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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

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 7

Testing the Well

Testing the well after it has been drilled is the most important part of the process. It costs more to complete the well than to drill it. Will this well produce enough oil or gas to make it worthwhile to complete the well? This is called the casing point. Is it worth casing (completing) the well? In the early days of drilling, if they struck oil, the oil would often gush out the well and sometimes above the top of the derrick during a gusher That's because the well was drilled with an early type of drilling rig called a cable tool drilling rig. There was no drilling mud in the well and they didn't have effective blowout preventers. There was nothing on an old cable tool drilling rig that would set the oil or gas afire during a gusher. Today, the well is drilled with a rotary drilling rig. The well is kept filled with heavy drilling mud and there are effective blowout preventers installed on top of the well. If you get a gusher today, it will catch fire and the well will blowout.

The modern well is evaluated using three types of logs. A sample log (Figure 7-1) is a long strip of paper which shows the composition of the rock layers such as sandstone, limestone and shale in the well. As the well is being drilled, a well site or well sitting geologist is at the rig. One of the geologist's responsibilities is to collect well cuttings at regular intervals. The geologist uses a binocular microscope to examine the well cuttings and make the sample log. A dark coating on the well cuttings could be oil staining. To test for oil, the cuttings are placed in a tube containing a solvent and then examined under a fluorescent light. Oil will produce a brilliant flash called a streaming cut. The most accurate source of information is to core the well. A core is a cylinder of rock, 3 to 6 inches in diameter, that is drilled from the well. Porosity, permeability and oil saturation can be measured from cores. Coring, however, takes longer that just drilling through the rock. This extra time on the drilling rig is expensive and, thus, cores are expensive. Because of the expense, usually only the reservoir rock is cored.

Figure 7-1 Sample Log

A mud logger is a service company that continuously samples and analyzes the drilling mud and well cuttings for oil and gas as the well is being drilled. A mud log is made in the mud logging trailer at the wellsite. Traces of oil and gas in the drilling mud are called oil cut and gas cut mud. Traces of oil and gas in the well cuttings are called a show of oil and a show of gas. The mud log does not determine the amount of oil and gas (it could be just a trace) but does identify potential reservoir rocks that deserve further tests.

The most important tests are made by wireline well logs that are run by a service company. A logging truck is driven out to the well and is backed up to the well. A long (20 to 70 feet) metal cylinder containing instruments called a sonde is unloaded from the truck and is lowered down the well on a wireline that is wound around a reel in the back of the logging truck. As the sonde is brought back up the well, the instruments in it measure the electric, acoustic and/or radioactive properties of the rocks along the wellbore. These measurements are recorded in the logging truck on a long sheet of paper called a wireline well log (Figure7-2).

Figure 7-2 Wireline Well Log

Some types of wireline well logs include electrical, induction, gamma ray, neutron porosity, formation density, sonic and caliper. By interpreting the curves on the wireline well log, the geologist or engineer can determine the exact elevation and composition of each rock layer, whether the rock layer has porosity and how much and what is the fluid (water, gas or oil) in the pores of the rock.

Wireline well logs are made after the well has been drilled. A recent development is MWD (measurement while drilling) and LWD (logging while drilling) . These are real time logs that are recorded as the well is being drilled. The geologist on the floor of the drilling rig can use this to identify the rocks through which they are currently drilling. The system also makes a directional log that shows the driller where the drill bit is going. MWD or LWD is used on all directional wells.

A drillstem test is similar to a temporary completion of the well (Figure 7-3). A pipe (drillstem) is run down the well which is still filled with drilling mud. Two, doughnutshaped devices made of a rubberlike substance called packers are attached to it. The drillstem is lowered until one packer is just below and the other packer is just above the zone to be tested. The packers are then expanded to seal off that zone from the rest of the well.

Figure 7-3 Drillstem Test

Even though the well is still filled to the top with drilling mud, there is no pressure on the zone being tested because of the packers. Any fluids can flow out of the zone and into the well. A valve is opened in the drillstem and the fluids from that zone flow into the drillstem. Gas will usually flow onto the surface where it is passed through an orifice ( choke) of a certain size and the amount and pressure is measured. Oil usually will not flow all the way up the drillstem to the surface. The valve in the drillstem is closed and the height of the oil in the drillstem is measured. Fluid pressures are also continuously recorded during the drillstem test. A drillstem test will identify the fluids (water, gas and/or oil) in the zone tested and determine the rate at which they can be produced.

Coming Next: Chapter Eight: Completing a Well

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