State-of-the-Art Well Stimulation
Authors: Holman George B. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

The intent of this article is not to review the state of The art in wellstimulation so much as to provide a vehicle for reducing the gap betweenresearch-oriented SPE papers and the needs of field operating engineers.Because of the volume and wealth of research-oriented papers it would not bepossible to give a creditable papers it would not be possible to give acreditable review in a short article; therefore, I hope to present a "roadmapor route" by which this gap can be reduced. Before discussing the "hows" ofstimulation, let's review the "whats." Obviously, stimulation is done toincrease profitability by increasing productivity. What makes stimulation work?The fundamental reasons for stimulation are impaired or low productivity due todamaged or low permeability. For the case of damage, its removal is morerelated to cleaning a filter bed than to actually increasing the near-wellborepermeability and is accomplished by either matrix acidizing or acid washing. Inthe case of low permeability, stimulation is accomplished by either acidfracturing or hydraulic fracturing with a sand-laden fluid; both result in anenlargement of the effective wellbore. The point is that stimulation isgenerally related to a restoration of permeability to its original state or anenhancement of permeability to its original state or an enhancement of flowarea (wellbore enlargement) by creating highly conductive fractures.

Acidizing. Since its first commercial use in 1932, hydrochloric acid hasremained the primary acid treating agent for oil wells because of itseffectiveness and relatively low cost. However, several other acids, includingformic, acetic, and hydrofluoric have been used for special applications suchas deep, hot wells or wellbore treatments in sandstone formations. Although thebasic chemistry of acid treating has been established for many years, theeconomics and effectiveness of the treatments are strongly dependent on localconditions in the wellbore and formation. A review of acidizing practice andtechnology was made by Krueger in 1973. A more complete and comprehensiveexpression of state of the art in acidizing is given in the SPE MonographAcidizing Fundamentals by Williams, Gidley, and Schechter. The followingsummation of that monograph reflects the need to provide a document whichserves as a basic reference for both field and research engineers. Stimulationtechniques have become increasingly complex and a better understanding of theindividual mechanisms that contribute to the overall treatment effectivenesshas evolved. Much of this development has been at the research level and is nottotally assimilated into general field practice. The SPE monograph was createdto serve as a basic reference for both field and research engineers interestedin developing a fundamental understanding of the acidizing process and ofprocedures for designing acid treatments to increase well productivity. Somechapters provide a comprehensive review of the science and technology thatserves as a basis for understanding the fundamentals of acid stimulation. Otherchapters are written for the field engineer and stress design procedures andthe proper selection of fluids and additives. Since my purpose here is tonarrow the gap between research-oriented SPE papers and the needs of fieldoperating engineers, I commend these authors for making my job simpler byproviding a current document on the state of the art in acidizing through 1979.To complete this presentation of the state of the art for acidizing, thefollowing comments add "what is new and what is needed." In the past, one ofthe main limitations of acid fracturing has been the difficulty of achievingdeep penetrating etched fractures. However, there is a new system which I feelwill improve etched fracture penetration. This system involves the use ofnitrogen to create foamed acid. Nitrogen has been used for many years as anoilfield tool. Volz discusses the uses of a gelled polymer solution withnitrogen to achieve deeper acid penetration because of the shielding effect ofthe foamed acid. penetration because of the shielding effect of the foamedacid. JPT