Advances in Matrix Stimulation Technology
Authors: Paccaloni Giovanni , Tambini Mauro 
Published: December 2007

Summary
This paper presents the results of extensive technical and statistical analyses of more than 650 matrix stimulation jobs. Seventy-eight failures were identified with absolute reliability by second stimulations successfully repeated on the same pay zones of the same wells. Only one significant parameter of the stimulation design differentiated the first failed stimulations from the second successful ones. This paper illustrates these failure/success factors and gives novel field-stimulation-response graphs.

Introduction
The only reliable means of measuring the effectiveness of a stimulation technique is to prove that the technical and economic objectives were reached. The most frequent goal of matrix stimulations is to optimize both productivity and recovery-in other words, to remove formation damage (s = 0) efficiently from the whole exposed pay, especially in multilayered reservoirs. The achievement of such an important objective should be checked by the combined use of evaluation tools, such as pressure-buildup analysis and flow meter and temperature surveys (more generally, production logs). When an operator can show negligible skin factor over all perforated intervals following a matrix stimulation job, the perforated intervals following a matrix stimulation job, the technique adopted (fluids, additives, and operating procedures) was undoubtedly the right one for the situation. Surprisingly, this logical statement is not yet fully assimilated; many new techniques and chemicals seldom are validated with reliable field data. The ratio of published theoretical and laboratory work to postjob evaluation is still exceptionally high. The field results presented in this paper have been used to validate or to confute a number of stimulation technologies and procedures. New ideas and techniques have also been tested in the, procedures. New ideas and techniques have also been tested in the, field; the increased success rate has compensated for the effort of carefully evaluating more than 650 jobs.

Statistical Survey
Table 1 shows the results of an extensive, in-depth analysis and statistical review of more than 650 matrix stimulation jobs over the past 11 years in 9 different countries. The table shows the well status (wildcat, appraisal, development, or water injection) and formation type (sandstone, limestone, or poorly consolidated sand). Of the 662 matrix stimulation jobs performed and evaluated, 82 (12 %) were well-documented failures; the apparent success rate was 88 % overall. The lowest success rate (78 % to 80 %) pertains to poorly consolidated sands often characterized by peculiar mineralogical compositions. The highest rate (91 %) was achieved for limestone/dolomite reservoirs.
Table 2 shows the main reasons for the failures. Surprisingly enough, 73% of the failures were imputed to incorrect field stimulation procedures, whereas 27% were attributed to poor design of stimulation-fluid types, volumes, additives, and sequences. This means that priority must be given to improved job supervision, real-time evaluation, and on-site quality control. Tables 3 through 5 list the reasons for failure, validated by a second successful stimulation (i.e.. a stimulation repeated on the same perforated interval of the same well where the First one failed) for sandstones, limestones, and poorly consolidated sands, respectively. Only one relevant parameter of the stimulation design differentiated the first failed stimulations from the second successful ones. The decision to absorb the additional costs for second stimulations was based on (1) the high technical and economic importance of the objective; (2) the fact that the first jobs were poorly supervised; and (3) our stimulation specialists' belief, poorly supervised; and (3) our stimulation specialists' belief, after careful analysis of the first job, that modified stimulation design and execution would have had a high probability of success. The fact that in 78 cases a second job was successful in the same reservoir and the same well where the first stimulation failed allowed us to identify the failure factors with absolute reliability. In most cases, the second successful stimulations were designed and supervised by our stimulation experts.