Modeling formation resistivity changes during Leak-Off Tests (LOT’s)

Leak-Off-Tests (LOT’s) are performed shortly after drilling into the new formation below a cased interval in order to determine the strength of the formation below the cased interval. This helps determine the upper limit of mud weight that can be used safely while drilling the next section, without risking formation breakdown and lost circulation. During LOT, well is shut in, and drilling fluid is pumped into the wellbore by a surface pump. As wellbore pressure increases due to pumping, the entire wellbore system including the formation first responds by expanding. When the wellbore pressure goes beyond a critical value called Leak-Off Pressure (LOP), drilling fluid starts to leak into the formation in the open hole section, first through porous flow and then, through fractures that are induced at the wellbore due to high pressure. This leakage of drilling fluid from the wellbore into the formation along with formation deformation can cause many changes in formation resistivity. Typically, formation resistivity depends on formation water saturation and salinity, and porosity through Archie’s equation. Hence, any change in resistivity can be modeled by modeling deformation and invasion during LOT. In this study, a poro-elastic model has been developed to investigate the resistivity change around wellbore during LOT by coupling the effects of deformation and invasion that occur as pressure builds up in the wellbore. Having a model to obtain resistivity around the wellbore during LOT is a prerequisite to predicting the resistivity tool response of a given tool during LOT. By predicting resistivity response during LOT and matching with measured field data, important properties of the formation like permeability and compressibility can be determined. The model developed assumes porous flow around the wellbore to determine the true formation resistivity during initial LOT pressure buildup by coupling the effects of deformation and invasion manifested in the formation in terms of change in formation porosity, water salinity, and water saturation. The results show that the effects of deformation on formation resistivity are relatively small, and while salinity mostly controls formation resistivity in a completely water-saturated zone, water saturation becomes a more important variable in an arbitrarily saturated zone with changing water saturation.