Rock Physics and Reservoir Characterization through Surface Measurements

The Guest Lecture by Mr. N. K. Khatri on 31st August 2012 in PDPU on “Rock Physics and Reservoir Characterization through Surface Measurements” The discussions on rock properties, log and rock fluids interpretation.

The Density, Gamma ray, Compressibility, NHPI and Resistivity logs from a well show the dependence of Neutron logs on the Hydrogen Index of a Rock as well its variation with Rocks and formation fluids.

The Vs and Vp curve generally shows a linear trend but anomalies occur due to the presence of Hydrocarbon gases. With the progress of the discussion it was concluded that Hydrocarbon bearing oil and Gas formations show anomalies but water bearing sands do not show any deviations from this linearity. Thus an overview of the final interpretation done by the geophysicist sitting on the surface was witnessed by the students.

Mudrock line after deselecting points from porous and tight gas sands. Vp/Vs is in colour bar axis. Deselected points are shown in grey colour on log curve for corresponding interval 

Relations for the density of the rock was shown to be dependent on the liquid saturation as well as their independent densities. The importance of changes in Poisson’s ratio versus the water saturation was graphically portrayed. The students were introduced to the Backward Gassmann equation and the Woods equations which were used to determine the bulk porosity of a rock using parameters like Bulk modulus of formation fluids, their saturation and the porosity. A glimpse of software used to determine the Rock parameters and Fluid parameters was provided where students could identify the required input and output parameters.

                        Effect of fluid compressibility and porosity on AVO response

The velocity of P-waves is found to be higher in wet sand as compared to the Gas sand due to its higher density. A modification of the Akki-Richards equation was analyzed where the focus revolved around two parameters namely, Reflection Coefficient (A) and Gradient (B). The methods for finding the values of these parameters was described along with the usefulness of it to determine the presence of Gas, Oil or brine was shown using Graphical Analysis.