A modifier is a phrase that describes or gives detail about a subject. Usually, a modifier clause has as its subject the subject of the main clause.
Having reached total depth, the drilling crew started pulling out of the hole.
Here, the entity that “reached total depth” is “the drilling crew,” and such usage is correct.
A dangling modifier, however, modifies something other than the subject of the main clause, and such usage is incorrect.
Having reached total depth, the drilling mud was changed over to completions fluid.
Now, the drilling mud was not the entity that “reached total depth,” so the first part of the sentence is “dangling,” i.e., has no subject in the main clause.
Let me state it another way.
“Having reached” is a participle that expresses action, but the doer is not the subject of the sentence. Since the doer of the action expressed in the participle has not been clearly stated, the participial phrase is said to be a dangling modifier.
Here are several different ways to fix dangling modifiers:
A) Name the doer of the modifier action as the subject of the main clause.
Original: Having arrived late for the meeting, his cup of coffee had to wait.
Who arrived late? Certainly not his cup of coffee.
Revised: Having arrived late for the meeting, Peter decided his cup of coffee had to wait.
B) Change the phrase that dangles into a complete clause with its own subject or doer of the action.
Original: Without having the production data, it was difficult to do a history match.
Who didn’t have the production data? Certainly not “it,” but “it” was still difficult.
Revised: Because we didn’t have the production data, it was difficult to do a history match.
The phrase is now a complete introductory clause; it does not modify any other part of the sentence, so it is not considered “dangling.”
C. Combine the phrase and main clause into one.
To improve gas production, the well was stimulated using hydraulic fracturing.
What improved gas production? This sentence says that the well improved its own production. (Oh, if only it were so!)
Revised: The well was stimulated using hydraulic fracturing to improve gas production.
Two more examples of dangling modifiers and their revisions:
Dangling: After reading the Capex report, the budget was changed.
Who read the report? Not the budget!
Revised: After reading the Capex report, the project manager changed the budget.
Dangling: The well was a dry hole, not having done a 3D seismic survey.
Who didn’t do a seismic survey? Certainly not the well!
Revised: The well was a dry hole because management decided not to do a 3D seismic survey.
In short, always check to see that your modifier phrases have the correct subject.