Upload
christian-smith
View
48
Download
0
Embed Size (px)
DESCRIPTION
.
Citation preview
Christian Smith English 202C De3inition
Neutron Logging to generate Porosity Graphs
Basic Overview The Neutron Logging tool is an essential tool sent down wellbores to generate logs that determine if a well will be pro3itable. This tool is a long rod, about the length of a car, that is sent down a hole to take porosity measurements (see Figure 1 below). Porosity refers to the tiny spaces in rocks that are capable of holding oil, or gas. If a formation has no porosity than there is no chance of 3inding oil or gas within said formation and would be a waste of money to drill. It varies between companies but generally each company has a porosity cutoff. This is a porosity percentage that a formation must have in order for a company to turn a pro3it on the well. For example a coal bed might have a porosity cutoff of 50% but a gas formation would probably have a lower cutoff around 10%.
Figure 1. Neutron Logging tool taken from my class with Dr. Morgan
Starting the Process To start taking measurements the logging tool is 3irst lowered to the desire depth in the hole. Once in position generally 1-‐3 prongs (a 3 prong logger is depicted below) will open up to keep the Neutron Logger in the same place throughout the entire process. This is where the 3irst problem of borehole washouts occurs. A washout refers to the sections of the well which are not uniform in diameter which can be seen below.
Figure 2 Shows the 3 prongs used to Figure 3 depicts a washout Stabilize the tool during the process is the uneven wellbore radius Image taken from Dr. Morgan’s class Image taken from Dr. Morgan
While the prongs do a good job of keeping the tool in place for accurate results, if the hole gets too large the tool has the potential to move around. There is no way to avoid this however later on when the graphs are generated it is easily to recognize where a washout has occurred.
Detection/Measurement The next step in the process is to slowly pull the logging tool back up the hole. While the tool is being pulled out of the hole it is emitting neutrons which allow engineers to evaluate the hydrocarbons in the formations. Typically Americium Beryllium (Am-‐Be) or Plutonium Beryllium (Pu-‐Be) are the radioactive elements used to launch neutrons into the formations. The diagram below is taken from Dr. Wang’s Formation Evaluation class and shows how the Neutron Logger takes measurements.
Figure 4 shows a Neutron Logging tool 3iring off Neutrons into a formation
When the neutrons enter the formation they collide with other atoms causing the neutrons to change paths. Particle physics tells us that if two atoms of the same size collide they will have an elastic collisions. What this means is that when the neutrons collide with Hydrogen because they are the same size the neutrons encounter a drastic change in path. The more Hydrogen the neutrons hit the greater the change they will return to the detectors on the Neutron Logging tool. The logging tool generally has two detectors, a near detector and a far detector which average the number of neutrons that return to the tool. Oil and gas are both hydrocarbons which mean they contain large amounts of Hydrogen atoms which can be seen by a high count of neutrons by the detectors. If a formation has no Hydrogen atoms than very few neutrons will return to the detectors. Note Hydrogen atoms are not the only atoms that can change the neutrons path, any atom can, Hydrogen just has the most profound effect.
Interpreting the Data Once the tool has completed taking measurements of the hole a computer uses this data to make a log. A Neutron Porosity log is a percentage based upon the number of hydrogen in the formations and generally looks
something like Figure 5. There are usually several logs combined into one; in this case we are looking at the PHIN log which stands for Neutron Porosity.
Figure 5 Porosity Log Image from Dr. Morgan
The way to read this log is to determine at what depth your formations is at and then 3inding that related depth on the porosity log. Next you determine where the line on the log is with respect to the minimum (far right) and maximum (far left). Be careful to read the log headings because sometimes the minimum and maximum are switch depending upon who did the logs. Higher quality graphs have gridlines which allow for more accurate readings but for now it is easier to see the log without a grid system. Once the porosity is determined for a given formation this is where the cutoffs are applied. Larger companies can sometimes have lower cutoffs because they can front the money and survive from smaller pro3its. Smaller companies however usually have higher cutoffs in order to make higher returns on their investments to keep their investors. One thing to keep in mind here is that this is not the sole log used to determine pay zones, it is merely one of the core logs used to predict pro3itability. This is because Hydrogen can be found in other compounds such as water this is why other graphs are needed.
Conclusion The Neutron Logging process is one of the core tests used to determine if a well can turn a pro3it. The tool is 3irst sent down the well to the desired depth where it then emits neutrons used to take measurements as it is pulled out of the well. Next the measurements are uploaded onto a computer where a log is generated. Finally a Petroleum Engineer evaluates the log to determine if a well is pro3itable based on the cutoffs that the companies uses.