Pore throat size and connectivity
|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Chapter||Predicting reservoir system quality and performance|
|Author||Dan J. Hartmann, Edward A. Beaumont|
Even very large pores contribute nothing to fluid flow unless they connect to other pores. Connectivity increases with the size of pore throats and with increasing number of pore throats surrounding each pore. The number of pore throats that connect with each pore is the coordination number.
Pore shape, throat size, and throat abundance
How do pore shape, pore throat size, and pore throat abundance affect the flow dynamics of a reservoir? Visualize a room with a door in each wall. The number of people who can fit into the room is a product of the size and shape of the room. The movement of people into or out of that room is a product of the size, shape, and number of doors. A large cubeshaped room with many small doors allows the people to leave the room at a given rate relative to a smaller tubular-shaped room with a few large doors.
A particular pore type has similar entrance/exit dynamics. Pore throats are the doors (ports) to the pore. Along with water saturation (Sw), pore throats control permeability to hydrocarbons in reservoir rocks.
Characterizing pore systems by size
How does one characterize the size of a pore system: by pore size or by pore throat size? Characterizing the size of a pore system by pore size presents problems. For example, how do we accurately measure and average pore size in rocks with poorly sorted pore sizes?
Pore systems are easily characterized by size using pore throat size. Pore throat sizes can be measured using capillary pressure curves. A capillary pressure curve is converted to a distribution profile of pore throat size, and a pore throat size that characterizes the rock is determined by picking a certain saturation level.
Which saturation level should we use? Work by Dale Winland and Ed Pittman shows a statistical correlation between optimal flow through rocks and the radius of the pore throats when 35% of the pore space of a rock is saturated by a nonwetting phase during a capillary pressure test. They call the size of pore throats at 35% nonwetting phase saturation r35, also called port size. Port size is convenient for characterizing the size of a pore system. Pore systems can be subdivided into “port types” by port size. (See Characterizing rock quality for a discussion of port size and r35.) The table below shows port types and size ranges for those port types.
|Port category||Port size range (r35), μ|
- Pore systems
- Pore system fundamentals
- Pore system shapes
- Pore and pore throat sizes
- Capillary pressure (Pc) curves: pore throat size determination
- Wardlaw, N. C., and J. P. Cassan, 1978, Estimation of recovery efficiency by visual observation of pore systems in reservoir rocks: Bulletin of Canadian Petroleum Geology, vol. 26, no. 4, p. 572–585.
- Pittman, E. D., 1992, Relationship of porosity to permeability to various parameters derived from mercury injection–capillary pressure curves for sandstone: AAPG Bulletin, vol. 76, no. 2, p. 191–198.