Sand Control in an Ultra-Fine Sand Environment
Authors: McLeod N.J. 
Published: December 2007

Over the years, gravel packing has emerged as one of the most successful techniques in controlling the production of sand where formations are poorly consolidated. The technique is based on the successful placement of a homogeneous sand or gravel layer between the formation and a porous mechanical retainer (screens).

Key to the effectiveness of the technique is the selection of the gravel pack sand and its placement against the formation. However, the challenge of effectively controlling sand while maximizing hydrocarbon production is inversely proportional to the size of the formation sand grains.

The success of sand control in the Dolphin gas field (offshore Trinidad) has significant implications in the tolerance that can be accommodated by gravel packs when used in conjunction with the latest screens in ultra fine sand environments and it extends the accepted limits associated with particle bridging concepts.

The Dolphin gas field comprises a stacked sequence of sand and shale intervals where the sands are poorly consolidated and of ultra-fine composition (D50 grain size less than 10 m).

As a result, a program of work was undertaken to select and validate through laboratory testing, the effectiveness of a 40/60 gravel pack sand used in conjunction with Pall 'Stratapack' and Baker 'Excluder' screens.

In order to provide maximum productivity with minimum skin effects, open-hole, high-rate gravel packs were chosen as the preferred method of sand placement. Problems with screen placement in the first well dictated the need for uncemented pre-drilled liners in order to facilitate the transportation of the screens to the bottom of the well. Gravel packs in lengths from 200 – 1800 feet at well deviations ranging from 0-73 degrees were successfully conducted with placement volumes exceeding 85% of estimated hole volume. To date, 8 wells have been completed using this concept.

Productivity of each well has begun with skin values ranging from 10 – 25, however, the skin values have decreased with time eventually stabilizing below 10 within the first 60 days of production. Rates as high as 60 MMscf/d have been recorded from individual wells with no sand production and stabilized pressures.

Introduction

A number of gas fields have been discovered and are on production in an offshore area to the south and south-east of the island of Trinidad. The area is locally known as the East Coast Marine Areas or ECMA (Figure 1). Many of these fields exhibit common physical features with reservoirs comprising fine laminated sands which are often loosely consolidated in nature. The sands are geologically very young of Pleistocene age with deposition being linked to flood episodes from a major river system to the south west. The Dolphin field was discovered in 1976 under a partnership of Texaco and Tenneco. This was followed by a period of appraisal drilling around 1980 when three more wells were drilled. Tenneco's holding in the field was acquired by British Gas in 1989 who, together with Texaco, elected to embark on full scale development in 1993.

From this point in time, the development schedule was kept intentionally tight in order to achieve production start up by early 1996. As a consequence, this dictated that equipment procurement needed to commence no later than September 1995 thus compressing the duration of the design period.

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