Advanced Carbonates Interpretations

Carbonates present a major challenge in interpretations. Interpretations in carbonates are mistakenly viewed as simple because of the absence of shale. However, other variables make the interpretations far more complicated than for clastics formations. These variables include vug porosity and fractures, microporosity, and large variations in the size of the interparticle porosity. This results in large variations in the porosity exponent (m). (m) is controlled by the tortuosity of the current path which in turn is a function of pore and grain distributions. Of special note is that most of the vug porosity is connected to the interparticle porosity. This was concluded by viewing data from 100’s wells after waterflooding and filtrate invasions which show that the fluids are also swept from the vugs after the invasion. Microporosity behaves like shales in clastic formations by reducing the resistivity resulting in unusually high estimated water saturation. Unlike shale, microporosity cannot be detected directly from the GR log and from the density-neutron plot. Carbonates exhibit mixed wettability that defines the porosity exponent (n). Rock wettability changes with exposure to different fluids such as water flooding, filtrate invasion, and imbibition. The exponent (n) is a true chameleon and there is a big question mark regarding the accuracy of the core derived (n) as compared to the true values of (n) at reservoir conditions. Rock typing is an important tool to address porosity heterogeneity. Rock typing equations are based on capillary pressure evaluations or theory. They directly address the variations of pore throat radius (PTR) as a function of porosity and permeability. Rock typing provides a good platform for carbonate zonations and permeability estimations. All the above variables are discussed in this book and supported by field examples.