Forward modeling of electric and electromagnetic resistive logs with piston and annular invasion types in pre-salt carbonate reservoirs of the Santos Basin, Brazil

Abstract The invasion of drilling fluid is a highly complex issue in well logging interpretation and is not well understood. Choosing the appropriate geoelectric model that provides responses to the real data, is critical for elucidating this process. For effective modeling, it is necessary to consider the resistivities of the fluids contained into the well and in the reservoir (washed zone) as well as the petrophysical properties of the geological formation (virgin zone). Traditionally, to analyze the resistivities of the virgin zone (Rt) and the invasion diameter (Di), a step profile or piston model is assumed, which is the simplest model of invasion. But This model does not consider the presence of a transition zone or the gravitational effects of fluids, which can give rise to another type of fluid distribution of fluids into the reservoir called annulus and affecting the electrical logging responses (noise) causing significant error in the geophysical interpretation. In this work, we simulated the responses of piston invasion and annulus models, using two algorithms for induction (ILD – deep and ILM – mean) and laterolog (LLD– deep and LLS – shallow) responses. For such purpose, we performed forward modeling using these algorithms based on the finite element technique for 2.5D symmetry. Finally, the real data obtained in carbonate formations of the Santos Basin, Brazil, were compared with the responses, showing a good fit.