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Abstract
Matrix acidizing has been a successful stimulation method to enhance production in carbonate reservoirs. One of the challenges is to distribute the acid evenly along the interest zone to achieve a maximum stimulation effect, due to the differences in porosity, permeability and degree of damage along the interest zone.

Well-S delineation gas well, located in Senoro Field Sulawesi-Indonesia, is drilled through a carbonate reservoir. It was decided to use an acid soluble diversion agent instead of conventional proppant. This diversion agent is an acid system that contains a polymer that crosslinks as the acid spends to near completion, with this crosslink then disappearing as the acid spends more completely over time. The non-crosslinked spent acid can then be flowed back from the formation with relative ease.

Acid solubility test for 15% HCl and crosslinked acid was performed to design the acid job and the result showed that Senoro carbonate has 90% solubility. This crosslinked acid serves as a diversion agent that will facilitate the acid to be evenly distributed along the interest zone. The well test result before stimulation was 13 MMSCFD gas with skin value of +3 (obtained from PBU test), while permeability was 4 md. The well was tested again after stimulation. Significant increased gas rate was observed while the skin damage decreased.

This paper describes the planning and implementation of crosslinked acid application as an effective diversion agent in long interval matrix acidizing stimulation. Furthermore, it explains the criteria to determine the efficiency of this application.

Introduction
Well-S located at Senoro Field, Senoro - Toili Block, Central Sulawesi, which operates under concession of JOB Pertamina - Medco E&P Tomori Sulawesi (JOB PMTS). Well-S is a delineation well drilled to confirm gas/condensate reserves at the southern part of Senoro - Toili structure

This well is a directional well with 7100-ft MD of total depth and 32o of maximum inclination and penetrates Senoro structure – anticline structure with a large gas trap. Reservoir target of this well is Minahaki formation which was successfully penetrated.

DST is used to test Minahaki formation with perforation length of 75-ft using TCP gun. Matrix acidizing was chosen to be the stimulation technique, but a quite long perforation length being an obvious challenge for stimulation. The well then being temporary abandoned after stimulated and re-tested for future development scenario.