One of the toughest challenges to improving carbonate stimulation through acidizing has been the violent rate of reaction between hydrochloric acid (HCl) and limestone. Much of the research work devoted to carbonate acidizing has always been targeted at decreasing the acid spending rate, there by increasing the depth of penetration by the live acid. No economically successful technique has been discovered that will alter the reaction rate of hydrochloric acid in contact with limestone, so alternative methods have been devised. Many of these involve establishing a barrier between the acid and the rock that will interfere with their ability to make contact. One of the most successful techniques is to introduce the acid into the formation as the internal phase of an oil-outside emulsion. While this approach has no effect on the acid-rock reaction rate, it is very effective at delaying the acid's contact with the rock. The strong interfacial tensions, created by the surfactant phase of the emulsion, must be overcome in order for the acid to react with the rock.
A significant improvement in emulsified acid system has recently been developed and proved successful in matrix carbonate treatments. The Improvement involves the introduction of a second internal phase that results in a further delay in the acid's ability to contact the formation walls. Preliminary laboratory Investigation indicated the technical and practical feasibilities of the tri-phase system, and subsequent field applications proved its superiority over even the highly successful conventional emulsified system.
The objectives of matrix stimulation vary significantly depending on the type of producing formation being treated. In sandstone formations, only a very small portion of the mineral content is soluble in acids or other commonly used solvents. Matrix treatments under such conditions can do very little in the way of "stimulation". Their economic success Is dependent upon the removal or prevention of permeability damage in the formation around the wellbore. permeability damage in the formation around the wellbore. Matrix treatments should only be considered for sandstone zones when the formation's natural permeability is capable of providing profitable production levels, but deliverability has been reduced by damage. Since the formation is virtually insoluble, little improvement over natural, undamaged productivity can be achieved.
In carbonate formations, however, acids can dissolve a very large portion of the mineral content. This means porosity can be significantly increased, so the portion of the formation contacted by acid will portion of the formation contacted by acid will exhibit higher permeability after a matrix acid treatment. Unlike sandstone applications, matrix acidizing can result in true "stimulation" of production from carbonates. A well producing from a limestone pay should produce at a level higher than its natural, undamaged capacity after a successful matrix acid job.
Designing the optimum matrix treatment is difficult in the case of either sandstone or carbonate, but for different reasons. In a sandstone, the difficulty lies in determining the type and extent of damage present. Since natural productivity cannot be present. Since natural productivity cannot be significantly stimulated by the job, the volumes and types of solvents used should be dependent only upon the damaging materials. Since damage can result from several sources, caution is required to insure that the treating fluid is compatible with all potential downhole conditions. With a carbonate, the difficulty lies in deciding what volume of acid is best. Hydrochloric acid is normally the preferred solvent, but determining the optimum radius of acidization is not a simple process.