Field-Scale Simulation Of Horizontal Wells

Abstract

Abstract A technique for the efficient modelling of horizontal wells in reservoir simulation is presented. This technique treats the horizontal well as a second "porosity" as in the dual-porosity approach for naturally fractured reservoirs. The well "permeability" and "relative permeabilities" life adjusted to yield the pressure drop and phase slip predicted from multiphase flow correlations. Because the wellbore flow equations are cast into the same form as the reservoir flow equations, efficient techniques that have been developed for reservoir simulation can readily be applied. Examples illustrating the application of this method are given for the prediction of the performance of a horizontal well in a reservoir where water and gas coning are important. A validation of the model is provided by comparing oil rates and cumulative oil produced with results obtained from a line source-sink representation of the wellbore and comparing pressure drop and slip with results from a two-phase flow correlation for a case where the wellbore pressure drop is relatively small. Introduction Recent interest in horizontal wells has been rapidly accelerating because of improved drilling technology, and the inereased efficiency and economy of oil recovery operations. A recent report by Karisson and Bitto(1) indicated that since the early 1980s, there have been more than 700 horizontal wells drilled with approximately 200 of those in 1988. The improvement in recovery and economics obtained with a horizontal well results From the extensive contact with the reservoir. This results in lower fluid velocities around the wellbore, while providing total flow rates which are economic. Typical applications of a horizontal well include:Reservoirs where conventional wells have low productivity; the use of horizontal drilling can be viewed as a method for well stimulation.Reservoirs where recovery is limited by water coning or gas cusping. This occurs usually when a thin oil column is sandwiched between a gas cap and an aquifer. The use of horizontal wells in this case lowers the pressure gradient near the wellbore and therefore reduces the water coning and gas cusping tendencies while allowing an economical production rate.Reservoirs with vertical fractures. Horizontal drilling allows the intersection of many vertical Fractures which Form the main now paths in the reservoir(2).Heavy-oil and tar-sands reservoirs where steam-assisted gravity drainage (SAGD) is practical. The process consists of injecting steam into an upper well and using a lower horizontal well as a producer to collect the draining oil and condensate from the steam chamber. Because the drilling costs of horizontal wells are 1.4 to 4 times more than those of vertical wells, it is imperative to conduct a reservoir engineering study of the recovery economics of horizontal wells before drilling. A reservoir simulator with horizontal-well capabilities can provide guidance into the design of well lengths, locations, optimal flow rates to prevent water coning or gas cusping and can predict the increa5c in recovery over that of conventional wells.