RU2280762C1 - Method for hydraulic coal bed fracturing - Google Patents

Abstract

FIELD: mining, particularly to weaken coal bed and to increase recovery of desorbed methane.

SUBSTANCE: method involves drilling well from well surface; casing the well; exposing coal bed; injecting working liquid in the coal bed, wherein the working liquid has pressure enough to perform hydraulic fracturing of the bed and flow rate providing elongated crack forming. Salt solution, gel with propane or gel decomposition substance are used as the working liquid. Propane content in gel is gradually increased from 50-100 kg/m³ to 500-600 kg/m³ as cracks are formed and fixed. The salt is potassium chloride or propant having particle diameters of 20-40 meshes. Coal bed exposing is performed by cutting casing pipe and cement ring in central bed zone within interval of 100 mm so that the ready hole has height of 50-100 mm. The coal bed may be also opened by coal bed drilling below casing pipe for depth of 50-100 mm.

EFFECT: increased formation weakening and increased methane recovery.

3 cl, 2 ex

Description

The invention relates to the field of mining, in particular to the problem of softening a coal seam for intensive extraction of desorbed methane.

A known method of hydraulic fracturing of a coal seam with a liquid, first implemented in the Donbass in 1957-58. for the purpose of creating the initial channels for underground coal gasification [1]. This technical solution for the first time formulates the fundamentals of the technology of hydraulic fracturing of coal seams with liquids of various viscosities.

The limitations of the known method lies in the lack of technological recommendations for effective softening of the coal seam.

There is also known a method of hydraulic fracturing of a coal seam for the purposes of its degassing by pulsed hydroprocessing of a rock mass [2]. The disadvantage of this known technical solution is the locality of the impact, limited only by a small area around the injection well.

The closest technical solution is the method of hydraulic fracturing of a coal seam, in which a suspension consisting of an acid solution and solid particles is used as a hydraulic fracturing fluid [3]. The disadvantage of this technical solution is the lack of technological recommendations for the main stages of the process. As a result of this, the softening of the coal seam is not effective enough, which determines the production of coal methane from the well not more than 5-8 thousand m ³ per day.

The aim of the present invention is to provide a method of hydraulic fracturing of a coal seam, characterized by its maximum softening, and therefore, increased methane recovery.

This goal is achieved by the fact that in the known method of hydraulic fracturing of a coal seam, which consists in drilling a well from the surface, casing it, opening the coal seam, injecting the working fluid into the coal seam at a pressure sufficient for hydraulic fracturing, with an injection rate that ensures the creation of extended cracks, the use of a solution of salt, a gel with propant, a substance that destroys the gel as a working fluid, with a gradual increase in the amount of propant in the gel as it develops Ia and fixing the formed cracks is used as the salt potassium chloride, 20-40 mesh proppant diameter, increase in the amount of proppant in the gel is carried out gradually from 50-100 to 500-600 kg / m ^3. In addition, the opening of the coal seam is carried out by milling the casing and cement ring in the middle part of the formation in the range of 100 mm to a height of 50-100 mm, or by drilling it below the casing of the well by 50-100 mm.

A comparative analysis of the proposed solution with the prototype shows that the hydraulic fracturing of the coal seam is carried out initially using potassium chloride KCl and the use of proppant, and the increase in the amount of proppant in the gel is carried out gradually from 50-100 to 500-600 kg / m ³ .

In addition, hydraulic fracturing and injection of proppant into the formed gap is carried out through a special zone of a limited surface, formed either by drilling a coal seam or by milling a casing string and a cement ring. All these distinguishing features meet the criterion of "novelty" for the proposed method.

The inventive method was partially implemented at an experimental site in the South Kuzbass during hydraulic fracturing on coal seams. The method is practiced on 4 experimental wells (UM-1.1; UM-1.2; UM-5.1 and UM-5.2).

In each well, several coal seams were subjected to hydraulic fracturing at depths from 550 to 950 m. The pressure of hydraulic fracturing of coal seams at a depth of 940-950 m was 28.0 - 30.0 MPa, and at a depth of 550-560 m - 15.0 - 16 , 0 MPa. Hydraulic fracturing pressure of intermediate coal seams (4-5 coal seams were opened), depending on the depth of their occurrence, ranged from 15-16 to 28-30 MPa.

Based on physical and technical considerations, the process of hydraulic fracturing of a coal seam began with the injection of a salt solution (in our case, KCl). The main objective of this operation is to rinse the surface of the microcrack and clean it of possible inclusions of rock and other substances that can reduce the flow area.

After that, a gel with different proppant contents was injected into the wells. Ceramic balls with a diameter of up to 1.5-2.0 mm, mainly 0.4-0.8 mm after sieving on a sieve, which corresponded to 20-40 mesh, were used as proppant. At the first moment of opening the gap, the content of proppant was limited to 50-100 kg per 1 m ^{3 of} gel with proppant. With a higher proppant content, plugs (excessive proppant) formed in the well, which necessitated emergency cleaning of the well.

As the gap opened and ceramic balls moved along it, the proppant concentration gradually increased to 500-600 kg / m ³ . With a proppant content of more than 600 kg / m ³ , as a rule, plugs also appeared on the bottom of the well. This was the basis for recommendations on the content of proppant in the gel from 50-100 to 500-600 kg / m ³ .

As a gel, an aqueous solution of guar gum was used (resin concentration - 1 kg per 3800 l of solution).

After completion of the “forcing” of the calculated amount of proppant, the gap was washed with a special solution that destroys the gel. Ethylene glycol or glycol ether was used as the solution destroying the gel. The main goal of this stage is to prevent "clogging" of the created gap and environmental pollution of groundwater.

In order to develop the optimal technology for opening the coal seam, two experiments were conducted. In one case, at the bottom of a vertical well (after cementing), a coal seam was drilled 60 mm below the casing. In another case, in the same well, the overlying coal seam was discovered by milling the casing and cement ring in the middle of the seam in the range of 100 mm to a height of 60 mm.

In contrast to the method of opening the coal seam by perforating the column in these opening options, problems with the formation of proppant plugs practically disappeared. At the same time, the identified trend in the content of proppant in the gel was observed from 50-100 to 500-600 kg / m ³ .

The technological parameters of the process of hydraulic fracturing of the coal seam worked out in the experiment can be considered optimal. The developed technological methods will be used to create coal-fired industrial fields in Kuzbass. In 2-3 years it is planned to produce up to 3 billion m ^{3 of} coal methane in Kuzbass annually.

Example 1. The UM-1.1 well was drilled, cased, the formation was opened at a depth of 947 m by milling the casing string and cement ring in the middle of the formation in the range of 100 mm to a height of 60 mm. Conducted hydrodynamic tests of the coal seam at this depth. Water was injected into the well, one pump was operating at an injection rate of 0.1 to 0.6 m ³ / min. Up to a pressure of 27 MPa, the injection rate was minimal (0.1 m ³ / min). After reaching a pressure of 27 MPa, the injectivity of the well increased sharply, and the water injection rate was set equal to 0.6 m ³ / min at a pressure of 30 MPa. This period corresponded to the creation of an extended crack in the coal seam (hydraulic fracture micro-cracks).

The potassium chloride (KCl) solution was pumped through the microslit to clean the walls of the slit of possible substances that reduce its cross section. The well was filled with a 10% KCl solution (well volume - 16.7 m ³ ) and pumped through the slot for 0.5 hours (injection rate 0.6 m ³ / min).

Fracking microcracking was carried out with ceramic balls with a diameter of 20-40 mesh (proppant). An aqueous solution of guar gum was used as a gel carrier.

A mixture of propant and gel was prepared on the surface in a special mixer placed next to the well head. The increase in the concentration of proppant in the gel is carried out gradually. In particular, during hydraulic fracturing at the UM-1.1 well, the concentration of proppant was increased in 4 stages: 80, 210, 400 and 550 kg / m ³ . Accordingly, these propant-gel mixtures were prepared in a mixer.

Each stage was carried out for approximately 10 min at an injection rate of 3 m ³ / min (4 pumps worked) and a discharge pressure of 30 MPa.

And, finally, in the last operation, the proppant gap was washed with a glycol ether solution that destroys the guar gum and prevents environmental pollution of groundwater and the “plugging” of the hydraulic fracture (micro fracture gap secured by proppant).

Example 2. Analogously to example 1, but we open the underlying layer - at the bottom of a vertical well (after cementing) we drilled a coal seam below the casing by 60 mm.

Information sources

1. Fedorov N.A. etc. The use of the process of hydraulic fracturing of coal seams at a depth of more than 250 m. Proceedings of VNIIIPromgaz, 1962, issue 6, pp. 62-78.

2. A.S. No. 1550174, E 21 F 7/00, 1990.

3. French patent No. 2567955, 4 E 21 In 43/26, 1984 (prototype).

Claims (3)

1. The method of hydraulic fracturing of a coal seam, which consists in drilling a well from the surface, casing it, opening the coal seam, injecting the working fluid into the coal seam at a pressure sufficient for hydraulic fracturing, with an injection rate that ensures the creation of extended cracks, used as a working liquids in series with a salt solution, a gel with a proppant, a substance that destroys the gel, with a gradual increase in the amount of proppant in the gel as the cracks develop and consolidate, distinguishing the fact that they use potassium chloride as a salt, proppant with a diameter of 20-40 mesh, the increase in the amount of proppant in the gel is carried out gradually from 50-100 to 500-600 kg / m ³ , and the opening of the coal seam is carried out by milling the casing and cement ring in the middle part of the formation or by drilling it below the casing of the well. 2. The method according to claim 1, characterized in that the milling of the casing and cement ring in the middle part of the reservoir is carried out in the range of 100 mm to a height of 50-100 mm 3. The method according to claim 1, characterized in that the drilling below the casing of the well is carried out at 50-100 mm