Many horizontal wells have been drilled in low- to moderate-permeability formations and completed in a manner that assumed there would not be a need for a true "stimulation" treatment. This has been a widespread occurrence in both the U.S.A. and Canada, and to a lesser extent for other locations around the globe.
Completing the horizontal section either openhole or with a noncemented liner is economically attractive. However, in many low-permeability reservoirs, and some with moderate-permeability values, production has often been below expectations and sometimes commercially unfeasible, even following efforts to overcome near-wellbore damage problems. This is especially true when considering only completions in reservoirs other than fractured chalks. In more permeable reservoirs, the higher investment costs of underbalanced drilling have often allowed low-damage completions with good economic return. In lower permeability reservoirs, this approach has not often been successful.
Previous studies offer numerous success stories for stimulating cased/cemented horizontal completions in low-permeability reservoirs.1-8 To achieve proper stimulation with a technique such as fracture-acidizing or hydraulic fracturing with proppants, operators must incorporate an economical, effective method for controlling fluid or proppant placement. However, operators faced with limited producing potential need to reduce well costs, so they often consider openhole or simple liner completions a necessity. Unfortunately, few proven technologies exist for controlling fracture placement in noncemented wellbores. In some cases, high risk or very high cost is associated with these technologies, depending on reservoir conditions or the potential for production improvement.
In attempting to solve these problems, operators have used several approaches. Some have succeeded both mechanically and economically; some have only been mechanical successes, while others have fallen short on both considerations. To achieve significant long-term production improvement, conventional proppant-laden fracturing treatments, waterfracs (those using very little proppant), and fracture-acidizing of carbonate formations all need some type of control over fracture placement along the horizontal. This paper examines several approaches used in attempting to solve these problems and analyzes the reasons why each succeeded or failed mechanically and/or economically.