Well Logging

Some time in the early 1920's, logs began being kept by drillers. These driller logs recorded the depth, kind of rocks, fluids, and anything else of interest that occurs while drilling the well. This type of information was useful when compared with nearby wells to interpret the subsurface geology.

More useful are cores and mud logs. A core is a cylindrical cut of a formation while a mud log is a record of the drilling mud which includes a description of the drill cuttings that are brought to the surface in the drilling mud.

A core sample contains the most information since it is a direct measurement of a large piece of the rock. Either a core barrel attached to the drill pipe is used or a device attached to a wireline takes a sidewall core. Once the sample is brought to the surface, it is packaged and sent to a laboratory for analysis. Core samples can provide a clear understanding of the strata's lithology, porosity, permeability and, most importantly, hydrocarbon content. This information helps determine the oil-bearing potential of the sampled beds.

As technology advanced, electric logging came into widespread use. An instrument called a sonde is lowered into the bore on a conductor line or electric wireline. The sonde measures and records electrical, radioactive or acoustic properties of the various drilled formations and transmits its information up the wire to a recorder. Porosity, permeability and fluid content are the primary objectives of well logging, especially for target reservoir rocks.

A Spontaneous Potential (SP) log records the electrical currents that flow in rock formations. Most minerals are non-conductors of electricity when dry. However, some, like salt, are excellent conductors when dissolved in water. As drilling fluids invade a permeable formation, spontaneous potential causes weak current to flow from the un-invaded saltier rock into the invaded rock. The SP log can be used to visually identify bed boundaries and calculate formation water salinity.

Resistivity logging devices measure and record the resistance of a formation to the flow of electricity. High saltwater saturation lowers resistivity, while oil and gas raise resistivity, since hydrocarbons are poor conductors. Common resistivity logs include the lateral focus log, the induction log, and the micro resistivity log.

Radioactivity logging devices, including gamma ray and neutron logs, measure natural and induced radioactivity. Gamma ray logs record the emissions of naturally radioactive elements in formation sediments. Since these elements leach out of porous and permeable rock, a gamma ray logging device can identify impermeable formations such as shale and clay-filled sands. The neutron log emits radiation from the sonde, bombarding the rock around the wellbore to primarily determine porosity.

Acoustic logging devices are also called sonic logs and operate on the understanding that sound travels better through dense rock than through more porous rock.

Correlating data provided by several different logging methods can provide a clear picture of the target reservoir rocks. It is important to note that, due to the expense of logging a well and the overlapping information provided, not all types of logs are run on each well being drilled.

Formation Pressure Data

As more wells are drilled and logged in a given field, it becomes easier to predict and determine where the productive petroleum reservoir will extend and end. However, with a new discovery, it is imperative to take some pressure readings to help estimate the lateral extent of the reservoir. Pressure can be taken through a DST or drill-stem test or wireline. Both involve isolating the potential reservoir to recover a sample of fluids and take pressure readings. What is recovered and the pressure data gained helps determine if a commercial reservoir has been found.

Offshore Drilling

By the 1930's petroleum exploration companies realized that oil and gas reservoirs existed in shallow waters offshore. However, the problem remained how to drill when your drilling rig must be above water and at the same time stand steady against any heavy wave action. The solution was a type of rig known as a submersible.

The earliest form of submersible rig was a posted barge. It consisted of a barge with several steel posts attached and anchored. A deck was laid across the top of the posts, and the drilling equipment was installed on the deck. Posted barges cannot be used in waters exceeding 30 feet. Later improvements on this concept resulted in ship-shaped barges and drill ships. While the ship-shaped barge must be towed into place, the drill ship travels under its own power. In deep waters, drill ships and ship-shaped barges are anchored much

like an ocean-going boat may be anchored or may be held into position by dynamic positioning. Here computer-controlled thrusters are used to maintain the ships position.

Another early drilling platform was the bottle-type submersible rig, which has several steel cylinders or bottles that when flooded with water come to rest on the ocean floor. When it comes time to move the rig, the water is pumped out, and the rig is moved by tugboats to the new location. Bottle-type rigs are usually designed to operate in maximum water depths of 100 feet, although some have been built that can work in up to 175 feet of water.

Continuing exploration to further offshore drilling in deeper waters resulted in new submersible designs. The Jackup rig made it possible to drill in waters up to 350 feet with a few operable in up to 600 feet. Jackups are bottom-supported rigs that can be either column- or truss-supported. Columnar legs are steel cylinders while open truss legs resemble a derrick. Both types have water-tight hulls that can float on the surface of the water while being moved into position.

Today the most common type of offshore rig is the steel-jacket platform. This consists of the jacket, which is a tall vertical section manufactured from tubular steel. The steel jacket is pinned to the ocean floor using driven piles. Additional sections of tubular steel are placed on top of each other. Above the water level are quarters for the drilling crew and the drilling rig. This system has been used to drill wells in up to 1,000 feet of water.

There are other ocean drilling rig designs that are used in special situations, including the concrete gravity platforms used in the North Sea and the steel-caisson platform used in the Cook Inlet of Alaska.

Directional Drilling

Directional drilling techniques were employed in the 1970's. Normally wells are drilled vertically; however, there are many occasions when it is helpful to be able to drill at an angle. Directional wells are drilled straight to a predetermined level and then gradually curved. By changing the direction of the drill bit in small increments of no more than 2 to 3 degrees at a time, it is possible to drill many wells into a reservoir from a single offshore platform. Directional wells may also be deflected from a shoreline to reach a reservoir under nearby water. In addition, directional wells are very useful in avoiding fault lines, which can cause hole problems, as well as in instances where it is undesirable to set a rig in a given spot because of an obstruction or for environmental reasons.

Directional well bits can be used to straighten a hole, deflect the hole from the original dry well to intersect a reservoir, kill a wild well that is burning, or sidetrack around a "fish" (an object that has become lodged in the hole and cannot be removed).

Several special tools are available to assist in directional drilling. The most common involves the use of a bent sub and a downhill motor. A bent sub is a short piece of pipe that is threaded on both ends and bent slightly in the middle. It is installed in the drill stem between the bottommost drill collar and the downhole motor. A downhole motor is driven by drilling mud, thus eliminating the need to rotate the drill stem.