Acidizing and Completion Strategy for a Deepwater Subsea 'Completion
Authors: Blauch M.E. , Cooke J.C. , Cavender T.W. 
Published: October 2008

Subsea completions designed to extract reserves from "conventionally" marginal economic offshore reservoirs require effective prevention and removal of formation damage and present difficult decisions during initial well completion. Subsequent stimulation and workovers may be prohibitive and may not be operationally feasible in the future life of the well. In addition, maximizing production potential through effective preproduction acidization, gravel packing, and completion techniques can enable the conversion of otherwise unrecoverable reserves into recoverable reserves and positive cash flow.

This paper presents a case study describing the strategy and design basis for preproduction stimulation and completion decisions based on laboratory core evaluation, reservoir unconsolidated sand characterization, and optimization of stimulation fluid selection. This information is used to develop a systematic evaluation of formation damage prevention criteria based on the integration of well and laboratory data.

Treatment and completion processes discussed include 1) perforating/completion strategy with respect to gravel pack design, 2) fluid loss control, 3) sand diverted, pregravel-pack acidizing with chemically retarded HF acid, 4) slurry gravel-packing, 5) quality control, and 6) fluid/additive selection. Post-job calculated bottomhole treating pressure results derived from computer acquired surface data are also presented.

INTRODUCTION

Santa Fe's Garden Bamks Block 224 No. 5 is located in 745-ft of water, 140 miles offshore in the U.S. Gulf of Mexico south of the Sabine Pass area of Texas (Fig. 1). This is a subsea well completed in two gas-bearing unconsolidated sands. The subsea tree is tied back to High Island's A-384 production platform, 14.6 miles away in 358 ft of water, currently giving this well the longest subsea flowline in the U.S. Also a U.S. first is the installation of the subsea tree and layaway flowline which is discussed in more detail in OTC Paper 7004. Additional detail of this platform tie-in project development can be found in other recent publications.1-3 A single-string, dual gravel pack completion was determined by the operator to be the optimal method for extracting the estimated 30 Bcf of recoverable high quality gas reserves. Conventional development of the field was not economically feasible, leading to the decision by the operator and design team to elect the satellite field development strategy.

Optimization of a cost effective well completion was a primary objective for designing the well's completion scheme with emphasis on formation evaluation and prejob planning with regard to gravel packing and acidization decisions. Due to the minimal number of deeper water completions in the Gulf of Mexico, an historical basis had not been built for the deeper water regarding completion alternatives for improving long-term well economics and risk prevention. Nearby offset evaluation wells completed previously did not produce to potential because of prior formation damage problems, proving to the design team the importance of the precompletion design strategy. Conventionally, matrix stimulation decisions are made independent of the well completion and are typically performed only when a well is deemed "below expectations" following initial gravel pack completion and production.