High-Angle Gravel-Pack Completion Studies

Abstract

Summary A two-part study was conducted to define optimal gravel-pack procedures for some high-angle well completions in an area operated by Chevron U.S.A. Inc. In the first part of the study, gravel slurries were pumped through a 1,080-ft [329-m] tubing string to simulate actual slurry transport conditions in high-angle wells. The tubing string had an inclination of 80 deg. [1.4 rad] from vertical. Measurements were made to determine suitable viscosity and solids concentration for effective gravel transport. In the second part of the study, a frill-scale cased-hole completion model was constructed. Gravel slurries that had satisfactory transport performance were tested for packing characteristics in the model wellbore (completion interval). The need for special completion-interval geometry to obtain satisfactory packs was investigated. Results showed that high-viscosity carrier fluids (600 to 700 cp [0.6 to 0.7 Pas]) with high gravel concentrations (15 lbm/gal [1797 g/dm3]) provide good transport, but they are unsuitable for use in completion intervals in wells with angles of 80 deg. [1.4 rad] from vertical. Satisfactory transport and improved packing were achieved with lower carrier viscosity and concentration (300 to 400 cp, 4 lbm/gal [0.3 to 0.4 Pas, 479 g/dm ]). Special liner-tailpipe (washpipe) geometry considerations reported by previous investigators are required in conjunction with the optimal slurry properties defined in this study. Completion operations designed from results of this study have satisfactorily met general placement criteria. Field experience to date has been in wells with inclinations up to 80 deg. [1.4 rad] from vertical. Introduction The experimental study described in this report was designed to simulate actual high-angle gravel-pack completion operations scheduled for offshore wells in the U.S. gulf coast. Costs for the study were justified on the basis of high installation cost and the practical limitations of gravel-packing high-angle wells with then-existing techniques. The subject wells are generally directional and highly deviated, some with maximum deviation angles of 80 deg. [1.4 rad] from vertical. Our experience with high-angle wells such as these indicates that successful gravel-pack operations require special completion-design considerations. Previous studies, including one simulating large-diameter completions (11 in. [28 cm]), demonstrated placement problems as well as procedures required to overcome them and to place a gravel pack successfully in an 80 deg. [1.4-rad] wellbore. Tests were scaled to the dimensions of the actual wellbores because of closer tolerances (casing/liner) of the planned completions. Transport studies were made to identify problems in getting gravel to the completion zone. Three transport tests were performed in a 1,080-ft [329-m] tubing string, and five gravel-pack tests were run in an 80 deg. [1.4-rad] inclined, frill-scale, transparent 7-in. [17.8-cm] diameter model wellbore. The pumping tests were designed to show the effects of fluid viscosity and gravel concentration when they were pumped through a long, highly deviated tubing string. Pumping services were provided by oilfield service companies. The gravel-pack tests were run in a model wellbore having the same dimensions as downhole hardware commonly used by Chevron in one area of the gulf coast. The purpose of these tests was to determine the packing characteristics of the slurries studied in the long-tubing-string transport tests. The smaller casing size used to construct the model for this study distinguishes it from a previous study. All slurries were prepared with hydroxyethel cellulose (HEC) polymer. Low-, intermediate-, and high-viscosity carrier fluids up to 600 to 700 cp [0.6 to 0.7 Pa s] and gravel loads up to 15-lbm/gal [1797-g/dm ] concentration were studied. Use of such viscous fluids is state of the art for gravel packing. Because of their high carrying capacity, viscous carrier fluids result in shorter placement times. Primary job-design considerations for slurry packing include eliminating blank sections in the wellbore (leakoff through a liner or screen is required for pack dehydration). For somewhat vertical wells, a reserve of gravel above the perforations or open hole is required to accommodate after-pack settling, a characteristic of slurry packing. Routine success associated with slurry packing vertical and near-vertical wellbores is not obtained in highly deviated wellbores. Premature sandouts and after-pack settling often cause completion failure in deviated wells. Process variables available for overcoming these problems include carrier viscosity and liner-tailpipe annulus restriction. JPT P. 69^