Design, Execution and Evaluation of Matrix Acid Stimulation Jobs using Chemical Diversion and Bullheading
Authors: Subero D. , Holder G. 
Published: December 2007

Design, Execution and Evaluation of Matrix Acid Stimulation Jobs using Chemical Diversion and Bullheading.

Abstract
A successful matrix acid stimulation job depends on various critical factors from the design of the treatment to the actual on - site job execution. Post job evaluation is also essential to learn from any shortcomings for the benefit of planning future jobs. The technical literature on the subject of matrix acid stimulation is very extensive. However, it is extremely difficult to locate material which describes the 'art' of the subject, rather than discussions on the science of separate aspects. This paper presents valuable information to practicing engineers on the salient features to be considered when designing, executing and evaluating a matrix acid stimulation job.
There has always been much debate on the relative merits of bullheading a matrix acid stimulation treatment with chemical diverters versus the placement of the acid treatment via coiled tubing. The authors agree that the acid design and job execution must be dictated by the particular 'well specific' situation. However, the authors' mega success with the bullheading technique provided the basis for this paper. The engineering techniques leading up to a successful matrix acid stimulation job by bullheading with chemical diverters are revealed from the authors' experience. An actual case example is illustrated together with engineering design techniques for the removal of various types of formation damage.

Introduction
Matrix acidizing is a stimulation technique involving the injection of an acid solution at pressures below the parting pressure of the formation. The objective of the exercise is to remove any formation damage which is determined to be present in the near wellbore area, thus restoring the well to its true productivity. The presence of formation damage could be determined by the separate or combined use of evaluation tools, such as pressure build analysis, flowmeter data and temperature surveys (more generally, production logs). The presence of formation damage could also be determined by analysis of a series of field data.
When injected into the formation, the acid dissolves the damage and some of the minerals present in the near wellbore area and hence restores all or a fraction of the original permeability. In limestone reservoirs, original permeability could be enhanced from acidizing. When compared to other oilfield operations such as hydraulic fracturing, significant failure rates have been reported for matrix stimulation applications. The only reliable means of measuring the effectiveness of a stimulation technique is to prove that the technical and economic objectives were achieved. The technical aspect deals with being able to reduce the skin factor (s, representing the formation damage) to a minimum value (ideal is s = 0) efficiently from the whole exposed pay, especially in multilayered reservoirs. When an operator can show negligible skin factor over all perforated intervals following a matrix stimulation job, the technique adopted (fluids, additives, and operating procedures) was undoubtedly the right one for the situation.
Historically an acid treatment was regarded as a relatively cheap operation. As a result, few efforts to improve acidizing technology were undertaken. However, the industry's position has significantly changed due to the ever increasing number of wells to be stimulated in general, partly from the need to optimize well performance.
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