Chemical Stimulation Experience on a Naturally Fractured Gas Condensate Reservoir - Field Case
Authors: Aguirre He , Bonilla Ricardo 
Published: December 2007

Abstract
Diverting technique is one of the most challenging factors on natural fractures reservoir stimulation. Results are not always as desired but detailed analysis of the results may reverse a negative outcome. Detrimental rate resulted after pumping high rate diverted chemical stimulation in a gas condensate reservoir. Lab studies and reservoir modeling determined that trapped fluids and increased condensate banking resulted from the chemical fluids combination. Methanol was then proved as a successful treatment to remove the damage. This field case was experienced in Liria YR4 (LRYR4) well, Cupiagua field, Colombia Eastern Cordillera Foothills. Cupiagua field produces gas condensate from the clastic low porosity quartz-arenite Mirador formation, where natural fractures play key role for wells productivity.
The well case is 836 ft reservoir length high angle well, which was completed on August 2001, testing 5600 bopd and large total skin (+116). On Nov. 2001, a chemical stimulation (Solvent/HCL) was pumped to reduce the observed large skin resulting on well productivity improvement. During 2002, welltesting showed continue oil rate increment in spite of 400 psi pressure depletion and reservoir pressure dropping below dew point with the corresponding Kh detriment due to condensate banking.
High skin was still measured advising room to improve the well performance. A second Solvent/HCl stimulation was pumped to maximize rate diversion including high viscosity gelled diesel as diverting agent, however the well lost production and skin increased. A single well, fully compositional, fine grid radial model was built in VIP to understand well and skin behavior. Blending the reservoir model, production history, PBU's and lab data analysis concur on identifying the failure cause. Methanol remedial job was executed to remove the undesirable rock-fluid interaction and successful results were then obtained reducing the skin and recovering the well productivity lost.

Introduction
Reduce the reservoir damage is critical to maintain wells productivity and recover the original estimated reserves. In most of the fields, life of the well is affected by chemical or physical factors that constrain production and diminished well potential. Several stimulation practices are implement to eliminate or reduce the damage targeted on enhancing productivity.
Factual approach and deep understanding of the mechanics that generate the damage are critical to select the right stimulation procedure and deliver the promised reservoir performance. Multidisciplinary teamwork is required for comprehensive reservoir evaluation; petroleum engineers, reservoir engineers and geoscientists should come out together in sensible models to explain reservoir behavior and identify possible solutions. Results presented in this paper are the outcome of petrotechnical disciplines integration in the understanding of the reservoir damage and successful chemical stimulation designed to improve well performance.
Cupiagua field is one of the giant Colombia's hydrocarbon accumulations. The field was discovered in 1992 but formal production started just in 1998. Cupiagua is an elongated compressive faulted anticline, about 25 km long by 3 km wide, (figure 1) which was formed when the Andes Cordillera uplifted during Paleogene times1. A series of inverted structures are developed in the Andian foothills, some of them have been proved important hydrocarbon accumulations, Cupiagua is one of those largest successfully proved structures. Surface terrain roughness and the complex geology are translated into complicated structural understanding (shortage seismic image definition) and well drilling cost as high as $40 million2. Unstable fissile shales, irregular borehole shape, abrasive formations and expensive holes are the common operational challenges to deal with when drilling the Andean foothills.