Characterization of Spontaneous Water Imbibition into Gas-Saturated Rocks

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. To this end, a method has been developed. Water relative permeability and capillary pressure can be calculated separately from water imbibition data using this method. A linear relationship between imbibition rate and the reciprocal of the gas recovery by spontaneous water imbibition was found and confirmed both theoretically and experimentally, even at different initial water saturations. The effect of initial water saturation on imbibition rate, residual gas saturation, and the gas recovery has been investigated. 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 and the ultimate gas recovery. It was found that the capillary pressure did not vary with initial water saturation in a certain range. The capillary pressure calculated using the new method was approximately equal to the values measured using an X-ray CT technique in a glass-bead pack. The computed water relative permeability was consistent with published experimental results. The method developed in this paper is also of importance for scaling-up experimental data. Introduction A lot of oil and gas reservoirs are being developed by water injection or associated with active aquifers. The reinjection of produced water into geothermal reservoirs, which are usually highly fractured, is also a practical solution to the problems of reservoir pressure decline and environment pollution. Spontaneous water imbibition phenomena exist in most of these reservoirs except those that are not water-wet; it is an important process driven by capillary forces. The study of spontaneous water imbibition is essential to predict the production performance in these reservoirs developed by water flooding, especially in highly stratified formations and fractured reservoirs where the amount and rate of mass transfer between the matrix and the fracture influence the recovery and the production rate. Another significance is to determine the wettability of the rock-fluid systems from the spontaneous water imbibition measurents. Since the spontaneous imbibition is a capillary pressure dominated process, the imbibition rate is significantly dependent on the properties of the porous media, fluids, and their interactions. These include permeability and relative permeability of the porous media, pore structure, matrix sizes, shapes and boundary conditions 5, , fluid viscosities , initial water saturation, the wettability of the rock-fluid systems, and the interfacial tension between the imbibed phase and the resident phase . Most of the studies in the literature focused mainly on oil-water-rock systems. There are a lot of naturally fractured gas reservoirs positioned over active aquifers. The water coning in these gas reservoirs often results in excessive water production, which may kill a well or severely curtail its economic life due to water handling costs. The understanding of the mechanisms that govern spontaneous water imbibition in gas-water-rock systems is important to the development of naturally fractured gas reservoirs with active aquifers. Few methods, however, are available for characterizing the process of spontaneous water imbibition into gas-saturated rocks. The method used usually is the Handy equation, that is, the weight or the volume of the imbibed water is proportional to the square root of the imbibition time: