Horizontal Wells: Still Appealing in Formations With Discontinuous Vertical Permeability Barriers?
Authors: Lion S.C. , Haldorsen H.H. , Manner Morten 
Published: December 2007

Summary
The appeal of horizontal wells often relies on increased productivitycompared to conventional vertical wells. A major productivity compared toconventional vertical wells. A major uncertainty is whether this productivityincrease is as great in formations containing discontinuous shales. With anumerical model study and a stochastic shale-modeling approach, the influenceof discontinuous shales on performance of vertical and horizontal wells wasstudied. We concluded that discontinuous shales cause a decrease in the Piratio between horizontal and vertical wells (compared with wells withoutshales) and that simulation studies are necessary to predict this decrease withan acceptable degree of accuracy. In a two-phase (oil/water) situation, weobserved that discontinuous shales increase oil recovery by decreasing watercut in both horizontal and vertical wells (compared -with wells withoutshales). Because of the large variability in the two-phase system results, long-term production testing may be the only means to obtain reliablepredevelopment data on the influence of discontinuous shales,

Introduction
The costs of a horizontal well rapidly approach the costs of a verticalwell; a horizontal well now typically costs only 1.2 to 1.5 times more per footdrilled. Thus, the expected and demonstrated productivity and coning benefitsof a horizontal well make it an increasingly attractive alternative toconventional vertical wells. Most North Sea sandstone reservoirs containsignificant vertical permeability barriers, such as shales. The shales affectthe reservoir transport properties because they may act as local no-flowbarriers within sand units or may subdivide the sands into separatehydrodynamic units. It is interesting, therefore, to investigate how thepresence of discontinuous permeability barriers will affect the appeal ofpresence of discontinuous permeability barriers will affect the appeal ofhorizontal wells compared with that of vertical wells, in terms ofproductivity. productivity. Analytical studies of horizontal-well productivityshow that the ratio of horizontal- to vertical-well productivity, J /J, decreases with decreases in the anisotropy ratio, k /k . From Fig. 1, it isapparent that the anisotropy ratio is of primary importance in screening theattractiveness of a horizontal well. Depending on the magnitude of thisparameter, a horizontal well may provide substantially increased productivitycompared with a provide substantially increased productivity compared with avertical well, or it may provide little or no increase in productivity, productivity, Where discontinuous vertical permeability barriers are presentit; the reservoir, two conceptually different strategies present it; thereservoir, two conceptually different strategies exist for estimation of theanisotropy ratio.Use of core plug measurements of k /k . This approachassumes that the influence of barriers on horizontal-well productivity isinsignificant. productivity is insignificant.Use of large-scale averagingfor the anisotropy ratio k /k.
This approach assumes that the barriersinfluence the horizontal-well productivity by reduction of effective verticalpermeability. The shales are assumed to be zero-thickness sheets permeability. The shales are assumed to be zero-thickness sheets that do not affect k. Notethat if a well has been drilled and tested, a large-scale (k /k), may beavailable directly from the test interpretation. In Fig. 1, where we assumethat k/k = 1 from core measurements, we see that Strategies I and 2 willpredict either excellent or poor horizontal-well productivity benefit. Thispaper investigates poor horizontal-well productivity benefit. This paperinvestigates which of the two strategies to use to screen horizontal wells or, alternatively, whether either strategy is valid. The effect of discontinuousshales on water-coning behavior also has been investigated. The shales areexpected to disturb the water-cone advancement toward a horizontal well. Ifthis action reduces total water production, it would reduce the need forwater-treatment capacity on platforms. This, again, will be important toconsider when field development solutions are evaluated.

Approach
The study was restricted to investigation of two different reservoirsituations: a singlephase oil reservoir and a two-phase oil/water reservoirwith a bottomwater drive. In the single-phase situation, both analyticalmethods and numerical simulations were used to estimate productivity ratiosbetween horizontal and vertical wells. In the twophase situation, the study wasrestricted to numerical simulations. There are basically two different ways torepresent discontinuous vertical permeability barriers in a simulation model:explicit inclusion of transmissibility modifiers at the interface betweengridblocks in the simulation grid and implicit representation of the barriersthrough reduced effective vertical permeability, k . These methods werecompared in this study. permeability, k . These methods were compared in thisstudy. The explicit method gave the "correct" results. A single set ofshale (statistical) data was assumed and used throughout the study. In thesingle-phase case, three different horizontal-well lengths were considered:100, 250, and 500 m [328, 820, and 1,640 ft]. In the two-phase case, only thetwo longest horizontal wells (250 and 500 m [820 and 1,640 ft]) wereconsidered. For all stochastic shale realizations, the mean shale length rangedfrom 100 to 150 m [328 to 492 ft], depending on the shale density for thespecific realization. This limited the variety of shale-length vs. well-length, L /L cases studied.