VAVRA, CHARELS L., Arco Exploration and Production Tech., Plano, TX

ABSTRACT: Capillary Pressure, Fluid Distributions & Diagenesis in Hydrocarbon Reservoirs: Interrelationships and Implications

Because many diagenetic reactions require substantial mass transfer of ions, the distribution of formation water in hydrocarbon reservoirs may be important in controlling the extent and location of these reactions after hydrocarbon emplacement. Fluid saturations and distributions in hydrocarbon reservoirs are controlled in part by capillary pressure properties including displacement pressure, pore throat sorting and minimum wetting-phase saturation. Because different rock types have different capillary properties, saturation profiles and fluid distributions will differ between rock types in a hydrocarbon reservoir.

Where buoyancy pressure is less than displacement pressure (i.e. below the hydrocarbon/water contact of a particular rock type) all pore space is filled with formation water, and the water/rock ratio and effective water permeability are maximum. Where buoyancy pressure exceeds displacement pressure, hydrocarbons begin filling pore space at the expense of brine, and the effective permeability to water decreases with increasing hydrocarbon saturation. At the minimum wetting-phase saturation, water/rock ratio is at a minimum, with immobile water being restricted to micropores, surface irregularities and pendular rings. Effective permeability to water approaches zero, impairing diagenetic reactions that require significant mass transfer.

The relationships among capillary pressure, fluid saturation and distribution and diagenesis imply that: (1) rocks may display different degrees and types of diagenesis above and below their hydrocarbon/water contact; (2) formation waters may have different compositions above and below the hydrocarbon/water contact; and (3) rocks with different initial capillary properties may undergo different diagenetic reactions due to the differences in fluid distributions.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.