Abstract

A well at an Underground Gas Storage location of NAM was identified to be impaired, this resulted in a significant reduction of well performance and jeopardized the integrity of the well, and as such, it classified as a stimulation candidate. However, an acid stimulation treatment in a business-critical environment such as the Grijpskerk Underground Gas Storage in the Netherlands (total peak capacity 2000 MM Scf/D), can only be executed when well and facility integrity are fully ensured.

Corrosion in wells and at production facilities due to acid stimulation treatments are being reduced by the use of appropriate corrosion inhibitors. However, due to the introduction of oxygen in the treatment fluids, for example during loading, transportation, mixing and pumping down a well, fear for corrosion still remains. In order to minimize the risk of corrosion by oxygen, a process was developed, in which all treatment fluids were prepared such that oxygen levels were considered essentially oxygen-free. The process involved the use of purging tanks, mixing the treatment fluids with nitrogen, the addition of oxygen scavengers and dilution of back-produced fluids.

The efficiency of acid treatments in pre-packed screens completions can be enhanced by the use of coiled tubing and the right choice of jetting method. Pre-packed screens generally do not resist high differential pressures, and therefore, the impact pressure of the jetting fluid on the screen needs to be kept low. At the same time, thorough treatment coverage onto the screen and into the pores of the pre-packed sand is desirable in order to achieve optimal stimulation results. A novel rotating/pulsating coiled tubing jetting tool was used that produces, apart from the non-damaging low impact pressure, hydraulic vibrations created by the pulsating effects of the jets. This pulsating effect dislodges insoluble fines in the pre-packed screen, and opens pore space normally unaffected by chemical action of the treatment fluid. The rotating nozzles ensure complete wellbore coverage.

This paper presents a case study of an oxygen-free acid stimulation treatment, performed in a well, completed with pre-packed screens, at the Grijpskerk Underground Storage facility in the Netherlands. Contributing to the success of the operation was the cross-discipline preparation, the selection of the optimum coiled tubing jetting tool, the investigation of impairment mechanism, selection of treatment recipe in combination with screen integrity tests, development of a process to create an essentially oxygen-free environment and spent-acid back production strategy. A dedicated production and test program was designed, for which chloride ions, oxygen and pH levels were monitored at the production facility against pre-set criteria.

Introduction

The Netherlands is home to underground gas storage wells that are used to inject and store gas during low-usage summer months, and produce the gas during peak delivery periods throughout the winter. To ensure that the contractual obligation to deliver adequate gas supplies during the peak period is met, the production and injection rate of one of its 7–5/8″ completed gas storage wells, GRK-47, in Grijpskerk, was to be restored from its impairment. In order to restore the well's capacity. a coiled tubing acid stimulation intervention was to be carried out.

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