Improved Acid Treatments By Use Of The Spearhead Film Technique
Authors: Davis J.J. , Mancillas G. , Melnyk J.D. 
Published: December 2007

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Abstract
A new treating procedure has been developed which uses additives in a spearhead fluid ahead of the acid treatment to create new fractures and to form an acid-dispersible seal on all exposed formation surfaces. The specially compounded additives are only slowly dispersible in acid and their presence on the exposed formation and fracture surfaces inhibits the chemical reaction until after the entire treatment volume has entered the formation. Additives used with the spearhead technique improve the over-all efficiency of an acid treatment.
This paper discusses laboratory development, mechanics of job performance and the results of a number of jobs involving the use of the spearhead technique. This new approach to acid treatment design and performance is a big step forward in the search for better methods.

Introduction
One of the commonly used approaches to acid treatment design is based on the reaction time of the acid with the formation. After the elapse of this reaction time, the extended formation fracture will receive only spent acid and only those fractures relatively near the wellbore will be enlarged. [Fig. 1] The needed volume of acid cannot be accurately predetermined because there is no general agreement as to the correct laboratory reaction time testing procedures.
A method similar to fracturing treatment design is also being used to design acid treatments for optimum formation penetration. The major problem encountered with this method is the difficulty in determining accurate constant leak-off coefficients.
Fluid loss control additives added to the acid soon lose their effectiveness because fresh acid is constantly leaking off past the fluid-loss particles that are blocking the formation permeability. As a result, the formation around the particles is eroded and the effectiveness of the fluid-loss additive is reduced or completely eliminated.
If the chemical reaction of the acid with the formation could be prevented until all of the acid were in place, the treatment could be designed as though the acid were a normal non-reacting fracturing fluid. Plating the formation fracture face with a protective film prior to the introduction of the acid into the formation would allow the use of normal fracturing treatment design calculations. [Figs. 2 and 3]