Abstract

This paper presents a new process for development of a novel cementing composition to provide ductility and thermal stability to the cement sheath in thermal recovery wells subjected to high temperature cycling during ISC process. In this novel composition a carbon-containing natural mineral fiber and an aluminum silicate additive are introduced along with Portland cement to perform effectively in wellbores having conditions prone to stress-induced cement failure.

ONGC's heavy crude oil-fields in north Gujarat, India necessitate the inroduction of thermal recovery process for improving the oil recovery. However the durability of the cement sheath consisting of API Class G cement admixed with silica is endangering the success of EOR process. The developed composition with carbon-containing natural mineral fiber (graphite) and an aluminum silicate additive (metakaolin) because of its durability at high temperature thermal cycling (<450°C) and compatibality with of Portland cement is an ideal composition for application in thermal recovery wells. The formulated thermally stable cement slurry with graphite and metakaolin has no significant effect on compressive strength and liquid permeability at high temperature thermal cycling (<450°C), thereby making it suitable for placement against combustion zone.

The field evaluation of this cement system in ISC wells of Balol & Santhal fields of ONGC in Mehsana asset is under-way. The developed novel cement system will solve the problem of charging of the subsurface shallow layers because of cement sheath failured during the in-situ combustion process.

Introduction

The heavy oil narrow belt of North Gujarat covered an area of about 45 km2 is marked by the western boundary of Mehsana

horst and eastern boundary by Jotana field has in place oil reserves of 12–18° API with viscosity ranging between 60 cp to more than 550 cp. Primary recovery produces at most 15 –18 per cent of such in-place oil due to adverse mobility contrast between oil and water. This necessitated the need to identify an appropriate Enhanced Oil Recovery (EOR) technique, to recover more residual oil. The screening of EOR techniques by the Institute of Reservoir Studies (IRS) of ONGC favors the application of thermal recovery processes such as steam flooding and in-situ combustion which is based on the principle of improving oil-mobility by reducing the viscosity of oil by heating it within the reservoir. Based on encouraging laboratory results, the pilot project was started in 1990. The flue gas analysis indicated the high concentration of nitrogen (70%) and carbondioxde(14%) with low concentration of oxygen(around 0.2%), establish the utilization of air in combustion process. During the wet combustion phase the concentration of these gases persisted indicating sustenance of fire- front. In-situ combustion process was successfully commercialized in Balol and Santhal fields in mid- 1997 and, commercial scheme extended for Lanwa and Becheraji field1.

In Situ Combustion Process

The in-situ combustion recovery requires the injection of sufficient air to support and sustain the combustion front in the reservoirs. The high temperature combustion front proceeds slowly through the reservoir to the production wells. The reservoir temperature may normally fluctuated upto 300–450°C and the fire front may reach up to 650–800°C. The combustion drive is converted into wet combustion by the alternate injection of water along with air. Wet combustion offers the advantages over the dry combustion by higher oil recovery2. During in situ process periodical quenching combustion front by water causes temperature falls to normal as a result of which the cement behind the casing and the casing are subjected to thermal cycling.

The success of in-situ combustion process depends on the reservoir and geological considerations. Since in situ combustion is an inter-well drive process of good horizontal continuity and is critical for the success of the project. Lack of good sand continuity, fractures and joint trends may create preferential flow channels causes of failure of in-situ process. Gaps in formation overburden or leaky inter-zonal seals in stratified reservoirs can allow fluid to leak into overlying strata and reduce the effectiveness of the injectant.

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