One
One of the most controversial problems in formation
evaluation is the shale effect in reservoir rocks. An accurate determination of
formation porosity and fluid saturation is subjected to many uncertain
parameters, all are induced by the existence of shale in pay formation.
Aside from shale effects on porosity and permeability, the electrical properties of reservoir rocks, consequently their fluid saturation are sensitively affected by the existence of shale. The way shaliness affects log responses depends on the proportion of shale, the physical properties of shale, and the way it is distributed in the host layer.
Shaly material can be distributed in the host layer in three ways :
- Laminar: Thin streaks of clay deposited between units of reservoir rock. They do not change the effective porosity, the water saturation or the horizontal permeability of the reservoir layer, but destroy vertical permeability between reservoir layers.
- Structural: clay particles constitute part of the rock matrix, and are distributed within it. It has similar general properties to laminar clays, as they have been subjected to the same constraints. However, they behave more like dispersed clays in respect of their permeability and resistivity properties.
- Dispersed: clay in the open spaces between the grains of the clastic matrix. The permeability is significantly reduced because clays occupy the pore space and the water wetness of clays is generally higher than that of quartz. The result is an increased water saturation and a decreased fluid mobility.
There are two types of shale:
- Effective shale ( montmorillonite and bentonite ) : has significant CEC (cation exchangecapacities), and can be identified by most of the shale indicator tools.
- Passive shale ( kaolinite and chlorite) : has essentially zero CEC, and recognized only by neutron tool.
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable.
Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. Mudstones, on the other hand, are similar in composition but do not show the fissility.