Water absorption capacity is a parameter to measure whether a water injection well is good or bad, which mainly uses the indicator curve, water absorption indexApparent water absorption index、Relative water absorptionTo represent.
Chinese name
Water absorption capacity
Foreign name
water absorbing capacity
Expression
Water absorption index, apparent water absorption index, etc
The curve of injection pressure changing with water injection volume under stable flow condition is called injection well indicator curve.
In the case of layered water injection, the layered indicator curve is the curve showing the relationship between the injection pressure of each layer (the pressure after passing the water nozzle) and the layered water injection volume.
(2) Water absorption index
The water absorption index refers to the daily water injection volume under the unit injection pressure difference, which is usually expressed in K, and the unit is mthree/(d·MPa)。 The water absorption index reflects the water absorption capacity of the formation.It is impossible to shut in the well frequently for pressure measurement during normal water injection. In order to obtain the water absorption index, the method of measuring the indicator curve is often used to measure the daily water injection volume under different flow pressures.
Relative water absorption refers to the percentage of water absorption of a small layer in the total water absorption of the whole well under the same injection pressure.The expression is:
Relative water absorption=water absorption of a single layer/water absorption of the whole well × 100%
The relative water absorption capacity is an indicator of the relative water absorption capacity of each layer.With the relative water absorption of each layer, the layering indicator curve of each layer can be drawn from the whole well indicator curve without layering test.
influence factor
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1. Factors related to downhole operation and management of water injection wells mainly include: plugging of water injection layer caused by kill fluid during operation;The water injection layer is blocked due to improper measures during acidizing, well washing and other operations.
2. Factors related to water quality mainly include:
1) Injection water and corrosion products of equipment and pipelines, such asFerric hydroxideBlockage caused by Fe (OH) 3, ferrous sulfide FeS, etc., and scale (CaCO3, BaSO4, etc.) generated by water in the pipeline.
2) Some microorganisms contained in the injected water (such asSulfate reducing bacteria, iron bacteria, etc.), in addition to its own blocking effect, its metabolites will also cause blocking.
3) The fine mud sand and other impurities in the injected water block the reservoir.
4) The injection water contains unstable salts that may precipitate in the oil layer.For example, the bicarbonate dissolved in the injection water may generate carbonate precipitation in the oil layer due to the change of temperature and pressure during the water injection process, blocking the reservoir pores and reducing the water absorption capacity of the reservoir.
3. Clay minerals in the oil layer expand after encountering water.
4. Formation pressure in water injection well area rises.Due to the rise of formation pressure in the water injection well area, the injection pressure difference is reduced and the water injection volume is reduced.
5. Bacterial blockage.[1]
Measures to prevent the decline of water absorption capacity
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To prevent the decline of water absorption capacity, corresponding measures should be taken against the reasons for the decline of water absorption capacity.During water injection, preventive measures shall be taken to prevent formation plugging.In order to avoid mud damage to the oil layer or bottom hole sand blockage caused by improper operation, during downhole operation of water injection wells, the operation without well killing and blowout is generally adopted, and acidizing is carried out carefully and correctly. The water injection operation should be stable.
The practice in the oilfield shows that the main factors that reduce the water absorption capacity in the process of water injection are water quality and the management of water injection system.Therefore, in the process of water injection, in order to prevent the decline of water absorption capacity of water injection wells, it is necessary to ensure that the water quality meets the requirements, and try to avoid all kinds of blocking caused by unqualified water quality.
In the daily management of water injection wells, attention should be paid to the following issues:
(1) Take water samples for test and analysis in a timely manner, and take immediate measures to ensure that unqualified water is not injected into the oil layer when unqualified water is found;
(2) Flush the surface pipeline, water storage equipment and well flushing as required to keep the pipeline, water storage equipment and well clean;
(3) Ensure stable water injection and reduce fluctuation, so as not to damage the formation structure and prevent the corrosives on the pipe wall from polluting the water quality and blocking the formation.
Improvement measures
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In the process of water injection, preventive measures should be taken to prevent formation plugging.The main reason for the decline of water absorption capacity is poor water quality or imperfect management of water injection system.
First of all, ensure that the water quality meets the requirements and avoid all kinds of blocking caused by unqualified water quality.The second is to strengthen the daily management of water injection wells. For this reason, water samples should be taken regularly for testing and analysis. If the water quality is found unqualified, immediate measures should be taken to regularly flush the surface pipelines, water storage equipment and wells, inject water steadily so as not to damage the oil layer structure, and prevent the corrosives on the pipe wall from polluting the water quality and blocking the formation.
（1） Fracturing stimulation
fractureIt is a common method to improve the water absorption capacity of the reservoir, which includes ordinary fracturing and separate layer fracturing.
Ordinary fracturing is applicable to low permeability formation with low water absorption index and high water injection pressure and formation with serious pollution. For the target formation, use a packer to block it as far as possible.
For the formation with thick oil layer, large lithologic difference within the layer or large difference between multiple layers, layered fracturing can be used to improve the water absorption contradiction between layers.
（2） Acidizing injection enhancement
Acidizing is another effective measure to improve the water absorption capacity of water injection wells.On the one hand, acidizing can be used to remove bottom hole blockages, and on the other hand, it can be used to improve theAbsolute permeability。
（3） Clay anti swelling
Clay anti swelling agents mainly include:[1]
1. Inorganic salts, such as KCl, NH4Cl, etc.Although this kind of agent can prevent the dispersion and migration of non expansive clay and the expansion of expansive clay, its validity period is short.
2. Inorganicsurface active agent , such as iron salts.Such agents have strict requirements for construction conditions, high cost and short validity period.
3、Ionic surfactant, such as polyamine.This kind of medicament has long validity period, low cost and easy construction.Due to the difference of clay mineral composition and reservoir rock, there is no fixed ready made anti swelling agent that can be used in all kinds of oil layers. To achieve ideal anti swelling effect, careful indoor screening must be carried out.[2]
