Abstract

The primary importance of plugging to abandon a well is to prevent contamination of groundwater aquifers by surface water, oil or gas seepage, or brine formations below the groundwater aquifers. Cement grout is the present material used in plugging. Fly ash is known to have properties very similar to cement and can be produced with strength similar to cement. Presently, only about half of the fly ash produced by the various coal-fired power plants is used and the rest must be treated as a waste product and disposed of in landfill. Much of this fly ash is the higher lime content, more cementitious, Class C fly ash.

The present research has been funded by Integrated Petroleum Environmental Consortium (IPEC). This research reveals Class C fly ash can be retarded like cement when slurried and it can be pumped and placed like cement with coiled tubing instead of employing a rig. To ensure a comprehensive understanding fly ash grout characteristics as a cement material, the extensive experiments were performed. These involved chemical compositions, size distribution, compressive strength, thickening time, durability, and rheology of fly ash slurry. Effects of the various additives on the characteristics of fly ash slurries were investigated by the extensive experiments. Frictional pressure loss tests were also performed with field size equipment in order to apply coiled tubing technology in placing fly ash slurry to the downhole of wellbores. Fly ash formulations chosen as the best candidates were pumped through a 2000 ft reeled of 1 ½-in. tubing and a 20 ft straightened 1 ½-in. tubing to determine frictional pressure characteristics. This paper specifically describes a process developed by the Well Construction Technology Center at the University of Oklahoma to formulate the best fly ash grout. This research results provide a cheaper, environmentally friendly, and more economical method of plugging wells.

Introduction

The use of fly ash has been studied to a certain extent in the past, however, pursuit of its use was not of extreme importance due to availability, familiarity and cost of cement. Its' use as an additive only has been studied extensively, however, stand-alone cementation with fly-ash has not been studied due to regulations specifying cement grout as the material that must be used. Much of this fly ash is the higher lime content, more cementitious, Class C fly ash. It is important to highlight the fact that fly ash does not have any harmful material among its components, which could make it hostile to subsurface environment.1–3 Fly ashes or any of their components are not included as harmful materials in the Identification and Listing of Hazardous Waste from the Environmental Protection Agency (EPA).4

A fly ash grout formulation has been developed based on the fundamental research. The physical characteristics of this formulation have been verified and it is found that the formulation can be applied as a plugging material and can be a substitute for cement.

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