A common method used to design acid treatments on acid-soluble formations isbased upon the use of "reaction times" and fracture calculations. Two of thedifficulties involved in utilizing this technique are the variable reactionrates of acid under different well conditions and the problem of determiningleak-off coefficients.
One approach to controlling these factors is to prevent reaction with theformation and restrict leak-off until all the acid is in place. This can be accomplished by plating the formation fractures with a dispersible film priorto the introduction of acid to the fracture system. Laboratory data and field results using this approach have indicated favourable results.
Laboratory tests were made on several types of spearhead fluids and additives. This test information is shown and discussed in the way it pertains to the particular spearhead and acid chosen for use in the field.
Also discussed are the advantages and use of the spearhead film technique inthe placement of acid in a formation. It is expected that this approach will, in many cases, provide more reliable fracture calculation for more effective treatment design.
Hydrochloric acid has long been one of the prime materials used instimulating limestone and dolomite reservoirs. Of the many acid treatmentsdone, most can be conveniently divided into two basic classifications:
matrix acidizing, in which the acid flow is confined to the naturalpermeability and porosity of the formation; and
fracture acidizing, in which the primary avenue of acid travel is through natural or induced formationfractures.
This paper is concerned with the aspects of fracture acidizing.
In a well, a zone of local wellbore damage can be responsible for a severeloss of productivity. Once this is removed or by-passed, substantial penetration of the acid is required to give a productivity increase of two tothreefold over that obtainable from the untreated formation. Radial permeability improvement, as obtained with matrix acidizing, would require prohibitive quantities of treating fluids to reach such a distance from the well bore. It has long been recognized that more reasonable volumes are required if the acid is used to create and etch a highly conductive fracture to the desired depth in the formation.
