Production

Primary Recovery Techniques

The drilling job is finished when the drill bit penetrates a reservoir trap and the trap is evaluated to see whether the well is a discovery or a dry hole. This evaluation is often started by examining the cuttings from the well bore. The cuttings are examined for traces of hydrocarbons while the drill bit passes through a reservoir trap. After the well is drilled the evaluation of these cuttings helps pinpoint the possible producing intervals in the well bore. At this time, an electric log is run; it is a special tool that is attached to a wire-line, lowered into the hole. It collects various data from the well bore. This data helps define possible producing intervals, presence of hydrocarbons, and detailed information about the different formations throughout the well bore. Well logging is not an exact science, comparison data from similar formations are essential in log evaluation. Further tests can also be run on individual formations within the well bore such as pressure tests, formation fluid recovery and sidewall core analysis. All are very effective tools to help evaluate the well, but by no means are any 100% precise.

If hydrocarbons are detected, the completion process begins. The only thing visible at the well head after the drill rig leaves the site is a series of valves and gauges connected vertically to each other and attached to the top of the well. This allows the amount of hydrocarbons to flow from the well and it prevents leakage at the surface. This structure is referred to as the Christmas Tree (see image).

If the hole is dry, it is plugged and abandoned. Production wells, also called Completion Wells, present their own set of problems. Hydrocarbons come in varying densities and viscosities; reservoir traps also have variations in porosity, permeability, pressures, and temperatures. All of these factors exert an influence on how easily hydrocarbons can be removed from a trap. Every reservoir has a certain volume of natural pressure associated with the hydrocarbons. When a producing well is established in a reservoir trap and the product is withdrawn, pressure drops (discussed earlier).

It is this differential pressure between the trap and the open hole that moves the hydrocarbons out of the reservoir, into the well, and up to the surface. The pressure may be the result of a number of forces. For example, water located below the oil layer may be pressing upward; when this occurs, it is referred to as a water drive system (see image). If the gas cap located above the oil is causing a downward pressure, it is referred to as a gas cap drive system (see image).

In most reservoir traps, initial pressure is sufficient to push the oil to the surface of the production well with only minimal help from a down hole pump. But, with declining well pressures, it becomes more difficult to get the hydrocarbon to the surface. Sometimes, artificial OIL lift is needed.

All of the techniques discussed thus far for removing the hydrocarbons from the reservoir and bringing them to the surface are referred to as Primary Recovery Techniques. Primary techniques rely entirely on natural forces within the reservoir trap. And primary recovery accounts for a large portion of the total volume of hydrocarbons in the trap, but not all of it. Less than 40% of hydrocarbons present are recoverable by means of Primary Recovery.

When production begins to drop off, it may be time for the well to receive a work-over (a major repairing and cleaning out of all pipes). Producing wells are like anything else; they require periodic maintenance. Corrosion can roughen pipe walls or cause failure, allowing product to leak onto the surface. Pieces of rock from the side of the well may break off and fall into the well clogging it. Natural gas pipes tend to accumulate paraffin (hydrate compounds that build up inside the pipe causing restrictions). Maintenance can result in everything from cleaning fluids being injected into the pipes to wire brushes being inserted to brush the pipes clean. Residues are flushed from the system before it is reconnected.

But work-over is not restricted only to the hardware; it may also be applied to the down-hole portion of the rock formation. Often, the formation through which the hydrocarbons are flowing becomes clogged which diminishes the volume of product reaching the well. Two processes used to improve formation characteristics are Acidizing and Fracturing. Acidizing involves injecting an acid into a soluble formation, such as a carbonate, where it dissolves rock. This process enlarges the existing voids and increases permeability.

Hydraulic Fracking involves injecting a fluid into the formation under significant pressure that makes existing small fractures larger and creates new fractures.