As a result of less than expected productivity from some wells in the El Furrial field in Monagas State in Eastern Venezuela, a systematic study became necessary. The results of the study were aimed at determining formation damage prevention guidelines for existing and future wells, and effective treatment measures for damaged wells.

The joint study was undertaken by PDVSA Exploración & Producción in Maturin, Venezuela and Core Laboratories’ Reservoir Optimization Services Group (ROSG) in Houston to determine the existing formation damage mechanisms. The primary objective was to recommend non-damaging drilling, completion and stimulation programs in this troublesome reservoir system.

The study included the review of drilling, completion and stimulation records as well as log analysis and well test evaluation for productivity potential estimation. Laboratory analysis to investigate possible sources of formation damage, evaluation of drilling mud systems and the design of a stimulation recipe was also a major part of this study.

A description of the laboratory tests employed in the evaluation of the potential and degree of impairment from the different mechanisms is presented along with interpretations and scaling of laboratory data to field operating conditions for pilot application design.

The study shows that the dominant damage mechanism in this field is organic scaling and associated problems. The crude oil contained in the reservoirs of this field has gravity ranging from 20° to 30° API, bubble points from 2,000 to 4,500 psi, asphaltene contents from 2 weight percent (at the top of the reservoir) to 20 weight percent (at the bottom), and about 4 mole percent of CO2.

The laboratory study shows that the current average reservoir pressures for the pertinent formations are dangerously close to their respective asphaltene flocculation onset and damaging conditions.

On the basis of the laboratory studies, two field pilots were undertaken. One to evaluate the effectiveness of the stimulation recipe designed for the system and the other to test the effectiveness of the drilling mud formulation determined in the laboratory.

Removal of asphaltene from the reservoir matrix through the use of InSol A/WTM mixed in xylene was found to be effective as a dispersant/solvent as well as an inhibitor.

Well selection and pre-stimulation activities for the first pilot are presented along with the chronological summary of activities. The paper presents the post treatment evaluation, indicating that the increase in productivity is in the range of 250%. The longevity of the treatment was monitored and is also discussed.

The drilling pilot was based on a laboratory study which indicated that an invert emulsion mud system, with CaCO3 and barite as bridging agents, would effectively reduce the amount of filtrate lost to the formation and hence reduce the damage caused during drilling. Paramount to the effectiveness of the mud system is the distribution of the solid bridging agents required through the zones of interest. The base line distribution and envelope to maintain this distribution was determined from laboratory studies of pore size and structure.

The methodology used and some of the drawbacks encountered with taking the laboratory results to actual practice in the field are highlighted. Post drilling and completion results are summarized with comparisons made to other similar wells in the field that were drilled and completed before and after the pilot.

We show that the benefit of this integrated systematic approach is that it enabled the design of a drilling mud and stimulation program based on laboratory experiments rather than field trials.

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