Abstract

While drilling in some oil fields of the world, loss circulation may be so severe that normal remedies that might heal the loss zone are not effective. Procedures have been developed to drill "blind" or without returns. The procedures have been refined enough so that stuck drill pipe may be avoided, but the most dangerous aspect of drilling "blind" is the lack of well control. Often when circulation is lost, the well kicks and gas can flow into the wellbore due to drastic reduction in hydrostatic pressure. Furthermore, normal well control operations are ineffective. The procedure generally employed involves pumping a viscous slug of gel-based mud into the annulus and this column of mud is called a "mud cap". This approach depends on when the lost zone is above or below a gas zone and either approach effectively limits the amount of fluid that can be used.

In order to properly place a mud cap, a better understanding of the relationship between the fluid rheological properties, column height, and the initial shut-in casing pressure is required. In this respect, no quick field procedure has been developed to help rig-site personnel to control these types of wells. A set of charts have been constructed to aid field personnel in blowout prevention, when they are confronted with a gas kick occurring while drilling without returns. This situation is common in much oil fields especially in Middle East and North Africa. The set of charts is based on the slot flow approximation for Bingham fluids. Two sets of charts are developed: one for the case in which pipe is the hole and the other when the casing is opened.

Each of the charts represents a different annular configuration. These charts make life easier for rig personnel especially in a very stressful situation, and this type of preplanning should help to avoid costly mistakes in judgment.

Introduction

Solutions for equations describing axial annular flow of Newtonian fluids can be determined directly. However, direct methods for solving annular flow equations for more complex rheological models, such as Bingham-Plastic and Power-Law, usually result in unwieldy expressions. To avoid this the slot flow approximation was developed. This method equates the geometry of an annulus to that of a narrow slot. Depending upon the geometric parameters used in the narrow slot approximation and the degree of accuracy required, some sort of correction for the curvature of the Newtonian curvature correction factor must be developed. Using the same correction factor as for Newtonian fluids, a slot flow approximation of the axial annular flow of Bingham-Plastic fluids should be relatively accurate. The primary purpose of this paper was to develop relations that describe the annular flow of Bingham fluids and the pressure gradient necessary to initiate flow. With an expression that relates pressure gradient to the Bingham plastic yield point, a set of practical charts were developed. These charts can be used by well-site personnel in determining mud properties and volumes needed to control gas kicks that often occur while drilling with lost circulation (no returns).

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