阅读说明：本技术 一种干热岩热能开发降温压裂方法 (Dry-hot rock heat energy development cooling fracturing method ) 是由 彭晓龙 惠城 任威严 查永进 汪海阁 于 2019-09-04 设计创作，主要内容包括：本发明公开了增强型地热发电系统(EGS)中干热岩热能利用技术的一种干热岩热能开发降温压裂方法。旨在通过向干热岩地层注入低砂比稠化水压裂液,对目标地层进行降温,由于低砂比稠化水压裂液的注入,井筒温度和裂缝内压裂液温度都逐渐降低,为了保证较好的降温效果,又达到不浪费的目的,需要精确计算低砂比稠化水压裂液的注入量。由于井筒周围地层温度分布状况与裂缝周围岩石温度分布不同,井筒周围地层温度随井深变化按地温梯度增加,而裂缝周围储层温度是一个均值,因此需要计算井下射孔位置温度变化和裂缝内压裂液的温度分布状况,以实现更好的干热岩层降温效果。(The invention discloses a dry hot rock heat energy development cooling fracturing method of a dry hot rock heat energy utilization technology in an enhanced geothermal power generation system (EGS). The method aims to cool a target stratum by injecting the low-sand-ratio thickened water fracturing fluid into a dry hot rock stratum, and due to the injection of the low-sand-ratio thickened water fracturing fluid, the temperature of a shaft and the temperature of the fracturing fluid in a crack are gradually reduced, so that a good cooling effect is guaranteed, the aim of avoiding waste is achieved, and the injection amount of the low-sand-ratio thickened water fracturing fluid needs to be accurately calculated. Because the temperature distribution condition of the formation around the shaft is different from the temperature distribution of the rock around the fracture, the temperature of the formation around the shaft is increased along with the change of the well depth according to the temperature gradient of the ground, and the temperature of the reservoir around the fracture is an average value, the temperature change of the position of the downhole perforation and the temperature distribution condition of the fracturing fluid in the fracture need to be calculated, so that the better dry-hot rock layer cooling effect is realized.)

1. A dry hot rock heat energy development cooling fracturing method is characterized by comprising the following steps and conditions:

1) injecting 27m of fracturing pad fluid (clear water) before injecting the sand-carrying fluid and the conventional fracturing fluid³The part of water is injected to the front end 55m of the crack;

2) injecting low sand ratio (sand carrying concentration is less than 10%) thickening water fracturing fluid, and cooling the stratum;

3) injecting conventional cross-linked fracturing fluid to carry out normal fracturing according to a normal sand carrying ratio;

4) the continuous fracturing construction process must be ensured, and the pump stop phenomenon must not occur. If the pump is stopped before the cross-linked fracturing fluid is pumped, the amount of the low sand ratio fracturing fluid is increased to ensure the effect of cooling the stratum.

The flow conductivity of the tail end of the crack formed by the temperature-reducing fracturing process is slightly lower than that of the conventional fracturing because the sand ratio is lower. However, because geothermal reservoirs are inherently strong, the reduction in proppant volume does not have a significant effect on fracture closure.

Technical Field

The invention relates to a hot dry rock heat energy utilization technology in an enhanced geothermal power generation system (EGS), in particular to a hot dry rock heat energy development, cooling and fracturing method.

Background

The geothermal energy is used as renewable new energy, has great resource potential and CO₂The advantages of low emission and wide distribution are achieved, and the renewable clean energy is mainly researched and developed in all countries of the world. Geothermal energy is divided into a hydrothermal type and a dry-hot rock type, geothermal resources are mined and utilized mainly in the world at present and are hydrothermal geothermal, but the potential of the future resources is the exploitation of the dry-hot rock type geothermal energy, namely an enhanced geothermal power generation system (EGS), and the enhanced geothermal power generation system (EGS) faces a series of engineering technical difficulties at present, so that no commercially successful example exists in the world at present.

Hydraulic fracturing is a key technology for achieving commercial success of an enhanced geothermal power generation system (EGS), and a water injection well and a water production well are communicated through artificial cracks, so that efficient development of geothermal energy is realized. However, due to the ultra-high formation temperature of the geothermal reservoir, great challenge is provided for the high temperature resistance of the fracturing fluid during fracturing, the temperature resistance of the conventional crosslinking fracturing fluid is below 160 ℃, but the temperature of the commercially-used dry hot rock formation is more than 250 ℃ and even reaches more than 300 ℃. The conventional crosslinking fracturing fluid cannot meet the temperature resistance requirement required by construction, and if the temperature resistance of the fracturing fluid is improved, the development cost is greatly increased, and a series of technical challenges are faced.

In view of the above problems, there is a need to design an effective solution.

Disclosure of Invention

Based on the analysis, the invention designs a dry hot rock heat energy development cooling fracturing method, which aims to cool a target stratum by injecting low-sand ratio thickened water fracturing fluid into a dry hot rock stratum. Because the temperature distribution condition of the formation around the shaft is different from the temperature distribution of the rock around the fracture, the temperature of the formation around the shaft is increased along with the change of the well depth according to the temperature gradient of the ground, and the temperature of the reservoir around the fracture is an average value, the temperature change of the position of the downhole perforation and the temperature distribution condition of the fracturing fluid in the fracture need to be calculated, so that the better dry-hot rock layer cooling effect is realized.

A method of reducing temperature and fracturing for thermal energy development of hot dry rock, in one embodiment, comprising the steps of:

1) injecting 27m of fracturing pad fluid (clear water) before injecting the sand-carrying fluid and the conventional fracturing fluid³The part of water is injected to the front end 55m of the crack;

2) injecting low sand ratio (sand carrying concentration is less than 10%) thickening water fracturing fluid, and cooling the stratum;

3) injecting conventional cross-linked fracturing fluid to carry out normal fracturing according to a normal sand carrying ratio;

4) the continuous fracturing construction process must be ensured, and the pump stop phenomenon must not occur. If the pump is stopped before the cross-linked fracturing fluid is pumped, the amount of the low sand ratio fracturing fluid is increased to ensure the effect of cooling the stratum.

The flow conductivity of the tail end of the crack formed by the temperature-reducing fracturing process is slightly lower than that of the conventional fracturing because the sand ratio is lower. However, because geothermal reservoirs are inherently strong, the reduction in proppant volume does not have a significant effect on fracture closure.

Compared with the prior art, the invention has the advantages that: (1) in the fracturing process, the problem of development cost increase caused by selecting high-temperature-resistant fracturing fluid due to ultrahigh formation temperature under a geothermal reservoir can be effectively solved, and the development cost of the whole reservoir can be greatly saved; (2) the temperature at the perforation location decreases more slowly as the injection time increases. When the temperature gradient of the fracturing fluid in the fracture is large, the temperature of the fracturing fluid in the range of 55m at the front end of the fracture is reduced by more than 80 ℃ for a high-temperature reservoir at 300 ℃, and the temperature resistance of the used fracturing fluid can meet the requirement of EGS ultrahigh-temperature stratum fracturing.

Drawings

The invention will be described in more detail hereinafter on the basis of embodiments and with reference to the accompanying drawings. Wherein:

FIG. 1 is a schematic representation of the steps of a method embodying the present invention;

FIG. 2 is a graph of temperature changes at the location of perforations in a wellbore during fracturing;

FIG. 3 is a temperature distribution image of liquid in a fracture 1-20 min before fracture termination;

FIG. 4 is a graph of the temperature change at the perforations under different geothermal gradients;

FIG. 5 is an image of the temperature distribution of the fluid in the fracture before fracture termination at different geothermal gradients;

Detailed Description

The invention will be further explained with reference to the drawings.

In one embodiment of the present invention, the geothermal gradient is 7 ℃/100 m. The temperature difference between the fluid in the fracture and the stratum reaches more than 80 ℃ within 55m of the front end of the fracture, and the volume of the fracturing fluid is 100m at the moment³。

As shown in figure 1, the dry hot rock thermal energy development cooling fracturing method comprises the following steps:

step 1: in one embodiment of the invention, the formation temperature at 4000m downhole is 300 ℃. The clear water injected into the pad fluid in the fracturing process has a good cooling effect on a shaft, and the cooling effect of the fracturing pad fluid on the stratum can be obtained by calculating the temperature distribution of the fracturing fluid in the fracture as shown in figure 3. Therefore, before injecting the sand-carrying fluid and the conventional fracturing fluid, 27m of fracturing pad fluid (clear water) is injected³This portion of water is injected at the front 55m of the fracture.

Step 2: in one embodiment of the invention, the heat transfer properties between the fracturing fluid and the reservoir are obtained by a temperature solution model of the perforation locations and a model of the heat transfer within the fracture during fracturing.

In one embodiment of the invention, COMSOL Multiphysics 5 is applied. 1, calculating by a software heat transfer Module (HeatTransfer Module) to obtain the temperature change of the perforation position as shown in figure 2, wherein the analysis result shows that the temperature of a shaft is rapidly reduced in a short time in the initial stage of fracturing construction, and the fracturing fluid has an obvious effect of reducing the temperature of the stratum. At t-20 min, i.e. at the end of the fracture, the temperature at the perforations is about 105 ℃, which is the initial temperature of the fracturing fluid in the fracture.

Further, the temperature change with time at the perforation position according to the geothermal gradient of 5 deg.C/100 m, 6 deg.C/100 m, and 7 deg.C/100 m, respectively, is shown in FIG. 4. As can be seen from fig. 4, the temperatures at the perforation locations before fracture termination were 80 deg.f each. 0,92. 4,105 ℃, the fracturing fluid has obvious effect of reducing the temperature of the shaft. The temperature distribution of the fracture internal fracturing fluid in the fracture is shown in figure 5 when the ground temperature gradient is respectively 5 ℃/100m, 6 ℃/100m and 7 ℃/100 m. When the reservoir temperature is 220 ℃, before fracturing is terminated, the temperature difference between the fracturing fluid at the position 50m from the front end of the fracture and the stratum reaches more than 70 ℃, and the temperature is reduced to 150 ℃ (figure 5 (a)); when the reservoir temperature is 260 ℃, before fracturing is terminated, the temperature difference between the fracturing fluid at the front end of the fracture 60m and the stratum reaches more than 80 ℃, the temperature is reduced to 180 ℃, and the performance requirement of the conventional fracturing fluid is met [ fig. 5(b) ]; when the reservoir temperature is 300 ℃, the temperature difference between the fracturing fluid at the front end of the fracture 55m and the stratum before fracturing is terminated reaches more than 80 ℃, the temperature is reduced to 220 ℃, and the performance requirement of the high-temperature-resistant fracturing fluid is met [ figure 5(c) ]. And injecting the low sand ratio (the sand carrying concentration is less than 10%) thickened water fracturing fluid according to the calculation result, and cooling the stratum.

And step 3: in one embodiment of the invention, a conventional cross-linked fracturing fluid is injected to perform normal fracturing at normal sand-carrying ratio.

And 4, step 4: in one embodiment of the invention, after the design displacement is reached, pumping is stopped and fracturing is completed.