A kind of acid-degradable and residue-free drilling fluid and preparation method thereof

Abstract

The invention discloses an acid-hydrolyzable residue-free drilling fluid and a preparation method thereof, and belongs to the technical field of drilling fluids for oil and gas reservoirs. The acid-hydrolyzable residue-free drilling fluid provided by the invention comprises the following components in parts by mass: 100 parts of water, 0.5 to 1.5 parts of a pH regulator, 0.5 to 1.0 parts of a flow pattern regulator, 1.0 to 4.0 parts of a fluid loss reducer, 0 to 10.0 parts of a temporary plugging agent, 1.0 to 3.0 parts of an inhibitor, and 5.0 to 200.0 parts of a weighting agent; the pH regulator is a mixture of sodium citrate, magnesium oxide, and sodium carbonate; the flow pattern regulator is polyethylene oxide and / or polyethyleneimine; the fluid loss reducer is one or more of waxy starch, waxy physically modified starch, and waxy chemically modified starch. The acid-hydrolyzable residue-free drilling fluid provided by the present invention overcomes the shortcomings of the prior art drilling fluid without solid phase or clay phase, such as residue and inability to degrade. It can be completely dissolved by acid completion fluid to form a transparent clear solution without suspended matter, achieving 100% degradation, without any damage to oil and gas reservoirs, and has good environmental performance.

Description

Technical Field

[0001] The invention belongs to the technical field of drilling fluid for oil and gas reservoirs, and particularly relates to an acid-degradable residue-free drilling fluid and a preparation method thereof. Background Art

[0002] my country has intensified its efforts in oil and gas exploration and development, and crude oil and natural gas production has continued to grow. Oil and gas layer protection is an important means of efficiently developing oil and gas reservoirs. How to reduce the damage of drilling fluid to the reservoir is a prerequisite for ensuring oil and gas production.

[0003] Drilling fluid refers to the drilling fluid used to drill the reservoir. Reservoir protection is closely related to the completion method. Openhole completion has higher requirements for reservoir protection of drilling fluid. Existing openhole completions include direct openhole completion, screen completion, perforated pipe completion, slotted pipe completion and other completion methods. The most important feature of these completion methods is that after drilling the reservoir, the drilling fluid is not put into the casing for perforation. The drilling fluid is in direct contact with the reservoir, which requires the drilling fluid to have extremely high reservoir protection performance. At present, most of the drilling fluids used in openhole completion are solid-free or clay-free drilling fluids, and some of them realize direct flowback completion. In addition, there is also the use of injecting gel-breaking fluid or completion fluid after the completion string to eliminate the reservoir damage caused by solid-free drilling fluid.

[0004] Directly return drilling fluid without solid phase or clay phase, the polymer or fluid loss agent in the drilling fluid invades the pores of the oil well and cannot be degraded, which poses a risk of damaging the reservoir. At present, the degradation method of return drilling fluid without solid phase or clay phase is usually gel breaking or delayed gel breaking technology, which can only reduce the viscosity of the polymer in the drilling fluid, and there will still be residual gel breaking residues and fluid loss agent particles. These gel breaking residues and fluid loss agent particles will still enter the pores of the oil well and damage the reservoir after the viscosity of the drilling fluid is reduced.

[0005] Therefore, it is of great significance and application value to develop a drilling fluid that can be completely acidified without any residue, completely avoid the reservoir damage problem caused by the drilling fluid, truly achieve no damage to the reservoir, and increase oil and gas production.

[0006] The Chinese patent with publication number CN114835917A provides a self-unblocking carbonate reservoir drilling fluid, the specific components of which include 100 parts of water, 5 to 16 parts of a composite self-unblocking temporary plugging agent, 2 to 6 parts of a fluid loss reducer, 0.1 to 0.8 parts of a viscosity enhancer, 0.1 to 0.5 parts of a pH adjuster, 0.1 to 0.5 parts of a waterproof lock agent, 0.1 to 0.5 parts of potassium chloride, and 0 to 230 parts of a weighting agent; the patent uses cationic starch, carboxymethyl starch, low-viscosity polyanionic cellulose, and low-viscosity carboxymethyl cellulose as fluid loss reducers; the fluid loss reducer disclosed in the patent cannot be dissolved by acid, and the viscosity enhancer cannot be degraded by acid or temperature, which means that the reservoir drilling fluid disclosed in the patent cannot be completely degraded and will cause damage to the reservoir. Summary of the invention

[0007] In view of this, the object of the present invention is to provide an acid-degradable residue-free drilling fluid and a preparation method thereof. The acid-degradable residue-free drilling fluid provided by the present invention can be 100% degraded without any damage to oil and gas reservoirs.

[0008] To solve the above problems, the specific implementation scheme of the present invention is as follows:

[0009] The present invention provides an acid-hydrolyzable residue-free drilling fluid, comprising the following components in parts by weight:

[0010] 100 parts of water, 0.5-1.5 parts of pH regulator, 0.5-1.0 parts of flow pattern regulator, 1.0-4.0 parts of fluid loss reducer, 0-10.0 parts of temporary plugging agent, 1.0-3.0 parts of inhibitor, 5.0-200.0 parts of weighting agent;

[0011] The pH regulator is a mixture of sodium citrate, magnesium oxide and sodium carbonate;

[0012] The flow pattern regulator is polyethylene oxide and / or polyethylene imine;

[0013] The fluid loss reducer is one or more of waxy starch, waxy physically modified starch and waxy chemically modified starch.

[0014] Preferably, the water is fresh water or sea water.

[0015] Preferably, based on 100 parts by mass of the pH adjuster, the pH adjuster includes: 20 to 40 parts by mass of sodium citrate, 30 to 50 parts by mass of magnesium oxide and 30 to 50 parts by mass of sodium carbonate.

[0016] Preferably, the waxy starch is one or more of waxy corn starch, waxy potato starch and waxy tapioca starch;

[0017] The waxy physically modified starch is one or more of pregelatinized waxy corn starch, pregelatinized waxy potato starch and pregelatinized waxy cassava starch;

[0018] The waxy chemically modified starch is one or more of carboxymethyl waxy starch, hydroxyethyl waxy starch, hydroxypropyl waxy starch and grafted modified waxy starch.

[0019] Preferably, the temporary plugging agent is calcium carbonate;

[0020] The acid solubility of the calcium carbonate is greater than 99.5%; the particle size of the calcium carbonate D10 is greater than 0.045 mm, and D90 is less than 0.09 mm.

[0021] Preferably, the inhibitor is a polyamine.

[0022] Preferably, the weighting agent is one or more of sodium formate, potassium formate and cesium formate.

[0023] The present invention also provides a method for preparing the acid-hydrolyzable residue-free drilling fluid described in the above technical solution, comprising the following steps:

[0024] A pH regulator, a flow pattern regulator, a fluid loss reducer, a temporary plugging agent, an inhibitor and a weighting agent are sequentially added to the water to obtain the acid-hydrolyzable and residue-free drilling fluid.

[0025] The invention provides an acid-hydrolyzable residue-free drilling fluid, comprising the following components in parts by mass: 100 parts of water, 0.5-1.5 parts of a pH regulator, 0.5-1.0 parts of a flow pattern regulator, 1.0-4.0 parts of a fluid loss reducer, 0-10.0 parts of a temporary plugging agent, 1.0-3.0 parts of an inhibitor, and 5.0-200.0 parts of a weighting agent; the pH regulator is a mixture of sodium citrate, magnesium oxide, and sodium carbonate; the flow pattern regulator is polyethylene oxide and / or polyethyleneimine; and the fluid loss reducer is one or more of waxy starch, waxy physically modified starch, and waxy chemically modified starch.

[0026] The acid-degradable and residue-free drilling fluid provided by the present invention fully overcomes the shortcomings of the existing drilling fluid without solid phase or clay phase, which has residue and cannot be degraded. The drilling fluid provided by the present invention can be completely dissolved by the acid completion fluid to form a transparent and clear solution without suspended matter, achieving 100% degradation, without harming the oil and gas reservoir, and having good environmental performance.

[0027] The acid-degradable and residue-free drilling fluid provided by the present invention can meet the reservoir protection requirements of open hole completion and achieve true harmlessness to the reservoir. After drilling the reservoir, the acid-degradable and residue-free drilling fluid can be pumped into the acid completion fluid. The acid-degradable and residue-free drilling fluid and the mud cake formed by it can be quickly dissolved by acid under the combined action of the acid completion fluid and the formation temperature. The dissolved liquid remains clear and transparent without any suspended residue, which fundamentally solves the problem of damage to the reservoir by the drilling fluid. The acid-degradable and residue-free drilling fluid provided by the present invention mainly has three types of raw materials for preparation: the first type is water-soluble materials, including pH regulators sodium citrate and sodium carbonate. Such materials are soluble in water, and the aqueous solution formed will not cause damage to the oil and gas reservoir. The second category is water-soluble polymers, including polyethylene oxide and polyethyleneimine. These two water-soluble polymers can form polymer solutions with high viscosity in aqueous solution, which fully meet the needs of drilling procedures. Within a certain period of time after drilling the reservoir, under the combined action of acidic completion fluid and formation temperature, these two polymers can be completely degraded into small molecule solutions, which do not harm the oil and gas reservoir. The third category is water-insoluble materials, including one or more of waxy starch, waxy physically modified starch and waxy chemically modified starch, magnesium oxide and calcium carbonate, which can be dissolved by acidic completion fluid to generate aqueous solutions. Waxy starch, waxy physically modified starch and waxy chemically modified starch can be quickly hydrolyzed into glucose under acidic conditions and temperature to form a transparent solution, and can also be 100% degraded; while ordinary starch and its physically and chemically modified starch cannot be quickly degraded into glucose by acid. Therefore, the raw materials used in the preparation of acid-degradable residue-free drilling fluid of the present invention can be acid-degraded and degraded, and will not cause damage to the reservoir.

[0028] The acid-hydrolyzable residue-free drilling fluid provided by the present invention has good rheological properties, fluid loss reduction and reservoir protection performance. The mud cake of the acid-hydrolyzable residue-free drilling fluid can be completely dissolved by a 1% methanesulfonic acid solution, and the acid solubility rate reaches 100%. The turbidity of the methanesulfonic acid solution after dissolution is less than 30NTU. The permeability recovery rate of the acid-hydrolyzable residue-free drilling fluid that has not been treated with acid reaches more than 96.5%, and the permeability recovery rate after acid treatment reaches 100%. All preparation materials of the acid-hydrolyzable residue-free drilling fluid of the present invention are environmentally friendly materials, which provides a guarantee for the safety and environmental protection of oil and gas development.

[0029] The present invention provides a method for preparing the acid-hydrolyzable residue-free drilling fluid, comprising the following steps: sequentially adding a pH regulator, a flow pattern regulator, a fluid loss reducer, a temporary plugging agent, an inhibitor and a weighting agent to water to obtain the acid-hydrolyzable residue-free drilling fluid. The preparation method provided by the present invention is simple to operate and has industrial application prospects. DETAILED DESCRIPTION

[0030] The present invention provides an acid-hydrolyzable residue-free drilling fluid, comprising the following components in parts by weight:

[0031] 100 parts of water, 0.5-1.5 parts of pH regulator, 0.5-1.0 parts of flow pattern regulator, 1.0-4.0 parts of fluid loss reducer, 0-10.0 parts of temporary plugging agent, 1.0-3.0 parts of inhibitor, 5.0-200.0 parts of weighting agent;

[0032] The pH regulator is a mixture of sodium citrate, magnesium oxide and sodium carbonate;

[0033] The flow pattern regulator is polyethylene oxide and / or polyethylene imine;

[0034] The fluid loss reducer is one or more of waxy starch, waxy physically modified starch and waxy chemically modified starch.

[0035] In the present invention, unless otherwise specified, the raw materials used in the present invention are preferably commercially available products.

[0036] The acid-hydrolyzable residue-free drilling fluid provided by the present invention comprises 100 parts by mass of water. In the present invention, the water is preferably fresh water or sea water, more preferably fresh water.

[0037] Based on the mass fraction of water, the acid-hydrolyzable residue-free drilling fluid provided by the present invention includes 0.5 to 1.5 parts by mass of a pH regulator, more preferably 0.8 to 1.2 parts. In the present invention, the pH regulator is preferably a mixture of sodium citrate, magnesium oxide and sodium carbonate. In the present invention, the mass fraction of the pH regulator is 100 parts, and the pH regulator preferably includes: 20 to 40 parts by mass of sodium citrate, 30 to 50 parts by mass of magnesium oxide and 30 to 50 parts by mass of sodium carbonate. In the present invention, sodium citrate has chelating and buffering effects, and can chelate with high-valent ions contained in water to ensure that other materials play a role; magnesium oxide has good acid solubility on the one hand, and has a pH buffering effect on the other hand; sodium carbonate provides basic alkalinity.

[0038] Based on the mass fraction of water, the acid-dissolvable residue-free drilling fluid provided by the present invention includes a flow pattern regulator of 0.5 to 1.0 parts by mass, more preferably 0.6 to 0.8 parts. In the present invention, the flow pattern regulator is preferably polyethylene oxide and / or polyethylene imine. In the present invention, when the flow pattern regulator is a mixture of polyethylene oxide and polyethylene imine, the mass ratio of polyethylene oxide to polyethylene imine is preferably 50 to 70:30 to 50. In the present invention, the polyethylene oxide and polyethylene imine have good coating inhibition, which can adjust the viscosity and rheology of the drilling fluid on the one hand, and can wrap the mud shale encountered during the drilling process to prevent it from dispersing. Polyethylene oxide has good temperature degradability. Under the action of the downhole formation temperature, it can be completely degraded into small molecules within a certain period of time to form a non-viscous transparent solution. Polyethylene imine can also be degraded into small molecules under the combined action of acidic conditions and formation temperature to form a non-viscous transparent solution.

[0039] Based on the mass fraction of water, the acid-hydrolyzable residue-free drilling fluid provided by the present invention includes a fluid loss reducer of 1.0 to 4.0 parts by mass, more preferably 2.0 to 3.0 parts. In the present invention, the fluid loss reducer is one or more of waxy starch, waxy physically modified starch and waxy chemically modified starch. In the present invention, the waxy starch is preferably one or more of waxy corn starch, waxy potato starch and waxy cassava starch. In the present invention, the waxy corn starch, waxy potato starch and waxy cassava starch can be compounded in any proportion. In the present invention, the waxy physically modified starch is preferably one or more of pregelatinized waxy corn starch, pregelatinized waxy potato starch and pregelatinized waxy cassava starch. In the present invention, the pregelatinized waxy corn starch, pregelatinized waxy potato starch and pregelatinized waxy cassava starch can be compounded in any proportion. In the present invention, the waxy chemically modified starch is preferably one or more of carboxymethyl waxy starch, hydroxyethyl waxy starch, hydroxypropyl waxy starch and grafted modified waxy starch. In the present invention, the carboxymethyl waxy starch, hydroxyethyl waxy starch, hydroxypropyl waxy starch and grafted modified waxy starch can be compounded in any proportion. In the present invention, the production process of the modified starch is not limited, and the existing production process can be used. In the present invention, the waxy chemically modified starch raw material is preferably one of waxy corn starch, waxy potato starch and waxy cassava starch, or compounded in any proportion.

[0040] In the present invention, the amylopectin content in the waxy corn starch, waxy potato starch and waxy cassava starch is preferably 100%. In the present invention, the waxy starch, waxy physically modified starch and waxy chemically modified starch can be rapidly hydrolyzed into glucose, and the hydrolyzed solution is clear and transparent without suspended residue. However, ordinary starch and its physically and chemically modified starch cannot be rapidly acid-hydrolyzed into glucose at a formation temperature of 60 to 140°C.

[0041] Based on the mass fraction of water, the acid-hydrolyzable residue-free drilling fluid provided by the present invention includes a temporary plugging agent in an amount of 0 to 10.0 parts by mass. In the present invention, the temporary plugging agent is preferably calcium carbonate. In the present invention, the acid solubility of the calcium carbonate is preferably greater than 99.5%; the particle size of the calcium carbonate is preferably D10>0.045mm, and D90<0.09mm. The high purity of calcium carbonate ensures that it can be completely dissolved by acid, and the particle size of calcium carbonate is controlled within an extremely small range, on the one hand to improve the temporary plugging performance, and on the other hand to ensure that all the calcium carbonate used are large particles to prevent calcium carbonate from invading deep into the formation.

[0042] Based on the mass fraction of water, the acid-hydrolyzable residue-free drilling fluid provided by the present invention includes 1.0 to 3.0 parts of inhibitor by mass, more preferably 1.2 to 2.5 parts. In the present invention, the inhibitor is preferably a polyamine. In the present invention, the inhibitor polyamine is soluble in water and is easy to form an aqueous solution, which will not cause damage to the oil and gas reservoir. In the present invention, the type of the polyamine is not limited, and commercially available ones are sufficient.

[0043] The acid-hydrolyzable residue-free drilling fluid provided by the present invention comprises 5.0 to 200.0 parts by weight of a weighting agent. In the present invention, the weighting agent is one or more of sodium formate, potassium formate and cesium formate. In the present invention, the weighting agent is soluble in water and is easy to form an aqueous solution that will not cause damage to oil and gas reservoirs. In the present invention, the type of the weighting agent is not limited, and commercially available weighting agents are sufficient.

[0044] The present invention also provides a method for preparing the acid-hydrolyzable residue-free drilling fluid, comprising the following steps:

[0045] A pH regulator, a flow pattern regulator, a fluid loss reducer, a temporary plugging agent, an inhibitor and a weighting agent are sequentially added to water to obtain the acid-hydrolyzable and residue-free drilling fluid.

[0046] In the present invention, the addition of pH regulator, flow pattern regulator, fluid loss reducer, temporary plugging agent, inhibitor and weighting agent is preferably carried out under stirring, and the stirring rate is preferably 3000-10000 r / min. In the present invention, the same stirring rate is preferably used in the entire preparation process.

[0047] In the present invention, after the pH adjuster is added, stirring is preferably performed for 10 minutes.

[0048] In the present invention, after the flow pattern regulator is added, it is preferably stirred for 30 minutes.

[0049] In the present invention, after the fluid loss reducer is added, it is preferably stirred for 30 minutes.

[0050] In the present invention, after the temporary plugging agent is added, it is preferably stirred for 20 minutes.

[0051] In the present invention, the inhibitor and the weighting agent are preferably added simultaneously, and then stirred for 40 to 60 minutes to obtain the acid-hydrolyzable and residue-free drilling fluid.

[0052] The technical solution of the present invention is further illustrated by specific implementation methods below. Those skilled in the art should understand that the embodiments described below are only part of the embodiments of the present invention, which are used to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0053] Example 1

[0054] A drilling fluid capable of acid hydrolysis and having no residue comprises the following preparation steps: adding 100 parts by mass of fresh water into a container, turning on a stirrer at a speed of 3000 r / min, adding 1.5 parts by mass of a pH regulator into the water, stirring for 10 minutes, slowly adding 1.0 parts by mass of a flow pattern regulator, polyethylene oxide, stirring for 30 minutes, slowly adding 1.0 parts by mass of a filtrate reducer, waxy corn starch, stirring for 30 minutes, adding 3.0 parts by mass of an inhibitor, polyamine, and 200.0 parts by mass of a weighting agent, cesium formate, stirring for 40 minutes, and obtaining a drilling fluid capable of acid hydrolysis and having no residue.

[0055] The pH adjuster includes 20 parts by mass of sodium citrate, 50 parts by mass of magnesium oxide and 30 parts by mass of sodium carbonate based on 100 parts by mass.

[0056] Example 2

[0057] A drilling fluid capable of acid hydrolysis and having no residue comprises the following preparation steps: adding 100 parts by mass of seawater into a container, turning on a stirrer at a speed of 10000 r / min, adding 0.5 parts by mass of a pH regulator into the water, stirring for 10 minutes, slowly adding 0.5 parts by mass of a flow pattern regulator, polyethyleneimine, stirring for 30 minutes, slowly adding 1.0 parts by mass of a filtrate reducer, waxy pregelatinized potato starch, stirring for 30 minutes, adding 10.0 parts by mass of a temporary plugging agent, calcium carbonate, stirring for 20 minutes, adding 1.0 parts by mass of an inhibitor, polyamine, and 5.0 parts by mass of a weighting agent, stirring for 40 minutes, and obtaining a drilling fluid capable of acid hydrolysis and having no residue.

[0058] The pH adjuster includes 40 parts by mass of sodium citrate, 30 parts by mass of magnesium oxide and 30 parts by mass of sodium carbonate based on 100 parts by mass.

[0059] Example 3

[0060] A drilling fluid capable of acid hydrolysis and having no residue comprises the following preparation steps: adding 100 parts by mass of fresh water into a container, turning on a stirrer at a speed of 6000 r / min, adding 1.0 parts by mass of a pH regulator into the water, stirring for 10 minutes, slowly adding 0.8 parts by mass of a flow type regulator, stirring for 30 minutes, slowly adding 3.0 parts by mass of a waxy chemically modified starch as a fluid loss reducer, stirring for 30 minutes, adding 5.0 parts by mass of a temporary plugging agent calcium carbonate, stirring for 20 minutes, adding 1.2 parts by mass of an inhibitor polyamine and 100.0 parts by mass of a weighting agent potassium formate, stirring for 40 minutes, and obtaining a drilling fluid capable of acid hydrolysis and having no residue.

[0061] Wherein, the pH adjuster comprises 30 parts by mass of sodium citrate, 30 parts by mass of magnesium oxide and 40 parts by mass of sodium carbonate, based on 100 parts by mass;

[0062] The flow pattern regulator comprises 70 parts by mass of polyethylene oxide and 30 parts by mass of polyethylene imine based on 100 parts by mass;

[0063] The acid solubility of the calcium carbonate is greater than 99.5%, and the particle size is tested by a wet laser particle size analyzer, with D10 being greater than 0.045 mm and D90 being less than 0.09 mm.

[0064] The waxy chemically modified starch is carboxymethyl waxy starch; the production process of the waxy chemically modified starch can be based on the existing public production process, and the raw material of the waxy chemically modified starch is waxy cassava starch.

[0065] Example 4

[0066] A drilling fluid capable of acid hydrolysis and having no residue comprises the following preparation steps: adding 100 parts by mass of seawater into a container, turning on a stirrer at a speed of 8000 r / min, adding 0.6 parts by mass of a pH regulator into the water, stirring for 10 minutes, slowly adding 0.7 parts by mass of a flow type regulator, stirring for 30 minutes, slowly adding 2.0 parts by mass of a fluid loss reducer, stirring for 30 minutes, adding 8.0 parts by mass of a temporary plugging agent calcium carbonate, stirring for 20 minutes, adding 2.0 parts by mass of an inhibitor polyamine and 50.0 parts by mass of a weighting agent, stirring for 40 minutes, and obtaining a drilling fluid capable of acid hydrolysis and having no residue.

[0067] Wherein, the pH adjuster comprises 20 parts by mass of sodium citrate, 30 parts by mass of magnesium oxide and 50 parts by mass of sodium carbonate, based on 100 parts by mass;

[0068] The flow pattern regulator comprises 50 parts by mass of polyethylene oxide and 50 parts by mass of polyethylene imine, based on 100 parts by mass;

[0069] The mass parts of the fluid loss reducer, calculated as 100 parts, include 40 parts by mass of waxy starch and 60 parts by mass of waxy physically modified starch; the mass parts of the waxy starch, calculated as 100 parts by mass, include 30 parts by mass of waxy corn starch, 50 parts by mass of waxy potato starch and 20 parts by mass of waxy cassava starch; the mass parts of the waxy physically modified starch, calculated as 100 parts by mass, include 50 parts by mass of pregelatinized waxy corn starch, 20 parts by mass of pregelatinized waxy potato starch and 30 parts by mass of pregelatinized waxy cassava starch;

[0070] The acid solubility of the calcium carbonate is greater than 99.5%, and the particle size is tested by a wet laser particle size analyzer, with D10 greater than 0.045 mm and D90 less than 0.09 mm;

[0071] The weighting agent comprises 30 parts by mass of sodium formate and 70 parts by mass of potassium formate based on 100 parts by mass.

[0072] Example 5

[0073] A drilling fluid capable of acid hydrolysis and having no residue comprises the following preparation steps: adding 100 parts by mass of seawater into a container, turning on a stirrer at a speed of 4000 r / min, adding 1.2 parts by mass of a pH regulator into the water, stirring for 10 minutes; slowly adding 0.9 parts by mass of a flow type regulator, stirring for 30 minutes; slowly adding 2.5 parts by mass of a fluid loss reducer, stirring for 30 minutes; adding 4.0 parts by mass of a temporary plugging agent, calcium carbonate, stirring for 20 minutes; adding 2.5 parts by mass of an inhibitor, polyamine, and 150.0 parts by mass of a weighting agent, stirring for 40 minutes, and obtaining a drilling fluid capable of acid hydrolysis and having no residue.

[0074] Wherein, the pH adjuster comprises 20 parts by mass of sodium citrate, 40 parts by mass of magnesium oxide and 40 parts by mass of sodium carbonate, based on 100 parts by mass;

[0075] The flow pattern regulator comprises 60 parts by mass of polyethylene oxide and 40 parts by mass of polyethylene imine based on 100 parts by mass;

[0076] The mass parts of the fluid loss reducer are calculated as 100 parts, including 20 parts by mass of waxy starch, 30 parts by mass of waxy physically modified starch and 50 parts by mass of waxy chemically modified starch; the waxy starch is waxy potato starch; the waxy physically modified starch is pregelatinized waxy cassava starch; the waxy chemically modified starch is hydroxypropyl waxy starch;

[0077] The acid solubility of the calcium carbonate is greater than 99.5%, and the particle size is tested by a wet laser particle size analyzer, with D10 greater than 0.045 mm and D90 less than 0.09 mm;

[0078] The weighting agent comprises 60 parts by mass of potassium formate and 40 parts by mass of cesium formate based on 100 parts by mass.

[0079] Comparative Example 1

[0080] The fluid loss reducer in Example 2 was replaced with ordinary corn starch to obtain a solid-free drilling fluid.

[0081] Comparative Example 2

[0082] The fluid loss reducer in Example 2 was replaced by pregelatinized common cassava starch to obtain a drilling fluid without a clay phase.

[0083] The drilling fluids prepared in Examples 1 to 5 and Comparative Examples 1 to 2 were tested according to GB / T16783.1-2014 "Field Test of Drilling Fluids in Petroleum and Natural Gas Industry Part 1: Water-Based Drilling Fluids". The test results are shown in Table 1.

[0084] Table 1 Test results of drilling fluids obtained in Examples and Comparative Examples

[0085]

[0086]

[0087] Note: T: aging temperature of drilling fluid, °C, PV: plastic viscosity of drilling fluid, mPa·s, YP: dynamic shear force of drilling fluid, Pa, Φ3: reading of 3 revolutions of six-speed rotary viscometer, dimensionless, API: medium-pressure water loss of drilling fluid (0.7MPa, 30min), mL, HFL-1: permeability recovery rate after drilling fluid directly contaminates core, %, HFL-2: permeability recovery rate after drilling fluid contaminates core and then contaminates core with 1% methanesulfonic acid, %.

[0088] The drilling fluids prepared in Examples 1 to 5 of the present invention and Comparative Examples 1 to 2 all have good rheological properties and filtration properties. The permeability recovery rate of the drilling fluids in Examples 1 to 5 of the present invention after directly contaminating the core reached more than 96.5%, and the permeability recovery rate after re-contaminating the core with a methanesulfonic acid solution was able to recover to 100%, or even higher, and the reason for exceeding 100% is that the acid solution has corroded the inherent pores in the core. The permeability recovery rate HFL-1 of direct backflow in Comparative Examples 1 and 2 is only 70.82% and 68.15%, and even after being treated with methanesulfonic acid, the permeability HFL-2 recovery rate is slightly improved, which is only about 80.35% and 78.17%. Comparative Examples 1 and 2 use ordinary starch, which cannot be completely acid-lyzed, and there are residual starch particles, indicating that the starch particles not dissolved by the acid have damaged the core.

[0089] The mud cake obtained from the drilling fluid prepared in Examples 1 to 5 and Comparative Examples 1 to 2 was immersed in a 1% by mass methanesulfonic acid solution and allowed to stand for a certain period of time at the test temperature. The turbidity and pH value of the methanesulfonic acid solution were tested, and the solution was filtered and dried to obtain a residue. The acid solubility of the mud cake was calculated. The test results are shown in Table 2.

[0090] Table 2 Test results of mud cakes obtained from drilling fluids prepared in Examples and Comparative Examples after soaking in methanesulfonic acid solution

[0091]

[0092]

[0093] Note: pH value: pH value after the mud cake is soaked in methanesulfonic acid solution, ZD: turbidity of the mud cake after soaking in methanesulfonic acid solution, the unit is NTU, SRL: acid solubility rate of the mud cake after soaking in methanesulfonic acid solution, 1% methanesulfonic acid is the blank control ratio, that is, there is no mud cake immersed, and the test data of 1% methanesulfonic acid solution itself.

[0094] The mud cake of the drilling fluid was soaked in a methanesulfonic acid solution and allowed to stand at test temperatures of 100° C. and 130° C. The mud cakes of Examples 1 to 5 of the present invention were all dissolved by methanesulfonic acid, and the turbidity of the dissolved methanesulfonic acid solution was less than 30 NTU, which met the completion fluid reservoir protection standard, indicating that it would not damage the reservoir, and the acid solubility of the mud cakes reached 100%, achieving true no residue; while after the mud cakes of Comparative Examples 1 and 2 were placed in a methanesulfonic acid solution, the turbidity of the methanesulfonic acid solution reached more than 100 NTU, which was relatively turbid, and the mud cakes showed flocculent precipitation, which would damage the reservoir, and the acid solubility of the mud cakes was only about 60%, which was caused by the starch fluid loss reducer in the mud cakes, indicating that the starch was not dissolved by the acid.

[0095] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. An acid-degradable, residue-free drilling fluid, characterized in that: The composition includes the following components in parts by weight: 100 parts of water, 0.5-1.5 parts of pH regulator, 0.5-1.0 parts of flow pattern regulator, 1.0-4.0 parts of fluid loss reducer, 0-10.0 parts of temporary plugging agent, 1.0-3.0 parts of inhibitor, 5.0-200.0 parts of weighting agent; The pH regulator is a mixture of sodium citrate, magnesium oxide and sodium carbonate; The flow pattern regulator is polyethylene oxide and / or polyethylene imine; The fluid loss reducer is one or more of waxy starch, waxy physically modified starch and waxy chemically modified starch; The waxy physically modified starch is one or more of pregelatinized waxy corn starch, pregelatinized waxy potato starch and pregelatinized waxy cassava starch; The waxy chemically modified starch is one or more of carboxymethyl waxy starch, hydroxyethyl waxy starch and hydroxypropyl waxy starch.

2. The acid-hydrolyzable residue-free drilling fluid according to claim 1, characterized in that: The water is fresh water or sea water.

3. The acid-hydrolyzable residue-free drilling fluid according to claim 1, characterized in that: The pH adjuster comprises 20 to 40 parts by mass of sodium citrate, 30 to 50 parts by mass of magnesium oxide and 30 to 50 parts by mass of sodium carbonate, based on 100 parts by mass of the pH adjuster.

4. The acid-hydrolyzable residue-free drilling fluid according to claim 1, characterized in that: The waxy starch is one or more of waxy corn starch, waxy potato starch and waxy cassava starch.

5. The acid-hydrolyzable residue-free drilling fluid according to claim 1, characterized in that: The temporary plugging agent is calcium carbonate; The acid solubility of the calcium carbonate is greater than 99.5%; the particle size of the calcium carbonate D10 is greater than 0.045 mm, and D90 is less than 0.09 mm.

6. The acid-hydrolyzable residue-free drilling fluid according to claim 1, characterized in that: The inhibitor is a polyamine.

7. The acid-hydrolyzable residue-free drilling fluid according to claim 1, characterized in that: The weighting agent is one or more of sodium formate, potassium formate and cesium formate.

8. The method for preparing the acid-hydrolyzable residue-free drilling fluid according to any one of claims 1 to 7, characterized in that: The following steps are involved: A pH regulator, a flow pattern regulator, a fluid loss reducer, a temporary plugging agent, an inhibitor and a weighting agent are sequentially added to the water to obtain the acid-hydrolyzable and residue-free drilling fluid.

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