Successful Acid Stimulation in Limestone - Tobo-Monserrate Formation in Gigante Field - Colombia
Authors: Zheng Zhenguo , Jaramillo Oscar Julian , Patiño Jhon , Reina John Fredy , Pacheco Carlos Francisco 
Published: September 2021

Abstract
This paper presents the successful stimulation workflow case for a carbonate acidizing pilot project performed in Gigante field in the Matambo block of the Upper Magdalena Valley basin in Colombia. An adecuate diagnostic, laboratory testing, treatment fluid selection, on-site QAQC and placement technique selection was fundamental to obtain a successful design and an optimized application.
Gigante field production is mainly coming from Tetuan and Caballos units producing 32-degree API crude oil during several years. Specifically, in Gigante-2 ST well, oil production has declined until its commercial limit.
During a candidate wells review, it was identified a previous acid stimulation treatment performed in Tobo-Monserrate formation in Gigante-1 with very poor results in oil production –short production time in natural flow– giving to this zone a low potential as oil producer and it was not considered as a primary target zone. The well was completed in its main target at Tetuan formation leaving Tobo-Monserrate behind an intermediate 7 in casing with no future expectatives to produce.
After a reservoir evaluation of Tobo-Monserrate formation done in Gigante-2 ST well, it was selected as a candidate for an intermedia matrix stimulation job to evaluate the real potential of this formation in Matambo block. During this phase, reservoir samples were tested against different acid treatments in the laboratory.
A gelled HCL based acid was selected based on their laboratory testing performance to delay acid reaction –improving acid penetration– and having fluid loss control to enhance reservoir coverage.
The complete chemical formula was customized to match the oil-treatment compatibility. An organic solvents treatment was added to dissolve organic scale prior to the acidizing. Acid was deployed directly through a TCP string to optimatize the operational time and managing treatment rate according to the pressure behavior.
During the acid pumping, a pressure drop is observed and treatment rate was increased to generate rate diversion. Gigante-2 ST well came in production at natural flow reaching 502 BOPD and 105 MSCFD evaluated after 35 days of the stimulation job proving and adding important hydrocarbon reserves from Tobo-Monserrate formation. A post job evaluation using a specialized chemical stimulation simulator shows a significant skin removal. Measured treatment pressure and rate were matched with the simulated parameters resulting in −3.24 of skin value post acid stimulation having a productivity improvement factor of 4.35 and an average wormhole penetration estimated from 60 to 75 in into the reservoir. A correct diagnostic, reservoir understanding, design, laboratory testing, execution and post job evaluation was the right route to obtain a successful stimulation job in operational terms and production results.
This paper is intended as a guideline for stimulation jobs in future interventions where the exact reservoir mineralogy is unknown. It shows a step-by-step methodology, a customized acid formula and finally, stimulation results as well as recommendations and learned lessons.