CN112940704A

CN112940704A - Method for bridging channels by using desert sand, slag and steel slag coupling fibers

Info

Publication number: CN112940704A
Application number: CN202110152914.XA
Authority: CN
Inventors: 李骏; 吴明移; 周利华; 何思源; 邹瑞萍; 官长平; 陈忠华; 张渝苹; 韩旭; 郭玉杰
Original assignee: Ganjiang New Area Aobo Particle Technology Research Institute Co ltd; Sichuan Nonferrous Metals Mining And Metallurgy Technology Co ltd; Sichuan Pattikos Energy Technology Co ltd
Current assignee: Ganjiang New Area Aobo Particle Technology Research Institute Co ltd; Sichuan Nonferrous Metals Mining And Metallurgy Technology Co ltd; Sichuan Pattikos Energy Technology Co ltd
Priority date: 2021-02-04
Filing date: 2021-02-04
Publication date: 2021-06-11

Abstract

The invention discloses a method for bridging a channel by using desert sand, slag and steel slag coupling fibers, wherein at least one of the desert sand, the slag and the steel slag which are subjected to surface modification is used as a propping agent to be compounded with special fibers for hydraulic fracturing construction; the surface modifier used by the proppant is a composition of one of 1, 3-propane sultone, 1,3, 2-dioxathiacyclopentane 2-oxide and 4-methyl-1, 3, 2-dioxathiacyclopentane 2-oxide and a nitrogen-containing silane coupling agent; the special fiber is a conventional fiber subjected to surface modification, and the surface modifier is a composition of one of 1, 3-propane sultone, 1,3, 2-dioxathiacyclopentane 2-oxide, 4-methyl-1, 3, 2-dioxathiacyclopentane 2-oxide and a nitrogen-containing silane coupling agent. The invention can apply desert sand, slag and steel slag without using value to hydraulic fracturing technology, and changes waste into valuable.

Description

Method for bridging channels by using desert sand, slag and steel slag coupling fibers

Technical Field

The invention relates to the technical field of yield increase transformation of oil and gas fields, in particular to a technical method for hydraulic fracturing by coupling desert sand, furnace slag and steel slag waste raw materials serving as a propping agent and fibers.

Background

With the vigorous exploitation of unconventional oil and gas resources, water fracturing technologies are used on a large scale. The hydraulic fracturing process is that a high-pressure large-displacement pump is adopted to inject fracturing fluid into a stratum on the ground, high pressure is suppressed at the bottom of a well, and when the pressure is greater than the ground stress near the well wall and the tensile strength of stratum rocks, cracks are generated in the stratum near the bottom of the well; and then injecting a propping agent, and when construction is stopped, closing the fracture on the injected propping agent so as to form a propping fracture with high flow conductivity in the stratum near the bottom of the well. The amount of proppant has also increased dramatically year by year, having grown from 95.75 million tons in 2010 to 261.75 million tons in 2018. At present, the commonly used propping agents for domestic oil fields are quartz sand and ceramsite, the quartz sand mainly comes from mining and river salvage, but since 2017, along with the increase of environmental protection, the sandstone industry experiences unprecedented environmental supervision storms, the shutdown and maintenance of part of sandstone mines and the closing of small and medium sandstone factories cause the great reduction of the supply quantity of sandstone markets, and the price of the quartz sand is increased accordingly. The ceramsite is prepared by sintering, and the cost of the ceramsite is higher than that of quartz sand due to the preparation process of the ceramsite.

The desert is mainly distributed in western regions of China, wherein, two desert regions of Takara Ma and Guerbantong are distributed in the province of Xinjiang, the area of the desert regions accounts for about 30 percent of the total area of the desert in China, and the desert sand resources are rich.

The method is characterized in that China is a large country in the iron and steel industry, blast furnace slag and steel slag are main byproducts in the iron and steel smelting process, 300-350 kg of blast furnace slag is generated every 1t of pig iron is smelted, the slag yield reaches about 32000 ten thousand tons according to 100000 ten thousand tons of annual pig iron yield in China.

If desert sand, slag and steel slag can be used for replacing sand for fracturing, the method has important significance for reducing the construction cost, protecting the local environment and reasonably developing and utilizing natural resources.

However, the performance of the existing desert sand, slag and steel slag can not meet the requirement of fracturing construction, because the propping agent is used for fracturing a hydraulic fracture, the high pressure of a stratum needs to be borne, the self strength of the desert sand and the slag is insufficient, the bearing performance of the desert sand and the slag can not meet the requirement of fracturing, and meanwhile, the transport performance of the desert sand, the slag and the steel slag is poor, and the desert sand, the slag and the steel slag can not be transported for a long distance in the stratum. Therefore, based on the above disadvantages, how to effectively develop desert sand, slag and steel slag which can be applied to hydraulic fracturing is very important.

Disclosure of Invention

The invention aims to provide a method for utilizing a bridge channel of desert sand, slag and steel slag coupling fibers aiming at the problem that the performances of the desert sand, the slag and the steel slag can not meet the fracturing construction requirements, so that the desert sand, the slag and the steel slag without utilization values can be applied to a water conservancy fracturing technology, and waste is turned into wealth.

The invention provides a method for bridging a channel by using desert sand, slag and steel slag coupling fibers, which is used for hydraulic fracturing construction by compounding a propping agent and special fibers. The dosage ratio of the propping agent to the special fiber is (990-999): (10-1).

The proppant is prepared by taking at least one of desert sand, slag and steel slag as a raw material and performing surface modification on the raw material. The surface modifier is selected from one of 1, 3-propane sultone, 1,3, 2-dioxathiacyclopentane 2-oxide and 4-methyl-1, 3, 2-dioxathiacyclopentane 2-oxide and the composition of the nitrogen-containing silane coupling agent.

The proppant is prepared by modifying the following method: (1) and cleaning the proppant raw material to remove surface impurities. (2) Drying the raw materials at 110 ℃, cooling to 60-80 ℃, soaking the dried raw materials in an ethanol-water mixed solution containing a nitrogen-containing silane coupling agent for 5-10 minutes, filtering, and drying in the shade at 70-80 ℃. (3) Soaking the proppant dried in the shade in a solution containing one of 1, 3-propane sultone, 1,3, 2-dioxathiolane 2-oxide or 4-methyl-1, 3, 2-dioxathiolane 2-oxide (the solvent is dichloromethane, toluene, acetonitrile and the like), maintaining the solution at 25-60 ℃ for 20 minutes, filtering out the proppant, drying the proppant at 80 ℃, and then heating to 110 ℃ for constant temperature for 30 minutes to obtain the modified proppant.

The proppant is one of desert sand, slag, a mixture of desert sand and steel slag and a mixture of desert sand and slag which are subjected to modification treatment. The treated proppant can enhance the interaction with the fibers.

The special fiber adopts conventional fiber with modified surface. The surface modifier is also a composition of one selected from 1, 3-propane sultone, 1,3, 2-dioxathiolane 2-oxide, 4-methyl-1, 3, 2-dioxathiolane 2-oxide and a nitrogen-containing silane coupling agent.

The special fiber is prepared by the following method: firstly, putting the conventional fiber into a plasma vacuum ion cleaning agent for cleaning for 5-10 minutes; then the fiber is put into ethanol/water solution containing nitrogen silane coupling agent to be soaked for 5 to 10 minutes, filtered and dried in the shade at 70 to 80 ℃; finally, soaking the fiber dried in the shade in a solution containing one of 1, 3-propane sultone, 1,3, 2-dioxathiolane 2-oxide or 4-methyl-1, 3, 2-dioxathiolane 2-oxide (the solvent is dichloromethane, toluene, acetonitrile and the like), maintaining the solution at 25-60 ℃ for 20 minutes, filtering the fiber, and drying in the shade or drying at the temperature ranging from room temperature to 60 ℃ to obtain the special fiber. The conventional fiber is selected from one or at least two of viscose fiber, acetate fiber, cuprammonium fiber, polyethylene fiber, polypropylene fiber, polyvinyl acetal fiber, polyester fiber, polylactic acid fiber, polyamide fiber, cellulose fiber and basalt fiber, and the length of the fiber is 1-19 mm. Preferably 3mm in length.

The treated fibers can be better dispersed in the low-viscosity fracturing fluid, the interaction between the fibers and the proppant is enhanced, the sedimentation of the proppant in the low-viscosity fracturing fluid can be slowed down, and even the suspension of the proppant in the low-viscosity conventional fracturing fluid is realized.

The nitrogen-containing silane coupling agent is one selected from N- (beta-aminoethyl) -gamma-aminopropyltrimethoxysilane, N- (beta-aminoethyl) -gamma-aminopropyltriethoxysilane, 3- (methacryloyloxy) propyltrimethoxysilane, N-aminoethyl-3-aminopropylmethyldimethoxysilane, bis (3-trimethoxysilylpropyl) amine, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, gamma-ureidopropyltriethoxysilane and 3-aminopropylmethyldiethoxysilane.

The invention relates to a method for bridging channels by using desert sand, slag and steel slag coupling fibers, which comprises the following construction steps: (1) determining the position of cluster perforation of a fracturing layer section and perforation parameters; (2) determining fracturing construction parameters; (3) performing perforation operation on the fracturing layer section according to the determined cluster perforation position and perforation parameters; (4) injecting hole cleaning liquid into the cluster perforation to perform acidification pretreatment on the cluster perforation; (5) mixing the special fiber and the propping agent in proportion and injecting the mixture into the stratum under the carrying of the sand carrying liquid; (6) after sand pack is completed, a displacement fluid is injected into the target fracturing interval to force a sand-carrying fluid in the wellbore into the fracture.

Compared with the prior art, the invention has the advantages that:

(1) the proppant used in the invention refers to one or more of the desert sand or the slag after modification treatment, or the desert sand mixed steel slag, or the desert sand mixed slag. The sand paving state with the channel can be formed through the technology, so that the seepage channel is changed from the original pore space to the channel space, the channel with high flow conductivity is formed, and the supporting effect of the channel is better than that of the conventionally used proppant. Meanwhile, the coupling between the propping agent and the special fiber can improve the transport distance of the propping agent, so that the problems of large density and insufficient transport distance of the desert sand, the slag and the steel slag are solved, meanwhile, the high-strength steel slag is introduced into the whole propping agent, so that the pressure-bearing strength of the whole propping agent can be improved, and the problem of insufficient strength of the desert sand and the slag is solved. The desert sand, the steel slag and the furnace slag which cannot be applied to the fracturing propping agent originally can be recycled.

(2) The invention solves the performance problems of desert sand, steel slag and furnace slag. By treating and mixing the desert sand, the steel slag and the furnace slag and combining the special fibers, proppant clusters are formed in the stratum, the seepage space of oil gas is changed from the original gap into a large channel, and the integral support property of the sand bank can be improved. Therefore, the method can reduce the crushing rate of the desert sand proppant cluster and greatly reduce the influence of the crushing of the proppant on the flow conductivity of the propped fracture.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

Drawings

FIG. 1 is a diagram showing the results of a bridge channel sanding experiment of modified desert sand coupled fibers.

FIG. 2 is a diagram showing the results of a sand laying experiment for a modified steel slag coupled fiber bridging channel.

FIG. 3 is a diagram showing the results of a sand laying experiment for a modified slag-coupled fiber bridging channel.

Fig. 4 is a diagram of the results of a sand laying experiment of common desert sand.

FIG. 5 is a diagram showing the results of a sanding experiment using ordinary steel slag.

Fig. 6, sand paving and mixing of mixed proppant coupled fibers.

Fig. 7, graph of fracture rate analysis of mixed proppant and fiber.

FIG. 8 is a desert sand fracture rate analysis diagram.

Detailed Description

The preferred embodiments of the present invention will be described in conjunction with the accompanying drawings, and it will be understood that they are described herein for the purpose of illustration and explanation and not limitation.

Example 1

The modification preparation method of the proppant comprises the following steps: (1) and cleaning the proppant raw material desert sand to remove surface impurities. (2) Drying the desert sand at 110 ℃, cooling to 60 ℃, soaking the dried desert sand in an ethanol-water mixed solution of N- (beta-aminoethyl) -gamma-aminopropyltrimethoxysilane for 5-10 minutes, filtering and drying in the shade at 70-80 ℃. In the solution, the dosage ratio of N- (beta-aminoethyl) -gamma-aminopropyl trimethoxy silane, ethanol and water is 1:2: 2. (3) Soaking the proppant dried in the shade in dichloromethane solution containing 1, 3-propane sultone, wherein the dosage ratio of 1, 3-propane sultone to dichloromethane solution is 1:3, maintaining the temperature at 25 ℃ for 20 minutes, filtering out the proppant, drying the proppant at 80 ℃, and then heating to 110 ℃ and keeping the temperature for 30 minutes to obtain the modified proppant.

Example 2

The modification preparation method of the proppant comprises the following steps: (1) cleaning the proppant raw material steel slag to remove surface impurities. (2) Drying the desert sand at 110 ℃, cooling the temperature to 70 ℃, putting the dried steel slag into ethanol-water mixed solution containing gamma-urea propyl triethoxysilane for soaking for 5-10 minutes, filtering, and drying in the shade at 70-80 ℃. In the solution, the gamma-urea propyl triethoxysilane, ethanol and water are used in a ratio of 6: 7(3), the proppant dried in the shade is soaked in an acetonitrile solution containing 4-methyl-1, 3, 2-dioxathiolane 2-oxide, the ratio of the 4-methyl-1, 3, 2-dioxathiolane 2-oxide to the acetonitrile is 1:4, the proppant is filtered out after being maintained at 60 ℃ for 20 minutes, the proppant is dried at 80 ℃, and then the temperature is raised to 110 ℃ and kept constant for 30 minutes, so that the modified proppant is obtained.

Example 3

The modification preparation method of the proppant comprises the following steps: (1) and cleaning the proppant raw material slag to remove surface impurities. (2) Drying the slag at 110 deg.C, cooling to 80 deg.C, soaking the dried slag in mixed solution of 3- (methacryloyloxy) propyl trimethoxy silane and ethanol in water for 5-10 min, filtering, and drying in the shade at 70-80 deg.C. In the solution, the ratio of the 3- (methacryloyloxy) propyl trimethoxy silane to the ethanol to the water is 1:2: 2. (3) Soaking the proppant dried in the shade in a toluene solution containing 1,3, 2-dioxathiolane 2-oxide, wherein the using amount ratio of the 1,3, 2-dioxathiolane 2-oxide to the toluene is 1:4, maintaining the temperature at 50 ℃ for 20 minutes, filtering out the proppant, drying the proppant at 80 ℃, and then heating to 110 ℃ for constant temperature for 30 minutes to obtain the modified proppant.

Example 4

The preparation method of the special fiber comprises the following steps: firstly, putting the conventional basalt fiber into a plasma vacuum ion cleaning agent for cleaning for 5-10 minutes; then putting the fiber into an ethanol water solution of N- (beta-aminoethyl) -gamma-aminopropyltrimethoxysilane for soaking for 5-10 minutes, wherein the dosage ratio of N- (beta-aminoethyl) -gamma-aminopropyltrimethoxysilane, ethanol and water in the solution is 1:2:2, filtering and drying in the shade at 70-80 ℃; and finally, soaking the dried fiber in a dichloromethane solution containing 1, 3-propane sultone, maintaining the temperature at 60 ℃ for 20 minutes, filtering the fiber, and drying in the shade or drying at the temperature ranging from room temperature to 60 ℃ to obtain the special fiber.

Example 5

The preparation method of the special fiber comprises the following steps: firstly, putting the conventional copper ammonia fiber into a plasma vacuum ion cleaning agent for cleaning for 5-10 minutes; then putting the fiber into an ethanol water solution containing bis (3-trimethoxysilylpropyl) amine to be soaked for 5-10 minutes, wherein the using ratio of the bis (3-trimethoxysilylpropyl) amine to the ethanol to the water is 1:2:2, filtering and drying in the shade at 70-80 ℃; and finally soaking the dried fiber in the shade in acetonitrile solution containing 1,3, 2-dioxathiolane 2-oxide, wherein the ratio of the 1,3, 2-dioxathiolane 2-oxide to the acetonitrile is 1:3, maintaining the temperature at 25 ℃ for 20 minutes, filtering the fiber, and drying the fiber in the shade or in the oven at the temperature ranging from room temperature to 60 ℃ to obtain the special fiber.

Example 6

The preparation method of the special fiber comprises the following steps: firstly, putting the conventional viscose into a plasma vacuum ion cleaning agent for cleaning for 5-10 minutes; then putting the fiber into an ethanol water solution containing 3- (methacryloyloxy) propyl trimethoxy silane, soaking for 5-10 minutes, wherein the using ratio of the 3- (methacryloyloxy) propyl trimethoxy silane to the ethanol to the water is 1:2:2, filtering and drying in the shade at 70-80 ℃; and finally soaking the dried fiber in a toluene solution containing 4-methyl-1, 3, 2-dioxathiolane 2-oxide, maintaining the temperature at 40 ℃ for 20 minutes, filtering the fiber, and drying the fiber in the shade or in the oven at the temperature of between room temperature and 60 ℃ to obtain the special fiber.

Example 7

Proppant system laying effect evaluation experiment: the proppant prepared in examples 1, 2 and 3 and the specialty fiber prepared in example 4 were evaluated for their performance by adding civil tap water to the fracturing fluid and other additives.

The evaluation method is as follows:

(1) assembling and testing the dynamic sand-carrying evaluation device and corresponding experimental recording equipment.

(2) Preparing a base fluid of the fracturing fluid: draining water to total volume of 2L, stirring, measuring, and adding into the stirring tankStirring in a stirring tank for 5 min until fully mixed. The formula of the fracturing fluid is as follows: drag reducer, KCl and NaCO are added into civil tap water₃Glutaraldehyde, silicon-containing defoaming agent and fluorocarbon cleanup additive.

(3) Pumping a preflush: and opening an outlet valve of the stirring tank, pumping 0.5L of the pre-liquid into the visual crack, and closing the outlet valve of the stirring tank after the pre-liquid pump finishes the injection.

(4) Preparing a sand carrying liquid: the fibers and proppant were added simultaneously to the stirred tank and stirred for 1 minute to mix well.

(5) Pumping sand-carrying liquid: and opening an outlet valve of the stirring tank, opening a main pump to inject the sand-carrying liquid into the device pump, and closing the main pump after the sand-carrying liquid pump finishes injecting. The entire experimental procedure was recorded.

Experimental results as shown in fig. 1 to 5, fig. 1 to 3 are the results of the bridge-channel sanding experiments of the proppant prepared in examples 1 to 3 and the specialty fiber prepared in example 4, respectively. FIGS. 4 and 5 show the results of sand-laying experiments for common desert sand and conventional steel slag. The contrast shows that the interaction between the modified desert sand, the modified steel slag, the modified furnace slag and the special fiber is enhanced, the mutual coupling between the modified desert sand and the special fiber is realized, the modification is obviously different from the common desert sand, and the laying height migration distance is obviously improved. Meanwhile, a larger channel is formed in the sand bank, and the seepage space is greatly improved.

Figure 6 is a sanding graph of an equal proportion mixture of two proppants prepared in examples 1 and 3 with specialty fibers. It can be seen that the mixed proppant can be fully mixed in the cracks, obvious layering cannot occur, and the high-strength proppant is fully mixed in the sand bank to achieve the effect of the proppant.

Example 8

Proppant strength evaluation experiments: the proppant fracturing experiment was used, and the evaluation method was as follows:

(1) assembling and testing a proppant fracturing device;

(2) 40g of proppant was placed into a fracturing chamber using API standards, then gradually pressurized to the design pressure on the fracturing chamber and maintained at that pressure for 3 minutes.

(3) The proppant is removed, the crushed proppant is screened out, the weight of the remaining proppant is weighed, and the proppant fracture rate is calculated.

Experimental results as shown in fig. 7 and 8, fig. 7 is a graph of the breakage of fibers versus an equal proportion mixture of two proppants prepared in examples 1 and 2. Fig. 8 is a graph showing the breakage rate of common desert sand. It can be seen that the pressure-bearing strength of the mixed proppant and the fibers is obviously enhanced, the crushing rate of about 40% can still be maintained under the high pressure of 75MPa, and the crushing rate of the conventional desert sand reaches more than 65%. Therefore, the crushing rate of the whole propping agent can be reduced by about 25% after the technology of mixing the high-strength propping agent and the fiber with the desert sand is adopted.

Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A bridge connection channel method using desert sand, slag and steel slag coupling fibers is characterized in that a propping agent and special fibers are compounded for hydraulic fracturing construction; the proppant is prepared by taking at least one of desert sand, furnace slag and steel slag as a raw material and performing surface modification; the special fiber adopts conventional fiber subjected to surface modification; the surface modifier used by the proppant and the surface modifier used by the fiber are both selected from one of 1, 3-propane sultone, 1,3, 2-dioxathiacyclopentane 2-oxide, 4-methyl-1, 3, 2-dioxathiacyclopentane 2-oxide and a composition of a nitrogen-containing silane coupling agent.

2. The method for bridging the channel by using the desert sand, the slag and the steel slag coupled fiber as claimed in claim 1, wherein the dosage ratio of the propping agent to the special fiber is (990-999): (10-1).

3. The method for bridging passages by using desert sand, slag and steel slag coupling fiber as claimed in claim 1, wherein the nitrogen-containing silane coupling agent is selected from one of N- (beta-aminoethyl) -gamma-aminopropyltrimethoxysilane, N- (beta-aminoethyl) -gamma-aminopropyltriethoxysilane, 3- (methacryloyloxy) propyltrimethoxysilane, N-aminoethyl-3-aminopropylmethyldimethoxysilane, bis (3-trimethoxysilylpropyl) amine, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, gamma-ureidopropyltriethoxysilane and 3-aminopropylmethyldiethoxysilane.

4. The method for bridging channels by using desert sand, slag and steel slag coupled fibers as claimed in claim 1, wherein the special fibers are prepared by the following method: soaking the conventional fiber in an ethanol water solution containing a nitrogen-containing silane coupling agent for 5-10 minutes, and drying in the shade; then soaking the dried fiber in a solution containing one of 1, 3-propane sultone, 1,3, 2-dioxathiolane 2-oxide or 4-methyl-1, 3, 2-dioxathiolane 2-oxide, maintaining the solution at 25-60 ℃ for 20 minutes, filtering the fiber, and drying to obtain the special fiber.

5. The method for bridging passages in desert sand, slag and steel slag coupling fiber as claimed in claim 4, wherein the conventional fiber is one or at least two selected from the group consisting of viscose fiber, acetate fiber, cuprammonium fiber, polyethylene fiber, polypropylene fiber, polyvinyl acetal fiber, polyester fiber, polylactic acid fiber, polyamide fiber, cellulose fiber and basalt fiber, and the fiber has a length of 1-19 mm.

6. The method for bridging passages by using desert sand, slag and steel slag coupled fibers as claimed in claim 1, wherein the proppant is prepared by modifying the following method: when the temperature of the proppant raw material is 60-80 ℃, the raw material is placed in an ethanol water solution containing a nitrogen-containing silane coupling agent for soaking for 5-10 minutes, filtered and dried in the shade at 70-80 ℃; then soaking the dried raw material in a solution containing one of 1, 3-propane sultone, 1,3, 2-dioxathiolane 2-oxide or 4-methyl-1, 3, 2-dioxathiolane 2-oxide, maintaining at 25-60 ℃ for 20 minutes, filtering out the proppant, drying at 80 ℃, and then heating to 110 ℃ for constant temperature for 30 minutes to obtain the surface-modified proppant.

7. The method of bridging passages using desert sand, slag, steel slag coupled fibers as claimed in claim 6, wherein the proppant is one of desert sand, slag, a mixture of desert sand and steel slag, and a mixture of desert sand and slag.

8. The method for bridging the channel by using the desert sand, slag and steel slag coupled fiber as claimed in claim 1, wherein the construction steps are as follows: (1) determining the position of cluster perforation of a fracturing layer section and perforation parameters; (2) determining fracturing construction parameters; (3) performing perforation operation on the fracturing layer section according to the determined cluster perforation position and perforation parameters; (4) injecting hole cleaning liquid into the cluster perforation to perform acidification pretreatment on the cluster perforation; (5) mixing the special fiber and the propping agent in proportion and injecting the mixture into the stratum under the carrying of the sand carrying liquid; (6) after sand pack is completed, a displacement fluid is injected into the target fracturing interval to force a sand-carrying fluid in the wellbore into the fracture.