Horizontal Wells: Successes And Failures
Authors: Joshi S.D. 
Published: March 2010

Introduction
To date, about 6500 horizontal wells have been drilled around the world in various field applications. In Canada alone, about 750 horizontal wells were drilled from 1988 until 1992. In 1993, about 700 horizontal wells were estimated to be drilled in Canada(1,2). Thus, a large amount of data exists to review the field performance of horizontal wells.
The objective of this article is to briefly review the field applications of horizontal wells under various reservoir conditions, especially for Canadian reservoirs. A review of economically successful and unsuccessful wells is used to develop a list of important parameters that need to be considered for selecting commercially successful applications of horizontal wells.

Field Applications
To date, horizontal wells have been used in naturally fractured reservoirs to enhance well productivity by intersecting natural fractures. Moreover, horizontal wells have also been used in oil reservoirs to minimize water and gas coning, and in gas reservoirs (with bottom water) to minimize water production. Additionally, in Canada alone, over five hundred horizontal wells have been drilled in high permeability, heavy oil reservoirs. Horizontal wells have also been used in low permeability gas reservoirs to enhance drainage volume and therefore, to enhance cumulative gas production per well, so as to make them economic. Horizontal wells have also been used in thermal oil recovery(3), not only to produce highly viscous bitumen in the Cold Lake area(4), but also to produce heavy oil in the Tangleflags North area(5). Other applications of horizontal wells include miscible flooding in the Keg River and Pembina Nisku reef reservoirs to produce a thin oil zone sandwiched between the top solvent zone and the bottom water zone(6,7). Recently, horizontal wells have also been used in waterflood applications. Relative to oil applications, only a few gas production wells are drilled horizontally, probably due to low gas prices over the past few years. Recently, in Canada, horizontal wells have also been used in gas storage reservoirs to enhance short term productivity to meet peak gas demand.
The main applications in Canada are significantly different from those in the U.S. In the U.S., out of about 4500 wells, over 2500 horizontal wells were drilled in the low permeability Austin Chalk formation and about 200 wells have been drilled in the fractured Bakken Shale formation. The objective here was to intersect natural fractures and improve well productivity. In the U.S., except in Alaska, California, and the Gulf-coast, very few horizontal wells are drilled for water and gas coning applications. Thus, the typical U.S. application is for a low permeability, naturally fractured formation.
In contrast the majority of Canadian applications are for water or gas coning situations. Almost 45% of the Canadian wells to date have been drilled in heavy oil fields in Saskatchewan and Alberta. About 40% of the wells have been drilled in medium to light oil fractured carbonate reservoirs in Southeastern Saskatchewan. Several heavy oil pools with bottom water zones (which could not be produced economically using vertical wells) have been made economic by drilling horizontal wells.