Abstract

The new analysis technique of Annular Flow Dynamics is a simple, practical way to predict and control rheology and hydraulics in drilling fluid systems. It is based on field experience in the sloughing shale areas of Canada and on a recently developed method of calculating annular pressure losses with greater accuracy and more simplicity than previous methods.

Annular Flow Dynamics is primarily oriented towards improving drilling practices in sloughing shale situations. However, as holes become deeper, additional restrictions are imposed on casing programs and hole size. These make it even more important to be able to determine and possibly predict certain drilling fluid parameters for maximum performance. By use of a logarithmic plot of viscometer data, the following can be determined:

  1. Effective annular viscosities at any pump rate and for any flow properties.

  2. Flow type (laminar or turbulent).

  3. Accurate annular pressure losses.

  4. Approximate bit nozzle shear rates.

  5. Upper and lower limits on both flow properties and pump rates for optimum well bore stabilization and minimum E.C.D. (Equivalent Circulation Density).

  6. Approximate ascending velocity of cuttings.

In the past year, this new technique has been used on a field basis in Canada with very positive results.

Introduction

Well bore instability occurs primarily when shales absorb water. Controlled activity oil muds are presently the only way not to satisfy the shales affinity for water. The mechanism of sloughing shales due to actual change in the physical and chemical properties of the shale have been thoroughly researched and discussed by the industry. Less attention has been paid to the mechanical aspect of stabilizing shales by well planned and flexible rheological and hydraulics programs. The combination rheology/hydraulics program must be flexible so that when out of gauge sections create hole cleaning problems, the rheological properties and/or pump rate can be raised without over-stressing the well bore and "tearing up" the hole around the drill collars.

In Canada, the most severe sloughing problems occur along the Eastern slope of the Rocky Mountains in the Foothills of Alberta, British Columbia and the Yukon. The most troublesome shales occur in the Cretaceous Period (Blackstone, Brazeau, Wapiabi, Led Park and Fort St. John Group to name a few).

The sloughing shale problems are compounded by the highly faulted and disturbed nature of the Foothills of the Rocky Mountains. This warping, up-bowing and folding of over-thrust faults and sheets are recognized features of foothills drilling and result in untold drilling and completion expenses.

The result of formation being forced up along the fault line is high formation dip angles, fractured and crushed sections of shale and stressed shale that is sensitive to any form of physical disturbance. These old and dry shale beds with their basically weak, wafer-like, laminated structures are extremely susceptible to annular flow erosion.

When drilling in areas where relatively old, dry sloughing shale is a problem, the basic objectives of a rheology/hydraulics program should be to minimize annular velocities and effective annular viscosities to minimize annular pressure drops, reduce to a minimum over-sized collars and stabilizing equipment, and maintain laminar flow in the annulus.

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