Higher Permeability Sandstone
Matrix Acidizing or Hydraulic Fracture
The effective permeability (reservoir fluid transmissibility) will determine whether a matrix treatment or a short, highly conductive fracture is required. Generally, when permeability is less than 5 to 10 md, a fracturing treatment is performed. With permeabilities greater than 5 to 10 md, a matrix treatment using a solvent (usually acid) is done. Matrix treatments do not part (fracture) the rock. Solvents are pumped into the porosity to remove or clean up damage.
Sometimes, a reservoir rock has very little permeability. A reservoir with low natural permeability can present a problem because the hydrocarbons in the reservoir, though ample, cannot be extracted at reasonable rates.
One way in which to simulate a reservoir is to break or fracture it. Fracturing is done after the well has been completed, that is, the well has casing, tubing, and has been perforated.
Fracturing actually splits open the rock reservoir. The technique uses several powerful pumps arranged on the surface near the well to be fractured. The pumps are connected by high - pressure lines to the well tubing. A special fluid mixture, often composed of mainly water and sand, is pumped down the tubing to the bottom of the well. Pumping continues, and pressure forces the fracturing fluid into the perforations. Soon, the pressure becomes so high it overcomes the strength of the reservoir rock and cracks it open. Pumping continues until the fracture is of the desired length. Then pumping ceases, the pressure is bled off, and the fracturing equipment is removed. The sand in the fracturing fluid holds the fracture apart so that reservoir fluids can flow into the fracture and to the well.
Fracturing a reservoir is similar to splitting a log with a hammer and a wedge. Muscle power swings the hammer to drive the wedge into the log to split it open. In a fracturing job (often shortened to frac job), powerful engines replace muscles, and high-pressure pumps take the place of the hammer. Fracturing fluid becomes the wedge and the formation behaves the same as the log - it splits open.
Now picture the split log being held open by the wedge (the log is not split in two). What happens if the wedge is taken out? The split comes back together, or heals. A fracture in the reservoir behaves in the same way. When the pumps are stopped and pressure bled off, the fracture comes back together - or will if allowed to. However, the sand in the fracturing fluid holds, or props, the fracture open, preventing it from healing. Because sand serves this function, it is often called a proppant. The propped fracture serves as a pathway for reservoir fluids to flow into the well and to the surface.
Production versus Time
As well which contains a productive product (oil or gas) can produce naturally at some producing rate. In some wells, this producing rate is too low to be economical due to the natural flow capacity being damaged by the drilling fluids, cementing or the perforating processes. Understanding the reservoir helps the oil companies and/or service companies to determine the type of stimulation treatment or method to use to improve the productivity. This graph illustrates production versus time which might be expected from a well. (1) Natural production (no stimulation). (2) Production for the normal stimulation job. (3) Production for a better stimulation method aimed at optimization for a particular reservoir condition.
Certain acids, when put into contact with suitable reservoir rocks, etch or dissolve them. The fact that acid reacts with rock and forms the basis of a stimulation method known as acidizing. Two types of oil well acidizing are fracture acidizing and matrix acidizing.
Fracture acidizing involves pumping acid into the reservoir at a pressure high enough to cause it to fracture. Fracture acidizing is similar to fracturing; however, acid instead of water is used as a fracturing fluid. The acid etches the face (each side) of the fracture to create flow channels for reservoir fluids. Fracture acidizing is utilized mainly in limestone formations.
Matrix acidizing does not involve fracturing. The body of the rock in which hydrocarbons and other fluids occur is often known as the rock matrix. In this method acid is injected down the tubing and into the permeable channels of the reservoir. The injection pressure is kept low enough to prevent fracturing the formation. Certain acids can remove substances that block reservoir permeability, especially mud solids.
Many different kinds of acids are used in acidizing oil wells. One of the more common is Hydrochloric acid. Hydrochloric acid is a strong acid that works best on carbonate rocks. Carbonate rocks include limestone and dolomite. Limestone is mostly calcium carbonate, and dolomite is mostly calcium magnesium carbonate. Limestone and dolomite reservoirs are called carbonate reservoirs.
Acetic acid and formic acid are sometimes used to acidize carbonate reservoirs that have high temperatures - 250 degrees Fahrenheit or more. Another acid used is hydrofluoric, or mud, acid. Hydrofluoric acid reacts with quartz, sand, and clay. Also, various acids can be mixed in order to obtain the desired results in an acidizing job. Regardless of the acid used, the idea behind acidizing is to enable reservoir fluids to flow out of the reservoir and into the wellbore.