Abstract

Till the recent times, covering the entire hydrocarbon bearing zones and simultaneously attaining quality cement bondage was a practical operational problem in the low fracture gradient and depleted fields of ONGC as the conventional lightweight slurries required for such conditions have low compressive strength, high porosity and high permeability which limit their application. Finding a feasible solution by developing lightweight cement slurry having adequate compressive strength was being investigated at the Institute of Drilling Technology (IDT) in ONGC for the past couple of years. Extensive studies performed culminated in the development of lightweight cement slurry systems for application in low fracture gradient and depleted formations to eliminate the losses during cementing and subsequent zonal isolation problem.

Formulated slurries inspite of low density have inherent properties of high compressive strength, low fluid loss, improved rheology, high stability, exceedingly low permeability and gas tight property. Field implementation results confirmed raising of long cement column without any losses and effectively encountering the perennial problem of cementing casings in low fracture gradient wells of Mumbai High in western offshore and CBM fields in India. A comparison of cementation with conventional and lightweight slurry system in the same field revealed better results with the latter.

This paper established the significance of application of these innovative lightweight slurry systems in terms of better well completion, reduced cost, improved productivity and less safety/environmental hazards in cementing across highly depleted zones and weaker formations. The details of the development, case studies of successful field implementation alongwith conclusions and recommendations derived from this endeavour are presented in this paper.

Introduction

Casing cementation of wells in depleted reservoirs & low fracture gradient formations is a challenging operation. Lightweight cement slurry is required for cementation in such wells to reduce the hydrostatic pressure of the fluid column as the hydrostatic head exerted by conventional slurries induces losses and permeability damage of reservoirs during cementation in such conditions. Such lightweight slurries must perform satisfactorily during the placement and also after the cement has set. While lightweight cement compositions have been developed and are being used but these slurries have low compressive strength, high porosity and high permeability, due to their high water-to-solid ratio, which limits their application.

Thus, there are continuing needs for low density cement compositions which have enhanced compressive strengths and low permeability upon setting for cementing pipe in well bore. Finding a feasible solution by developing lightweight cement slurry having adequate compressive strength was being investigated at the Institute of Drilling Technology (IDT) in ONGC and a range of low/ ultra lightweight cement slurries (1.60 - 1.05 sg) having adequate compressive strength and other improved slurry properties have been developed. These new cement systems are designed to provide successful isolation where conventional cement densities or placement technique would not work.

This paper presents the laboratoriy data showing unique feature of these developed light/ultralight weight slurry systems alongwith several recent field case histories demonstrating the benefits of using these new cement systems in low fracture gradients and depleted fields.

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