Grid System Requirements in Numerical Modeling of Pressure-Transient Tests inHorizontal Wells

摘要

The common approach of using refined grids and small timernsteps usually does not provide accurate and efficient numericalrnmodels to simulate horizontal-well pressure-transient tests. Inrnmost cases, this approach requires impracticably longrncomputational times or creates artifacts on pressure andrnderivative responses that may be confused with therncharacteristics of certain flow regimes. The results of thisrnstudy indicate that the conventional well index should be usedrnwith log-distributed grid and time steps to obtain closer matchrnto the analytical solution at early times. There is, however, arnlimit to grid refinement beyond which the results do notrnimprove. This paper introduces the transient well index thatrnallows the use of larger grids while improving the accuracy ofrnsimulations especially at early times.rnIntroductionrnThe need for numerical simulation of transient pressurernbehavior of horizontal wells often arises in the interpretationrnof complex structures and calibrating the static data used inrnreservoir models. Several techniques1, 2 have been proposed inrnthe literature to improve the grid system requirements inrnmodeling pressure-transient responses of vertical wells. Thesernreferences have implied the extension of the same techniquesrnto horizontal wells but no conclusive information has beenrnpresented.rnWe first document the effect of grid size selection and thernrepresentation of horizontal wellbore within the grid blocks.rnNext, we present guidelines to determine the grid size andrndiscuss modifications of the Peaceman’s probe radius formula1rnto improve the simulation of horizontal well pressure-transientrnresponses. These techniques include the use of the appropriaternwell indices and transmissibility modifiers for transient flowrnin horizontal wells. The transient numerical well indexrn(TNWI) presented in this paper is shown to be essential tornreproduce the characteristics of the early-time pressuretransientrncharacteristics of horizontal wells. We demonstraternthat with the correct choice of the equivalent well grid radius,rnthe proposed techniques considerably improve the simulationrnof the early time responses without requiring significant gridrnrefinement. The techniques proposed can be easilyrnimplemented in the conventional simulation codes to simulaternboth short-term and long-term performances of horizontalrnwells.