Key attributes and selection criteria of a stabilizer

Stabilizer specifications and standards are defined by API 7-1. Key attributes of a stabilizer are indicated in the figure below:

A.    Wrap Angle: Wrap angle is measured in degrees and is defined as the total contact that all the stabilizer blades make with the borehole wall. Based on the stabilizer design and drilling operations requirements, the wrap angle can vary from 180o to 600o. Wrap angle does not apply to straight-blade stabilizers. Based on the wrap angle, stabilizers can be defined as

·       Open spiral: 180o – 220° wrap angle

·       Full spiral: 300o – 350° wrap angle             

·       Tight Spiral: 500o – 600° wrap angle

Straight blade stabilizers are preferred on bearing housing and motor stabilizers and help slide. The wrap angle provides stiffness and stability to the drill string. It results in efficient drilling by reducing vibrations and allows an efficient transfer of weight to the bit. It is an important consideration for spiral blade stabilizer selection as excessive wrap angle could have negative effects as well.

·       While drilling, efficient fluid flow and transportation of cuttings is an important requirement. Excessive wrap angle can restrict the flow and interfere with the smooth movement of cuttings, which could lead to the balling up of the stabilizer.

·       The rotation of the drill string creates constant contact between the stabilizer blade and the borehole walls. A high wrap angle increases the contact area and excessive contact area can lead to higher torque and vibrations that could damage the drill string components. The wrap angle is important to provide stabilization to the drill string. However excessive wrap angle could reduce string stability due to excessive vibrations, which may compromise hole quality and directional control.

·       More than the optimum wrap angle creates greater force, and the scrapping action of the stabilizer could damage the borehole walls.

Hence it is important to carefully consider the wrap angle while selecting a stabilizer. A wrap angle between 270o and 360o is preferred for spiral blade stabilizers. In this range of wrap angles, the stabilizer provides the best combination of optimum BHA stability with low downhole vibration, required cutting transport efficiency, and the minimum possible borehole damage.

An easy way to visually verify are:

·      The blades are angled with a right-hand wrap.

The blades are arranged in a manner that there is a clear line of sight between the leading top edge of a blade and the trailing bottom edge of the next blade as shown in the figure below.

B. Blade Taper Angle: The gauge diameter of a stabilizer blade is more than the mandrel or the main body of the stabilizer. A taper is provided from the main stabilizer body to the blade pad. The taper angle is defined as the angle of the taper to the main body of the stabilizer. The taper angle serves the purpose of a transition from the body of a stabilizer to the blade of the stabilizer.

Having a smooth transition is an important design consideration of a drilling stabilizer. An abrupt transition would result in a shoulder that will make the stabilizer hang up while tripping or drilling. It will increase the amount of shock and vibrations which could lead to potential damage to the drill string and the borehole.

A taper angle ≥ 45o should not be used as it will result in an abrupt transition to the blade. A smooth transition with a taper angle ≤ 30o is ideal. Anything between 30o and 45o should be carefully reviewed.

C.    Bypass Area: The Bypass area of a stabilizer refers to the space between the blades of the stabilizer. Having a sufficient bypass area is important to ensure the efficient removal of cuttings. A lower bypass area will result in sub-optimal flow of cuttings which will tend to ball up the stabilizer.

The ‘Bypass Area’ is expressed in percentage and is defined as the ratio of the ‘Junk Slot Area’ of the stabilizer and the ‘Hole Area’. Since the volume of cutting generation depends on the size of the hole being drilled, the optimum Bypass area also depends on the hole size. Bypass area ≥ 35% for hole sizes 10-5/8” or more and ≥ 25% for hole sizes below 10-5/8” is ideal for having optimum clearance for efficient cuttings removal. The bypass area of some RSS and MWD tools may not meet these criteria and could constrain the ROP.

D.    Pad Pressure: Pad pressure is defined as the pressure exerted by the stabilizer blades on the borehole wall during drilling operations. The two contributing factors to pad pressure are the total side load and the pad area in contact with the borehole wall. The load on pads depends on several factors as below:

a.      While drilling a directional well, the vertical component of the weight acts on the pads under gravity.

b.    While drilling a directional well, the bit needs to be steered from time to time for inclination and azimuthal correction. This steering load is applied on the borehole walls through stabilizer pads.

c.     The hole curvature and the Dog Leg Severity (DLS) also affect the side load.

Pad pressure is expressed in force per unit area and is a ratio of ‘Total Side Load’ and ‘Pad Area for 180o of Contact’. Excessive pad pressure could cause high torque and damage to the borehole walls. This can lead to poor quality of the hole and result in higher vibration levels.  A pad pressure ≤ 300 psi is recommended for efficient drilling. If high side loading is estimated based on hole configuration, a review of stabilizer selection with longer and wider blades could provide a higher pad contact area and help reduce the pad pressure.

Stabilizer Selection Guideline Summary:

Reference:

(1) API SPEC 7-1

(2) Stabilizer Selection Based on Physics and Lessons Learned, Paul E. Pastusek, Paper presented at the IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, March 2018, Paper Number: SPE-189649-MS