heres some light reading, shamelessly stolen from...

heres some light reading, shamelessly stolen from www.couchoil.com FAQ

What is a mud log?

Once drilling reaches a depth at which formations of interest may be encountered mud logging begins. A professional mud logger, who usually has his own special trailer on the drill site, monitors many things that are of interest:

* The rate of penetration of the drill bit. Generally, as we drill deeper the rate of penetration will decrease as the density of the formation increases. However, the rate of penetration of a sand is usually faster than the rate at which we penetrate shale. Thus, an increased rate of penetration may be an early indication of the entry into a sand.
* The mud weight in and out. Under normal circumstances the mud going into the drill string will be the same as the weight of the returning mud. When the returning mud is lighter it is probably because the mud is being cut by the entry of gas into the well bore.
* Gas units. The mud logger constantly measures the amount of gas that is being brought up along with that the mud that is being circulated back to the surface.
* Gas character. The gas that is contained in the mud is constantly measured and classified into its various components (methane, ethane, butane, etc.)
* Analysis of cuttings. The mud logging is constantly analyzing the cuttings that are brought up from the bottom. These cuttings are classified in a number of ways, the most of important of which are the type of material being drilled through, that is, sand, shale, limestone, etc. The mud logger is constantly monitoring the percentage of the cuttings that are classified in these ways.

The results of all of the above measures are simultaneously depicted in the form of a presentation called a mud log

What is a wireline log?

After each section of interest is drilled, and before casing is installed in that section, a wireline log is run. The wireline log takes several different measures that are related to the presence of hydrocarbons:

* Spontaneous potential (Gamma Ray) – all of the materials that are drilled through give off small amounts of radioactivity. Sands give off less radioactivity than shales. A decrease in spontaneous potential may signal the entry into a sand. An increase may be related to moving back into a shale.
* Resisitivity – One of the logging tools has an emitter that creates an electrical voltage. A pickup device measures this voltage after it travels through the material. A sand bearing hydrocarbons is a poor conductor of electricity and will have high resistivity. Shales conduct electricity better than sands. The best conductors that might be measured are wet sands. Salt water (the type of water in a wet sand) is a very good conductor of electricity and will have resistivity close to 0 ohms. Several resistivity measures are taken, each of which looks a little deeper into the formation.
* Neutron porosity - low values indicate the likely presence of hydrocarbons
* Density Porosity - high values indicate a higher degree of porosity within the material being measured
* Caliper readings - measure the diameter of the hole. Holes drilled through shales will generally have larger diameters because they are more easily "washed out" by the action of the high pressure drilling mud.

What is completion?

Completion of a well is comprised of four stages: casing, testing, building production facilities and, for natural gas, laying a pipeline. Once wireline logs have been run, a decision will be made to proceed with completion. First, casing is run to the bottom of the hole and cemented in place. If a decision were made to not complete the well, this casing would not be necessary and the hole would be plugged and abandoned.

Next, the formations of interest would be tested by perforating the casing to measure the flow of hydrocarbons. As a general rule, formations will be tested from the bottom up. If a formation is deemed to not be productive, a cast iron bridge plug may be set above it to isolate it from the other zones that are up hole. If upper zones were tested first, it would be necessary to seal them off with a cement squeeze prior to moving lower. A cement squeeze is much more complex and expensive than merely setting a bridge plug. It could also damage the formation rendering it less productive in the future if a decision is made to produce it.

What is Fracture Stimulation ("Frac")?

In order for a well to produce hydrocarbons it is, first, necessary that they be present (good porosity). However, it is also necessary that they be able to flow through the formation to the perforations (good permeability). It is not uncommon to see a well that comes on with a high pressure and rate of flow but declines quickly. The gas near the perforations flows out and the more distant gas only works its way slowly from pore to pore.

One remedy for this situation is to fracture the formation. Water is pumped down the well bore, through the perforations and into the formation under very high pressure (for example, 10,000 psi). The purpose is to fracture the sandstone and open up fissures out to a distance of hundreds of feet. Frac fluid is also loaded with hardened grains of sand suspended in a gel. The purpose of this frac sand is to hold the fissures open once the pumps are shut off and the pressure declines back to its normal level. Without the frac sand to hold them open, the fissures would simply close back up.

After a successful fracture, gas that is distant from the perforations does not need to travel through hundreds of feet of sand. Rather, it only needs to flow to the nearest fissure and can then follow its path to the perforations.

When the logs show low permeability the well may be frac'd right after the testing phase. Other times the well is allowed to flow for awhile, allowing the formation pressure to decline enhancing the likelihood of a successful frac.