Factors Affecting Water Breakthrough Time of a Vertical Well Subject to Double-Edge Water Drive Mechanism

Abstract

Water production in an oil well is undesirable both economically and environmentally. Therefore water arrival pattern into a well has to be understood clearly where a reservoir is surrounded by an aquifer. However, predicting the breakthrough time of water in a producing well has been a controversial issue and a major challenge to petroleum engineers. This paper investigates water arrival pattern into a vertical well completed in reservoir with double-edge water external lateral boundaries to be able to understand important factors affecting water breakthrough. Water breakthrough time is the time it takes for the first droplet of water cut to reach the wellbore under the prevailing production rate regime. This marks the end of the production of clean oil from the well. Once the breakthrough time is approached, depending on the strength of the external boundary (supplying aquifer) the pressure responses in the reservoir first attain a steady state condition. Further production then leads to steady rise in produced water-oil ratio. Real time dimensionless pressure distribution, using appropriate source functions for a vertical well in a reservoir with double-edge water as its lateral external boundaries was utilized for the study. Well responses were computed for a wide range of reservoir, wellbore and fluid properties. Results show that water breakthrough time is strongly affected by the lateral distance of the external edge water from the perforations according to the relation tDx2eD/π2(kh/ kv). Not too far edge water would yield earlier breakthrough than far away edge water for a well completed in an isotropic reservoir. In a largely horizontally anisotropic reservoir therefore, water breakthrough may be delayed if the water from the edges respond unequally to a production transient in the well. For a reservoir with reasonable horizontal isotropy, only central well location may delay water breakthrough. If this does not yield the desired delay, then the perforation should be located farther vertically above the points opposite to the pay zone.