A drilling fluid system for offshore oil and gas drilling and its preparation method and application

Abstract

The present invention relates to a drilling fluid system for marine oil and gas drilling, a preparation method and application thereof, and belongs to the technical field of deepwater natural gas hydrate drilling. The drilling fluid system includes the following components: natural seawater, sodium chloride, potassium chloride, density regulator, fluid loss reducer, viscosity enhancer, reservoir bridge plugging agent, micro-nano plugging agent, anti-mud balling lubricant, hydrate kinetic inhibitor and hydrate decomposition inhibitor. According to the key technical problems of natural gas hydrate drilling and production in the South China Sea, the present invention optimizes the combination of viscosity enhancer, fluid loss reducer, lubricant and hydrate inhibitor, determines the various treatment agents required for the configuration of the drilling fluid system, and forms a drilling fluid system for marine oil and gas drilling with excellent rheological fluid loss performance, hydrate formation hydration expansion inhibition performance, plugging performance, lubrication performance and hydrate inhibition performance.

Description

Technical Field

[0001] The invention relates to a drilling fluid system for marine oil and gas drilling, a preparation method and application thereof, and belongs to the technical field of deepwater natural gas hydrate drilling. Background Art

[0002] In the low-temperature and high-pressure environment of the shallow deep sea, natural gas hydrates are easily formed, which leads to problems such as the inability to release BOP, directly threatening well control safety. In addition, hydrates will form crystals on the seabed, occupy the gaps in the sediments, resulting in a reduction in the volume of the sediments and reducing the efficiency of drilling operations. In the pipeline, hydrates form blockages, causing serious damage to the pipeline system and even endangering life and property. When drilling into natural gas hydrate formations, hydrates will decompose, mainly due to the thermal effect of the drilling fluid, changes in formation pressure, drilling disturbances, chemical factors and mechanical crushing. The heat transferred by the high-temperature drilling fluid causes the hydrates to heat up, the pressure reduction destroys their stability, the drilling operation disturbs the formation, the chemical composition changes the hydrate conditions, and the mechanical crushing directly destroys their structure. Therefore, effectively inhibiting the formation and decomposition of hydrates has become one of the important ways to improve the efficiency and safety of drilling operations.

[0003] Deep-sea drilling fluid systems often use drilling fluid additives such as salts and organic matter to inhibit the formation of hydrates. However, the use of these additives will increase the density of the drilling fluid, increase drilling costs, and cause environmental pollution. More importantly, in the deep-sea shallow environment, the difference between the formation pore pressure and the formation fracture pressure is small, and the density window of the safe drilling fluid is extremely narrow, which increases the possibility of risks such as downhole leakage and well wall rupture. Therefore, seeking a deep-sea shallow drilling fluid system that can effectively inhibit the decomposition and formation of hydrates and maintain a low density of the drilling fluid has become one of the hot spots in today's drilling fluid system research.

[0004] The development of natural gas hydrate drilling fluid system has been relatively mature, but most of its systems are developed mainly for the inhibition of hydrate secondary formation. There are few studies on the decomposition of reservoir hydrates and wellbore collapse in horizontal well drilling. Secondly, there are few studies on the regulation of drilling fluid density, and the problem of narrow safety window density cannot be effectively solved. Therefore, it is of great significance to develop a high-performance water-based drilling fluid that can regulate density and inhibit the formation and decomposition of hydrates.

[0005] Restricted by the phase equilibrium conditions of natural gas hydrates and geothermal gradient, natural gas hydrates in my country are mainly distributed in the deep-sea shallow sedimentary layers in the South China Sea. The sediments have poor cementation, and natural gas hydrates play a strong cementation role in the reservoir. The disturbance caused by drilling and production projects will inevitably destroy this cementation effect, leading to the degradation of the mechanical properties of the reservoir, causing well wall instability, sand production, formation subsidence, submarine landslides and other reservoir structural stability problems.

[0006] The geological conditions of the deep-water shallow strata where the South China Sea natural gas hydrate reservoir is located are complex. Hydrates are easy to decompose and the formation has poor cementation ability. The drilling density window is narrow and the mud line temperature is low. During the on-site application of drilling fluids, problems such as well wall instability, difficulty in controlling the low-temperature rheological properties of drilling fluids, and serious environmental pollution are often faced. Therefore, more stringent requirements are placed on the drilling fluid performance for drilling and production of deep-water shallow hydrate reservoirs.

[0007] The main problems of drilling fluid for hydrate gas in the South China Sea are: ① Seabed natural gas hydrates are extremely sensitive to temperature and pressure. During the drilling process, the drilling fluid and natural gas hydrates interact with each other in mass transfer and heat transfer, which can easily induce the decomposition of natural gas hydrates in the reservoir, causing redistribution of formation stress around the wellbore, reduced formation strength, and leading to well wall instability; ② The safe density window of the natural gas hydrate reservoir is narrow and the pore pressure is low, which requires high low-density control of the drilling fluid.

[0008] Based on the actual natural gas hydrate reservoir and drilling conditions in the South China Sea, the drilling fluid density needs to be controlled at 1.05~1.06g / cm 3 The drilling fluid system has a high hydrate resistance and good wellbore stability. However, there is no drilling fluid system suitable for the actual natural gas hydrate reservoirs and drilling conditions in the South China Sea. Summary of the invention

[0009] In view of the deficiencies of the prior art, the present invention provides a drilling fluid system for marine oil and gas drilling, a preparation method thereof and an application thereof. The components of the present invention have good compatibility with each other, can be applied to hydrate formation drilling, inhibit the formation and decomposition of hydrates while maintaining low density, and also have good rheological properties, fluid loss reduction performance, lubrication performance, plugging performance, hydrate formation hydration expansion inhibition performance, etc., and can play a role in stabilizing the well wall.

[0010] The technical solution of the present invention is as follows:

[0011] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator, fluid loss reducer, viscosity enhancer, reservoir bridge plugging agent, micro-nano plugging agent, anti-mud balling lubricant, hydrate kinetic inhibitor and hydrate decomposition inhibitor;

[0012] Wherein, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 5 to 10 parts by weight, the content of potassium chloride is 3 to 5 parts by weight, the content of density regulator is 1 to 3 parts by weight, the content of fluid loss reducer is 0.3 to 2 parts by weight, the content of viscosity enhancer is 0.1 to 0.3 parts by weight, the content of reservoir bridging agent is 2 to 3 parts by weight, the content of micro-nano plugging agent is 1 to 3 parts by weight, the content of anti-mud balling lubricant is 1 to 2 parts by weight, the content of hydrate kinetic inhibitor is 0.5 to 1.5 parts by weight, and the content of hydrate decomposition inhibitor is 0.5 to 1.5 parts by weight;

[0013] The micro-nano plugging agent is modified polystyrene latex (HFSD).

[0014] Preferably according to the present invention, in the drilling fluid system, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator is 3 parts by weight, the content of fluid loss reducer is 1 part by weight, the content of viscosity enhancer is 0.15 parts by weight, the content of reservoir bridging agent is 2 parts by weight, the content of micro-nano plugging agent is 2 parts by weight, the content of anti-mud balling lubricant is 1 part by weight, the content of hydrate kinetics inhibitor is 1 part by weight, and the content of hydrate decomposition inhibitor is 1 part by weight.

[0015] Preferably according to the present invention, the density regulator is glass microspheres.

[0016] Preferably according to the present invention, the fluid loss reducer is one or a mixture of two of modified starch (PF-FLOTROL) and polyanionic cellulose (PF-PAC-LV).

[0017] Preferably according to the present invention, the viscosity increasing agent is xanthan gum XC.

[0018] Preferably according to the present invention, the reservoir bridging plugging agent is an acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB).

[0019] Preferably, according to the present invention, the modified polystyrene latex (HFSD) is prepared according to the following method:

[0020] 2-Acrylamido-2-methyl-1-propanesulfonic acid and dimethyldiallylammonium chloride are dissolved in pure water, the pH value is adjusted to neutral, and then reactive emulsifiers allyloxyhydroxysulfonate sodium (Cops-1) and styrene are added, and emulsification is carried out at 3500-4500 rpm for 25-35 minutes to obtain an emulsion; then the emulsion is heated to 65-75°C, a mixed solution of ammonium persulfate and sodium bisulfite is added, and the reaction is carried out at a constant temperature for 4-6 hours to obtain a modified polystyrene latex (HFSD).

[0021] Further preferably, the mass volume ratio of the 2-acrylamido-2-methyl-1-propanesulfonic acid, dimethyldiallylammonium chloride, reactive emulsifier sodium allyloxyhydroxysulfonate (Cops-1), styrene and pure water is 5:4:1:50:100, unit: g / g / g / g / mL.

[0022] Further preferably, the volume ratio of the emulsion to the mixed solution of ammonium persulfate and sodium bisulfite is 30:1.

[0023] Further preferably, in the mixed solution of ammonium persulfate and sodium bisulfite, the concentration of ammonium persulfate is 1 mol / L, and the concentration of sodium bisulfite is 0.5 mol / L.

[0024] Preferably according to the present invention, the anti-balling lubricant is a surfactant mixture (PF-HLUB).

[0025] Preferably according to the present invention, the hydrate kinetic inhibitor is polyvinyl pyrrolidone (PVPK90).

[0026] Preferably, according to the present invention, the hydrate decomposition inhibitor is prepared according to the following method:

[0027] Add acrylamide and dimethyldiallylammonium chloride to pure water, stir and mix evenly to obtain a mixed solution; then, under nitrogen protection, add a composite aqueous solution of ammonium persulfate and sodium bisulfite dropwise to the mixed solution, react at 45-75°C and 100-300rpm for 5-7 hours, and after shearing, washing, drying and crushing, obtain a hydrate decomposition inhibitor (ADP).

[0028] Further preferably, the mass volume ratio of acrylamide, dimethyldiallylammonium chloride and pure water is 9:1:50, unit: g / g / mL.

[0029] Further preferably, the volume ratio of the mixed solution to the composite aqueous solution of ammonium persulfate and sodium bisulfite is 10:1.

[0030] Further preferably, in the composite aqueous solution of ammonium persulfate and sodium bisulfite, the concentration of ammonium persulfate is 1 mol / L, and the concentration of sodium bisulfite is 1 mol / L.

[0031] Further preferably, the dropwise addition speed of the composite aqueous solution of ammonium persulfate and sodium bisulfite is 3 seconds per drop, and the dropwise addition is completed within 15 to 25 minutes.

[0032] The method for preparing the drilling fluid system for offshore oil and gas drilling comprises the following steps:

[0033] The natural seawater is stirred at 2000-3000 rpm and 25° C. for 3-6 minutes, and then a hydrate kinetic inhibitor and a hydrate decomposition inhibitor are added to the natural seawater according to a ratio, and the mixture is stirred at 4000-6000 rpm for 15-25 minutes. Subsequently, the rotation speed is increased to 8000-10000 rpm, and at intervals of 10-20 minutes, a fluid loss reducer, a viscosity enhancer, a reservoir bridging agent, a micro-nano plugging agent, sodium chloride, potassium chloride and an anti-mud balling lubricant are added in sequence according to a ratio. Finally, a density regulator is added according to a ratio at a rotation speed of 5000-6000 rpm, and the mixture is stirred for 15-25 minutes to obtain a drilling fluid system for offshore oil and gas drilling.

[0034] The above-mentioned drilling fluid system for offshore oil and gas drilling is used as a drilling fluid system in deep-water shallow drilling. It inhibits the secondary generation of natural gas hydrates in the wellbore and the decomposition of natural gas hydrates in the reservoir while maintaining low density, thereby stabilizing the well wall.

[0035] The technical features and beneficial effects of the present invention are as follows:

[0036] 1. The various drilling fluid treatment agents in the drilling fluid system for offshore oil and gas drilling provided by the present invention have good compatibility with each other, the system is convenient to prepare, simple to use, cheap, environmentally friendly and non-toxic, and meets the specific requirements of on-site construction of hydrate formations.

[0037] 2. Based on the key technical difficulties in drilling and producing natural gas hydrates in the South China Sea, the present invention optimizes the combination of thickeners, filtration reducers, lubricants and hydrate inhibitors, determines the various treatment agents required for the configuration of the drilling fluid system, and forms a drilling fluid system for marine oil and gas drilling with excellent rheological filtration performance, hydrate formation hydration expansion inhibition performance, plugging performance, lubrication performance, and hydrate inhibition performance.

[0038] 3. The drilling fluid system for offshore oil and gas drilling provided by the present invention has added glass microbeads as a density regulator, and the density of the drilling fluid system can be controlled by controlling the amount of glass microbeads added to adapt to deepwater formations under different environments. At the same time, the present invention also contains unconventional, laboratory-made micro-nano plugging agents modified polystyrene latex (HFSD) and hydrate decomposition inhibitors (ADP), which have improved rheology, filtration, plugging, inhibition and other properties. In addition, the present invention does not require the addition of additional shale inhibitors (such as polyamines), because the hydrate decomposition inhibitors contain a large number of cationic inhibitory groups and amide adsorption groups, which can play a certain shale inhibition role and save costs.

[0039] 4. The drilling fluid system for offshore oil and gas drilling of the present invention is subjected to indoor experiments to evaluate the rheological properties, filtration loss reduction performance, lubrication performance, plugging performance, hydrate formation hydration expansion inhibition performance, and hydrate inhibition performance of the optimized drilling fluid system. The experiment shows that the low-density strong hydrate inhibition drilling fluid system has good performance: the system AV is 35mPa·s at room temperature, the filtration loss is 5.8mL, and it has good low-temperature rheology, the lubrication coefficient is 0.09281, the medium-pressure sand bed plugging experiment (60~80 mesh) has an intrusion depth of 1.3cm, the core immersion experiment has good results, the linear expansion rate is only 0.8%, and it has good hydrate inhibition performance: under 9MPa and 1.3℃, hydrates are not generated within 24 hours, the average hydrate decomposition rate is 0.124mol / h, and the hydrate decomposition inhibition rate is increased by 36.08% compared with pure water. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a graph showing the core immersion test results of Example 1, Comparative Example 1 and Comparative Example 3.

[0041] Figure 2 It is a linear expansion result diagram of Example 1 and Comparative Examples 1-3 and Comparative Example 5.

[0042] Figure 3 This is a graph showing the results of the hydrate inhibition experiment of pure water in comparative example 1.

[0043] Figure 4 This is a graph showing the hydrate inhibition test results of the drilling fluid system in Example 1.

[0044] Figure 5 This is a graph showing the results of the hydrate decomposition inhibition experiment for the drilling fluid system of Comparative Example 5.

[0045] Figure 6 This is a graph showing the results of a hydrate decomposition inhibition experiment for the drilling fluid system of Example 1. DETAILED DESCRIPTION

[0046] The present invention is further described below with reference to specific examples, but is not limited thereto. The raw materials used in the examples are all conventional raw materials and can be obtained commercially; the methods described are all prior art unless otherwise specified.

[0047] The micro-nano plugging agent modified polystyrene latex (HFSD) and hydrate decomposition inhibitor (ADP) used in the examples and comparative examples were homemade in the laboratory and were prepared by the following methods respectively.

[0048] The specific preparation method of the micro-nano plugging agent modified polystyrene latex (HFSD) is as follows:

[0049] 10 g of 2-acrylamido-2-methyl-1-propanesulfonic acid and 8 g of dimethyldiallylammonium chloride were dissolved in 200 mL of pure water, and the pH value was adjusted to neutral with a 20 wt % NaOH solution, and then 2 g of reactive emulsifier sodium allyloxyhydroxysulfonate (Cops-1) was added, and emulsified at 4000 rpm for 30 minutes using a high shear mixer emulsifier, during which 100 g of styrene was slowly added to obtain an emulsion; 300 mL of the emulsion was transferred to a three-necked flask, and placed in a constant temperature water bath for heating. When the temperature rose to 70°C, 10 mL of a mixed solution of ammonium persulfate and sodium bisulfite was added to initiate the reaction, and the reaction was carried out at a constant temperature of 70°C for 5 hours to obtain a modified polystyrene latex (HFSD);

[0050] Wherein, in the composite aqueous solution of ammonium persulfate and sodium bisulfite, the concentration of ammonium persulfate is 1 mol / L, and the concentration of sodium bisulfite is 0.5 mol / L.

[0051] The specific preparation method of the hydrate decomposition inhibitor (ADP) is as follows:

[0052] 18 g of acrylamide and 2 g of dimethyldiallylammonium chloride were added to 100 mL of pure water, and stirred with a magnetic stirrer until the solid was uniformly dissolved to obtain a mixed solution; the mixed solution was added to a 250 mL three-necked flask, placed in a constant temperature water bath, and nitrogen was introduced for 15 minutes to exhaust the air. After the air was exhausted, a composite aqueous solution of ammonium persulfate and sodium bisulfite was added dropwise to the three-necked flask at a rate of 3 seconds / drop, and the addition was completed within 20 minutes. The mixture was reacted at 55°C and 200 rpm for 6 hours; after the reaction was completed, the reactant was sheared, washed with anhydrous ethanol, dried in an electric constant temperature blast drying oven at 80°C for 24 hours, and crushed to obtain a hydrate decomposition inhibitor (ADP).

[0053] Example 1

[0054] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), fluid loss reducer polyanionic cellulose (PF-PAC-LV), viscosity enhancer xanthan gum XC, reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), micro-nano plugging agent modified polystyrene latex (HFSD), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90), hydrate decomposition inhibitor (ADP);

[0055] Among them, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator glass beads is 3 parts by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 1 part by weight, the content of filtrate reducer polyanionic cellulose (PF-PAC-LV) is 0.3 parts by weight, the content of thickener xanthan gum XC is 0.15 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 2 parts by weight, the content of micro-nano plugging agent modified polystyrene latex (HFSD) is 2 parts by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 1 part by weight, the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 1 part by weight, and the content of hydrate decomposition inhibitor (ADP) is 1 part by weight.

[0056] The method for preparing the drilling fluid system for offshore oil and gas drilling comprises the following steps:

[0057] The natural seawater was stirred at 2500 rpm and 25° C. for 5 minutes, and then a hydrate kinetic inhibitor and a hydrate decomposition inhibitor were added to the natural seawater according to a ratio, and the mixture was stirred at 5000 rpm for 20 minutes. Subsequently, the rotation speed was increased to 9000 rpm, and at intervals of 15 minutes, a fluid loss reducer, a viscosity enhancer, a reservoir bridging agent, a micro-nano plugging agent, sodium chloride, potassium chloride and an anti-mud balling lubricant were added in sequence according to a ratio. Finally, a density regulator was added according to a ratio at a rotation speed of 5500 rpm, and the mixture was stirred for 20 minutes to obtain a drilling fluid system for marine oil and gas drilling.

[0058] Example 2

[0059] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), viscosity enhancer xanthan gum XC, reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), micro-nano plugging agent modified polystyrene latex (HFSD), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90), hydrate decomposition inhibitor (ADP);

[0060] Among them, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator glass beads is 3 parts by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 2 parts by weight, the content of thickener xanthan gum XC is 0.15 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 2 parts by weight, the content of micro-nano plugging agent modified polystyrene latex (HFSD) is 2 parts by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 1 part by weight, the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 1 part by weight, and the content of hydrate decomposition inhibitor (ADP) is 1 part by weight.

[0061] The preparation method of the drilling fluid system for offshore oil and gas drilling is the same as that of Example 1.

[0062] Example 3

[0063] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), fluid loss reducer polyanionic cellulose (PF-PAC-LV), viscosity enhancer xanthan gum XC, reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), micro-nano plugging agent modified polystyrene latex (HFSD), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90), hydrate decomposition inhibitor (ADP);

[0064] Wherein, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 3 parts by weight, the content of density regulator glass beads is 1 part by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 2 parts by weight, the content of filtrate reducer polyanionic cellulose (PF-PAC-LV) is 0.3 parts by weight, the content of thickener xanthan gum XC is 0.15 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 2 parts by weight, the content of micro-nano plugging agent modified polystyrene latex (HFSD) is 2 parts by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 1 part by weight, the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 1 part by weight, and the content of hydrate decomposition inhibitor (ADP) is 1 part by weight.

[0065] The preparation method of the drilling fluid system for offshore oil and gas drilling is the same as that of Example 1.

[0066] Example 4

[0067] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), fluid loss reducer polyanionic cellulose (PF-PAC-LV), viscosity enhancer xanthan gum XC, reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), micro-nano plugging agent modified polystyrene latex (HFSD), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90), hydrate decomposition inhibitor (ADP);

[0068] Among them, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 5 parts by weight, the content of potassium chloride is 3 parts by weight, the content of density regulator glass beads is 1 part by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 0.3 parts by weight, the content of thickener xanthan gum XC is 0.1 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 2 parts by weight, the content of micro-nano plugging agent modified polystyrene latex (HFSD) is 1 part by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 1 part by weight, the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 0.5 parts by weight, and the content of hydrate decomposition inhibitor (ADP) is 0.5 parts by weight.

[0069] The method for preparing the drilling fluid system for offshore oil and gas drilling comprises the following steps:

[0070] The natural seawater was stirred at 2000 rpm and 25° C. for 6 minutes, and then a hydrate kinetic inhibitor and a hydrate decomposition inhibitor were added to the natural seawater according to a ratio, and the mixture was stirred at 4000 rpm for 25 minutes. Subsequently, the rotation speed was increased to 8000 rpm, and at intervals of 20 minutes, a fluid loss reducer, a viscosity enhancer, a reservoir bridging agent, a micro-nano plugging agent, sodium chloride, potassium chloride and an anti-mud balling lubricant were added in sequence according to a ratio. Finally, a density regulator was added according to a ratio at a rotation speed of 5000 rpm, and the mixture was stirred for 25 minutes to obtain a drilling fluid system for marine oil and gas drilling.

[0071] Example 5

[0072] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), fluid loss reducer polyanionic cellulose (PF-PAC-LV), viscosity enhancer xanthan gum XC, reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), micro-nano plugging agent modified polystyrene latex (HFSD), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90), hydrate decomposition inhibitor (ADP);

[0073] Among them, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 10 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator glass beads is 3 parts by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 1 part by weight, the content of filtrate reducer polyanionic cellulose (PF-PAC-LV) is 1 part by weight, the content of thickener xanthan gum XC is 0.3 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 3 parts by weight, the content of micro-nano plugging agent modified polystyrene latex (HFSD) is 3 parts by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 2 parts by weight, the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 1.5 parts by weight, and the content of hydrate decomposition inhibitor (ADP) is 1.5 parts by weight.

[0074] The method for preparing the drilling fluid system for offshore oil and gas drilling comprises the following steps:

[0075] The natural seawater was stirred at 3000 rpm and 25° C. for 3 minutes, and then a hydrate kinetic inhibitor and a hydrate decomposition inhibitor were added to the natural seawater according to a ratio, and the mixture was stirred at 6000 rpm for 15 minutes. Subsequently, the rotation speed was increased to 10000 rpm, and at intervals of 10 minutes, a fluid loss reducer, a viscosity enhancer, a reservoir bridging agent, a micro-nano plugging agent, sodium chloride, potassium chloride and an anti-mud balling lubricant were added in sequence according to a ratio. Finally, a density regulator was added according to a ratio at a rotation speed of 6000 rpm, and the mixture was stirred for 15 minutes to obtain a drilling fluid system for marine oil and gas drilling.

[0076] Comparative Example 1

[0077] Pure water without any drilling fluid additives.

[0078] Comparative Example 2

[0079] Prepare a pure water solution containing 3.5% sodium chloride.

[0080] Comparative Example 3

[0081] Prepare a pure aqueous solution containing 1% shale inhibitor polyamine.

[0082] Comparative Example 4

[0083] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), viscosity enhancer xanthan gum XC, reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), micro-nano plugging agent modified polystyrene latex (HFSD), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90), hydrate decomposition inhibitor (ADP);

[0084] Among them, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator glass beads is 3 parts by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 2 parts by weight, the content of thickener xanthan gum XC is 0.5 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 2 parts by weight, the content of micro-nano plugging agent modified polystyrene latex (HFSD) is 2 parts by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 1 part by weight, the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 1 part by weight, and the content of hydrate decomposition inhibitor (ADP) is 1 part by weight.

[0085] The preparation method of the drilling fluid system for offshore oil and gas drilling is the same as that of Example 1.

[0086] Comparative Example 5

[0087] A drilling fluid system for offshore oil and gas drilling, the drilling fluid system comprising the following components: natural seawater, sodium chloride, potassium chloride, density regulator glass microspheres, fluid loss reducer modified starch (PF-FLOTROL), fluid loss reducer polyanionic cellulose (PF-PAC-LV), viscosity enhancer xanthan gum XC, reservoir bridging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB), anti-mud balling lubricant surfactant mixture (PF-HLUB), hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90);

[0088] Among them, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator glass beads is 1 part by weight, the content of filtrate reducer modified starch (PF-FLOTROL) is 1 part by weight, the content of filtrate reducer polyanionic cellulose (PF-PAC-LV) is 0.3 parts by weight, the content of thickener xanthan gum XC is 0.15 parts by weight, the content of reservoir bridge plugging agent acid-soluble temporary plugging agent ultrafine calcium carbonate (PF-EZCARB) is 2 parts by weight, the content of anti-mud balling lubricant surfactant mixture (PF-HLUB) is 1 part by weight, and the content of hydrate kinetics inhibitor polyvinyl pyrrolidone (PVPK90) is 1 part by weight.

[0089] The preparation method of the drilling fluid system for offshore oil and gas drilling is the same as that of Example 1.

[0090] Test Example 1

[0091] Drilling fluid density is one of the most important parameters in the drilling process. Reasonable drilling fluid density can effectively avoid well control problems. In deepwater shallow hydrate drilling, in order to prevent problems such as well kick, blowout and hydrate decomposition due to pressure reduction, a higher density drilling fluid is generally used for drilling. Since the deepwater shallow strata where the hydrate reservoir is located have poor cementing ability and low mechanical strength, excessive drilling fluid may cause the formation to leak, causing a large amount of hydrate to decompose and overflow, thus endangering drilling safety. In addition, excessive drilling fluid density during drilling will reduce the mechanical drilling speed and facilitate the secondary formation of hydrates, so a reasonable drilling fluid density should be selected. Experiments have found that adding low-density glass beads to the drilling fluid can effectively reduce the density of the drilling fluid. Every 1% of glass beads added can reduce the density by about 0.1g / cm 3 The density of drilling fluid is increased, and its filtration loss is also reduced. This is also an effective way to reduce the density of the system in the solid-free composite salt drilling fluid system. However, excessive addition of glass beads will lead to a decrease in the stability of the drilling fluid system. After standing for a period of time, the drilling fluid will show obvious stratification, indicating that if too much glass beads are added, the system's ability to suspend its solid particles will decrease, causing changes in the performance of the drilling fluid. Therefore, the amount of glass beads needs to be controlled to not exceed 3%.

[0092] Referring to the national standard "GB / T29170-2012 Laboratory Test of Drilling Fluids in the Petroleum and Natural Gas Industry", the density, rheological parameters at different temperatures and filtration loss at normal temperature and pressure of the drilling fluid systems of Examples 1 to 3 and Comparative Examples 4 to 5 were tested. The experimental results are shown in Table 1.

[0093] Table 1. Rheological filtration test of base slurry added with drilling fluid system of Examples 1 to 3 and Comparative Examples 4 to 5

[0094]

[0095] It can be seen from Table 1 that, compared with the drilling fluid systems of Comparative Examples 4 and 5, under the premise of meeting the low density, the viscosity, shear force, and filtration loss of the drilling fluid systems of Examples 1 to 3 all meet the requirements, and the low-temperature viscosity changes less, which is more conducive to carrying cuttings and cleaning the wellbore, and is more suitable for the actual natural gas hydrate reservoirs and drilling conditions in the South China Sea.

[0096] Test Example 2

[0097] The sand bed filtration test was used to evaluate the plugging performance of the drilling fluid system of Example 1 in a 60-80 mesh sand layer. The specific experimental steps are as follows:

[0098] (1) Add 300 mL of sand with a mesh size of 60-80 mesh into the medium-pressure sand bed filter loss tester, carefully compact the sand and pour in 150 mL of drilling fluid;

[0099] (2) After ensuring safety, open the gas cylinder and pressurize it to 0.7 MPa. Pay attention to observe the phenomena during the experiment;

[0100] (3) After 30 minutes, the depth of the drilling fluid penetrating into the sand layer was recorded, and the plugging performance of Example 1 on the sand layer was tested by using 60-80 mesh sand to simulate the deep-sea shallow pore development formation.

[0101] Compared with the penetration depth of 5.6 cm in the sand bed of Comparative Example 5, the penetration depth of the drilling fluid system of Example 1 is 1.3 cm, indicating that it has better permeability leakage plugging ability. The lubricity of the drilling fluid system of Example 1 is measured using an EP-R type extreme pressure lubrication instrument. The measurement results are calculated to show that the lubricity coefficient of the drilling fluid system of Example 1 is 0.09281, which is less than 0.15, indicating that the system has good lubrication performance.

[0102] Test Example 3

[0103] The artificial core formed by pressing the hydrate in-situ soil was placed in the drilling fluid system of Example 1 and the pure water of Comparative Example 1 and the pure water solution containing 1% shale inhibitor polyamine of Comparative Example 3 to conduct core immersion experiment to test the stability. The results are as follows: Figure 1 shown.

[0104] from Figure 1 It can be seen that the core begins to lose stability as soon as it is immersed in Comparative Example 1. After 10 minutes, it collapses and disperses severely. The solution becomes turbid due to the diffusion of clay particles. After 24 hours, the hydrated and dispersed clay particles settle to the bottom of the cup, the solution becomes clear, and the core structure is completely destroyed.

[0105] In the presence of the pure water solution containing 1% shale inhibitor polyamine in Comparative Example 3, the collapse of the core was not significantly improved. Compared with the pure water in Comparative Example 1, the particle size of the deep-sea soil dispersed in the solution became larger and more angular after 24 hours, indicating that the pure water solution containing 1% shale inhibitor polyamine in Comparative Example 3 had a certain effect of inhibiting the hydration of the hydrated in-situ soil and maintaining the core shape. At 0 minutes and 10 minutes, the morphology of the core immersed in the drilling fluid system of Example 1 did not change significantly. After 24 hours, several fine cracks appeared on the surface of the core, and the overall structure remained intact. It can be intuitively seen that the drilling fluid system of Example 1 has good performance in inhibiting the hydration expansion of the hydrated soil and has the ability to stabilize the well wall.

[0106] Test Example 4

[0107] The artificial core formed by pressing the hydrate in-situ soil was put into the drilling fluid system of Example 1 and Comparative Examples 1 to 3 and Comparative Example 5 to conduct linear expansion experiments to test the linear expansion rate. The results are as follows: Figure 2 shown.

[0108] Depend on Figure 2 It can be seen that the expansion rate of the hydrate in-situ soil artificial core in the drilling fluid system of Example 1 is much smaller than that in Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 5. The 16-hour linear expansion rate of the drilling fluid system of Example 1 is only 0.8%, which can indicate that the drilling fluid system of Example 1 can effectively inhibit the hydration expansion of hydrate formation clay.

[0109] Test Example 5

[0110] The hydrate inhibition performance of the drilling fluid system of Example 1, the pure water of Comparative Example 1, and the drilling fluid system of Comparative Example 5 was evaluated. The specific experimental steps are as follows:

[0111] The effective volume of the reactor is 1L. After repeated cleaning with pure water, the configured system is added to the reactor and sealed and installed. Observe the data acquisition system and record data including liquid temperature, gas temperature, stirring torque, etc. Set the initial jacket temperature to 24°C, open the gas injection valve, pass 1MPa of methane, and release it after waiting for 10 minutes. This is to exclude the remaining air in the kettle and place it in a gas environment where only methane exists. Turn on the stirring paddle and set the stirring speed to 500rpm, and inject methane until the pressure in the kettle reaches 9MPa. Under this setting, the hydrate generated is mainly dispersed in the solution in particles, which is conducive to uniform heat transfer of the jacket. Keep it at 24°C for one hour. When the temperature and pressure balance is stable, adjust the jacket temperature so that it is evenly reduced to 0°C within one hour. When the jacket temperature decreases, the temperature in the kettle decreases to a certain value and hydrates begin to be generated in large quantities. At this time, it can be observed from the pressure-temperature curve that the interaction between methane molecules and water molecules in the process of natural gas hydrate generation will cause heat release. Therefore, before the generation, the temperature and pressure change with time in a linear relationship. When hydrates begin to form in the device, the temperature in the kettle will rise due to heat release. At the same time, the hydrate generation process will consume a large amount of methane gas. At this time, the pressure will also drop, which is manifested on the curve as the temperature curve rises instead of falling, and a peak will appear. The pressure curve will suddenly increase in slope. Set the jacket temperature at 0℃ for 16 hours to ensure that the hydrate generation is complete and the temperature and pressure reach a stable equilibrium state. Turn off the stirring and set the jacket temperature to rise rapidly from 0℃ to 14℃ to simulate the process of drilling fluid contacting with formation hydrates during drilling to trigger their decomposition. The hydrate decomposition rate is calculated according to the PR equation. When the pressure in the kettle changes by less than or equal to 0.01MPa in half an hour, it can be considered that the hydrate decomposition is complete.

[0112] 600 mL of the drilling fluid system of Example 1 and Comparative Example 1 were injected into the fully automatic hydrate evaluation reactor to evaluate the formation inhibition and decomposition inhibition. The results are as follows: Figure 3 , 4 shown.

[0113] Depend on Figure 3 , 4 It can be seen that compared with the comparative example 1 which generates a large amount of hydrates at 4.6°C, the drilling fluid system of Example 1 can not generate hydrates within 24 hours under the conditions of 9MPa and 1.3°C, and has good hydrate formation inhibition.

[0114] Then, the hydrate decomposition inhibition was evaluated. Under the condition of initial pressure of 9 MPa, the jacket temperature was rapidly reduced from 24°C to -6°C to generate hydrates. The results are as follows: Figure 5 , 6 shown.

[0115] Depend on Figure 5 , 6 It can be seen that under the conditions of -4°C and 7.8MPa, hydrates begin to form, and the temperature and pressure change amplitude is small, indicating that the amount of hydrates generated is small, which can also indirectly reflect the good hydrate generation inhibition of the drilling fluid system of Example 1. According to the temperature and pressure changes during the hydrate decomposition process, the average hydrate decomposition rate of Example 1 is calculated by using the PR equation to be 0.124mol / h, compared with 0.194mol / h of Comparative Example 1 and 0.176mol / h of Comparative Example 5, the hydrate decomposition inhibition rate is increased by 36.08% and 29.55%, respectively. It can be concluded that the drilling fluid system of Example 1 has good hydrate decomposition inhibition.

[0116] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A drilling fluid system for offshore oil and gas drilling, It is characterized in that The drilling fluid system comprises the following components: natural seawater, sodium chloride, potassium chloride, density regulator, fluid loss reducer, viscosity enhancer, reservoir bridging agent, micro-nano plugging agent, anti-mud balling lubricant, hydrate kinetic inhibitor and hydrate decomposition inhibitor; Wherein, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 5 to 10 parts by weight, the content of potassium chloride is 3 to 5 parts by weight, the content of density regulator is 1 to 3 parts by weight, the content of fluid loss reducer is 0.3 to 2 parts by weight, the content of viscosity enhancer is 0.1 to 0.3 parts by weight, the content of reservoir bridging agent is 2 to 3 parts by weight, the content of micro-nano plugging agent is 1 to 3 parts by weight, the content of anti-mud balling lubricant is 1 to 2 parts by weight, the content of hydrate kinetic inhibitor is 0.5 to 1.5 parts by weight, and the content of hydrate decomposition inhibitor is 0.5 to 1.5 parts by weight; The density regulator is glass microspheres; the fluid loss reducer is a mixture of one or two of modified starch PF-FLOTROL and polyanionic cellulose PF-PAC-LV; the viscosity enhancer is xanthan gum XC; the reservoir bridging agent is an acid-soluble temporary plugging agent ultrafine calcium carbonate; the micro-nano plugging agent is modified polystyrene latex; the anti-mud lubricant is a surfactant mixture PF-HLUB; the hydrate kinetic inhibitor is polyvinyl pyrrolidone; The modified polystyrene latex is prepared as follows: Dissolve 2-acrylamido-2-methyl-1-propanesulfonic acid and dimethyldiallylammonium chloride in pure water, adjust the pH value to neutral, then add reactive emulsifiers sodium allyloxyhydroxysulfonate and styrene, emulsify at 3500-4500 rpm for 25-35 minutes to obtain an emulsion; then heat the emulsion to 65-75° C., add a mixed solution of ammonium persulfate and sodium bisulfite, react at a constant temperature for 4-6 hours to obtain a modified polystyrene latex; The hydrate decomposition inhibitor is prepared according to the following method: Acrylamide and dimethyldiallylammonium chloride are added to pure water, and the mixture is stirred and mixed to obtain a mixed solution; then, a composite aqueous solution of ammonium persulfate and sodium bisulfite is added dropwise to the mixed solution under nitrogen protection, and the mixture is reacted at 45-75° C. and 100-300 rpm for 5-7 hours, and a hydrate decomposition inhibitor is obtained after shearing, washing, drying and crushing.

2. The drilling fluid system for offshore oil and gas drilling according to claim 1, It is characterized in that In the drilling fluid system, relative to 100 parts by weight of natural seawater, the content of sodium chloride is 7 parts by weight, the content of potassium chloride is 5 parts by weight, the content of density regulator is 3 parts by weight, the content of fluid loss reducer is 1.3 parts by weight, the content of viscosity enhancer is 0.15 parts by weight, the content of reservoir bridging agent is 2 parts by weight, the content of micro-nano plugging agent is 2 parts by weight, the content of anti-mud balling lubricant is 1 part by weight, the content of hydrate kinetic inhibitor is 1 part by weight, and the content of hydrate decomposition inhibitor is 1 part by weight.

3. The drilling fluid system for offshore oil and gas drilling according to claim 1, It is characterized in that The mass volume ratio of the 2-acrylamido-2-methyl-1-propanesulfonic acid, dimethyldiallylammonium chloride, reactive emulsifier sodium allyloxyhydroxysulfonate, styrene and pure water is 5:4:1:50:100, unit: g / g / g / g / mL; The volume ratio of the emulsion to the mixed solution of ammonium persulfate and sodium bisulfite is 30:1; In the mixed solution of ammonium persulfate and sodium bisulfite, the concentration of ammonium persulfate is 1 mol / L, and the concentration of sodium bisulfite is 0.5 mol / L.

4. The drilling fluid system for offshore oil and gas drilling according to claim 1, It is characterized in that The mass volume ratio of acrylamide, dimethyldiallylammonium chloride and pure water is 9:1:50, unit: g / g / mL; The volume ratio of the mixed solution to the composite aqueous solution of ammonium persulfate and sodium bisulfite is 10:1; The concentration of ammonium persulfate in the composite aqueous solution of ammonium persulfate and sodium bisulfite is 1 mol / L, and the concentration of sodium bisulfite is 1 mol / L; The drop speed of the composite aqueous solution of ammonium persulfate and sodium bisulfite is 3 seconds per drop, and the drop addition is completed within 15 to 25 minutes.

5. The method for preparing the drilling fluid system for offshore oil and gas drilling according to claim 1, It is characterized in that The steps include: The natural seawater is stirred at 2000-3000 rpm and 25° C. for 3-6 minutes, and then a hydrate kinetic inhibitor and a hydrate decomposition inhibitor are added to the natural seawater according to a ratio, and the mixture is stirred at 4000-6000 rpm for 15-25 minutes. Subsequently, the rotation speed is increased to 8000-10000 rpm, and at intervals of 10-20 minutes, a fluid loss reducer, a viscosity enhancer, a reservoir bridging agent, a micro-nano plugging agent, sodium chloride, potassium chloride and an anti-mud balling lubricant are added in sequence according to a ratio. Finally, a density regulator is added according to a ratio at a rotation speed of 5000-6000 rpm, and the mixture is stirred for 15-25 minutes to obtain a drilling fluid system for offshore oil and gas drilling.

6. Application of the drilling fluid system for offshore oil and gas drilling according to claim 1, It is characterized in that It is used as a drilling fluid system in deepwater shallow drilling.

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