient than the lower limit.

Harvesting net power and desalinating water by pressure-retarded membrane distillation

Pressure-retarded membrane distillation(PRMD)can convert low-grade heat to useful work by harvesting the pressure energy of the condensation liquid on the cold side.In this study,a new type of PRMD system for combined freshwater and power production was proposed.For this configuration,the thermal energy of the phase change is transported to the interface mainly by conduction rather than convection,which significantly reduces the pump power loss of the liquid flow.In addition,it also utilizes a multistage structure to regenerate heat with low pump power loss.The experimental results showed that,for a module with a heating area of 1.0 m^(2),this system can produce 188 L of freshwater and 27.8 kJ of power each day when operating between 80℃ and 40℃.The water and power densities of the PRMD configuration would be affected by saline ions in the feed liquid,air resistance in the evaporation chamber,membrane wetting,and membrane compaction.The experimental and molecular dynamics simulation results indicated that a higher temperature difference or working temperature will significantly improve the desalination and power generation rates because of the increased mass transfer driving force of the vapor gradient.This study proved that it is possible for a PRMD system to simultaneously obtain net power and freshwater using low-grade heat as the only energy input.Nevertheless,the liquid supply,working pressure,and membrane properties should be improved to achieve better performance.