Primary Recovery Techniques

Onshore or offshore, 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 is often done by examining the mud and cuttings. Both will contain traces of hydrocarbons if the drill bit passes through a reservoir trap. At this point, a drill stem test is performed; it is a special tool that is attached to the end of the drill stem, lowered into the hole, and collects any fluids at the bottom of the hole. The sample is brought to the surface and analyzed. If hydrocarbons are detected, the exploration well is replaced with a development well and production 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. 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 and 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 When a producing well is established in a reservoir trap and product withdrawn, pressure drops (discussed earlier).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. If the gas cap located above the oil is causing a downward pressure, it is referred to as a gas cap drive system. If the primary driving pressure is the result of pressure release from dissolved gas in solution with the oil, then it is referred to as a solution gas drive system.

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 lift is needed. This is accomplished by injecting the escaping gas back into the well to mix with the oil at depth. This impregnation by gas causes the oil to become lighter and thus the fluid column weighs less and is more easily pumped to the surface. When this process is utilized, it is referred to as gas lift.

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.

QUESTION: How much of the total volume of hydrocarbons present in a reservoir trap are removed by Primary Recovery Techniques?
ANSWER: Less than 40%

Tertiary Methods

Getting to the remaining 60% left in the formation requires other Enhanced Oil Recovery techniques, often referred to as Secondary or Tertiary Recovery. There are three general techniques used to recover the residual oil left behind: Miscible Recovery, Thermal Recovery, and Chemical Recovery. Miscible (ability of fluids to mix with each other to form a single phase ) recovery uses light (low density) liquid hydrocarbons such as propane or ethane that has been extracted from natural gas or CO₂. Gas is injected into the reservoir trap through another well into the water that exists below the oil - water interface. Remember: oil and water separate into distinct layers and are not miscible. So fluids that do not mix well with the oil will be much more effective at displacing, or flushing it, from the reservoir.

Thermal recovery uses heat to aid in flushing more hydrocarbons from the reservoir. As oil is heated, it becomes less dense and therefore flows more easily. This is accomplished by injecting high pressure steam into the reservoir through a nearby well; this heats up the hydrocarbons in the reservoir which in turn causes them to move up the completed well. A corollary to this technique that is sometimes used is called fire flooding. Hydrocarbons in the reservoir rock are actually set ablaze in an effort to induce flow.

Chemical recovery injects polymers into the water phase of the reservoir trap. Polymers are chemicals characterized by large molecules which when injected into water, add bulk to the water. The water is "thickened." This water is able to sweep or wash the hydrocarbons from the pores within the reservoir. Surfactants (reduces the adhesive forces of the water molecules to solid substances) and detergents are sometimes injected (Surfactant flooding) or sometimes in combination with polymers to recover residual oil. The surfactant causes the oil to flush more easily through the porous materials thereby freeing up hydrocarbons that were adhering to the rock grains.

All of these secondary recovery methods are expensive and add cost to the production of the hydrocarbons. And they do not always work well or have the desired affect. Measuring the expected benefits against the costs (additional wells may have to be drilled in order to inject the fluids) can be a major challenge.

When production begins to drop off, especially when compared to other wells in the vicinity, it may be time for the well to receive a work-over (a major repair and clean out of all pipes). Producing wells are like anything else; they require periodic maintenance. Any number of things can go wrong. Corrosion can roughen pipe walls or cause failure allowing product to leak onto the surface of the earth. 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 snow (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 also. 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 Fracturing involves injecting a fluid into the formation under significant pressure that makes existing small fractures larger and creates new fractures. These fractures extend outward from the well bore into the formation containing the hydrocarbons thereby increasing the permeability of the formation which enhances to flow of product.

QUESTION: Would the resulting increase in permeability in the formation due to either Acidizing or fracturing be classified as Primary or Secondary permeability? Why?
ANSWER: Secondary because it was induced by man; it is artificial.

QUESTION: What is the purpose of the Packer in this illustration?
ANSWER: It prevents the acid or fracture fluid from flowing up the annular space of the well thereby losing its effectiveness.

QUESTION: How many reservoir traps can one vertical well be removing hydrocarbons from?
ANSWER: Any multiple number. Wells may be removing hydrocarbons from more than one subsurface zone, but doing all of this from within one hole. These are called multiple completions.

QUESTION: How many oil wells can be drilled/completed from one platform offshore?
ANSWER: Most offshore platforms can accommodate up to 50 or more wells, many of which are directionally drilled. This approach allows an operator maximum flexibility to drain a large portion of an oil field from one location.

Petroleum flowing from a well is not refined product. It is a mixture of crude oil and gas, and impurities such as brine, sand, and other suspended solids. The mixture flows into a separator where gravity is utilized to separate the mixture into product (oil, gas) from the impurities. The product is then moved, generally by pipelines into storage tanks that are close to the production well(s). From here it is transported to refineries for processing.

____________, 1988, Oil: Shell Corporation, Houston, TX., 47 pp.