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About the Author

Norman J. Hyne is Professor of Petroleum Geology at the University of Tulsa in Tulsa, Oklahoma. He is also president of NJH Energy, a company that owns and manages the operation of oil wells. Norman is well known for his "Basic Petroleum Geology for Non-Geologists" short course that he has taught through out the world. He is the author of "Non-Technical Guide to Petroleum Geology, Exploration, Drilling and Production" and "Dictionary of Petroleum Exploration, Drilling and Production" published by PennWell Books. Norman has also edited four books on the petroleum geology of the mid-continent.

Finding and Producing Oil and Gas

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Contents:

Chapter 1 - Gas and Oil

Chapter 2 - Petroleum Geology

Chapter 3 - Petroleum Exploration - The Prospect

Chapter 4 - Land - Leasing and Site Preparation

Chapter 5 - Drilling the Well

Chapter 6 - Testing the Well

Chapter 7 - Completing the Well

Chapter 8 - Producing Gas and Oil

Chapter Six

Drilling Problems and Techniques

Usually, a well is drilled down to contract depth with no problems.  As the well gets deeper, both the temperature and pressure increase. Both are usually predictable.  The temperature increases at a rate of about 1.4 F per 100 feet and the pressure on the fluids in the pores of the rocks increases at a rate of 45 pounds per square inch. For example, in Oklahoma where the surface temperature averages 55 F, the temperature in a well at 10,000 feet would be 195 F and the pressure would be 4,500 pounds per square inch.

Every so often, however, there is a problem during drilling. The most common problem is that something will break and fall to the bottom of the well.  For example, the drillpipe twists off and falls down the well or a cone falls off the tricone bit. This is metal and you cannot drill through it and it is called junk or fish at the bottom of the well. Drilling must be suspended and a special tool called a fishing tool has to be leased from a service company. The fishing tool is then lowered down the well to grapple for the fish in a process called fishingFishing can take days. While you are fishing, you are not drilling the well but you are still being charged for the drilling rig.  This charge can be avoided if fishing insurance is bought before the well is drilled.

Another problem, called lost circulation, is caused by drilling into a very permeable formation.  The drilling mud is flowing into the lost circulation zone without building up a filter cake along the wellbore.  Very little, if any drilling mud is circulating back to the surface. This is bad for two reasons. First, the drilling mud cost money.  Secondly, there is only a limited amount of drilling mud at the wellsite.  When you run out of drilling mud, you have to stop drilling.  Service companies sell finegrained, fibrous particles called lost circulation material that can be pumped down the well to clog up the lost circulation zone.

The most serious problem is caused by drilling into an abnormal high pressure zone where the pressure on the fluid (water, gas or oil) in the pores of the rock is much higher than expected. The drilling mud pressure cannot control it and fluids will flow into the well in what is called a kickUsually the driller has ample warning that there is a kick on the bottom of the well because the drilling mud will be flowing out of the top of the well at a very high rate and automatic devices warn him.  The driller kills the well by closing the blowout preventer. Very heavy drilling mud called kill mud is then pumped down the well to bring the well back under control. The well is then drilled deeper using the kill mud.  Sometimes, however, the well is not killed in time and natural gas flows onto the floor of the drilling rig and catches fire during a blowoutThe rig and well are usually lost during a blowout.

When a well is drilled nearly straight down, it is called a straight holeThe well, however, never goes straight down because the drillstring is being turned clockwise (to the right) on a rotary drilling rig.  Instead, the well corkscrews down.  If it is kept in a 5 cone, it is considered a straight hole. The well can be surveyed using a compass or gyroscope to make a directional log, a map of the well location at various depths.

Rotary drilling today is such an art that the drilling contractor can almost always guarantee a straight hole that is drilled almost straight down, but also can make the well go off at an angle and end up exactly where you want it by using directional drillingDirectional drilling is done with a turbine or mud motor and diamond bit on the bottom of the drillstring. A turbine or mud motor, located just above the bit, is driven by the drilling mud that is pumped down the center of the drillstring. A diamond bit is a solid metal bit with no moving parts (Figure 6-1). There are hundreds of small, industrial diamonds that are attached to the bottom and sides of the bit in geometic patterns.

There are many uses for directional drilling and it is very commonly done today. One of the must common uses is sidetracking, to drill around a fish in the well. An extended reach well has a very large horizontal displacement (horizontal reach) from the surface to the bottom of the well (Figure 6-2). The world's record for a horizontal reach is over 6 miles.

Horizontal drain holes (Figure 6-3), drilled parallel to the oil or gas pay zone, are common today. They often produce 3 to 5 times more oil or 5 to 20 times more gas than a straight hole. A double wing horizontal drain hole (Figure 6-4) has two horizontal branches going out in opposite directions in the pay zone.

Many wells are not drilled as straight holes and there are two ways to measure the depth of a well (Figure 6-5). Total depth (TD ) is measured along the wellbore.  True vertical depth (TVD) is measured straight down. Horizontal reach is the horizontal displacement of a directional well from the surface to bottom hole location of the well.

Drilling offshore is very similar to drilling on land. The drilling rig, technique and crew are the same except that only two 12hour crews of workers are used each day. In shallow, protected waters such as a lagoon or canal, the drilling rig is located on a barge.  For exploratory drilling in shallow water (up to three hundred feet deep), the drilling rig is mounted on a jackup rig (Figure 6-6), a platform that has legs and sits on the sea floor.  The jackup rig is towed to the drillsite. The lower hull is flooded with water and sits on the ocean bottom. The upper hull is then jacked up the legs. In deeper water, the drilling is done from a floating drilling platform (semisubmersible) that is anchored to the bottom in water depths up to one to two thousand feet. In very deep waters, drilling is done from a drillship that floats above the drillsite. It is kept on station by a computer that constantly recalculates the ship's exact location. Propellers on the side of the drillship called thrusters are used to constantly readjust the ship's location above the well being drilled.  This is called dynamic positioning. Drillships are very expensive to operate but can drill in any water depth.  To connect the drillship or semisubmersible to the well on the bottom of the ocean, a marine riser, a flexible, metal tube is used. The drillstring runs down the center of the marine riser.  The marine riser gives a closed system to circulate drilling mud just as on land.

Offshore wells are very expensive. It costs about 5 times more to drill the same well to the same depth offshore than it does on land.

Coming Next: Chapter Seven: Testing the Well

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Copyright © 1997, 1998, 1999, and 2000 by Lewis G. Mosburg, Jr. and Ogden, the Invisible English Sheep Dog

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