CS237901B1 - Hydraulic cracking fluid - Google Patents

Abstract

The solution relates to a water-based hydraulic fracturing fluid for intensification of oil and gas wells. The essence of the solution is that the hydraulic fracturing fluid is formed by a carboxyalkyl starch gel in an aqueous solution. The carboxyalkyl starch content is preferably in the range of 0.3 to 5.0 %.

Description

The invention relates to a water-based hydraulic fracturing fluid for intensifying oil and gas wells.

As is known, the hydraulic fracturing method is used to intensify production, injection and UGS wells. In this case, fluids for creating cracks and fractures are used. Fluids for creating cracks are known as gels and emulsions, the main components of which contain oil fractions. .It is more common to use aqueous fluids, which are given considerable viscosity with additives. This high viscosity gives the fluids the ability to retain sand and other protective materials in suspension for so long that they are deposited in the broken fractures. Only then is it desirable to reduce the viscosity to the extent that they can be easily removed from the formations. The decrease in viscosity should occur after a certain period of time under given p,T conditions in the soil. This period should not significantly exceed the processing time.

At the same time, a small loss of fluid is required at the fracture-matrix interface.

In addition to starches, other viscosity-increasing additives are known, such as gelatin, pectin, guar or alginates.

It is known that the water-based cracking fluids used up to now have the disadvantage that they can lead to increased damage to the matrix due to their relatively high solids content. On the other hand, they do not have a sufficiently stable viscosity above 80 °C, so that premature removal of the protective material and increased initial liquid loss must be expected. This means that a larger amount of cracking fluid must be used beforehand to compensate for this loss.

The decomposition times at storage temperatures until complete loss of viscosity are long, meaning that it is only after several hours or even days that the viscosity of the fracturing fluid is reduced to the point where it can be easily removed from the wells. This blocks the flow of host rock to the probe for a longer period of time.

The purpose of the invention is to shorten the time lag between the action of the fracturing fluid and the start of full extraction and at the same time prevent long-term blockages, or to reduce the coefficients of the fracturing fluid to such an extent that the additional amount of fracturing fluid that must be additionally available due to fluid loss can be reduced to a minimum.

The invention has as its basic task to create a water-based fracturing fluid with a low fluid coefficient and low initial loss, the viscosity of which can be reduced after a relatively short time.

According to the invention, this task is solved by forming a gel of carboxyalkyl starches in water. The starting product is starch, which is etherified with haloalkylcarboxylic acids, preferably monochloroacetic acid.

The carboxymethyl starch thus formed is cold soluble in water and forms a highly viscous gel. It has been found that the required concentration of carboxymethyl starch in water ranges between 0.3 and 5.0 θ/o, preferably 0.4 to 2.4 percent.

It was further found that carboxymethyl starch is usable at a temperature at the probe tip of 60 °C to 200 °C. The preferred temperature range is 80 to 150 °C. No significant decrease in viscosity was observed during the 3-day aging period. The experiments showed that the carboxymethyl starch gel is completely degradable in aqueous solution within the recommended temperature range.

By using carboxyalkyl starches in water according to the invention, while maintaining the salt content in the water, the viscosity, the bearing capacity of the sand and the gel blocking time under the action of the cracking fluid can be controlled as much as possible or shortened.

The invention will be explained in more detail below with the help of examples.

Example 1

Carboxymethyl starch is soluble in cold water. When mixed into water, it immediately increases the viscosity considerably. The product is completely dissolved after 5 to 10 minutes.

After 15 minutes of mixing, a homogeneous, highly viscous liquid is obtained, which swells for an additional 1 hour.

Table 1 shows the dependence of viscosity on concentration at room temperature, after aging times of 1 h, 1 d, 2 d and 3 d.

Table 1 final viscosity in cP at room temperature trace lh ld 2 d 3 d

% 480 sec ¹ 160 sec. ¹ 480 s- ¹ 160 sec. ¹ 480 sec ¹ 160 sec ¹ 400 sec. ¹ 160 s ^-1 0.8 50 54 34 40 32 38 32 38 1.0 157 238 171 254 175 270 184 281 1.2 373 610 349 605 288 475 265 427 1.5 540 1026 540 1129 540 1048 540 1064 1.8 540 1399 540 1512 540 1139 502 923

Example 2 methyl starch in water under heat stress for more than 3 hours.

Table 2 shows the behavior of the carboxy gel.

conc. processing temperature % °C 0.8 25 80 120 1.0 25 80 120 1.2 25 80 120 1.5 25 80 120 1.8 25 80 120

measurement temperature viscosity at °C d = 480 s ¹ cP

25 50 80 40 25 12 25 157 80 110 25 13 25 373 80 288 25 14 25 >540 80 243 25 16 25 >540 80 301 25 24

Example 3

The ability of carboxymethyl starch gel in water to carry sand changes with thermal stress.

Experiments to measure the rate of sand fall were carried out according to the standard test for fracturing fluids (Recommended Practice Procedure for the Evaluation of Hydraulic Fracturing Fluids, API-Test RP 39, July 1960).

The measurement results at room temperature, at 80 °C and at 120 °C are shown in Tables 3 and 4.

Tempering was carried out at 120 °C in a pressure vessel.

Table 3 concentration sand falling speed in cm/min at room temperature at 80 °G %

0.8 19 8 1.0 <3 <3 1.2 <3 <3 1.5 <3 <3 1.8 <3 <3

Table 4 final sand fall rate at 120 °G in cm/min trace 10 min 0.5 h 1 h 1.5 h 2 h 2.5 h 3 ih %

1.8 <3 <3 <3 9.4 20 75 300

Example 4

This example shows the dependence of the liquid loss properties of a carboxymethyl starch gel in water under given p-T-conditions (see Table 5).

The measurement method for determining the flow coefficients was carried out according to the standard method for testing fracturing fluids (Recommended Practice Procedure for the Evaluation of Hydraulic Fracturing Fluids, API-Test RP 39, July 1960).

A fluid consumption coefficient with values around 1.10“ ³ m/Vmin is considered good, lower values are considered excellent.

S = initial loss (ml/cm ² ), p-pressure (MPa)

Q25 = accumulated amount of filtrate after 25 min (ml)

C = fluid consumption coefficient (m/j/min)

Table 5 final temperature ΔΡ % °C MPa

1.2 20 7.0 1.5 20 7.0 1.8 20 7.0 1.2 70 7.0 1.5 70 7.0 1.8 70 7.0 Example 5

S ' Q25 C . 103 ml/cm ² ml m/Vmin

0.15 26 0.98

0.11 14 0.5

7 0.3

0.44 32 0.96

0.35 20 0.52

0.02 14 0.58

The effect on the permeability of a rock sample after the destruction of carboxymethyl starch gel in water was documented in the following way.

Sandstone cores were used. The geometric dimensions of the core were 3 cm in diameter and 3.4 cm in length. A gel made from 18 g/l carboxymethyl starch in water was used. The temperature of the experiment was 120 °C. After measuring the initial permeability with nitrogen, the gel was pressed. The increasing volume of the pressed liquid was 22.7 ml in 36 minutes. After two hours of pressing, the reversible permeability was measured with nitrogen. A 15% decrease was recorded. Repeated measurement of the reversible permeability after 10 hours showed that the decrease in permeability compared to the initial permeability was 0%.

Claims (2)

SUBJECT CLAIMS 1. Water-based hydraulic cracking fluid for intensifying oil and gas probes, including injection probes and UGS probes, characterized in that it is found on a carboxyalkyl starch gel in aqueous solution. 2. The fluid for hydraulic crack formation according to claim 1, characterized in that it contains 0.3 to 5.0% carboxyalkyl starch.