Abstract

This paper summarizes the systematic methodology & engineering process employed to identify and refine the highly effective fluid-train solutions used to drill, and install the highly productive, long horizontal gas well completions of the NCMA Hibiscus Project offshore Trinidad. It presents and discusses the unique fluids design, pre-project evaluation, and the integrated application efforts undertaken to:

  1. Minimize formation and completion damage; and

  2. Maximize gravel-pack placement and filtercake removal efficiencies.

The paper will identify important reservoir drilling and completion fluid service integration points (metrics), laboratory validation methods employed, and provide completion process details that led to the successful high-rate gas well installations in an unconsolidated sandstone reservoir.

Specific topics discussed will include: the design and implementation of: the optimized Reservoir Drilling Fluid (RDF), RDF to Completion Fluid displacement, gravelpacking process, and the filtercake removal treatment. Finally, the paper will present case histories of the five completions installed in the Hibiscus reservoir and provide comparisons of:

  1. RDF drilling performance,

  2. gravelpacking efficiency, and

  3. well performance (productivity) of a stand-alone screen completion versus the gravel packed wellscreen completions that employed the unique RDF and filtercake cleanup treatments.

Introduction
Project Background

The North Coast Marine Area (NCMA) lays North-northwest of Trinidad. (Figure 1) The original Hibiscus gas field development plan comprised a nine-well drilling campaign, utilizing a Minimum-Area Self-Elevating (MASE) rig aboard the Hibiscus platform. As a result of the high deliverability the first development phase was concluded after completing seven wells. Hibiscus was a "fast-track" gas development with the first completion to be installed less than 16 months after project sanction. First gas was required in July of 2002 at full contractual rates of 240 MMcfd/d. Initial well deliverability modeling predicted that a minimum of 4 horizontal wells would be required to meet this demand. The modeling suggested that the producing intervals in these wells would need to be 8.5" in diameter and between 1500 and 2000 ft in length to effectively exploit the reserves and meet the delivery requirements. The basis of design for the project imposed zero sand-production tolerance on the processing facilities. Although this permitted more economic design of the production facilities, it placed considerable emphasis on installing effective sand control.

Geological Description of the Reservoir

Reservoir formations in the Hibiscus field of the NCMA consist primarily of unconsolidated sand with interstitial clays and interbedded shales. The lithology of these formations is that of sandy, mixed and muddy turbidites. Extensive core and formation analyses including CAT-scan, XRD, granulometry, Hg porosimetry, and SEM micro structure were performed to characterize the rock and understand the potential damage mechanisms that might impact the target sands. This work indicated that the average clay & shale content ranges from approximately 7 to 10% with distribution being primarily intergranular, with occasional shale streaks. Porosity in these sands ranges from 10 to 21% with permeability from 10md to 1.0 Darcy. The average permeability is approximately 125 md. The net to gross for the Hibiscus wells was estimated to range from 75 to 100%. Granulometry of the prospective producing sands indicated that the uniformity coefficient (d40/d90) varied from 2.2 to as high as 16.4 with d90 values as low as 5μ.

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