Rules of thumb appear in every profession. They are developed when practitioners observe a repeated cause and effect relationship. This paper presents some rules of thumb used by practitioners in well testing, examines their validity and limits and, in some cases, develops their theoretical basis. The paper is presented as a forum for discussion among well testing practitioners.
Rules of thumb evolve in every civilization and culture as humans experience and observe cause and effect relationships. Rules that don't work are discarded; rules that do work become part of the culture, tradition, practice or science.
Rules may be very general ("Well groomed people are more pleasant than poorly groomed people.") or quite specific ("Sands in this area with less than 5 % porosity are not productive.") Rules, as illustrated in the above examples, can also lull us into false security or even devastating error.
Rules, whether general or specific, are empirical and may be based on sound, if not well understood, physical, economic, social or other principles. They allow us to short cut some thinking processes and, in doing so, can cause us to make costly mistakes. As we grow more and more experienced, we personally develop and adopt new rules of thumb. It is important that we periodically, or at least initially, examine those rules to make sure that they are appropriate for each new situation.
In well testing we use and misuse various rules of thumb. In this discussion, we investigate some of the more common and even private rules to discover their theoretical justifications and the limits of their applicability. Contributors to this paper were invited to submit their favorite rules and to present the physical justification for their existence.
In reviewing rules of thumb, we will examine one from the world of medicine to avoid bringing discredit on the practice of petroleum engineering. This rule was applied with sometimes disastrous results.
The practice of "bleeding" to cure various ailments was widespread prior to the 20th century. Bleeding involves draining not insignificant quantities of blood from a sick patient. Today we laugh at the thought that physicians routinely applied bleeding to patients and are glad that the practice has been discontinued. How did the practice come about and why was it popular?
Contemporary experiments on laboratory animals show that bleeding reduces fever in sick animals. We can speculate that someone, at some time in the past, observed this effect and applied it successfully to reduce fever in humans. Having success, he "published" his results, probably by word of mouth, and others tried it. The practice got out of control when practitioners applied it outside of the narrow window of symptoms which it effectively addressed.
Today we don't bleed people, even to reduce fever. We have developed modern pharmaceutical methods for fever control. Perhaps these too will someday be regarded as misapplications.
