Abstract

In the North Field, Qatar's giant offshore gas field, effective matrix acid stimulation of the thick carbonate gas reservoir is considered essential for ensuring optimum long-term well productivity and hydrocarbon recovery. This paper describes the selection, design, execution, results and evaluation of the acid stimulation methods utilised by the Qatargas 3&4 project in its gas production wells. Bullheaded acid matrix treatments were designed and applied under the boundary conditions of cased-and-perforated completions, lithology variations, and a thick, layered reservoir with large permeability contrasts and possible presence of fractures and thief zones. Achieving an effective stimulation, as indicated by skin values and inflow profiles of the stimulated wells, depends on the ability to achieve acid diversion into all zones of interest. The paper presents how acid diversion was achieved through application of limited entry perforating combined with high-rate bullhead acid treatments pumping alternating stages of concentrated hydrochloric acid and in-situ viscosifying chemical diverter stages. As an alternative diversion method, bullhead acid treatments consisting of alternating stages of concentrated hydrochloric acid and in-situ viscosifying surfactant-based acid mixed with a degradable fibre material were utilised.

Introduction

The Qatargas 3 and Qatargas 4 projects were executed by the Qatargas 3&4 Joint Asset Development Team (JADT), staffed with secondees from Qatargas, ConocoPhillips, Shell, and direct hires, over the time period 2005-2010. Since early 2011, the LNG trains of both projects are on line. The two projects have been fully handed over to and are being operated by Qatargas Operating Company Limited. Combined with the LNG output of earlier projects Qatargas 1 and Qatargas 2, this makes Qatargas Operating Company Ltd the world's largest LNG company, with a production capacity of 42 million tonnes per annum of LNG.

The Upstream part of the QG3&4 project consisted of construction of the gas production wells located on three offshore production platforms. This paper presents the case history of how effective acid stimulation of the reservoir in the production wells was achieved. It describes the initial stimulation design process, and how the stimulation job design was adapted and improved in the execution phase through a continuous improvement loop of Design-Execute- Evaluate.

More information on the Qatargas 3&4 project and specific detail on the formation evaluation aspects of the Upstream part of it, are presented in Ref. [1].

The production gas wells of QG3&4 were designed as 7" monobore wells, with a cased-andperforated completion over the reservoir. The vast majority of the wells are deviated to some 45-55° in the reservoir section. In order to achiev e optimum well productivity and long-term well deliverability, acid stimulation of the target carbonate reservoir layers is an essential requirement.

The gas-bearing formation consists of alternating layers of limestone, dolomite and anhydrite. The gross target reservoir has a thickness of some 1,400 ft, consisting of 4 sub-units. A small part of the production wells are completed as producers on sub-units 1-3, or one sub-unit 4. However the majority of the wells are commingled producers on all four sub-units.

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