Pretreatment process of gas field produced water for recycling drilling fluid and drilling fluid system

Abstract

The application discloses a pretreatment process of a gas field produced water for preparing a drilling fluid and a drilling fluid system, relates to the technical field of gas field produced water treatment and recycling, and is characterized by the following: the pretreatment process is simpler, and the construction cost and difficulty are lower, and the process is specifically proposed for the gas field produced water; the gas field produced water can be effectively treated; and the treated gas field produced water can be used to prepare a drilling fluid with good rheological property, high thermal stability and high inhibition, so that the problems of large quantity of the gas field produced water, difficulty in treatment and lack of a suitable treatment process are solved, and the sustainable development of oil and gas field development is realized.

Description

Technical Field

[0001] This invention relates to the field of gas field produced water treatment and reuse technology, specifically to a pretreatment process and drilling fluid system for gas field produced water reuse and drilling fluid preparation. Background Technology

[0002] With the increasing exploration and development of oil and gas fields in my country and the growing emphasis on environmental protection, the treatment of produced water during oil and gas field extraction has become a common challenge for Chinese oil and gas companies. Produced water refers to the associated water generated during gas extraction, including groundwater and condensate. During natural gas exploration, development, and production, formation water is carried to the surface by natural gas in the form of free water or gas-saturated water. Produced water typically exhibits high pollutant concentrations, complex composition, significant differences in properties between different types, and high hazard levels. Produced water is exposed to high-temperature, high-pressure gas layers underground for extended periods, resulting in the dissolution of large amounts of soluble salts, suspended solids, oil, harmful gases, and organic matter. Furthermore, the presence of numerous bacteria and microorganisms in the formation inevitably introduces these microorganisms into the produced water during long-term storage, leading to its exposure to the surface. Moreover, the frequent addition of chemical agents during gas extraction further complicates the composition of the produced water, increasing the difficulty of treatment. Produced water also contains corrosion inhibitors, hydrate inhibitors, demulsifiers, and other agents added during gas production and surface gathering and transportation. Due to the different geological conditions of the gas reservoir layers and the different fracturing gas production processes, the water quality and quantity characteristics of tight gas fields are complex. After only simple physical treatment, the produced water cannot meet the discharge water quality requirements.

[0003] Currently, the treatment methods for produced water from gas fields include deep well reinjection, internal reuse, external reuse, resource utilization of water components, and treated discharge. From the perspective of treatment technology, produced water from gas fields can be mainly divided into two types: one is a primary treatment process mainly based on demulsification, oil removal, flocculation sedimentation, and filtration; the other is a deep treatment process mainly based on oxidation, desalination, and advanced filtration. The primary treatment process mainly removes most of the condensate oil and suspended solids, and the treated water quality basically meets the requirements for reinjection. However, gas field produced water reinjection requires reinjection wells, and scaling is prone to occur during the reinjection process, clogging fractures and pores, affecting gas well production, and even leading to well abandonment. Deep treatment mainly uses advanced oxidation to remove organic matter, membrane treatment and reverse osmosis to reduce salt content, and ultrafiltration to remove fine impurities. The water quality after deep treatment is better, basically meeting the needs for discharge and greening irrigation; however, this treatment process is complex, costly, and inefficient, requiring the construction of dedicated treatment plants, occupying large amounts of land, and requiring significant investment in personnel and equipment, and cannot meet the needs of in-well treatment.

[0004] With the implementation of the new Environmental Protection Law, the cross-regional transfer of produced water between different construction areas is prohibited. The reinjection and reuse of produced water are subject to dual restrictions from local governments and gas production plants. In some areas, the lack of storage capacity for produced water at gas gathering stations has led to the shutdown of some wells, impacting production capacity. As gas fields continue to develop, water resources are becoming increasingly scarce, and the supply-demand imbalance is becoming more prominent. Water shortage may have become a bottleneck for the sustainable development and clean production of oil and gas resources. The reuse of produced water is key to overcoming this bottleneck, alleviating the pressure on water use for gas field development and solving the problem of produced water disposal. However, produced water has high salinity, and the calcium content in it is high. 2+ Mg 2+ and Fe 2+ The high content of [a substance] has a significant impact on the reusability of existing drilling fluid systems. However, there is very little research on related process technologies for the reuse of produced water in drilling fluid preparation. Therefore, it is necessary to develop a new technology to solve the technical problem of the difficulty in effectively reusing produced water.

[0005] It should also be noted that document CN110563194A discloses a pretreatment agent and method for preparing fracturing flowback fluid in drilling fluid formulation. It discloses a pretreatment process for fracturing flowback fluid by adding a hardness remover, antifoaming agent, bactericide, pH adjuster, and high-valence ion shielding agent. However, careful analysis reveals that… First, fracturing flowback fluid is a liquid generated during hydraulic fracturing operations in oil and gas fields. It mainly consists of the original fracturing fluid, along with some minerals and organic matter dissolved or carried from the formation. Produced water, on the other hand, refers to the wastewater discharged from the formation during oil and gas extraction as oil and gas are extracted from underground reservoirs. This wastewater primarily originates from the reservoir's own bottom water, edge water, condensate water generated after water injection, and formation water. In other words, the properties of fracturing flowback fluid and produced water are different, making this existing technology unsuitable for treating produced water from gas fields.

[0006] Secondly, the existing technology requires the addition of high-valence ion removal agents during the treatment process, which not only increases the construction procedures and difficulty, but also leads to increased costs.

[0007] Finally, the existing technology uses a mixture of soda ash, sodium tripolyphosphate, and disodium ethylenediaminetetraacetate as the hardness removal agent, which has the problems of high pretreatment raw material costs and complex processes. Furthermore, the hardness removal agent contains phosphorus and nitrogen ions, which can easily impact the environment.

[0008] Therefore, there is an urgent need to provide a new technology for the specific treatment of produced water from gas fields. Summary of the Invention

[0009] The purpose of this invention is to overcome the aforementioned problems in the prior art and to propose a pretreatment process and drilling fluid system for the reuse of produced water from gas fields in drilling fluid preparation. This invention proposes a simpler treatment process with lower construction costs and difficulty specifically for produced water from gas fields. It can not only effectively treat produced water from gas fields, but also enable the treated produced water to be used to prepare drilling fluids with good rheological properties, strong thermal stability, and high inhibition properties. This solves the problems of large produced water volume, difficult treatment, and lack of suitable treatment processes in existing gas fields, and realizes the sustainable development of oil and gas field development.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a pretreatment process for gas field produced water reuse in drilling fluid preparation, which includes the following steps: Step S1: Add 0.01%-0.1% defoamer to the produced water of the gas field by mass percentage and stir evenly to make the foaming rate of the produced water of the gas field less than 2%; Step S2: Add 0.1%-3% of ion removal agent to the gas field produced water by mass percentage and stir evenly to remove some metal ions from the gas field produced water; Step S3: Add 0.001%-0.05% of bactericide to the gas field produced water by mass percentage and stir evenly to kill iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the gas field produced water; Step S4: Add 0.01%-0.5% pH adjuster to the gas field produced water by mass percentage and stir evenly to adjust the pH value of the gas field produced water to 8-10, thus obtaining pretreated gas field produced water.

[0011] The defoamer is one of the following: organosilicon, aluminum stearate, fatty acid ester, phosphate ester, fatty alcohol, white oil, kerosene, or a mixture of several in any proportion.

[0012] The ion removal agent is one of soluble carbonates and soluble sulfates, or a mixture of two in any proportion.

[0013] The bactericide is one of aldehydes, cationic surfactants, and inorganic chlorine, or a mixture of several in any proportion.

[0014] The pH adjuster is sodium hydroxide or potassium hydroxide.

[0015] Secondly, the present invention provides a drilling fluid system for gas field produced water obtained using the above-described pretreatment process. This drilling fluid system is suitable for upper formations and comprises the following components by weight: The pretreated gas field produced water consists of 50-98 parts, selected soil 1-4 parts, water loss reducing agent 0.5-3 parts, soda ash 0.05-0.15 parts, sodium hydroxide 0.1-0.3 parts, and viscosity enhancer 0.2-0.3 parts.

[0016] Thirdly, the present invention provides a drilling fluid system for gas field produced water obtained using the above-described pretreatment process. This drilling fluid system is suitable for intermediate formations and comprises the following components by weight: The pretreated gas field produced water contains 50-98 parts, a fluid loss reducer 0.5-6 parts, a viscosity enhancer 0.2-0.5 parts, and an inhibitor 1-7 parts.

[0017] Fourthly, the present invention provides a drilling fluid system for gas field produced water obtained using the above-described pretreatment process. This drilling fluid system is suitable for lower formations and comprises the following components by weight: The pretreated gas field produced water consists of 50-98 parts, selected soil of 1-3 parts, fluid loss reducer of 4-7 parts, viscosity enhancer of 0.2-0.4 parts, inhibitor of 5-10 parts, and weighting agent of 10-30 parts.

[0018] The preferred soil is bentonite or salt-resistant bentonite.

[0019] The fluid loss reducing agent is one of the following, or a mixture of several in any proportion: polyanionic cellulose low viscosity, drilling fluid fluid loss reducing agent - modified natural polymer, sulfonated phenolic resin, modified starch, modified polyacrylic acid copolymer emulsion, anti-collapse sealing agent, and modified asphalt.

[0020] The viscosity enhancer is one of polyanionic cellulose high viscosity, xanthan gum, carboxymethyl cellulose, or a mixture of several in any proportion.

[0021] The inhibitor is one of salts, organic amine polymers, sodium formate, or a mixture of several in any proportion.

[0022] The weighting agent is one of barite powder and limestone powder, or a mixture of the two in any proportion.

[0023] The advantages of using this invention are: 1. The pretreatment process of the present invention includes four steps, wherein, Step S1 involves adding 0.01%-0.1% defoamer to the produced water from the gas field and stirring thoroughly. This reduces the foaming rate of the produced water to less than 2%, thus mitigating the impact of dissolved gases in the produced water on the viscosity, density, and other properties of the drilling fluid during preparation. Furthermore, several experiments have shown that if the amount of defoamer is less than 0.01%, the foaming rate will exceed the safe operating requirement of 2%, failing to achieve the desired defoaming effect. If the amount is greater than 0.1%, excessive defoamer will lead to increased costs and wasted raw materials. Moreover, some defoamers are surfactants, and excessive addition can negatively impact the performance of the subsequently prepared drilling fluid.

[0024] Step S2 involves adding 0.1%-3% of an ion removal agent to the produced water from the gas field and stirring thoroughly, which effectively removes Ca from the produced water. 2+ Mg 2+ and Fe 2+ Removing most metal ions not only helps prevent these ions from affecting the performance of drilling fluid treatment agents, but also improves the compatibility of produced water with other treatment agents in the drilling fluid system. Furthermore, several experiments have shown that the ion removal agent itself is an inorganic salt; if the addition amount is less than 0.1%, it will not effectively reduce the metal ion concentration to a level suitable for preparing the drilling fluid. If the addition amount is greater than 3%, it will introduce other anions and cations, which is detrimental to the reuse of produced water.

[0025] Step S3 involves adding 0.001%-0.05% of a bactericide to the produced water from the gas field and stirring thoroughly. This kills iron bacteria, saprophytic bacteria, and sulfate-reducing bacteria in the produced water, ensuring that the levels of these bacteria in the treated water meet the requirements for solution preparation. Furthermore, several experiments have shown that if the amount of bactericide added is less than 0.001%, the bacteria elimination effect in the produced water will be unsatisfactory; if the amount added is greater than 0.05%, it will result in waste of raw materials and be environmentally unfriendly.

[0026] Step S4 involves adding 0.01%-0.5% pH adjuster to the gas field produced water and stirring it evenly to adjust the pH value of the gas field produced water to 8-10, which is beneficial to obtaining gas field produced water that meets the requirements for preparing drilling fluid.

[0027] Furthermore, it should be noted that, firstly, the reagents used in this pretreatment process are all common treatment agents used in drilling fluid preparation, which are environmentally friendly, inexpensive, and readily available, resulting in lower construction costs. Secondly, this pretreatment process is simple and easy to operate, allowing for on-site pretreatment of produced water without the need for new pretreatment facilities; the treated produced water can be prepared and used immediately on-site. Thirdly, this pretreatment process does not rely on new equipment; existing equipment in conventional drilling teams can meet the pretreatment requirements, reducing the difficulty of pretreatment and increasing the likelihood of widespread adoption of this process.

[0028] In summary, this invention significantly reduces the concentration of ions in gas field produced water that negatively impact drilling fluid performance, and lowers the bacterial content in the water, thus meeting the requirements for drilling fluid preparation. Furthermore, it enables the preparation of drilling fluid using gas field produced water as a raw material, greatly reducing the need for clean water during on-site operations, alleviating water shortages in some water-scarce areas, and simultaneously solving the problem of produced water treatment.

[0029] 2. This invention uses one or more of the following as defoamers: silicone, aluminum stearate, fatty acid ester, phosphate ester, fatty alcohol, white oil, and kerosene. It has the advantage of effectively eliminating bubbles in the produced water and those dissolved in the produced water, which is beneficial to the stability of the solution preparation performance.

[0030] 3. This invention uses one or more of soluble carbonates and soluble sulfates as ion removal agents. Soluble carbonates and sulfates are both broad-spectrum materials, easy to purchase, and inexpensive.

[0031] 4. The present invention uses one or more of aldehydes, cationic surfactants, and inorganic chlorine as the bactericide, which effectively kills iron bacteria and corrosive bacteria, and inhibits the presence of organic matter, thus helping to eliminate the adverse effects of bacteria on the solution preparation.

[0032] 5. Based on the pretreated gas field produced water, the present invention can prepare three drilling fluid systems. These three drilling fluid systems are suitable for the upper, middle and lower formations, respectively. They all have the advantages of good rheological properties, strong thermal stability and high inhibition. Moreover, the drilling fluid prepared with the treated produced water has an apparent viscosity comparable to that prepared with clean water, and its inhibition is better than that of the drilling fluid system prepared with clean water.

[0033] 6. The present invention uses bentonite or salt-resistant bentonite to form a high-viscosity drilling fluid in the preparation of pretreated produced water, which meets the needs of on-site construction.

[0034] 7. This invention uses specific fluid loss reducing agents and viscosity increasing agents, which are easy to procure, have stable performance, can be formulated into low fluid loss drilling fluid, and are conducive to safe downhole construction.

[0035] 8. The present invention uses a specific inhibitor with good inhibitory properties, which can effectively stabilize the formation and thus ensure wellbore safety.

[0036] 9. The present invention uses one or a mixture of two of barite powder and limestone powder as a weighting agent, which is beneficial to subsequent acid fracturing and reservoir protection. Attached Figure Description

[0037] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0038] like Figure 1 As shown, this invention provides a pretreatment process for reusing produced water from gas fields to prepare drilling fluid, which includes the following steps: Step S1: Add 0.01%-0.1% of defoamer to the gas field produced water by mass percentage and stir evenly to make the foaming rate of the gas field produced water less than 2%, so as to reduce the impact of dissolved gas in the gas field produced water on the viscosity, density and other properties of the drilling fluid when it is prepared.

[0039] Step S2: Add 0.1%-3% of ion removal agent to the gas field produced water by mass percentage and stir evenly to remove some metal ions from the gas field produced water, further reduce the impact of metal ions on the performance of drilling fluid treatment agents, and improve the compatibility of produced water with other treatment agents in the drilling fluid system.

[0040] Step S3: Add 0.001%-0.05% of bactericide to the gas field produced water by mass percentage and stir evenly to kill iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the gas field produced water, so that the iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the treated gas field produced water meet the solution preparation requirements.

[0041] Step S4: Add 0.01%-0.5% pH adjuster to the gas field produced water by mass percentage and stir evenly to adjust the pH value of the gas field produced water to 8-10, thus obtaining pretreated gas field produced water.

[0042] The defoamer mentioned above is one of the following: organosilicon, aluminum stearate, fatty acid ester, phosphate ester, fatty alcohol, white oil, kerosene, or a mixture of several in any proportion.

[0043] The aforementioned ion removal agent is one of soluble carbonates and soluble sulfates, or a mixture of two in any proportion.

[0044] The above-mentioned bactericides are one of aldehydes, cationic surfactants, and inorganic chlorine, or a mixture of several in any proportion.

[0045] The pH adjuster mentioned above is sodium hydroxide or potassium hydroxide.

[0046] Based on the pretreated gas field produced water described above, this invention also proposes three drilling fluid systems suitable for formations at different depths, as detailed below: The first type: For upper formations, this invention provides a drilling fluid system, characterized by comprising the following components by weight: The pretreated gas field produced water consists of 50-98 parts, selected soil 1-4 parts, water loss reducing agent 0.5-3 parts, soda ash 0.05-0.15 parts, sodium hydroxide 0.1-0.3 parts, and viscosity enhancer 0.2-0.3 parts.

[0047] The second type: For the central formation, this invention provides a drilling fluid system, characterized by comprising the following components by weight: The pretreated gas field produced water contains 50-98 parts, a fluid loss reducer 0.5-6 parts, a viscosity enhancer 0.2-0.5 parts, and an inhibitor 1-7 parts.

[0048] The third type: For lower formations, this invention provides a drilling fluid system, characterized by comprising the following components by weight: The pretreated gas field produced water consists of 50-98 parts, selected soil of 1-3 parts, fluid loss reducer of 4-7 parts, viscosity enhancer of 0.2-0.4 parts, inhibitor of 5-10 parts, and weighting agent of 10-30 parts.

[0049] The preferred soils mentioned above are bentonite or salt-resistant bentonite.

[0050] The aforementioned fluid loss reducing agent is one of the following or a mixture of several in any proportion: polyanionic cellulose low viscosity PAC-L, drilling fluid fluid loss reducing agent - modified natural polymer NAT-20, sulfonated phenolic resin SMP-2, modified starch CMS, modified polyacrylic acid copolymer emulsion (modified paraffin), anti-collapse sealing agent FT401, and modified asphalt.

[0051] The aforementioned thickening agent is one of polyanionic cellulose high-viscosity PAC-H, xanthan gum XCD, and carboxymethyl cellulose, or a mixture of several in any proportion.

[0052] The above-mentioned inhibitors are one of the following: salts (potassium chloride, sodium chloride, sodium formate, etc.), organic amine polymers, sodium formate, or a mixture of several in any proportion.

[0053] The aforementioned weighting agent is one of barite powder and limestone powder, or a mixture of two in any proportion.

[0054] The present invention will now be described in detail with reference to specific embodiments.

[0055] Example 1 A pretreatment process for gas field produced water reuse in drilling fluid preparation includes the following steps: Step S1: Take 100 parts of gas field produced water and add 0.01 parts of defoamer (organosilicon) to the gas field produced water by mass percentage. Then stir evenly at 5000 r / min to make the foaming rate of the gas field produced water less than 2%.

[0056] Step S2: Based on step S1, add 0.1 parts of ion removal agent (soluble carbonate) to the gas field produced water by mass percentage, and then stir evenly at 5000 r / min to remove some metal ions from the gas field produced water.

[0057] Step S3: Based on step S2, add 0.001 parts of bactericide (aldehyde) to the gas field produced water by mass percentage, and then stir evenly at 5000 r / min to kill iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the gas field produced water.

[0058] Step S4: Based on step S3, add 0.01 parts of pH adjuster (sodium hydroxide) to the gas field produced water by mass percentage, then stir evenly at 5000 r / min, and then use pH test paper to measure the pH of the gas field produced water to be 8, thus obtaining pretreated gas field produced water A.

[0059] The process described in this embodiment was used to treat the produced water from the gas field at Gas Gathering Station No. 1. The treatment results are shown in Table 1 below: Example 2 A pretreatment process for gas field produced water reuse in drilling fluid preparation includes the following steps: Step S1: Take 100 parts of gas field produced water and add 0.05 parts of defoamer (aluminum stearate) to the gas field produced water by mass percentage. Then stir evenly at 5000 r / min to make the foaming rate of the gas field produced water less than 2%.

[0060] Step S2: Based on step S1, add 1.5 parts of ion removal agent (soluble sulfate) to the gas field produced water by mass percentage, and then stir evenly at 5000 r / min to remove some metal ions from the gas field produced water.

[0061] Step S3: Based on step S2, add 0.01 parts of a bactericide (a cationic surfactant) to the gas field produced water by mass percentage, and then stir evenly at 5000 r / min to kill iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the gas field produced water.

[0062] Step S4: Based on step S3, add 0.2 parts of pH adjuster (potassium hydroxide) to the gas field produced water by mass percentage, then stir evenly at 5000 r / min, and then use pH test paper to measure the pH of the gas field produced water to be 9, thus obtaining pretreated gas field produced water B.

[0063] The process described in this embodiment was used to treat the produced water from the gas field at gas gathering station No. 2. The treatment results are shown in Table 2 below: Example 3 A pretreatment process for gas field produced water reuse in drilling fluid preparation includes the following steps: Step S1: Take 100 parts of gas field produced water and add 0.1 parts of defoamer (fatty acid ester) to the gas field produced water by mass percentage. Then stir evenly at 5000 r / min to make the foaming rate of the gas field produced water less than 2%.

[0064] Step S2: Based on step S1, add 3 parts of ion removal agent to the gas field produced water by mass percentage. The ion removal agent is a mixture of soluble carbonate and soluble sulfate in a 1:1 ratio. Then stir evenly at 5000 r / min to remove some metal ions from the gas field produced water.

[0065] Step S3: Based on step S2, add 0.05 parts of a bactericide (inorganic chlorine) to the gas field produced water by mass percentage, and then stir evenly at 5000 r / min to kill iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the gas field produced water.

[0066] Step S4: Based on step S3, add 0.5 parts of pH adjuster (sodium hydroxide) to the gas field produced water by mass percentage, then stir evenly at 5000 r / min, and then use pH test paper to measure the pH of the gas field produced water to be 10, thus obtaining the pretreated gas field produced water C.

[0067] The process described in this embodiment was used to treat the produced water from the gas field at gas gathering station No. 3. The treatment results are shown in Table 3 below: Example 4 This embodiment provides a drilling fluid system suitable for upper formations. The drilling fluid system is prepared using pretreated gas field produced water A, and specifically includes the following components by weight: The pretreated gas field produced water A75 parts, selected soil 2 parts, water loss reducing agent 2 parts, soda ash 0.1 parts, sodium hydroxide 0.2 parts, and viscosity enhancer 0.25 parts.

[0068] Specifically, the preferred soil is salt-resistant bentonite, the fluid loss reducer is drilling fluid fluid loss reducer - modified natural polymer NAT-20, and the viscosity enhancer is xanthan gum XCD.

[0069] The drilling fluids were prepared using produced water A from the gas field and fresh water, respectively. The performance comparison of the prepared drilling fluids is shown in Table 4 below: As can be seen from the data in Table 4, the performance of the first-stage drilling fluid system prepared with pretreated produced water is basically equivalent to that of the six-speed rotational viscosity and filtration loss prepared with clean water.

[0070] Example 5 This embodiment provides a drilling fluid system suitable for upper formations. The drilling fluid system is prepared using pretreated gas field produced water B and specifically includes the following components by weight: The pretreated gas field produced water (B98 parts) contains 4 parts of preferred soil, 3 parts of dehydration reducer, 0.15 parts of soda ash, 0.3 parts of sodium hydroxide, and 0.3 parts of viscosity enhancer.

[0071] Specifically, the preferred soil is bentonite, the water loss reducing agent is sulfonated phenolic resin SMP-2, and the viscosity improver is polyanionic cellulose high viscosity PAC-H.

[0072] The drilling fluids were prepared using produced water B from the gas field and fresh water, respectively. The performance comparison of the prepared drilling fluids is shown in Table 5 below: As can be seen from the data in Table 5, the performance of the drilling fluid system prepared with pretreated produced water is basically equivalent to that of the six-speed rotational viscosity and filtrate loss prepared with clean water.

[0073] Example 6 This embodiment provides a drilling fluid system suitable for upper formations. The drilling fluid system is prepared using pretreated gas field produced water (C) and specifically includes the following components by weight: The pretreated gas field produced water contains 50 parts C, 1 part preferred soil, 0.5 parts fluid loss reducer, 0.05 parts soda ash, 0.1 parts sodium hydroxide, and 0.2 parts viscosity enhancer. Specifically, the preferred soil is bentonite or salt-resistant bentonite, the water loss reducing agent is polyanionic cellulose low-viscosity PAC-L, and the viscosity improving agent is polyanionic cellulose high-viscosity PAC-H.

[0074] The drilling fluids were prepared using produced water (C) from the gas field and fresh water, respectively. The performance comparison of the prepared drilling fluids is shown in Table 6 below: As can be seen from the data in Table 6, the performance of the first-stage drilling fluid system prepared with pretreated produced water is basically equivalent to that of the six-speed rotational viscosity and filtration loss prepared with clean water.

[0075] Example 7 This embodiment provides a drilling fluid system suitable for central formations. This drilling fluid system is prepared using pretreated gas field produced water B and specifically includes the following components by weight: The pretreated gas field produced water contains 50 parts B, 0.5 parts fluid loss reducer, 0.2 parts viscosity enhancer, and 1 part inhibitor.

[0076] Specifically, the water loss reducing agent is polyanionic cellulose low-viscosity PAC-L, the viscosity improving agent is a mixture of polyanionic cellulose high-viscosity PAC-H and xanthan gum XCD in a 2:1 ratio, and the inhibitor is potassium chloride.

[0077] The drilling fluids were prepared by mixing two portions of produced water (B) from the gas field with fresh water. The inhibition properties of the prepared drilling fluids are compared in Table 7 below. As can be seen from the data in Table 7, the drilling fluid system prepared with pretreated produced water exhibits better inhibition properties than the drilling fluid system prepared with clean water. Specifically, the cuttings recovery rate of the drilling fluid system prepared with pretreated produced water is 71.4% (average of two tests), which is significantly higher than the 53.6% recovery rate of the system prepared with clean water. This indicates that the produced water pretreatment method of the present invention meets the requirements for drilling fluid preparation, and the drilling fluid system exhibits better inhibition performance than the clean water drilling fluid.

[0078] Example 8 This embodiment provides a drilling fluid system suitable for central formations. This drilling fluid system is prepared using pretreated gas field produced water B and specifically includes the following components by weight: The pretreated gas field produced water contained 75 parts B, 2 parts fluid loss reducer, 0.25 parts viscosity enhancer, and 4 parts inhibitor.

[0079] Specifically, the aforementioned fluid loss reducer is drilling fluid fluid loss reducer - modified natural polymer NAT-20, the aforementioned viscosity enhancer is polyanionic cellulose high viscosity PAC-H, and the aforementioned inhibitor is sodium chloride.

[0080] The drilling fluids were prepared by mixing two portions of produced water (B) from the gas field with fresh water. The inhibition properties of the prepared drilling fluids are compared in Table 8 below. As can be seen from the data in Table 8, the drilling fluid system prepared with pretreated produced water exhibits better inhibition properties than the drilling fluid system prepared with clean water. Specifically, the cuttings recovery rate of the drilling fluid system prepared with pretreated produced water is 81.95% (average of two tests), which is significantly higher than the 64.5% recovery rate of the system prepared with clean water. This indicates that the produced water pretreatment method of the present invention meets the requirements for drilling fluid preparation, and the drilling fluid system exhibits better inhibition performance than the clean water drilling fluid.

[0081] Example 9 This embodiment provides a drilling fluid system suitable for central formations. The drilling fluid system is prepared using pretreated gas field produced water A, and specifically includes the following components by weight: The pretreated gas field produced water consisted of 98 parts A, 6 parts fluid loss reducer, 0.5 parts viscosity enhancer, and 7 parts inhibitor.

[0082] Specifically, the aforementioned water loss reducing agent is a modified polyacrylic acid copolymer emulsion (modified paraffin), the aforementioned thickening agent is carboxymethyl cellulose, and the aforementioned inhibitor is sodium formate, etc. The drilling fluids were prepared by mixing two portions of produced water (A) from the gas field with fresh water. The inhibition properties of the prepared drilling fluids are compared in Table 9 below. As can be seen from the data in Table 9, the drilling fluid system prepared with pretreated produced water exhibits better inhibition properties than the drilling fluid system prepared with clean water. Specifically, the cuttings recovery rate of the drilling fluid system prepared with pretreated produced water is 92.65% (average of two tests), which is significantly higher than the 84.8% recovery rate of the system prepared with clean water. This indicates that the produced water pretreatment method of the present invention meets the requirements for drilling fluid preparation, and the drilling fluid system exhibits better inhibition performance than the clean water drilling fluid.

[0083] Example 10 This embodiment provides a drilling fluid system suitable for central formations. The drilling fluid system is prepared using pretreated gas field produced water (C) and specifically includes the following components by weight: The pretreated gas field produced water contains 60 parts C, 1 part fluid loss reducer, 0.2 parts viscosity enhancer, and 3 parts inhibitor.

[0084] Specifically, the aforementioned water loss reducing agent is modified asphalt, the aforementioned viscosity improver is polyanionic cellulose high viscosity PAC-H, and the aforementioned inhibitor is an organic amine polymer.

[0085] The drilling fluids were prepared by mixing two portions of produced water (C) from the gas field with fresh water. The inhibition properties of the prepared drilling fluids are compared in Table 10 below. As can be seen from the data in Table 10, the drilling fluid system prepared with pretreated produced water exhibits better inhibition properties than the drilling fluid system prepared with clean water. Specifically, the cuttings recovery rate of the drilling fluid system prepared with pretreated produced water is 73.55% (average of two tests), which is significantly higher than the 59.7% recovery rate of the system prepared with clean water. This indicates that the produced water pretreatment method of the present invention meets the requirements for drilling fluid preparation, and the drilling fluid system exhibits better inhibition performance than the clean water drilling fluid.

[0086] Example 11 This embodiment provides a drilling fluid system suitable for lower formations. The drilling fluid system is prepared using pretreated gas field produced water (C) and specifically includes the following components by weight: The pretreated gas field produced water contains 70 parts C, 2 parts of preferred soil, 6 parts of water loss reducing agent, 0.3 parts of viscosity increasing agent, 7 parts of inhibitor, and 15 parts of weighting agent.

[0087] The preferred soil is salt-resistant bentonite; the fluid loss reducer is a mixture of drilling fluid fluid loss reducer - modified natural polymer NAT-20 and anti-collapse plugging agent FT401 at a ratio of 1:2; the viscosity enhancer is a mixture of polyanionic cellulose high viscosity PAC-H and xanthan gum XCD at any ratio; the inhibitor is sodium chloride; and the weighting agent is limestone powder.

[0088] The drilling fluids were prepared by mixing gas field produced water (C) with fresh water. The apparent viscosity and water loss performance of the prepared drilling fluids are compared in Table 11 below. The data in Table 11 show that the apparent viscosity of the drilling fluid prepared with pretreated produced water is 51.5 mPa·s, which is higher than the 47 mPa·s of the system prepared with clean water. This indicates that the pretreated produced water has good compatibility, and the performance of the selected drilling fluid additives is less affected by the produced water, thus ensuring the rheological properties of the drilling fluid. Furthermore, after hot rolling at 120°C for 16 hours, the apparent viscosity of the system prepared with pretreated produced water is still higher than that of the system prepared with clean water, indicating that the developed drilling fluid system has good temperature resistance, and the system prepared with produced water maintains excellent performance even after hot rolling. In terms of filtration loss, the filtration loss of the drilling fluid prepared with produced water before and after hot rolling is comparable to that of the system prepared with clean water. In summary, the drilling fluid system prepared with produced water exhibits excellent performance. The apparent viscosity of the drilling fluid is not negatively affected by the produced water, it has good temperature resistance, and its filtration loss is comparable to that of the system prepared with clean water, thus meeting the field requirements for produced water performance.

[0089] Example 12 This embodiment provides a drilling fluid system suitable for lower formations. The drilling fluid system is prepared using pretreated gas field produced water A, and specifically includes the following components by weight: The pretreated gas field produced water A98 parts, selected soil 3 parts, fluid loss reducer 7 parts, viscosity enhancer 0.4 parts, inhibitor 10 parts, and weighting agent 30 parts.

[0090] The preferred soil is bentonite, the water loss reducing agent is anti-collapse sealing agent FT401, the viscosity improver is carboxymethyl cellulose, the inhibitor is an organic amine polymer, and the weighting agent is a mixture of barite powder and limestone powder in a 1:1 ratio.

[0091] The drilling fluids were prepared using produced water A from the gas field and fresh water, respectively. The apparent viscosity, water loss performance, and linear expansion rate of the prepared drilling fluids are compared in Tables 12 and 13 below: The data in Table 12 show that the apparent viscosity of the drilling fluid prepared with pretreated produced water is 61.5 mPa·s, which is higher than the 57 mPa·s of the system prepared with clean water. This indicates that the pretreated produced water has good compatibility, and the performance of the selected drilling fluid additives is less affected by the produced water, thus ensuring the rheological properties of the drilling fluid. Furthermore, after hot rolling at 120°C for 16 hours, the apparent viscosity of the system prepared with pretreated produced water is still higher than that of the system prepared with clean water, indicating that the developed drilling fluid system has good temperature resistance, and the system prepared with produced water maintains excellent performance even after hot rolling. In terms of filtration loss, the filtration loss of the drilling fluid prepared with produced water before and after hot rolling is comparable to that of the system prepared with clean water. In summary, the drilling fluid system prepared with produced water exhibits excellent performance. The apparent viscosity of the drilling fluid is not negatively affected by the produced water, it has good temperature resistance, and its filtration loss is comparable to that of the system prepared with clean water, thus meeting the field requirements for produced water performance.

[0092] As shown in Table 13, throughout the experiment, the linear expansion of shale in drilling fluid prepared with clean water was consistently higher than that in drilling fluid prepared with pretreated produced water. This indicates that produced water drilling fluid provides better inhibition, and this advantage becomes more pronounced over time. The results of the linear expansion measurement are consistent with those of the cuttings recovery rate measurement, further confirming that drilling fluid prepared with pretreated produced water exhibits better inhibition. Moreover, in different drilling fluid systems, pretreated produced water consistently maintains superior inhibition compared to clean water, which is of significant importance for the field application of drilling fluid prepared with produced water.

[0093] Example 13 This embodiment provides a drilling fluid system suitable for lower formations. The drilling fluid system is prepared using pretreated gas field produced water B and specifically includes the following components by weight: The pretreated gas field produced water contains 50 parts B, 1 part preferred soil, 4 parts water loss reducer, 0.2 parts viscosity enhancer, 5 parts inhibitor, and 10 parts weighting agent.

[0094] Specifically, the preferred soil is bentonite, the water loss reducing agent is polyanionic cellulose low-viscosity PAC-L, the viscosity improving agent is polyanionic cellulose high-viscosity PAC-H, the inhibitor is sodium formate, and the weighting agent is barite powder.

[0095] The drilling fluids were prepared using gas field produced water B and fresh water, respectively. The apparent viscosity, water loss performance, and linear expansion rate of the prepared drilling fluids are compared in Table 14 below: The data in Table 14 show that the apparent viscosity of the drilling fluid prepared with pretreated produced water is 38 mPa·s, which is basically equivalent to the 35 mPa·s of the system prepared with clean water. This indicates that the pretreated produced water has good compatibility, and the performance of the selected drilling fluid additives is less affected by the produced water, thus ensuring the rheological properties of the drilling fluid. Furthermore, after hot rolling at 120°C for 16 hours, the apparent viscosity of the system prepared with pretreated produced water is basically equivalent to that of the system prepared with clean water, indicating that the developed drilling fluid system has good temperature resistance, and the system prepared with produced water still exhibits excellent performance after hot rolling. In terms of filtration loss, the filtration loss of the drilling fluid prepared with produced water before and after hot rolling is comparable to that of the system prepared with clean water. In summary, the drilling fluid system prepared with produced water exhibits excellent performance. The apparent viscosity of the drilling fluid is not negatively affected by the produced water, it has good temperature resistance, and its filtration loss is comparable to that of the system prepared with clean water, thus meeting the field requirements for produced water performance.

[0096] As can be seen from the specific content of the above embodiments, the performance of the gas field produced water after pretreatment in Examples 1-3 meets the requirements for drilling fluid preparation. According to different well sections (formations) during construction, Examples 4-13 use different parameters to characterize the performance of the prepared drilling fluid and obtain drilling fluid with excellent performance that meets the construction requirements.

[0097] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification, unless specifically stated otherwise, may be replaced by other equivalent or similarly purposed alternative features; all disclosed features or methods... The steps in the process, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A pretreatment process for reusing produced water from gas fields to prepare drilling fluid, characterized in that... Includes the following steps: Step S1: Add 0.01%-0.1% defoamer to the produced water of the gas field by mass percentage and stir evenly to make the foaming rate of the produced water of the gas field less than 2%; Step S2: Add 0.1%-3% of ion removal agent to the gas field produced water by mass percentage and stir evenly to remove some metal ions from the gas field produced water; Step S3: Add 0.001%-0.05% of bactericide to the gas field produced water by mass percentage and stir evenly to kill iron bacteria, saprophytic bacteria and sulfate-reducing bacteria in the gas field produced water; Step S4: Add 0.01%-0.5% pH adjuster to the gas field produced water by mass percentage and stir evenly to adjust the pH value of the gas field produced water to 8-10, thus obtaining pretreated gas field produced water.

2. The pretreatment process for gas field produced water reuse in drilling fluid preparation according to claim 1, characterized in that: The defoamer is one of the following: organosilicon, aluminum stearate, fatty acid ester, phosphate ester, fatty alcohol, white oil, kerosene, or a mixture of several in any proportion.

3. The pretreatment process for gas field produced water reuse in drilling fluid preparation according to claim 1, characterized in that: The ion removal agent is one of soluble carbonates and soluble sulfates, or a mixture of two in any proportion.

4. The pretreatment process for gas field produced water reuse in drilling fluid preparation according to claim 1, characterized in that: The bactericide is one of aldehydes, cationic surfactants, and inorganic chlorine, or a mixture of several in any proportion.

5. The pretreatment process for gas field produced water reuse in drilling fluid preparation according to claim 1, characterized in that: The pH adjuster is sodium hydroxide or potassium hydroxide.

6. A drilling fluid system for gas field produced water obtained using the pretreatment process described in claim 1, the drilling fluid system being suitable for upper formations, characterized in that: Includes the following components by weight: The pretreated gas field produced water consists of 50-98 parts, selected soil 1-4 parts, water loss reducing agent 0.5-3 parts, soda ash 0.05-0.15 parts, sodium hydroxide 0.1-0.3 parts, and viscosity enhancer 0.2-0.3 parts.

7. A drilling fluid system for gas field produced water obtained using the pretreatment process described in claim 1, the drilling fluid system being suitable for intermediate formations, characterized in that: Includes the following components by weight: The pretreated gas field produced water contains 50-98 parts, a fluid loss reducer 0.5-6 parts, a viscosity enhancer 0.2-0.5 parts, and an inhibitor 1-7 parts.

8. A drilling fluid system for gas field produced water obtained using the pretreatment process described in claim 1, the drilling fluid system being suitable for lower formations, characterized in that: Includes the following components by weight: The pretreated gas field produced water consists of 50-98 parts, selected soil of 1-3 parts, fluid loss reducer of 4-7 parts, viscosity enhancer of 0.2-0.4 parts, inhibitor of 5-10 parts, and weighting agent of 10-30 parts.

9. The drilling fluid system according to claim 6 or 8, characterized in that: The preferred soil is bentonite or salt-resistant bentonite.

10. The drilling fluid system according to any one of claims 6-8, characterized in that: The fluid loss reducing agent is one of the following, or a mixture of several in any proportion: polyanionic cellulose low viscosity, drilling fluid fluid loss reducing agent - modified natural polymer, sulfonated phenolic resin, modified starch, modified polyacrylic acid copolymer emulsion, anti-collapse sealing agent, and modified asphalt.

11. The drilling fluid system according to any one of claims 6-8, characterized in that: The viscosity enhancer is one of polyanionic cellulose high viscosity, xanthan gum, carboxymethyl cellulose, or a mixture of several in any proportion.

12. The drilling fluid system according to claim 7 or 8, characterized in that: The inhibitor is one of salts, organic amine polymers, sodium formate, or a mixture of several in any proportion.

13. The drilling fluid system according to claim 8, characterized in that: The weighting agent is one of barite powder and limestone powder, or a mixture of the two in any proportion.

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