BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
The porosity and permeability of carbonate rocks, like those of sands, are controlled by the currents
and waves in the original depositional environment. However, the original texture is vastly altered by
the solution and re-precipitation of calcium carbonate after burial.
Recently deposited lime muds consist mainly of aragonite (which is a different and more unstable
crystallographic form of calcium carbonate) and high-magnesium calcite. Consolidated limestones
consist of low-magnesium calcite and sometimes dolomite. Profound changes take place soon after
burial.
When originally deposited, lime muds have a porosity of 50 percent or more, but when they are
consolidated into limestone their porosity is generally less than two percent. Shales lose porosity by a
compaction process that involves flattening. However, limestones are formed from lime mud by recrystallization,
and the pores are filled by precipitation of calcite, apparently brought in from
elsewhere, because no compaction has occurred. Oolites and fossils are not squashed and flattened.
Consolidated limestones show abundant evidence of solution and precipitation. Micritic skeletal
limestones often have the original shells dissolved out, leaving cavities. Irregular channels and cavities
(vugs) formed by solution permeate some limestones. These, like fractures, are usually lined and are
sometimes filled with clear crystalline calcite.
Dolomitization. Limestones are often partially or completely changed to dolomite. Dolomite has
the composition CaMgCO 3 and it is crystallographically similar to calcite. However, it has a greater
density, less solubility in water, less ductility, and more brittleness. Obviously, waters enriched in
magnesium permeated the calcium carbonate deposits sometime after their burial. They laid down an
atom of magnesium and picked up one of calcium. Usually the dolomitization involved a recrystallization.
First, dolomite crystals (rhombi) form from the micrite in a random manner. Later, the
dolomitization micrite is dissolved, leaving intracrystalline porosity. Some of the Arabian fields
produce from beautiful crystalline dolomite, which resembles granulated sugar.
Types of Pores in Carbonates
The interstitial pores in carbonates basically resemble those in sandstones. They are the open cavities
between the grains, usually more or less clogged by mud or precipitated substances, or opened by
water solution. However, carbonates differ from sandstones in being soluble and brittle. Because they
are soluble, they often have large cavities called channels or vugs. Because they are brittle, they often
have fractures that, if they are open or enlarged by solution, are also large openings. Carbonates
therefore often – but by no means always – have a secondary porosity that may be greater than the
primary interstitial, or matrix, porosity. The fractures may contribute only slightly to porosity but
vastly increase the permeability. The behavior of fluids in carbonates containing only interstitial
porosity resembles that in sandstones, but in vuggy or fractured carbonates it is entirely different.
Permeability in Carbonates
Carbonates are characterized by different types of porosity and have unmodal, bimodal and other
complex pore size distributions, which result in wide permeability variations for the same total
porosity. Although most sedimentary rocks possess significant porosity when freshly deposited, the
rate at which these vital properties undergo reduction with age is principally due to compaction. The
compatibility of sedimentary rock depends more on the texture than it does on composition. The
textures of dense limestones are not effective reservoir materials because of compaction, unless they
have been extensively fractured after they were formed. Fracturing produces secondary permeability
meaning it was created after the rock in question was formed. The fractures create new void spaces in
the rocks which allow fluids to move into the voids. Primary permeability occurs during the
compaction process when the original materials are forming the rock.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-13