This paper is based on a series of residual water analyses performed on Ekofisk Field chalk samples. The main objective of this work was to critically evaluate the importance of capillary suction as a cohesive mechanism in chalk. .
Capillary suction models have been proposed in the literature to explain the structural weakening that occurs when water is injected into chalk reservoir rocks. These models theorize that mineral grains are held together by the surface tension of water menisci. Water saturations of 5 - 10 % are generally considered optimum for this mechanism. As additional water is injected into the rock, the menisci disappear and the rock loses cohesive strength. Proponents of these models claim that the superior strength of "dry" chalk is also due to capillary water which is so tightly bound that it survives normal laboratory drying procedures. Despite the popularity of this theory, no evidence has been published that verifies the presence of capillary menisci in dried samples.
We used a thermal desorption - mass spectrometry technique to quantify the amount of adsorbed water that remains fn dried reservoir rock samples. The results of these analyses on Ekofisk Field chalks demonstrate that the capillary suction cohesion model is not a valid mechanistic description of chalk behavior. Our data show that the residual water content of chalk is at least twenty times too low to support capillary water menisci. An alternative weakening mechanism, such as mineral dissolution at grain-to-grain contact points, needs to be considered. This type of critical testing is necessary in order to develop a realistic and accurate model of waterflood response in chalk reservoirs.