Hydraulic fracturing (Fracturing) is one of the most effective geological and technical measures, the purpose of which is to intensify the flow of reservoir fluid to production wells. The use of this technology allows not only to increase the production of reserves in the radius of the drainage of the well, but also to expand this area, increasing the final oil recovery. Given this factor, the design of field development can be done with the arrangement of a rarer grid of wells.
Short description
The essence of hydraulic fracturing is described by the following process:
- the overburden is affected by excessive pressure (the flow rate of the process fluid is much greater than it can be absorbed by rocks);
- downhole pressure increases until it exceeds internal stresses in the collector;
- rocks are torn in the plane of least mechanical strength (most often in an inclined direction or vertically);
- newly formed and old cracks increase, their connection with the system of natural pores appears;
- increased permeability zone near the well increases;
- special granular proppants (proppants) are pumped into extended cracks for their fixation in the open state after the pressure on the reservoir is removed;
- the resistance to the movement of the reservoir fluid becomes practically equal to zero, as a result, the flow rate of the well increases several times.
The length of the cracks in the rocks can be several hundred meters, and the bottom of the well becomes associated with remote sections of the reservoir. One of the most important factors in the effectiveness of this treatment is crack fastening, which allows you to create a filter channel. However, well productivity cannot increase indefinitely with increasing fracture size. There is its maximum length, beyond which the flow rate does not become more intense.
Application area
This technology is used both for producing (increasing oil recovery) and injection (increasing injectivity), horizontal and vertical wells. The following areas of hydraulic fracturing are distinguished:
- intensification of the flow rate of wells with a contaminated bottomhole zone in formations with different permeability;
- development of heterogeneous deposits structure;
- improving the hydrodynamic connection of the well with the natural system of fractures in the reservoir;
- expansion of the reservoir fluid inflow zone;
- development of low permeability formations and low profit wells;
- change in filtration flows in injection wells;
- restoration of parameters of wells that are not amenable to influence by other methods.
The limitations for hydraulic fracturing technology are gas and oil zones, which are characterized by the following features:
- rapid cone formation (pulling formation water to the bottom of the well);
- sudden breakthroughs of water or gas into the wellbore;
- depleted reservoirs with low reserves, oil-saturated lenses of a small volume (due to economic unprofitability).
Most often, hydraulic fracturing is used as an intensification method for medium and high permeability formations. For them, the main factor in increasing the flow of formation fluid is the length of the formed fracture, and in the deposits with low permeability of the rocks - its width.
Hydraulic fracturing: advantages and disadvantages
The advantages of hydraulic fracturing are:
- the possibility of application in areas with a diverse geological structure;
- impact both on the whole deposit and on its section;
- effective reduction of hydraulic resistance in the bottomhole zone;
- the inclusion of weakly drained adjacent areas;
- cheap working fluid (water);
- high profitability.
The disadvantages include the following:
- the need for large stocks of water, sand, additional chemicals;
- uncontrolled process of creating cracks in the rock, unpredictability of the mechanism of crack formation;
- when wells with large flow rates are put into operation after hydraulic fracturing, proppant can be removed from the fractures, resulting in a decrease in the degree of their disclosure and a decrease in flow rate in the first months after the start of operation;
- the risk of uncontrolled gushing and environmental pollution.
Process varieties
Hydraulic fracturing methods differ in the type of formation of cracks, the volume of injected fluid and proppants, as well as other characteristics. The main types of hydraulic fracturing include the following:
- According to the area of โโinfluence on the reservoir: local (length of cracks up to 20 m) - has the greatest distribution; deep penetrating (crack length 80-120 m); massive (1000 m and more).
- By coverage of formations: single (impact on all formations and interlayers); multiple (for wells that have opened 2 or more layers); interval (for a specific formation).
- Special methods: acid fracturing; TSO technology - the formation of short cracks to prevent their spread to the oil-water contact and to reduce the proppant injection volume (this method shows high efficiency in sand reservoirs); pulsed (the creation of several radially diverging cracks in medium and high permeability rocks to reduce the skin effect - deterioration of the permeability of pores due to their contamination by particles contained in the filtered formation fluid.
Multiple break
Multiple fracturing is carried out using several methods:
- Initially, a crack is created using conventional technology. Then it is temporarily clogged by pumping substances (granular naphthalene, plastic balls and others), covering the perforations. After that, hydraulic fracturing is done in another place.
- The separation of the zones is carried out using packers or hydraulic shutters. For each of the intervals, hydraulic fracturing is performed according to the traditional scheme.
- Phased hydraulic fracturing with isolation of each underlying zone by a sand cork.
In clay sections, the most effective is the creation of vertical cracks, since they connect productive oil and gas bearing layers. Such cracks are produced by exposure to non-filtering liquids or by a rapid increase in the injection rate.
Preparation for hydraulic fracturing
Hydraulic formation technology consists of several stages. The preparatory work is as follows:
- Investigation of the well for formation fluid inflow, ability to absorb the working fluid and determine the pressure required for hydraulic fracturing.
- Sand or clay peel cleaning of the face (rinsing with water under pressure, hydrochloric acid treatment, sandblasting and other methods).
- Well check with a special template.
- The descent into the borehole of the pipe for supplying the working fluid.
- Install a sealing packer and hydraulic anchors to protect the casing.
- Installation of wellhead equipment (manifold, lubricator and other devices) for connecting pumping units to injection pipelines and sealing the well.
A schematic diagram of the strapping of technological equipment for hydraulic fracturing is shown in the figure below.
Fracturing sequence
Technique and technology of hydraulic fracturing consists of the following procedures:
- The working fluid is fed into the injection pipes (most often, oil - for the producing well or water - for the injection).
- Increase the pressure of the fracturing fluid to the maximum design value.
- Check the tightness of the packer (there should be no overflow of fluid from the annulus).
- Proppant is added to the working fluid after hydraulic fracturing occurs. This is judged by a sharp increase in injectivity of the well (pressure drop in the pumps).
- The last batch of proppant includes radioactive isotopes for subsequent verification of the absorption zone using nuclear logging.
- The squeeze fluid with the highest pressure is supplied for reliable wedging of cracks.
- The fracturing fluid is removed from the bottom to ensure the flow of formation fluid into the wellbore.
- Dismantle the technological equipment.
- Let the well into operation.
If the well is relatively shallow, then the working fluid may be fed through the casing. It is also possible to carry out hydraulic fracturing without a packer - through tubing and annular space. This helps to reduce hydraulic losses for highly viscous fluids.
Machines and mechanisms for hydraulic fracturing
Equipment for hydraulic fracturing includes the following types of equipment:
- Ground vehicles and devices: pumping units (ANA-105, 2AN-500, 3AN-500, 4AN-700 and others); sand mixing plants on the chassis of vehicles (ZPA, 4PA, USP-50, Kerui, Lantong and others); tankers for transporting liquids (ACN-8C and 14C, ATK-8, Sanji, Xishi and others); piping of the mouth (manifold, wellhead, shutoff valves, distribution and pressure manifold with check valves, pressure gauges and other equipment).
- Ancillary equipment: units for hoisting operations; Winches monitoring and control stations; pipe trucks and other equipment.
- Underground equipment: packers for uncoupling the formation in which hydraulic fracturing is planned from another part of the production string; anchors to prevent the rise of underground equipment due to high pressure; tubing string.
The type of equipment and the number of units of equipment are determined based on the design parameters of hydraulic fracturing.
Design characteristics
The following basic formulas are used to calculate hydraulic fracturing:
- Bottom-hole pressure (MPa) for hydraulic fracturing using a filtered fluid: p = 10 -2 KL c , where K is a coefficient selected from the interval of values โโ1.5-1.8 MPa / m, L c is the length of the well, m
- Liquid injection pressure with sand (to crack a crack): p p = p - ฯgL c + p t , where ฯ is the density of the sand carrier fluid, kg / m 3 , g = 9.8 m / s 2 , p t is the pressure loss sand-carrier fluid friction. The last indicator is determined by the formula: p t = 8ฮปQ 2 ฯL c / (ฯd B ) 2 , where ฮป is the hydraulic resistance coefficient, Q is the injection rate, m 3 / s, d B is the inner diameter of the tubing.
- Number of pumping units: n = pQ / (p p Q p K T ) + 1, where p p is the working pressure of the pump, Q p is its supply at a given pressure, K T is the coefficient of the technical condition of the machine (selectable within 0.5 -0.8).
- Amount of displacement fluid: V = 0.785d B 2 L c .
If hydraulic fracturing occurs using sand as a proppant, then its amount per 1 operation is assumed to be 8-10 tons, and the amount of fluid is determined by the formula:
V = Q s C s , where Q s is the amount of sand, t, C s is the concentration of sand in 1 m 3 of liquid.
Calculation of these parameters is important, since if the pressure is excessively high during hydraulic fracturing, fluid is pushed into the reservoir, and accidents occur in the production casing. Otherwise, if the value is too low, it will be necessary to stop the hydraulic fracturing due to the inability to reach the required pressure.
The design of hydraulic fracturing is as follows:
- Selection of wells according to the existing or planned field development system.
- Determination of the best crack geometry, taking into account several factors: rock permeability, well pattern, proximity to the oil-water contact.
- Analysis of the physical and mechanical characteristics of rocks and the selection of a theoretical model of crack formation.
- Determination of the type of proppant, its amount and concentration.
- The choice of fluid for hydraulic fracturing with suitable rheological properties and the calculation of its volume.
- Calculation of other technological parameters.
- Determination of economic efficiency.
Hydraulic fracturing fluids
Working fluids (displacement, fracturing and sand carrier) - this is one of the most important elements of hydraulic fracturing. The advantages and disadvantages of their various types are associated primarily with rheological properties. If previously only viscous compositions based on oil were used (to reduce their absorption by the reservoir), then an increase in the power of pumping units has now made it possible to switch to water-based liquids with a low viscosity. Due to this, the pressure on the mouth and losses on hydraulic resistance in the tubing string were reduced.
In world practice, the following main types of hydraulic fluids are used:
- Water with and without proppants. Its advantage is low cost. The disadvantage is the small depth of penetration into the reservoir.
- Polymer solutions (guar and its derivatives GPG, KMGPG; hydroxyethyl cellulose ether, carboxymethyl cellulose, xanthan gum). B, Cr, Ti, Zr and other metals are used to crosslink molecules. By price, polymers belong to the middle category. The disadvantage of such fluids is a high risk of negative changes in the reservoir. The advantages include a large penetration depth.
- Emulsions consisting of a hydrocarbon phase (diesel fuel, oil, gas condensate) and water (mineralized or fresh).
- Hydrocarbon gels.
- Methanol.
- Thickened carbon dioxide.
- Foam systems.
- Foams, consisting of cross-linked gels, nitrogen or carbon dioxide foams. They have a high cost, but do not affect the quality of the collector. Their other advantages are high proppant load bearing capacity and self-destruction with a small amount of residual liquid.
To improve the functions of these compounds, various technological additives are used:
- surfactants;
- emulsifiers;
- joints that reduce hydraulic friction;
- blowing agents;
- acid changing agents;
- thermal stabilizers;
- bactericidal and anticorrosive additives and others.
The main characteristics of hydraulic fracturing fluids include:
- dynamic viscosity required for crack opening;
- infiltration properties that determine fluid loss;
- the ability to transfer proppant without premature sedimentation from solution;
- shear and temperature stability;
- compatibility with other reagents;
- corrosive activity;
- environmental friendliness and safety.
Liquids with a low viscosity require injection of a larger volume in order to achieve the required pressure in the reservoir, and with a high pressure, a larger head developed by pumping technology, as this results in significant losses in hydraulic resistance. More viscous fluids are also characterized by lower filterability in the rocks.
Proppant materials
As proppants, or proppants, the following are most often used:
- Quartz sand. One of the most common natural materials, and therefore its cost is low. Fixes cracks in various geological conditions (universal). The size of the sand grains for hydraulic fracturing is selected 0.5-1 mm. The concentration in the sand carrier varies between 100-600 kg / m 3 . In rocks characterized by severe fracturing, the material consumption can reach several tens of tons per well.
- Bauxites (aluminum oxide Al 2 O 3 ). The advantage of this type of proppant is its greater strength compared to sand. It is made by grinding and roasting bauxite ore.
- Zirconium oxide. It has properties similar to the previous type of proppant. Widely used in Europe. A common disadvantage of such materials is their high cost.
- Ceramic granules. For hydraulic fracturing, granules from 0.425 to 1.7 mm in size are used. Relate to medium-strength proppants. Show high economic efficiency.
- Glass balls. Previously used for deep wells, it is now almost completely replaced by cheaper bauxite.
Acid fracturing
The essence of acid hydraulic fracturing is that at the first stage, a crack is artificially created (as with conventional hydraulic fracturing technology), and then acid is pumped into it. The latter reacts with the rock, there are long channels that increase the permeability of the reservoir in the bottomhole zone. As a result, the coefficient of oil recovery from the well increases.
This type of hydraulic fracturing process is especially effective for carbonate rocks. According to researchers, more than 40% of the world's oil reserves are associated with this type of reservoir. . . (, , ).
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