Abstract: Characterization of Spontaneous Water Imbibition into Gas-Saturated Rocks

HORNE, ROLAND, Stanford University, Stanford, CA

Spontaneous water imbibition into gas-saturated rocks is an important physical process during water injection into highly fractured petroleum and geothermal reservoirs. Few methods, however, are available for characterizing the process of spontaneous water imbibition into gas-saturated rocks. The method used usually is Handy’s equation, that is, the weight of the imbibed water is proportional to the square root of the imbibition time. There are three disadvantages to using Handy's equation for characterizing spontaneous water imbibition. First, effective capillary pressure and effective water permeability can not be calculated separately from the spontaneous water imbibition test. Secondly, the straight line between the square of weight gain and the imbibition time often does not go through the origin point, as it is supposed to. Thirdly, the relationship between the square of weight gain and the time was not a straight line during the later period of water imbibition, or even in the early period in some cases. In this paper, a method has been developed for characterizing the process of spontaneous water imbibition into gas-saturated rocks. Using this method, effective capillary pressure and effective water permeability can be calculated separately from spontaneous water imbibition data. A linear relationship between the water imbibition rate and the reciprocal of recovery by spontaneous water imbibition was found and confirmed both theoretically and experimentally, even at different initial water saturations. A lot of experiments of spontaneous water imbibition into air-saturated porous media (glass-bead packs, Berea sandstone, and chalk) have been conducted at room temperature. The effect of initial water saturation on imbibition rate, residual gas saturation, and imbibition recovery has been investigated. We found that there was almost no effect of initial water saturation on residual gas saturation by spontaneous water imbibition. The higher the initial water saturation, the lower the water imbibition rate. The capillary pressure curve of air-water in a glass-bead pack has been measured using an X-ray CT technique. The effective capillary pressure calculated using the new method is consistent with the values measured using the X-ray CT technique. The method developed in this paper is also of importance for scaling-up experimental data.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California