Drilling Technology has been continuously faced with new problems in drilling deeper and more difficult holes. A major problem is excessive friction caused by unstable hole conditions which in turn are due to sensitive clay formations or poor drilling practices.

In addition to improvement by mechanical means and sophisticated tools, the drilling fluid properties are of utmost importance in drilling such holes successfully to final depth.

Two important drilling fluid properties in this respect are lubricity and clay inhibition.

Lubrication can be obtained by adding liquid and solid lubricants to the circulating fluid, and thus greatly reduce torque, over-pull and casing wear. The inhibitive properties of the fluid may reduce or prevent swelling of clay and restrict the tendency of the clay to adhere to bit and stabilizers, the "balling up".

Clay preservation, moreover, prevents undue damage to the formation permeability in the pay zone. In order to keep the drilling fluid in permeability in the pay zone. In order to keep the drilling fluid in good shape it should be reasonably resistant to high temperatures and contaminants; drilled solids have to be removed continuously.

Drilling fluid properties have to be monitored continuously and immediate action has to be taken if sudden changes occur; automation of drilling fluid control is progressing fast.

The presentation reviews some recent developments in drilling fluid technology which greatly contributed to reducing friction.

Some case histories are given of successful applications.

Introduction

Excessive friction in an uncased hole is often caused by unstable hole conditions. When drilling a hole into the formation, the existing mechanical stress conditions are disturbed resulting in serious instability of the wall which may eventually lead to total collapse.

Plastic shales or rock salt may even "flow" into the hole Plastic shales or rock salt may even "flow" into the hole after the bit has passed, particularly when drilling too fast. Clays may swell by hydration with the same effect. In permeable sands, particularly in depleted zones, differential pressure sticking could cause stuck pipe. Balling of the bit and reamers in sticky clay formations is not only a nuisance because of the poor on pulling because of the piston effect of the balled bottom assembly.

Such problems are far more serious in deviated holes, when the pipe lies against the wall or is even into it when drilling below key-seat or dog-leg. Casing wear is another hazard leading to costly remedial measures, particularly in deviated holes. Such difficulties can be partly prevented by good drilling practices, such as proper selection of the assembly weight on the bit, rotary and tripping speed, casing schedule and whatever other mechanical measures are available. It is generally agreed that the drilling fluid could play a major role in preventing drilling problems as described.

DRILLING FLUID REQUIREMENTS

Part of good drilling practice is optimum drilling fluid Part of good drilling practice is optimum drilling fluid characteristics. Mud weight should not be higher than necessary to keep formation pressure under control, as higher mud weight reduces the rate of penetration and may lead to circulation losses or worse. Fluid viscosity has to be controlled when drilled clay is disintegrated and quickly increases clay content or when brine influx occurs, in order to maintain optimum hydraulics in the pumps and at bit nozzles.

The filtrate loss should be low enough to prevent excessive cake thickness and to reduce the chances of differential pressure sticking. At times the mud should have formation pressure sticking. At times the mud should have formation preserving properties to limit mud making or clay swelling. preserving properties to limit mud making or clay swelling. Drilling fluids should exhibit good lubricity, both in the casing and open hole, in particular when deviating. What was considered to be an excellent mud some decades ago, will not suffice today or in future.

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