Oil-fracture is an important technical measure for increasing oil and gas production and filling injection wells. Since 1949, it has become a method for increasing the production of oilfields at home and abroad. Today's fracturing equipment capacity can open deep wells of up to 6 000 m with a seam length of up to 1 km.
In order to achieve the desired effect of fracturing, fracturing wells, fracturing units, fracturing fluids, and proppants must be reasonably selected based on geological conditions.
Although hydraulic fracturing is a widely used stimulation measure, it is not effective for all wells. In general, the following wells are suitable for fracturing:
The oil layer rock is compact and has low permeability. For example, tight sandstone, limestone, etc., have better effects after fracturing.
Low-yield wells with high oil saturation and high oil well pressure. After fracturing, the output is often greatly increased.
Wells whose oil layers are blocked near the wellbore, reducing production and water absorption capacity. Small-scale fracturing is very effective in relieving clogging.
In order to improve the fracturing effect, oil-water wells can also be used for fracturing, mainly water wells. This can play a role in efficiency and efficiency.
In addition, the overall plan should be adjusted according to the geological conditions of the oilfield and the layout of the well pattern to give full play to the role of oil and water wells. For wells with high permeability, wells at the edge of oil and water and wells with poor cementing quality are generally not suitable for fracturing.
The fracturing fluid should be selected according to the fluid properties of the reservoir and the physical and chemical properties of the formation.
The type of fracturing fluid is essentially determined based on the chemical nature of the rock. For limestone and dolomite, acid-based fracturing fluid should be used. For sandstone and low-dissolved rock formation, water-based fracturing fluid or oil-based fracturing fluid should be used. Divalent cations can also be added to water-based fracturing fluid. (eg add 0.5% CaCl2). For the water injection well, salt water can be used as the fracturing fluid. If the production layer contains salt components that are easily soluble in water, fresh water can also be used.
The physical properties of rocks (temperature, pressure, permeability, porosity, presence or absence of primary pores and pores, etc.), especially temperature and pressure, need to be well considered. Generally, the viscosity of the fracturing fluid is greatly affected by the temperature. For example, gelled crude oil can only be used within 93 °C; thickened acid is diluted quickly at high temperatures, and the amount of thickener must be increased when the temperature is high. Under normal oil layer temperature, the viscosity of water-based fracturing fluid should be no less than 2 000 mPa·s. When the temperature is high, it is possible to use plant glue with better viscosity-temperature performance or increase the amount of vegetable gum.
In high-pressure well fracturing, high relative density fracturing fluids are required to overcome bottomhole pressure and reduce the power required by ground equipment. In the case of fracturing in low pressure wells, low viscosity, low relative density liquids should be used, which are easily discharged from the formation after fracturing.
The fracturing fluid selected must be compatible with the formation fluid and will not produce harmful emulsions or sediments. For oil layers containing heavy crude oil, bitumen or paraffin, it is preferred not to use low relative density crude oils, but water based fracturing fluids or aromatic crude oils.
The type of proppant choice depends on the nature of the formation and the depth of the well. For shallow wells with low rock embedding pressure, sand can be used. For deep wells with large embedded pressure, aluminum balls or copper balls that are not easily deformed or crushed are generally used. For medium and deep wells with medium embedded pressure, walnut shells or hard plastic balls can be used. . Due to the high cost of high-strength proppants, sometimes a deep well can be pressed into a portion of the sand and then a portion of the high-strength proppant.
The choice of sand to liquid ratio depends on the performance of the fracturing fluid and the displacement of the pump during construction. In general, the high sand-liquid ratio under certain conditions has a good fracturing effect. However, it is also restricted by other factors. If the displacement of the fracturing fluid is not affected, the sand-to-liquid ratio is simply increased, which often causes sand plugging during construction. Under current equipment and fracturing conditions, the sand ratio is generally between 10% and 20%. As the viscosity of the fracturing fluid increases, the sand to liquid ratio can be increased to 30% to 40%.
Size of proppant: The diameter of sand commonly used in China is 0.4 to 0.8 mm, 0.8 to 1.2 mm or 1.5 to 2.0 mm. There is currently a tendency for the diameter of the proppant to increase as the amount and viscosity of the fracturing fluid increase.
After each well is fractured, it should be summarized to find out the reasons for success or failure in order to sum up experience and facilitate further battles. A more comprehensive summary, an estimate of the downhole situation. For example, the cracks appearing in the formation are vertical or horizontal, the length of the crack and its ability to conduct flow. It is not enough to record these parameters only by the parameters in the fracturing process. Special methods are needed for measurement. For example, the application of the expansion printing or downhole TV method to find the shape of the crack, the seismic wave and geoelectric method are used to measure the crack orientation.
To evaluate the fracturing effect of a well, two indicators are currently used:
A multiple of the yield or water injection before and after fracturing under comparable conditions;
The length of the period of increase in production.
These two parameters reflect the conductivity of the fracture, the length of the fracture, and the ability of the formation to supply liquid. The evaluation of the fracturing effect can not only verify whether the parameters of the fracturing work are reasonable, but also indicate whether the well selection is appropriate.