What are the measures to increase production and injection of oil and water wells?

Oil extraction wells or water injection wells, due to certain factors, make the oil layer near the bottom of the well clogged, as a result of which the production of oil wells is reduced, or even no oil, or water injection wells can not be injected into the water, which affects the pressure of the oil reservoir and the effect of water driven oil, reducing the recovery rate of the oil reservoir. In this case, two technical measures have been proposed to transform the oil formation: fracturing and acidizing.

I. Fracturing

Fracturing, also known as hydraulic fracturing, is the use of high-pressure pumping units on the ground to pump highly viscous fluids (fracturing fluids) into wells at a displacement that exceeds the absorption capacity of the formation, generating high pressure at the bottom of the well. When this pressure exceeds the fracture pressure of the formation, one or several fractures are created at the bottom of the well. Fracturing fluid with proppant is then injected into the fracture, and when the pump is stopped, a fracture of sufficient length, width and height that is no longer closed can be formed in the formation. This kind of sand-filled fracture has high flow-conducting ability, which greatly improves the seepage conditions of oil and gas in the near-well zone, and achieves the purpose of increasing the production of oil wells or increasing the injection of water wells.

In recent years, with the continuous improvement of technology level, hydraulic fracturing has become an important means of low permeability reservoir reforming and increasing production and injection.

(I) Fracturing fluid

Fracturing fluid is the working fluid in the process of hydraulic fracturing to reform oil and gas reservoirs, and plays the roles of transmitting pressure, forming and extending fractures, and carrying proppant. Fracturing fluid and its performance have a close relationship with the size of the created fracture size and the conductivity of the fracture, so the fracturing fluid is an important factor affecting the effect of fracturing.

Fracturing fluid is a general term for fracturing construction fluid. According to the role of fracturing fluid in different stages of the fracturing process, it can be divided into:

Clearing fluid - 5% HCl and 0.2% surfactant aqueous solution in conjunction with plugging ball, to unclog the shot hole borehole of fractured well section.

Forward cushion fluid - when fracturing water-sensitive, scaling or high wax content formations, it is necessary to pump clay stabilizers, descalers or wax cleaners in advance; at the same time, this fluid can also play a role in lowering formation temperatures for high-temperature and deep-well formations.

Fracturing fluid - Generally, proppant-free fracturing fluid is used as a pre-fracturing fluid to open up the formation, lower the formation temperature and extend the fracture, and prepare the space for sand-carrying fluid to enter the fracture.

Sand Carrying Fluid - is used to further extend the fracture, carry proppant into the fracture, and fill the sand bed with high conductivity. The sand-carrying fluid is the main fluid used to complete the fracturing operation and evaluate the performance of the fracturing fluid.

Topping-off fluid--This fluid is used to top-off all the sand-carrying fluid into the fracture of the formation so as to prevent sand from sinking to the bottom of the well. The volume of topdressing fluid is the volume of the wellbore, and it cannot be overtopped.

With the continuous improvement of the fracturing process, superior fracturing fluids are constantly emerging. Nowadays, the frequently used fracturing fluids include water-based fracturing fluid, oil-based fracturing fluid, emulsion fracturing fluid, foam fracturing fluid and so on. In particular, the water-based freeze gel fracturing fluid developed in recent decades has the advantages of high viscosity, low friction resistance and good sand suspension performance, and has now become the most widely used fracturing fluid at home and abroad.

(1) Active water fracturing fluid (water-based): low viscosity fracturing fluid with surfactant added to the water solution. This fracturing fluid is simple to formulate, low cost, low viscosity, high filtration loss and weak sand-carrying capacity, and is suitable for small-scale unblocking fracturing of shallow wells with low sand volume and low sand ratio and fracturing of coalbed methane wells.

(2) Thickened water fracturing fluid (water-based): a viscous aqueous solution formulated with thickeners and surfactants. The viscosity of thickened water fracturing fluid is higher than that of activated water fracturing fluid, the sand-carrying capacity is a little stronger, and the performance of filter loss reduction is a little better. It is mainly used for small-scale fracturing in low temperature (less than 60℃), shallow wells (less than 1,000m) and low sand ratio (less than 15%).

(3) Water-based freeze-gel fracturing fluid (water-based): this is a kind of elastic, non-sticky and container jelly-like fracturing fluid. Water-based freeze-gel fracturing fluid has a strong sand-carrying capacity and minimal frictional resistance, making it a more desirable fracturing fluid.

(4) Thickened oil fracturing fluid (oil-based): it is a fracturing fluid made of polymer dissolved in oil. Its base fluid is crude oil, gasoline, diesel, kerosene, condensate. Its advantages are high viscosity, strong sand suspending ability, small filtration loss, and no harm to the oil formation; its disadvantages are high cost, high friction resistance when flowing, and the viscosity decreases very quickly with the temperature, so it is only suitable for low-pressure, oleophilic, and strong water-sensitive formations.

(5) emulsified fracturing fluid: a liquid is dispersed in another liquid that is not miscible with it to form a multi-phase dispersion system. The phase that exists in the form of liquid beads is called dispersant (or inner phase, discontinuous phase); the phase that plays a role in dispersion is called dispersant medium (or outer phase, continuous phase). In emulsions used as fracturing fluids, one phase is water or brine solution, polymer thickened aqueous solution, aqueous gel solution, acid solution, and alcohol solution; the other phase is crude oil, refined oil, condensate, or liquefied petroleum gas. In addition, surfactants must be added to the system to facilitate the formation of stable emulsions. The emulsified fracturing fluid is characterized by a certain viscosity, low filtration loss, and little damage to the formation, but its frictional resistance is generally higher than that of water or oil, and it is suitable for water-sensitive, low-pressure formations.

(6) Foam fracturing fluid: it is a dispersion system in which gas is dispersed in liquid. In order to stabilize the foam, a foaming agent is usually added. The gas phase in the system is CO2, N2, air; the liquid phase is thickened water, water jelly, acid, alcohol or oil; the foaming agent is mostly non-ionic surfactant. The characteristics of this fracturing fluid are: small friction resistance loss, low filtration loss, fast return speed, strong sand-carrying capacity, little damage to the formation, and it is suitable for gas-bearing sandstone or shale formations, low-permeability, low-pressure, and water-sensitive formations.

(2) Proppant

In hydraulic fracturing, the role of proppant is to fill the hydraulic fracture produced by fracturing, so that it will not be reclosed under the action of rock stress, and to form a flow channel with a certain degree of inflow capacity. Obviously, the greater the length and width of the propped fracture, the stronger the fracture's flow-conducting ability, and the better the fracture's production increase.

Proppants for fracturing can be broadly categorized into three major types: natural, man-made and natural modified. The natural one is represented by quartz sand, the man-made one is represented by ceramic granule, and the natural modified one is represented by resin-coated sand.

1. Quartz sand

Quartz is a widely distributed, hardness of the stability of the mineral, but also the first widely used proppant, so far at home and abroad still in the first place. Quartz sand hardness, brittle, when hard strata broken will greatly reduce the ability of the cracks of the inflow, when soft strata and easily embedded inside the cracks. However, the quartz density is low, easy to construction pumping; cheap, easy to get; good sphericity, strong infusion ability, is still the most commonly used proppant at home and abroad.

2. Artificial ceramic particles

Since the late 1970s, with the deep, dense layer of exploration and development needs, our country has developed a blowing bauxite high-strength proppant, medium-high-density high-strength sintered bauxite ceramic particles and low-density medium-strength sintered bauxite ceramic particles. These sintered or blown artificial proppants are collectively called ceramic grains, and their main features are: high strength, salt and temperature resistance, and low crushing rate; however, their relative density is high, which puts forward higher requirements on the performance of fracturing fluids and pumping conditions, and their processing is complicated and costly.

3. Resin-coated sand

Resin-coated sand is a kind of proppant made of modified phenolic resin wrapped on the surface of quartz sand by a special process and heat-setting treatment. According to the wrapping method of the resin, it can be divided into two kinds of cladding sand, pre-cured and (can be) cured, which undertake different tasks in fracturing. The former is wrapped with a layer of resin on the surface of quartz sand, so that even if the sand inside the cladding is crushed, the resin on the outside can still encapsulate the crushed pieces and particles, thus keeping the fracture with a high conductivity; the latter is pre-wrapped with a layer of resin on the surface of the quartz sand matching the temperature of the fracturing layer, and is placed as a trailing proppant on the near-well-seam section of the hydraulic fracture, and when the fracture is closed and the temperature of the formation is restored, this (??curable) resin cladding sand can be used as a proppant for the fracturing. When the fracture is closed and the formation temperature is restored, the (curable) resin-coated sand first softens into a glass ball under the formation temperature, and then the same (curable) resin-coated sand around it is cemented from soft to hard, thus forming a "barrier" in the depth of the fracture and the wellbore zone and preventing the proppant in the fracture from regurgitating and refluxing.

In addition to the above types, in the 1950s and 1960s used metal aluminum balls, plastic balls, walnut shells and glass balls and other proppant, due to their own shortcomings by the limitations of the proppant has been replaced by better proppant, is no longer in use.

(C) Fracturing process

The fracturing process includes a series of work, such as the selection of fracturing wells (layers), the selection of fracturing process, and the optimization and design of fracturing construction parameters. When the fracturing fluid, proppant and fracturing equipment have been determined, the effect of fracturing depends on the fracturing process.

The nature of the formation, pressure, temperature and other conditions are different in each region, and the well completion methods and technical equipment conditions are also different, so the fracturing process is also different. The following is a description of some of the more commonly used fracturing process methods.

1. Combined-layer fracturing technology

The production layer of oil and gas wells is often a layer group, and fracturing the various small layers of this layer group at the same time is called combined-layer fracturing, also called cage fracturing. For wells completed in barehole, the barehole section is difficult to be fractured into small layers, so this method is often used for fracturing. The specific construction is divided into three cases: tubing fracturing, casing fracturing and simultaneous tubing fracturing. Oil pipe fracturing is to squeeze the fracturing fluid into the bottom of the well from the oil pipe, and take protective measures such as bringing hydraulic anchor and casing with balanced pressure; casing fracturing is not to put down the oil pipe in the well, and install the wellhead to fracture directly; oil casing fracturing at the same time is to connect some fracturing cars to each of the oil pipe and casing outlet, and at the same time, injecting the fracturing fluid into the well, and adding sand from the casing.

2. Layered fracturing technology

Fracturing construction, when the purpose layer has multiple layers, in order to achieve the purpose of complete transformation, to use layered fracturing technology.

Currently, one of the more widely used fracturing techniques at home and abroad is packer fracturing. It is realized by layering pipe columns with packers. The packer is the key to the layered fracturing column, and its function is to isolate the destination layer from the upper and lower reservoirs, prevent the fracturing fluid from entering the upper and lower reservoirs, and connect the destination layer independently to the pressure system within the fracturing column. For the bottom layer, single packer can be used to fracture; for wells with multiple layers, double packer can be used to fracture any layer; for deep wells with multiple layers, "bridge plug + packer" can also be used to fracture in layers.

Second, acidizing

Acidizing is a chemical measure to increase production and injection by injecting acid liquid prepared according to requirements from the ground to the formation through the well, in order to remove the blockage in the near-well zone and restore the permeability of the formation, or to corrode certain components in the oil formation through the chemical reaction between acid and rock, so as to restore or improve the permeability of the oil formation.

(I) acid type

What kind of acid is used in acidizing must be selected according to the characteristics of the formation and blockage of the acidized well, the purpose of the measure and the construction requirements.

1. Hydrochloric acid

Acidizing, hydrochloric acid concentration is generally 6% to 15%, but with the emergence of efficient corrosion inhibitors, can be directly used in industrial hydrochloric acid (concentration of about 30%) acidizing. The use of concentrated hydrochloric acid can acidify the deeper layers, reducing the dilution of formation water and generating more CO2, which facilitates the discharge of residual acid.

Hydrochloric acid can dissolve the corrosion products that block water wells, thus restoring the permeability of the formation, e.g.

Hydrochloric acid can also dissolve carbonate minerals (calcite, dolomite, etc.) in the oil and water wells and in the formations, e.g.

Reactants can be soluble in water, and they can be discharged to the surface along with the waste acid, which increases the pore size of the formation and the permeability of the near-well zone.

If acidizing a high-temperature well or a deep well, you can't use hydrochloric acid directly because the reaction rate is too fast to act on the far-reaching formations. Then we can use potential acid, the so-called potential acid, refers to those substances that can produce acid under certain conditions, such as:

2. Hydrofluoric acid

Hydrofluoric acid (HF) can dissolve the clays (mainly minerals such as montmorillonite, ilmenite, kaolinite, etc.) that clog the strata or cement the strata, and also dissolve siliceous substances (quartz and feldspar) in the sandstone, so that it can restore or enhance the permeability of the strata.

Since hydrofluoric acid has the above properties, a certain amount of hydrofluoric acid can be added to improve the acidizing effect when acidifying sandstone formations with clay plugs or clay cementation. The commonly used soil acid acidizing solution in the oilfield is a mixture of 6% to 15% hydrochloric acid and 3% to 15% hydrofluoric acid.

Hydrofluoric acid is not used in all cases. For carbonate (limestone, dolomite) formations, if hydrofluoric acid is used, it can produce precipitates that clog the formation.

According to the conditions of the formation, the actual situation of site construction, and the different purposes of acidification, different acidifying liquids can be used for acidification, such as multi-component acid, emulsifying acid, thickening acid, formic acid and acetic acid, etc., which can have different acidifying effects.

(B) acid additives

Acid with the acid, in order to achieve a specific purpose of the chemical substances added to the acid is called acid additives. Commonly used acid additives are retardant, corrosion inhibitor and iron ion stabilizer.

1. retarder

To reduce the acid, rock reaction speed, improve the acidification radius of the substance called retarder. Addition of retardant acid solution is called retardant acid. Commonly used retardants are surfactants and thickeners.

Surfactants such as sodium dodecyl sulfate, they are adsorbed on the surface of the rock, the hydrophobic groups outward to prevent the acid from contacting the reaction with the rock, reducing the reaction rate. In addition, surfactant adsorption in the formation near the bottom of the well is large, and the reaction rate of acid and rock is small; when the acid enters into the deep part of the formation, the concentration of surfactant decreases, the adsorption is small, and the reaction rate of acid and rock is large. The addition of surfactant is also conducive to the return of residual acid. The amount of surfactant added is about 1%.

The thickener is commonly used xanthan gum, polyethylene glycol (used at low temperature), polymer (such as polycationic compounds). The addition of the thickener increases the viscosity of the acid, reduces the diffusion rate of H+ in the acid to the surface of the rock, and thus reduces the reaction rate of acid and rock.

2. Corrosion inhibitor

The chemical substances used to reduce the corrosion rate of acid on downhole metal equipment (such as tubing, casing) are called corrosion inhibitors. Corrosion inhibitors are divided into inorganic corrosion inhibitors, organic corrosion inhibitors. Commonly used in oil fields are organic corrosion inhibitors containing O, S, N heteroatoms, such as 7701, imidazoline and so on.

3. Iron ion stabilizer

When the acid, rock reaction, the acid pH is reduced, the acid in the iron salts (especially Fe3+) hydrolysis precipitation, resulting in a secondary blockage of the formation pore, so often in the acid add iron ion stabilizer. Commonly used iron ion stabilizers have two types: one is a complexing agent, such as citric acid, EDTA sodium salt, etc.; one is a reducing agent, such as isoascorbic acid, sulfurous acid, etc..

(C) Acid treatment methods and acidizing technology

The commonly used acid treatment methods are conventional acidizing and fracturing acidizing.

Conventional acidizing is acidizing in which the injection pressure is less than the rupture pressure of the formation, in order to unblock the formation near the bottom of the well, so it is also called unblocking acidizing.

Fracturing acidizing is acidizing in which the acid injection pressure is greater than the fracture pressure of the rock, i.e., acidizing is carried out on the basis of fracturing, in which the fracture is formed by hydraulic action on the one hand, and the wall of the fracture is dissolved into an uneven surface by the dissolving action of the acid on the other hand. After stopping the pump and unloading the pressure, the wall of the fracture can not be completely closed, and it has a high inflow capacity.

In recent years, with the development of the petroleum industry, acidification technology is also more and more advanced. In addition to ordinary hydrochloric acid, earth acid acidification, there are foam acid acidification, micellar acid acidification, emulsified acid acidification, thickened acid acidification and chemical retardation acid acidification and other technologies.

(D) the residual acid discharge

Acid construction is over, stay in the stratum of the residual acid water activity has basically disappeared, can not continue to erode the rock, and with the increase in its pH, the original will not be precipitated metal precipitation will be generated one after another metal hydroxide precipitation. In order to prevent the generation of precipitation secondary clogging stratum pores, affecting the effect of acidification, in general, the residual acid should be discharged as soon as possible. For this reason, should be done before the acidification and put into production preparatory work, acidification construction immediately after the end of the liquid discharge.

After the residual acid flows to the bottom of the well, if the residual pressure (bottom pressure) is higher than the back pressure of the wellbore liquid column, it can rely on the energy of the stratum to carry on the release and discharge of liquid; if the residual pressure is lower than the back pressure of the wellbore liquid column, then it is necessary to manually discharge the residual liquid from the wellbore to the surface. At present, the commonly used artificial liquid discharge method: first, reduce the pressure of the liquid column or reduce the density of the liquid, such as drawdown method, gas lift method; second is to increase the injection of liquids to help spraying, such as the injection of liquid carbon dioxide method and liquid nitrogen method.