Evaluation of Control Techniques for Unconsolidated Silty Sands
Authors: Holman G.B. 
Published: December 2007

In a laboratory study of sand-control techniques for unconsolidated silty sands in high-capacity wells, wellbore simulations were used to model a typical completion in the offshore Teak field. Among other results, it was found that the optimum control system was an open-hole gravel pack or a triple-wrap screen when used in open hole.

Introduction
Amoco Trinidad produces from three offshore fields near Trinidad in the West Indies. As shown in Fig. 1, these are the Samoan, Teak, and Poui fields. These fields produce from Miocene-age sands and contain both oil and gas wells. Although sand production exists, the Samoan and Poui fields do not have the same degree of sand-control problems as does the Teak field. After discovery of the Teak field, it became apparent that some type of sand control would be necessary; the two types of control chosen were triple-wrap screens and gravel packing.
Initially, wells completed with the triple-wrap screen were capable of high producing rates; therefore, emphasis was placed on this type of installation. At the same time, wells that were completed with gravel packs were performing poorly. Although attempts were being made to stop sand production down hole and the triple-wrap screens appeared to be working, trace amounts of sand were continually being produced from all wells. These traces of sand were sufficient to erode choke manifolds and flowlines and to fill up onshore and offshore separators. A triple-wrap screen was pulled after only 21 days on production. This screen, as pulled after only 21 days on production. This screen, as shown by Fig. 2, had multiple eroded holes. Shortly thereafter, a gravel-pack screen that was pulled after less than 2 weeks on production had similar eroded holes. Subsequently, most wells pulled had holes eroded in the screens. At this time, a project was initiated to determine the optimum sand-control technique for these operations. Using specially designed wellbore simulators, tests were run to evaluate conventional gravel packs, to determine the effectiveness of prepacking, and to compare the triple-wrap screen with prepacking, and to compare the triple-wrap screen with the gravel-pack system in either cased or open-hole completions.
Project Objectives Project Objectives Our basic approach to this sand-control problem had been to stop the migration of sand down hole by installation of either the conventional gravel pack or triple-wrap screen. Neither system was completely successful because of erosion of the screens, chokes, flowlines, and valves by produced sand. Disposal of the produced sand was also a problem. The primary purpose of the project was to find a way to stop sand production and project was to find a way to stop sand production and maintain high productive capacity.
The objectives of this project were to (1) evaluate a conventional gravel pack, where gravel is placed only in the annulus between the screen and casing; (2) evaluate the effectiveness of prepacking or placing gravel outside the casing in conjunction with a gravel pack; (3) compare the triple-wrap screen with gravel pack; (3) compare the triple-wrap screen with gravel packing to determine which would better retain sand packing to determine which would better retain sand and allow maximum productivity; and (4) compare open-hole and perforated-casing completions with gravel packs and triple-wrap screens.
Experimental Procedures
Apparatus - Wellbore Simulators
JPT
P. 979