While every effort is put forth before drilling begins to ensure a successful well, not every well hits hydrocarbons at a commercially productive level. Statistically, wildcat (or exploratory) wells have a one in seven chance of discovering oil or gas. While six may be dry holes, one in the group can make a big enough difference to outweigh all the risks.
Once drilling operations have been completed, it is important for drillers, engineers and geologists to determine whether to move on to the next phase: completion for production. Formation tests ascertain if there are enough hydrocarbons to produce from a well, as well as provide important information to design the well completion and production facilities.
Used to establish formation pressure, permeability, and reservoir and formation fluid characterization, there are three major methods of formation testing that help to reveal the downhole formation: well logging, core sampling and drill stem tests.
A way of retrieving and recording downhole information, well logging involves lowering measurement instruments into the well during or after the drilling process. Used as a journal of what has been encountered during drilling operations, well logs measure the electric, acoustic, radioactive and electromagnetic properties of a downhole formation.
These measurements help to determine the permeability, porosity and reservoir pressure, among other characteristics of the formation, and ultimately the presence of hydrocarbons downhole. Well logging tools can be lowered into the well and raised to retrieve the information, or they can be included in the drillstem and send the information to the surface in real-time.
Well logging is the first step in formation evaluation to determine whether hydrocarbons are present within the well.
Another method of formation testing is performed by obtaining core samples. Here, a small segment of the formation is retrieved from the well and analyzed to determine porosity, permeability and the presence of oil and gas - the capabilities and productivity of the well.
While core samples can be taken throughout the drilling process, core samples are also retrieved after drilling has been completed. In this case, the drill stem is pulled from the well, and the drillbit is removed from the end -- replaced by a special coring instrument called a core head. Next, the drillstem is introduced back into the well and the core head retrieves a long cylinder of rock from the bottom of the well.
The core sample is then analyzed and broken to determine the presence of hydrocarbons, the fluid makeup and reservoir qualities. Sometimes oil can be seen in oil staining of the rock fragments within the sample. Also, the sample can be put under an ultraviolet light, and if there is oil and gas, the hydrocarbons will glow.
Drill Stem Tests
Used to provide a more definitive idea of the production capacity of the well, drill stem tests identify the types of fluids within the well, as well as the flowrate of these fluids, formation permeability and reservoir pressure.
Drill stem tests involve connecting a measurement device to the bottom of the drill stem, also in place of the drillbit, and lowering the system into the well, all the way to the formation. The instrument is activated at the bottom of the well, measuring the flow of oil or gas for a specified amount of time, usually an hour.
The testing tool includes a perforated anchor at the bottom that allows fluids to enter the empty pipe. Also rubber packers expand against the sides of the hole to seal pressure. A series of valves open and close to control the flow of the hydrocarbons into the empty drill stem. Additionally, the tool contains a pressure-measuring device.
When the tool is opened, the oil and gas enter the drill pipe and are sent through a flowline to the reserve pit on the surface. While oil or gas can reach the surface during the specified testing time, many times hydrocarbons and water simply enter the drill pipe, but do not reach the surface. Nonetheless, the flow, pressure and volumes are recorded.
Important factors in determining the success of the drill stem test and, in turn, the well, include the depth of the tool; duration of the test; time required for hydrocarbons to reach the surface; fluids recovered in the drill pipe; initial and final flow pressures, indicating the increase in flowing capacity of the well; and the shut-in bottom hole pressure, which signifies the maximum reservoir potential.
While drilling a well can be expensive, sometimes completion operations can be even more expensive. It is important to decide whether a well is commercially productive or it is more logical to simply plug the well and move on to another location. Typically, one or more formation test is performed to determine if the well is productive or not.
If formation tests reveal that the well does not have enough hydrocarbons present to complete the well for production, the well is plugged and abandoned. However, if the formation tests prove the well productive, it is moved into the completion phase, which includes running completion strings the length of the well, casing the well and cementing it.