ABSTRACT

A non-acidic multifunctional iron sulfide scale dissolver has been developed as one of the efficient and cost-effective alternatives for removing iron sulfide dominated scale deposited both in the downhole tubular and near wellbore region. Compared with three typical conventional iron sulfide scale dissolvers (THPS, pH neutralized chelate and traditional chelate), this newly developed iron sulfide dissolver showed significantly better performance on pyrrhotite, calcite and field iron sulfide scales, and much less corrosive at high temperature. Different with the traditional HCl acid based iron sulfide scale dissolver, this new dissolver is non-corrosive, no H2S generation and iron sulfide re-precipitation problems, which is significantly improve the efficient and safe operations. In addition, the new developed iron sulfide dissolver showed multifunctional characteristic of well stimulation, addressed and confirmed by dynamic coreflood evaluation and CT scan technique.

This paper introduces a new developed non-acid iron sulfide scale dissolver with multi-function of well stimulation, and gives a guideline of selection criteria and procedure for iron sulfide dissolver for high temperature application. It can be applied to iron sulfide dissolution in both sour oil and gas wells.

INTRODUCTION

Iron sulfide scale deposition is one of the problems experienced in sour gas wells in the last few decades. It is deposited in downhole tubulars and topside pipelines, and causes operational problems, such as restriction of downhole surveillance and intervention, and significant losses in gas production.1 A typical iron sulfide scale deposited in downhole and topside tubulars is shown in Figure 1.2

Iron sulfide scale formation differs widely from well to well, in terms of severity and chemical composition; it also varies with the depth in the same well.2 Most deposits are a mixture of different types of minerals. One of the typical scale compositions in these sour gas wells is shown in Figure 2.

Mechanical de-scaling treatment has been used in the past to remove downhole iron sulfide scale in these wells.3-5 Mechanical remediation by way of jetting is possible if the affected areas contain a mineral scale build up that are readily accessible. Pipework with more tenacious deposits may require more aggressive milling operations. The mechanical descaling, though can effectively remove the iron sulfide scale deposited in the tubing, is time consuming and costly incurring costs in terms of deferred oil production and equipment rental. Also, it cannot address the potential scale deposited in the near wellbore region.

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