Abstract

Completing high-rate gas wells in areas where sand control is necessary ranks among the top challenges within the oil industry. High-velocity gas flowing through the downhole hardware combined with the abrasive nature of finer formation solids dislodged and produced can significantly shorten the completion life and have strong financial implications for the operator.

This paper describes the gravel-pack and filter-cake cleanup techniques applied to Mahogany MB-13, an offshore horizontal openhole completion and Trinidad's best producing well at the time of its production start-up. Optimizing these operations, along with correct well configuration and precise borehole placement in the reservoir structure, were fundamental to achieving high production rates.

The gravel placement method and the uniform coverage of the gravel-pack screens helped to deter the creation of high-velocity spots. This result allowed the operator to use higher drawdown pressure differentials without damaging the completion.

Aspects of the sand-control design, gravel-pack placement to ensure complete wellbore coverage, and the chemicals and procedures for filter-cake cleanup are analyzed in depth.

The new-generation sand-control service tools that were developed to preserve the optimum wellbore conditions from the end of drilling through the completion are also discussed.

Finally, production data from the MB-13 are presented to support the completion efficiency reported.

Introduction
Mahogany Field

The Mahogany field is located offshore Trinidad in the Columbus basin in 300 ft of water (Fig. 1). Although the field was discovered in the late 1960s, lack of demand for natural gas delayed its development. Exploratory drilling and delineation wells found additional gas and condensate accumulations in the mid 1990s.

Extensive information about the Mahogany field is available in the technical literature discussing wellbore geometry, completion architecture,1,2,3 and reservoir characterization.4,5 The field comprises over 30 hydrocarbon reservoirs (stacked and separated by 7 faults) with accumulations in excess of 2.6 Tcf of gas, 30 million bbl of condensate, and 20 million bbl of oil. The Mahogany field was generated during the Pleistocene period from shoreface sand deposited during the northeasterly progradation of the Orinoco Delta. It consists of intercalated sand deposits, coarsening upwards, interbedded with silts and shales. Two main gas compartments, the 20 and 21 Sand reservoirs, were identified through a 3D seismic survey in 1993. Fig. 2 details the cross-sectional structure of the field. The 20 Sand, where the horizontal section of the MB-13 well was placed, has typical permeabilities of 800 mD (up to 5,000 mD) in its upper section and 50 to 750 mD in the lower section. Rock porosity is in the 23% to 35% range.

Sand-Control Completions in the Mahogany Field

The potential for sand production was identified early in the Mahogany development, and several sand-control methods were utilized, from cased hole gravel pack, high-rate water pack (HRWP), and frac pack to openhole stand-alone screens and openhole gravel packing.

Waters et al.2 compared gravel-packed and HRWP wells with frac-packed offset wells in the Mahogany field and concluded that the improved contact area in the latter method resulted in a productivity index (PI) increase of 3.4- to 3.8-fold normalized for net pay and net perforated interval. Fracturing was, despite the production advantages, avoided in some wells because of a close oil/water contact.

An important piece of information presented by Waters is that the only other well producing hydrocarbons from the 20 Sand was a cased hole gravel-pack completion.

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