Abstract
Some faults or boundaries have been recognized to be sealing to the migration of underground fluids. Other faults are indicated to be non-sealing or leaky to fluid flow by providing a path for fluid migration. Determining whether a fault or barrier is sealing or non-sealing and their effects in the subsurface is a major problem for the engineer in petroleum exploitation.
From the transient pressure analysis point of view, few publications have been referred to this issue. The determination of the non-sealing degree has been performed by type-curve matching which leads to a limited number of type curves and involves a trial-and-error procedure.
This paper presents a pressure transient analysis method to characterize a leaky boundary by identifying a fault or boundary scalable dimensionless conductivity parameter which goes from −1 for constant pressure fault/boundary to +1 for a fully sealing barrier. The model used here assumes that the mobility and diffusivity ratios of reservoirs on two sides of the barrier are the same. The interpretation method used is the Tiab's Direct Synthesis Technique4 which basically utilizes characteristic points obtained of intersection of various straight line portions of the pressure and pressure derivative curve, slopes and starting times of these straight lines. These points, slopes and times are then used with appropriate equations to solve directly for permeability, wellbore storage, skin, boundary distance and leakage degree.
A step-by-step procedure to determine distance from wellbore to the barrier and the boundary scalable dimensionless conductivity is presented and successfully tested by means of simulated examples. The scalable dimensionless conductivity provides a degree, or even percentage, of the leakage condition of a boundary or fault. This is determined from the ratio between the radial line and the second radial line. The scalable dimensionless conductivity can be used to estimate the value of boundary conductivity.