vuggy porosity yakal and tindalo, page-4

PC43,

You are spot on about vugs. They are great if they're connected.

A reservoir is a rock formation in which hydrocarbons have accumulated.
Before producing hydrocarbons from the reservoir, it is usually desirable
to quantify the properties of the reservoir. Among the most important
properties are the porosity and permeability of the reservoir. The term
"porosity" refers to the volume of the pore space expressed as a percent
of the total volume of the rock mass, or that volume within the rock
formation that can contain fluids. The term "permeability" refers to a
measurement of the rock formation's ability to transmit fluids. Formations
that transmit fluids readily, such as sandstones and carbonates with
larger and well-connected pores, are described as permeable. Impermeable
rocks, such as shales and siltstones, tend to be finer-grained or of a
mixed grain size, with smaller, fewer, or less interconnected pores. The
ability to accurately quantify the porosity and permeability of a
reservoir volume is essential for production planning and ultimate
hydrocarbon recovery, i.e., the percentage of total hydrocarbons
producible from the reservoir over its entire lifespan.

Sandstones usually have a relatively homogeneous pore system. Therefore,
the way fluids flow in sandstones may be modeled or controlled so that the
hydrocarbon recovery is maximized. In contrast, carbonates often have a
heterogeneous pore system. Typically, carbonates have two types of
porosity systems: a micro (or matrix) porosity system with small
grain-size pores mostly in inter-crystal and intra-crystal forms and a
macro porosity system created by alteration of rock. In vuggy carbonates,
the macro porosity system is dominated by vugs. Vugs are cavities, voids,
or large pores in a rock. Vugs are typically caused by dissolution of the
rock. Hereafter, a macro porosity system dominated by vugs will be
referred to as a vug porosity system. When flow into a well occurs through
two porosity systems, such as a matrix porosity system and a vug porosity
system, the reservoir is known as a dual-permeability reservoir.

Historically, hydrocarbon recovery from dual-permeability reservoirs has
been low because of lack of understanding of their complex nature. In
vuggy carbonates, for example, well-connected vugs which result in very
high permeability may concentrate in particular zones and areas of the
reservoir. During production, it is common to inject water into the
reservoir to s eep the hydrocarbons in various zones of the reservoir. The
injected water may all flow into the super-permeability zones, also
commonly known as "thief" zones. As a result, only the hydrocarbons in
these thin super-permeability zones are swept and produced while the
majority of hydrocarbons in the lower permeable zone are left un-swept.
This is why hydrocarbon recovery from carbonates is generally much lower
than from sandstones. Therefore, it is critical to accurately identify
where the high-permeability zones and low-permeability zones are located
before any production programs.