Abstract

High degree of depletion and overbalanced drilling have caused ideal conditions for fracturing and lost circulation. Thief zone introduces severe lost circulation and well control problems, often resulting into loss of the interval or the entire well. In most cases, operators choose to drill such depleted sections with SBM or OBM systems rather than WBM. Dumping mica, nut hulls and cross-linked polymer pills, certainly still work, for some applications, but have narrow operating limits. Very few lost circulation remedies have been successful, especially when using invert emulsion drilling fluids. Controlling losses in this zone has proved to be extremely difficult.

A novel alternative LCM pill has been designed to seal temporarily or indefinitely critical intervals where drilling and cementing operations are very difficult to successfully accomplish. This formulation is prepared with invert emulsion, cross-linked with magnesia, up to 90% soluble, especially designed to be used in pay zones, a low cost alternative and is appropriate for sealing non-productive intervals. The phase change of invert emulsion and addition of light burned magnesia form the viscous fluid and remains stable till activator is not added. By addition of activator, control process of static gelation starts and forms the magnesia cement. Such formulations have sp gr 1.10–1.45 and stable at high temperature (< 120deg C). The apparent viscosity of this pill is kept in the range of 45–60cp to avoid any settling of magnesia. This pill can be retarded for desired time of placement and gives its vertical setting. The compressive strength of the molds is observed 300-1000psi within a short time and also soluble in 15% HCl.

The paper presents various formulations of innovative pills which can be used effectively and economically to combat circulation loss in depleted reservoir formations.

INTRODUCTION

Loss circulation is an age-old problem normally encountered during drilling. It is one of the most time-consuming and costly problem faced by oil well drilling industry. Loss circulation or loss of returns is defined as the drilling fluids into the formations which are usually lost to natural or induced formation fractures. After the lost circulation occurs, the level of the drilling fluid in the annulus may drop and then stabilizes at a particular level depending upon the formation pressure. The loss of drilling mud results in increased drilling time, loss of expensive mud, plugging of productive formations, and / or loss of well control. Normally, bridging agents and plugging materials are used to combat loss circulation. And another approach is used to reduce the density of the circulating fluid. In some instances, both approaches are combined to establish circulation. In many cases the lost circulation zones have fractures of different sizes which must be sealed of with bridging or plugging materials [Eric, Davidson, 2000]. Wide assortments of bridging or plugging materials, commercially available for reducing lost circulation during drilling, are classified as granular, flake and fibrous.

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