The well damage history in the fields operated by BP Colombia has showed an interesting performance in all producer and injector wells. The abnormal productivity decline detected since the start-up in 1991 was a clear evidence of the skin damage magnitude, in fact, the fields have not plateau history. The initial high oil and gas rates production added to strong geological structures in which a variety of fluids have been producing (critical volatile and gas condensate) have contributed to the development of a very complex damage scenario; main damage factors identified at early stage of production were fines migration and mineral scales (calcite, siderite, barite, celestite and iron compounds). Research regarding acid stimulation and mineral scale inhibition was the first step trying to mitigate the high oil rate decline. New damage mechanisms were detected as long as the fields were in production, water and condensate blockage, organic scales (wax and asphaltenes), iron sulfide scales, and wettablity changes were the main damage mechanisms detected. Engineering work, laboratory and geological studies, backflow analysis, third parties investigation and an extensive field trial program have been developed to determine the best stimulation program that should be applied in each one of producer/injector wells. Although high success removing formation damage has been obtained, it has been necessary to focus additional effort to find and test new technology addressed to increase both IIOR and stimulation lifetime.
Cusiana and Cupiagua fields constitute the SDLA association contract and are located in Colombian foothills - Colombia, South America. Mirador, Barco and Guadalupe are the producing formations with average properties summarized in tables 1 and 2. The recovery mechanism consists mainly of pressure sustainment and liquids vaporization through gas re-injection; water injection has been implemented for Barco formation in some areas of Cusiana field. As shown in fig 1, the formations in SDLA fields are unique in their porosity - permeability trends. These formations are characterized by very low porosities coupled with reasonable permeabilites. The low porosity of the formations means that the reservoirs have small pore throats that are very susceptible to damage by blockage (both fluid and particulate).
This paper is divided into two sections, first we briefly described how the main damage issues has been addressed historically; second, field cases are presented to illustrate the formulation and results of recent strategies aimed on damage control. Complementary information has been included in the annex section presenting a well ranking exercise that qualitatively estimates the degree of damage related to several mechanisms.
The following section summarizes how the stimulation strategy has been revisited through a detailed review of the primary data on formation damage, stimulation treatments and ongoing technology developments aimed at both improving and understanding the stimulation process; the stimulation with the highest level of impact will be described.
Completion Damage. Most of SDLA wells are 5' or 7' monobore type; invert emulsion mud (water in oil 20:80) plus calcium carbonate as weighting material is used for drilling while keeping overbalance pressures 1500 psi average. Cemented liner is set across producing intervals followed by underbalance perforating (2000 - 2600 psi UB, high penetration charges). In general, the extent of completion damage is variable depending on the presence of natural fractures and stability issues which can promote massive mud losses; normally, a varsol + HCl type fluid was used for mud cake dissolution, even though, fines destabilization problems took place in sensitive formations (Barco & Guadalupe) and fluid blockage (mud filtrate and cement filtrate) had not been treated effectively.
Currently, mitigation of completion damage is based on prevention and suitable contingency stimulation fluids; for the first, optimal mud particle size / pore size relationships has been achieved which has limited filtrate volumes through a very low K cake; on the other hand, diesel + alcohol followed by EDTA is being applied as stimulation contingency; mud´s calcium carbonate dissolution is being achieved without any collateral problems and fluid and emulsion blockages (caused by mud & cement filtrate) are being directly treated.