Effect of Areal Heterogeneity on Waterflood Performance Between Parallel Horizontal Wells
Authors: Ehlig-Economides C.A. , Laine R.E. , Oguntaee B. , Serpen E. 
Published: December 2007

Proposal

Advanced well architectures are widely considered for accelerating production. This work continues previous work on accelerating recovery via waterflood in application of what has been called p-mode production, in which the objective is to produce up to 50% of the oil in place in the drainage volume in less than 5 years. Previous studies considered only a homogeneous drainage volume. In this study, to evaluate the areal effect of heterogeneity in 2-D, many permeability fields corresponding to different standard deviation (or Dykstra- Parsons coefficient), and various horizontal maximum and minimum correlation lengths (or reservoir connectivity) were generated using geostatistical simulation techniques. Stochastic representation of the reservoir model offers the chance to observe reservoir performance under many equiprobable geological images or realizations. The effect of reservoir heterogeneity on the breakthrough oil recovery and on the pressure drop between the injection and production wells at 50% recovery has been investigated. The simulated cases exhibit a relationship between the breakthrough oil recovery and the watercut at 50% recovery.

Three permeability distributions are considered and the geometric mean value in all cases is kept constant. Fifteen sets of realizations were generated for five horizontal maximum correlation lengths and corresponding horizontal minimum correlation lengths. The realizations were then ranked by arithmetic mean permeability. High, medium, and low mean permeabilities were selected for fluid flow simulations that were then used to characterize effects on breakthrough oil recovery and pressure drop at 50% recovery. Results indicated that the maximum correlation length (chosen as perpendicular to flow) aids oil recovery at longer correlation lengths, and that increasing the minimum correlation length (chosen as parallel to flow) increasingly resembles channeling and reduces oil recovery. Greater permeability standard deviation caused larger pressure drop Results showed discernable trends that can be used to orient wells and adjust length and spacing based on expected value ranges for the heterogeneity parameters.

The heterogeneity parameters may be estimated in a given situation from actual seismic and log data as well as from expected value ranges associated with a particular analog geology. As such, the analysis presented in this paper can be used to design the well length and spacing in new developments based on data gathered from the appraisal process.

Introduction

The concept of p-mode production was described in the work by Guerithault, et al.1,2 The objective is to produce up to 50% of the oil in place in the drainage volume in less than 5 years. Instead of the waterflooding practice common in the Western hemisphere that commences only after primary production declines and targets a constant water injection rate or injection pressure, p-mode production targets a constant oil production rate that is ensured by injecting voidage-replacement water volumes from the beginning. In this work, the waterflood is conducted with two parallel horizontal wells, one an injector and one a producer. Because the reservoir pressure is maintained at the original value, a p-mode production well couplet does not disturb the surrounding reservoir or wells.

Refs. 1 and 2 provided a step-wise first order well construction design procedure that is summarized in the Appendix. The Guerithault work assumed a homogeneous anisotropic (kV kH) formation. A more recent study by Serpen,3,4 used the same configuration as Guerithault, et al., and introduced heterogeneity. Reference 4 describes variations in breakthrough oil recovery and pressure drop between the wells as a function of the heterogeneity parameters in the model. This work is summarized in the section below on the heterogeneous model.

The subject of this work is to apply what was reported in Ref. 4 to the design concept for a homogeneous formation so that heterogeneity can be taken into account in the design.