Hydroflouric Acid Stimulation Developments For Field Application
Authors: Smith C.F. , Ross W.M. , Hendrickson A.R. 
Published: December 2007

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Abstract
Use of hydrofluoric-hydrochloric acid mixtures in matrix treatments of sandstone reservoirs can provide economical and effective stimulation in many areas. Laboratory data of HF-HCl reactivity in many types of formations have been expanded and applied to actual field tests to obtain a correlation on which to base treatment recommendations. Knowledge gained has led to methods of determining the most efficient acid volume requirement for specific reservoir conditions and development of better techniques for uniform placement of acid in the formation.

Introduction
Matrix treatment of sandstone formations with Mud Acid has been the subject of continuing research and field development for the past two years. The use of this hydrofluoric-hydrochloric (HF-HCl) acid mixture in matrix treatment has been developed in two major areas:
a better understanding of the acid reaction capabilities, and development of better techniques for uniform placement in the porous structure of the formation.
An introduction of HF-HCl reactivity was published earlier and data from that paper is expanded in this one. In addition, this paper will report the results of a field research program which has led to improvements and new developments in matrix treatment of sandstone formations.

PRINCIPLES OF MATRIX TREATMENT
The purpose of matrix acidizing is to restore natural permeability through restoration of undamaged flow channels in the vicinity of the wellbore. This area of treatment is termed the "critical matrix," since the major pressure drop during production occurs here. Successful matrix acidizing presupposes that the reservoir has sufficient oil and/or gas and natural permeability at bottom-hole pressure to allow economic production provided the well is not damaged. Permeability may be reduced by any one or a combination of several types of damage:
Skin - Mud filter cake which clogs perforations or the face of the pay.
Induced - Interactions of filtrate from mud or cement with minerals in the "critical matrix."
Natural - Particle migration leading to"check valve effects"; natural deposits; water blocks or saturation changes which result from production.