STRENGTH VARIATIONS WITH DEFORMATION IN UNCONSOLIDATED ROCK AFTER WATER BREAKTHROUGH

摘要

Reservoir rock may be weakened significantly after water breakthrough because of capillary strength changes with water saturation,and by the effects of chemical reactions between formation water and rock cementation.Because of weakening,the rock will behave differently in single phase(oil)and in water-oil fluid environments.Two analytical models are developed to study the changes in rock capillary strength with water saturation and deformation in two different loading states: confined and detached(unconfined).Based on the model calculations,it is found that capillary strength increases rapidly to a peak value with water saturation at a low saturation state,and then decreases gradually to become zero if the sand becomes 100% water saturated.The greater the distance between particles in a detached state,the higher the water saturation needed to maximize capillary strength.Furthermore,the peak capillary strength is affected by several factors,including interparticle distance,liquid contact angle,particle size,and interphasic surface tension.The more the particles are detached or squeezed,the lower the peak strength.Small particle size,large interphase surface tension,and low contact angle result in high capillary strength.By introducing the concept of solid strain from rock mechanics into capillary models,it can be shown that,at the same amount of water in the liquid bridge,capillary strength keeps decreasing with rock compression.Conversely,it increases slightly with extensional deformation before reaching a peak,and then continuously decreasing as extensional deformation continues.The rate of decrease of rock strength with compressional deformation is much faster than in the case of extensional deformation.The models and results can be used to quantify the failure risk associated with loading a reservoir rock after water breakthrough,allowing more quantitative well management procedures.