New HF Acid System Produces Significant Benefits in Nigerian Sandstones
Authors: Martin A.N. , Smith K.L. 
Published: December 2007

New HF Acid System Produces Significant Benefits in Nigerian Sandstones A.N. Martin, SPE, BJ Services Co. (UK) Ltd.; and K.L. Smith, SPE, BJ Services Co. (Nigeria) Ltd.

Abstract
This paper details the results of several matrix acidizing treatments carried out in Nigerian sandstones, using a new Hydrofluoric acid system. This system, which contains a phosphonic acid complex, reduces the reaction rate of HF acid on clays, whilst at the same time keeping the reaction products in solution. The acid system was previously described by Di Lullo and Rae in SPE 37015. The case histories included in this paper are treatments carried out on several formations in Nigeria. They include remedial treatments carried out on zones which had experienced detrimental results due to poorly-designed conventional HF acid stimulations.
The new HF acid system increases the effectiveness of the treatment in two main ways. Firstly, it forms a thin layer of an aluminium silicate phosphonate compound, which is resistant to HF acid, over the clay. This layer diverts the HF acid away from the clays, increasing the tendency to react with the silica. Quartz dissolution is also improved and acid penetration is greatly increased. This action also has the benefit of reducing the production of insoluble compounds formed as a result of the HF acid/clay reaction. The other main benefit of the system is that the phosphoric acid complex acts to chelate metallic ions, keeping previously insoluble reaction products in solution. Secondary benefits include reduced inhibitor loading and safer handling, as the pH of the fluid never approaches the low pH of the conventional systems.

Introduction
HF acid stimulations are performed regularly by several operators in Nigeria. Because of the limited information available on large numbers of these wells, procedures have developed on a "hit and miss" basis, until a successful methodology was obtained. In particular, there is often very little data on the mineralogy of the formations to be treated. However, procedures have developed empirically so that a reasonably successful stimulation result can be expected, simply by pumping a fluid system that has been developed by trial and error. Of course, there are major disadvantages to this. What do we do for a formation we have not treated before? What happens if we have a well with an unexpected, unusual mineralogy?
The consequence of these problems is that a regrettable proportion of the wells that are treated, experience an increase in damage. In an attempt to reduce this incidence of damage, to increase the effectiveness of successful treatments and to provide remedial treatments on wells that have been damaged by conventional procedures, the new system was introduced to Nigeria.

Background to Matrix Acidizing in Sandstones
Hydrofluoric Acid has been used to successfully stimulate many different reservoirs around the world, for a number of years. The HF is used primarily to remove damage caused whilst drilling the well - or put another way, the reduction of a significantly positive skin factor. However, for a number of different reasons, true stimulation - which results in a negative skin factor - is usually beyond the capabilities of even a well designed HF treatment. The main reason for this is the presence in the formation of clay particles, such as illite, smectite and chlorite.
Typically, clay particles have a much higher surface area to mass ratio than sandstone grains, usually in the range of hundreds or even thousands of times greater. This is illustrated in figures 1 and 2. As the conventional HF systems are pumped into the formation, the clay particles consume a correspondingly large proportion of the acid. This means that most of the acid is spent in reactions with the clays, rather than on attacking the sandstone grains, which would increase the pore throat diameters and the volume of the pore spaces, and this in turn would increase the porosity and permeability. This effect, coupled with the fact that HF acid is extremely reactive, means that the live HF acid system rarely gets more than a few inches from the wellbore.
P. 309^