BOREHOLE GRAVITY AND GRADIOMETRY MODELLING – A FEASIBILITY STUDY

Abstract

Carbonate reservoirs are becoming the area of interest in the field of hydrocarbon exploration because of the huge extent of proven oil reserves. Carbonates are generally tight reservoirs and most of them inherently fractured. Working with fractured reservoirs needs a different approach when compared to depositional and diagenetic reservoirs. Carbonate rocks have low porosity and permeability. In carbonates, the migration of oil takes place through the fractures. Therefore, we need to be able to delineate the fractured rocks in carbonate reservoirs to extract the hydrocarbons. For identifying fractured reservoirs, we usually evaluate seismic data. However, for deep-seated anomalies, the signal gets attenuated, and the low frequency signal cannot provide us high-resolution images of the subsurface. By considering recent advances in tensorial gravity, we wish to study the feasibility of gravity gradiometry in boreholes to delineate the fractures serving as hydrocarbon reservoirs. For simulation purpose, we use the existing forward problem solutions to generate both the gravity and the gravity gradient responses of the sphere and rectangular parallelepiped. We propose the transformation matrix, which helps in generating the tensorial gravity responses for an arbitrarily oriented rectangular parallelepiped. We use these responses to create tensorial gravity logs at different boreholes with respect to the body and analyze them for body parameters. Accordingly, we report few such preliminary simulation studies. We have used Matlab software to generate the forward problem solutions and analyze the logs obtained from the boreholes. We integrated both the surface and vertical log profiles in the current study. We validated our software by considering few examples of simulated fractures to check the results achieved from the gravity gradient responses. Results achieved can be of practical importance in fracture characterization of carbonate reservoirs.