Sweep Efficiency Comparisons Of Horizontal And Vertical Wells
Authors: Claridge E.L. , Qui S. 
Published: March 2010

Abstract
The technology of horizontal well drilling, casing and cementing, logging and completion has developed rapidly, during the past five years. Theoretical relationships for calculating well productivity and for analyzing pressure transient tests are now available. It is well established that the productivity (or injectivity) of horizontal wells will be several times that of vertical wells in most cases. However, there has been very little published concerning comparisons of oil or gas recovery at an economic limit when horizontal wells are used instead of vertical wells.
In the study reported here, a series of typical reservoir simulations with light and heavy oils were used to make such comparisons. These include:a vertical bottom-water drive,a vertical gas cap drive,a tilted fault block edge-water drive,a tilted fault block gas cap drive,steam drive followed by water drive in a horizontal layered reservoir, andprimary recovery by solution gas drive in a reservoir model with four sealing faults, or alternatively,with four vertical high-conductivity fractures.
The models are idealized and do not represents any actual reservoirs. Computer simulation was used to make the comparisons, so that all conditions could be kept constant except for the changes which were made singly, thus, allowing strict comparability.
It was found that in most cases the differences in ultimate oil recovery for a given process at its economic limit between vertical and horizontal wells were small when the economic limit is determined by water cut or by gas-oil ratio, but is significantly better for horizontal wells if the economic limit is based on a minimum oil rate, regardless of cut or GOR; this is due to the rate advantage of horizontal wells. In practice only part of this additional ultimate oil recovery is likely to be realized. However, it is encouraging to see that the intuitive expectation of some improved recovery is borne out by these comparisons.

Introduction
A review of horizontal well technology was presented by Butler(1) at the 1988 Petroleum Society Meeting. He lists four situations where horizontal wells offer advantages over conventional (vertical) wells:reservoirs where conventional wells have low productivityreservoirs with vertical fracturesoil reservoirs where recovery is limited by water or gas coningthick continuous heavy oil and bitumen containing sands where steam-assisted gravity drainage is practical
Butler reviews drilling methods and relative productivity and cost of horizontal vs vertical wells, as well as the variety or circumstances under which horizontal wells appear to be most appropriate. He mentions certain matters of cementing and completion which are helpful in layered reservoirs, but does not discuss logging or pressure transient resting in horizontal wells. Sherrard, Brice and Wilkinson(2) do describe a pressure build-up test and difficulties in interpretation. Spreux, Louis, and Rocca(3) describe logging techniques for horizontal wells.
Most of the emphasis in Butler's paper is on the higher productivity of horizontal wells.