Abstract: With more and more deep wells, ultra-deep wells, highly deviated wells, directional wells and horizontal wells drilled worldwide, demands on the lubricity of water base drilling fluids are increasing. Drilling lubricants with polar adsorption ability can effectively enhance the adsorption of the lubricant molecules on the friction surfaces, thereby improving the lubricity of the drilling fluid. This paper summarizes the progress made in the research of drilling lubricants with polar adsorption ability in recent years, including alcohol ethers, vegetable oil esters and their derivatives, long chain alkyl glucosides as well as extreme-pressure lubricants. The development trend of lubricants that can be adsorbed on the surfaces of drilling tools and lubricants with multiple adsorption points are then introduced. Generally speaking, the lubricity of a drilling lubricant is closely related to the polarity, type and quantity of the adsorptive groups in its molecular structure. Amino group and amide group, which have stronger polarity, have better adsorption on the surfaces of drilling tools or borehole walls than hydroxyl group and ether bond which have weak polarity. Under extreme pressures, lubricants containing sulfur, borate or phosphate can be adsorbed on the surfaces of drilling tools through chemical reaction, producing extreme lubricity. Nitrogen-containing heterocyclic compounds, on the other hand, can be adsorbed firmly on the surfaces of drilling tools, forming a complex adsorption film. Lubricants with multiple polar adsorptive groups can form multi-point adsorption on the surfaces of friction, effectively improving the strength of the lubricating film.
Abstract: Most drilling fluids have various disadvantages such as low temperature stability, low density and poor circulating suspending capacity when used to drill ultra-high temperature deep and ultra-deep wells. An oil base drilling fluid with the highest density of 2.30 g/cm3 has been developed to solve these problems. The drilling fluid was formulated with newly developed emulsifiers, viscosifier and thinner with high temperature resistance of 200℃. Laboratory evaluation showed that the ultra-high temperature emulsifiers SD-HTPE and SD-HTSE have little effect on the rheology of the drilling fluid. After hot rolling at 220℃, the emulsion stability of the drilling fluid was 1201-1856 V. The ultra-high temperature viscosifier SD-OIV can increase the LSRYP from 3 Pa to 13 Pa and the dynamic settlement index from 0.2096 to 0.6466. HTHP filter loss of the drilling fluid can be reduced by 76.74%. The ultra-high temperature thinner SD-ORV can reduce the LSRYP of the drilling fluid by 85.71%. The drilling fluid had good dynamic circulating rheology and thermal stability at 220℃, 40 MPa and low shear rates. Application of this drilling fluid in Well Tatan-1 in south Sichuan showed that the ultra-high temperature high density oil base drilling fluid had good thermal stability, rheology, settlement stability and HTHP filter loss at 214℃. No tight spots and drag were encountered during drilling, and tripping was conducted smoothly. Field application of this drilling fluid proved that it satisfied the requirements of drilling ultra-high temperature deep and ultra-deep wells.
Abstract: A new lipophilic emulsifier, NGE-1, with high temperature resistance and stability has been developed with Bisphenol F, chlorosulfonic acid and ethanolamine as the main raw materials through etherification, sulfonation and esterification reactions. The molecular structure of the emulsifier NGE-1 was characterized by FTIR, and the performance and stability of NGE-1 were analyzed by measuring the oil-water interfacial tension and electric stability test. Pre-designed groups were found in the molecular structure of NGE-1. Laboratory experiment showed that NGE-1 can remarkably reduce interfacial tension and has good emulsification stability. Oil base muds treated with NGE-1 have electric stability of 580 V after aging at 200℃, and the rate of emulsification can be as high as 96% after standing for 24 h. An oil base mud was formulated with this emulsifier and other carefully selected additives. Performance evaluation of this mud showed that the mud can be weighted to a maximum density of 2.4 g/cm3, and has high temperature resistance of 200℃ and resistance to 15% salt contamination.
Abstract: A high temperature low density oil base drilling fluid was formulated with a newly developed organophilic clay MZ to protect low pressure reservoirs in high temperature deep wells. In laboratory experiment, the colloid fraction of MZ was greater than that of another three organophilic clays:A828, B500A and B38. The colloid fraction of MZ in diesel oil can be as high as 97.4%. The properties of the slurry made from MZ only change slightly before and after hot rolling at high temperatures, indicating that high temperature does not destroy the spatial network structure formed by the coalescing and crosslinking of the particles of MZ, and MZ has good colloidal stability and thermal stability. The crystal interlayer spacing of MZ is 2.05 nm. Substitution of quaternary ammonium ions for metal ions in the crystal layers of bentonite increases the crystal interlayer spacing. Testing on the high temperature low density oil base drilling fluid formulated with MZ showed that the viscosity o the drilling fluid changed only slightly before and after hot rolling. After hot rolling, the gel strengths of the drilling fluid increased, and the HTHP filter loss was less than 3 mL. Oil base drilling fluids formulated with MZ are resistant to high temperature to 220℃, contamination from invasion of 50% salt water or 15% shale cuttings, satisfying the needs of drilling deep wells penetrating complex formations.
Abstract: Studies were conducted on the testing items in 24 standards of organophilic clays. It was found that the frequency of occurrence of those items such as appearance, moisture, screening residue (particle sizes greater than a specified value), ratio of colloid, apparent viscosity, plastic viscosity and yield point is more than 60%. It is pointed, based on the investigation of relative literature and field operation with oil base drilling fluids, that there are 6 problems in the standards that need to be addressed. This paper discusses in detail the cause of the problems and the methods of solving these problems. Advices to the formulation of the standards of the organophilic clays are also presented.
Abstract: In the North Block in Pakistan, the geological condition of the formations is very complex, and the formations penetrated have repeated inversion. The petroleum resource is buried deep in high temperature high pressure formations. Wells drilled in this block penetrated long section of shales in the second interval and salt/gypsum sections in the third interval. The shales swell and slough when in contact with water, and tight spots and pipe sticking were encountered in the salt/gypsum sections. Contamination of salt water from the formation to the drilling fluid spoiled the mud and made the mud properties difficult to control. These problems have made the drilling operations in this block extremely difficult, and the North Block is thus famous for being one of the most complex drilling areas in Pakistan or even worldwide. To deal with these problems, a diesel oil drilling fluid was formulated with 0# diesel oil (the continuous phase), CaCl2 water solution (dispersed phase) and other additives selected. This drilling fluid has satisfied the requirements of the local operator company. Laboratory evaluation showed that the properties of the drilling were stable at temperatures up to 180℃. The drilling fluid can be weighted to a density of 2.20 g/cm3. The emulsion stability of the drilling fluid before and after aging was 1200 V. The drilling fluid had good suspending capacity and cuttings carrying capacity. The rheology of the drilling fluid remained stable at high temperature and high pressure. The drilling fluid has the ability to resist contamination from 15% salt water, 10% shale cuttings and gypsum. All these characteristics of the drilling fluid have satisfied the requirements of the drilling operations in the North Block in Pakistan.
Abstract: This paper introduces the characteristics of the tight oil reservoirs in the Shenbei block of Liaohe Oilfield and the studies performed on the hydration, swelling, sloughing and cave-in of the hard and brittle shales in the member 4 of the Shahejie formation in the block. Using clay mineral analysis, nano CT scan imaging and SEM, the tight oil shale was studied and the clay mineral content, average pore radius and the development of micro-fissures of the tight oil shale was determined. Laboratory tests were conducted to optimize a special compound nano plugging agent based on the characteristics of the tight oil shales. A high-performance drilling fluid with high plugging capacity, high inhibitive capacity and low erosion to borehole wall was formulated through laboratory study. The primary percent recovery of shale cuttings tested with the drilling fluid formulated was 93%, and the tertiary percent recovery of shale cuttings tested with the drilling fluid formulated was near 80%. The percent reduction of permeability was more than 50%. Field application of the drilling fluid on two wells showed that this "high viscosity high gel strength" drilling fluid successfully resolved downhole troubles such as borehole wall sloughing and cave-in, reaming and back-reaming in shale sections and pipe sticking that were previously encountered. Occurrence of downhole troubles was reduced by 17%. Time for well completion was reduced by150 d, demonstrating the effectiveness of ROP enhancement of using the drilling fluid.
Abstract: A low molecular weight polymer filtrate reducer PANAD was developed to overcome the disadvantages of conventional filtrate reducers use in deep and ultra-deep well drilling, such as poor stability at high temperatures, poor resistance to salt contamination and excessive impact on the rheology of drilling fluids. The PANAD was synthesized through chain transfer reaction with several monomers such as 2-acrylamide-2-methylpropanesulfonic acid (AMPS), N, N-dimethyl acrylamide (DMAM), N-vinyl pyrrolidone (NVP) and dimethyl diallyl ammonium chloride (DMDAAC). 2,2-azobis,2-methylpropylimid, dihydrochloride was used as chain initiator. PANAD functions normally at elevated temperatures,up to 230℃, and high salinity,up to 20% salt concentration). The optimum synthesis conditions determined by orthogonal experiment are as follows, molar ratio of DMAM:AMPS:DMDAAC:NVP=7:2:2.5:1, reaction temperature=65℃ and concentration of initiator=0.7%. The intrinsic viscosity of PANAD 58 mL/g, as determined by "one-point method". The molecular structure and thermal stability of PANAD, as characterized by FT-IR and thermalgravimetric analysis, showed that PANAD had excellent thermal stability, with its chain pyrolysis temperature of 314℃. Test of the filtration control performance of PANAD showed that PANAD worked normally in water base drilling fluids at a maximum temperature of 230℃. Aged fresh water base and 20% saltwater base drilling fluids treated with 1% PANAD had API filtration rate of 8.9 mL and 22.5 mL, respectively. Filtration rate of fresh water base drilling fluid at 180℃ was 35.6 mL, a better filtration control performance than Driscal D. PANAD has very weak effect on the rheology of the drilling fluid before and after aging at 230℃. The mechanisms of filtration control of PANAD was analyzed through zeta-potential measurement, adsorption test and SEM analysis.
Abstract: With more and more stringent environment protection requirements imposed on the petroleum exploration industry, drilling fluid additives are required to be nontoxic and have high performance. As the core additive of drilling fluids, filter loss reducer is required to have properties such as high temperature resistance, salt resistance, good filtration control performance, lowor nontoxicity, and good biodegradation. Using lignin as a raw material, a biomass synthetic resin filter loss reducer, LDR-501, was produced through biodegradation and chemical reaction. Evaluation of LDR-501 showed that the BOD5/CODCr value is 0.26, indicating that LDR-501 is biodegradable. The EC50 of LDR-501 is 440 000 mg/L, and the cloud point salinity of LDR-501 is 160 g/L. A base mud treated with LDR-501 had HTHP filtration rate of 18.6 mL after aging at 180℃ for 16 h, indicating that LDR-501 has good high temperature resistance, salinity resistance and good filtration control performance. The development of LDR-501 has accelerated the application of biomass resources in drilling fluid industry and laid the foundation of improving the environmental friendliness of water base drilling fluids.
Abstract: Drilling automation is an inevitable trend in the development of petroleum industry, and the automatic measurement of the drilling fluid properties is now finding more and more application in field operation instead of just being a laboratory tool. The Research Institute of Drilling Technology of Shengli Oilfield (Sinopec) has recently independently developed an instrument that is used to online measure the properties of drilling fluids. The instrument can be used to measure the properties of drilling fluids in a real-time, continuous and automatic manner, and the measuring data can be transmitted to remote area. Drilling fluid properties measured with variable diameter rheology test method, dual-tube serial vibrating tube and ion-selective electrode method are integrated by a data acquisition system. The software used with the measuring instrument has nine pages displaying intuitively the acquired data in graphical form. This instrument has been tried in the drilling of many wells in the Shengli Oilfield and three HTHP deep wells by CNOOC, the highest density of mud used was 2.30 g/cm3. Properties measured with the instrument are basically consistent with the manually measured data, with test errors being in the designed ranges. Using this instrument, drilling efficiency is enhanced and downhole safety is ensured. The development of this instrument is required by the national oil and gas safety strategy, and is of great significance to the acceleration of drilling automation, drilling informatization and intelligent upgrading and transformation of drilling operation.
Abstract: Severe mud losses into fractured formations seriously restrict the efficient development of oil and gas in formations with complex geological conditions. Bridging agents presently used for mud loss control have poor deformability, the commonly used water-adsorbing resin lost circulation materials (LCMs), on the other hand, have expansion rates that are too fast, therefore, it is difficult for these LCMs to form dense pressure-bearing barriers in the channels through which the mud is lost into the formations. To solve this problem, a new delayed expansion LCM SDSAP was developed. SDSAP has a density of 1.52 g/cm3 and particle sizes between 0.425 mm and 3.35 mm which are adjustable according to the rate of mud loss. It has excellent water-adsorbing expanding performance, pressure resistance and salt resistance capacity. When water is adsorbed, SDSAP becomes deformable, and is therefore suitable for control mud losses in loss channels with wide range of sizes. SDSAP has a certain delayed expansion effect and low rate of expansion at ambient temperatures. It also has temperature-response property; when excited by high temperatures in downhole condition, it swells quickly, which is beneficial to shorten the time required to seal off the loss channels and thus reduces the amount of mud lost. An LCM slurry formulated with SDSAP and other LCMs such as rigid, elastic LCMs and fibers to stop mud losses into fractures with opening sizes of 1-3 mm. The barrier formed by the LCM slurry had pressure bearing capacity of more than 7 MPa, and the amount of mud lost was greatly reduced. It can be used to solve mud losses into complex fractured formations.
Abstract: Most methods presently used to evaluate the performance of anti-sloughing agents lack the specificity to chemical bonding anti-sloughing agents (bonding, solidifying), and thus cannot be used to evaluate this kind of anti-sloughing agents. Based on the mechanisms of chemical bonding anti-sloughing agents in preventing sloughing (or collapsing) of borehole wall, a method has been established to evaluate the performance of the chemical bonding anti-sloughing agents. The setup of the new method has borrowed some ideas and standards from adhesive science, and has introduced the parameter of lap shear strength to measure the bonding strength between two interfaces. After many times of optimization and improvement, data tested with this new method showed that the errors of the measurement results are reasonable, and the evaluation data have the properties of being parallel, reliable and scientific. The new method can be used to specifically evaluate the performance of the chemical bonding anti-sloughing agents.
Abstract: The pore pressure, collapse pressure, leaking pressure and fracture pressure of a depleted gas zone declined remarkably with time. When drilling into a depleted gas zone, the differential pressure between the pressure exerted by the mud column and the pore pressure of the formation is high, resulting in mud losses and formation damage. Understanding of the mechanisms and nature of mud losses is key to solve these problems. Based on the extended reach horizontal drilling practice in the D gas field in the north of South China Sea, a set of technologies was established for analyzing, preventing and controlling mud losses into shallow depleted loose sandstone gas zones. Using these technologies, the nature of the mud losses was first accurately determined by analyzing the current four-pressure profile and mud hydraulics. A solids-free weak-gel drilling fluid EZFLOW-II was then developed. Using EZFLOW-II, the circulation pressure and ECD can be reduced, thereby minimizing the risk of mud losses. In developing the fluid EZFLOW-II, the density, rheology, plugging performance and lubricity of the drilling fluid were further optimized based on the current four-pressure profile of the depleted gas zone to satisfy the needs of hole cleaning in extended reach well and reservoir protection. In drilling operation, several measures, such as ROP control, periodic short trip and using of good SCE, were taken to prevent mud losses and pipe sticking, and to protect the gas reservoir. The use of these technologies helped solve mud losses in extended horizontal drilling and the resultant formation damage in the shallow depleted sandstone gas reservoir in Gas Field D in South China Sea. The work is of guiding significance to the prevention of mud losses into low pressure shallow depleted loose gas reservoirs of similar characteristics and to the protection of the gas reservoirs.
Abstract: Working fluids pumped into a low permeability gas reservoir cause damage to the production capacity of the gas well, and permeability recovery is generally used to evaluate the degree of reservoir damage. Based on field practice, a percent retention of productivity index is presented as an index for evaluating the gas reservoir damage by fluids injected into gas wells. An evaluation method, which is called productivity index method, was established for evaluating the damage caused by fluids injected into low permeability gas reservoirs. It was found that the productivity index method can determine the effects of differential pressure and time on the degree of reservoir damage by acquiring dynamic experimental data using pressure depletion displacement. The productivity index method can simulate the reservoir condition and tell the change of filtration capacity of rock specimen before and after being damaged by the working fluids. This method, in connection with pressure depletion method and industrial standard methods, provides a new clue for improving the evaluation of reservoir damage by working fluids and is helpful in optimizing working fluids for reservoir protection.
Abstract: The sedimentation stability of a high temperature high density drilling fluid plays a key role in stabilizing the property of the drilling fluid and in the safety of well control. Using small amplitude in-situ oscillation method, in which low shearing stress and low frequency oscillation were applied to a high density drilling fluid, the viscoelastic response of the drilling fluid constituents in gel solution was observed under the condition that the associative structure of the drilling fluid was not broken down, and analyses were performed on the interaction among the additives in the drilling fluid at rest and the effect of the stress environment on the barite in the drilling fluid. Experimental results showed that drilling fluid additives, such as copolymers, sulfonates and bentonite, can develop a weak gel structure with flexible network among them through hydrophobic association and bridging. The gel solution of the highdensity drilling fluid generally exhibits viscous characteristics. indicating that interaction among clay, sulfonate and copolymer in a high-density drilling fluid at elevated temperatures effectively hinders the sedimentation of barite, thereby improving the sedimentation stability of barite.
Abstract: Fracturing operation in Longdong, Changqing Oilfield produced very large amount of fracturing flowback fluids which were seldom reused because of technical obstacles. To minimize the amount of the fracturing flowback fluids discarded as waste fluid and to turn them into useful resources, four typical fracturing flowback fluid samples taken from oil wells in Longdong were studied for their basic properties, ionic composition, biochemical specification and foaming performance to determine the major factors affecting the formulation of a drilling fluid with the fracturing flowback fluids. Based on the affecting factors determined, desanders, biocides, foaming inhibitors and ion removers for the preprocessing of the fracturing flowback fluids were selected and evaluated. A preprocessing technical procedure for preprocessing the fracturing flowback fluids was developed, taking into account the field practice prevailing in Changqing Oilfield. As a pilot test, fracturing flowback fluids produced from two wells on the horizontal well platform Hua-H20-* in Longdong area (a block with tight oil reservoir) where water shortage has long been a problem were preprocessed and then used to formulate a drilling fluid. The results of the pilot test showed that the properties of the drilling fluid formulated were very close to the properties of the drilling fluids formulated with makeup water; the density of the drilling fluids formulated was 1.25-1.30 g/cm3, and the rate of loss of apparent viscosity was less than 15.0%. Borehole wall was stable in the sections drilled with the drilling fluid formulated with the fracturing flowback fluids, and the drilling fluid had filtration rate and foaming performance that were well under control, highly satisfying the needs of the drilling operation. This study has laid the foundation for formulating drilling fluids with fracturing flowback fluids and maximizing the overall utilization of water resource.
Abstract: drilled in Lijin block (Shengli Oilfield) penetrate two formation members, the shale Sasan member and the sandy conglomerate Sasi member interbedded with calcareous mud stone. These two members are easy to collapse and hard to drill. Geological data show that wells drilled nearby have experienced many times of mud losses, borehole wall collapse and pipe sticking. The well Lixie572 has well angle of at least 70° and horizontal displacement of 1911 m, requiring that drilling fluid should have good drill cutting carrying capacity, or the drill tools will got stuck by settled cuttings. A drilling fluid formulated KCl, NaCl and amino polyol was used to drill this well. KCl was used to inhibit shale formations by clay lattice fixation and NaCl to inhibit shale formations by flocculation. The amino polyol has strong shale inhibitive capacity. The drilling fluid had low solids and bentonite content throughout the drilling operation, and clay dispersion was kept at low level. The ROP was greatly increased, and total drilling time was saved by 20 d. Using high temperature filtration control agents and other sealing agents such as emulsified paraffin, polyether polyol and ultra-fine calcium carbonate, the borehole wall was maintained stable. Near-balanced drilling was adopted in the drilling process; the density of the mud was maintained at the lower limit of the designed values. In doing so, mud losses were avoided and the reservoir was protected from being damaged. Wireline logging, rotary side coring and casing running were all performed successfully at the first try. Average percent hole washout was only 5.20%, and there were no downhole troubles and no downhole accidents ever happened.
Abstract: To successfully cement wells penetrating low-pressure easy-to-leak formations in Ebei (north Hubei) area, Zhongyuan Well Cementing Company has developed a high strength ultra-low-density cement slurry suitable for use at temperatures between 60℃ and 120℃. The cement slurry was formulated with self-developed light-weight cement, high strength hollow glass beads (as light-weight material) and ultra-fine silica powder. With this three-level grading model, an ultra-low-density design framework was established. The performance of the ultra-low-density cement slurry was optimized with AMPS filter loss reducer, low-density strength enhancer, early-strength agent and retarder. The cement slurry had density of 1.15 g/cm3, filtration rate of 40-42 mL, free water of less than 1.0%, density difference of 0, and adjustable thickening time below 300 min. The thickening curve was right-angled. At atmospheric pressure and 40℃, the set cement had 72 h compressive strength of 12.1 MPa, which satisfies the needs of field operation. Staged mixing procedure was adopted in mixing the cement slurry. 98% of the bulk slurry samples had properties very close to the properties of the slurry samples made in laboratory, indicating that the cement slurry had stable properties. In cementing the well D12-P42, well D17-2 and well DK13-FP1 in Daniu block (north Hubei) with the ultra-low-density cement slurry, no loss of cement slurry happened, and pass rate of the cementing job quality was 100%, realizing the borehole integrity expected. The successful application of the ultralowdensity cement slurry has provided a technical support to cementing wells penetrating low-pressure easy-to-leak formations.
Abstract: Set cement from a low-density cement slurry has low strength and slow hardness development at the top of the cement slurry. Regular accelerating early strength materials, though can be used to enhance the strength of the set cement, will obviously shorten the thickening time of the cement slurry, hence reducing the strength of the set cement. To solve this problem, a "weak accelerating high early strength" compound strength enhancer was developed with a surface modified nano material and some high efficiency activators. The strength enhancer was evaluated in different low-density cement slurries by controlling the concentration of the strength enhancer, temperature, water/cement ratio and density. It was found in the evaluation experiments that the strength enhancer can greatly increase the compressive strength of the set cement, with only minor effect on the thickening time of the cement slurry. In these experiments, the percent strength increase of the top of the low-density cement slurry can be 40% or higher. The nano compound strength enhancer can be used in different low-density cement slurries to improve the compressive strength of the set cement. Application of this strength enhancer in Changqing Oilfield has proved its good performance.
Abstract: Many oil and gas wells in China are cemented with ultra-low-density cement slurries because these wells penetrated lowpressure and easy-to-leak formations. Design criteria, test methods and slurry mixing procedure for cement slurries of normal density can not be used in operation with ultra-low-density cement slurries because of the low density of the light-weight materials, difference in breaking resistance of different materials and low density ratio of the solid phase and liquid phase in the ultra-low-density cement slurry. All these factors negatively affect the property stability of the cement slurry. Thus, improvement should be made to the program of field testing the ultra-low-density cement slurry and the program of mixing cement slurry based on the physical properties of the light-weight materials and the designed density of the cement slurry. To investigate the problems encountered in using ultra-lowdensity cement slurry, laboratory studies were conducted from several aspects, such as light-weight material, ratio of liquid to solid, downhole condition, compressive strength experiment, quality of cement slurry, cement mixing and field mixing of cement slurry etc. Suggestions have been provided regarding the selection of microspheres, optimum window of liquid-solid ratio, correction of downhole environment, selection of strength experiment method, control of cement slurry properties, quality assurance of the dry cement and LVF dynamic slurry mixing system etc. These suggestions are of instructive significance for the increase of the success rate of cementing wells with ultra-low-density cement slurries.
Abstract: inhibitor for use in acidification fluids has been developed to deal with corrosion to downhole tubular goods by acids at 200℃. The synthesis of this corrosion inhibitor involved the use of Mannich base quaternary ammonium salt as the main additive, an alkynol derivative and an inorganic salt as the synergists, and an alcohol solvent and a dispersant as the auxiliary additives. The introduction of the alkynol derivative (to replace propynol) into the corrosion inhibitor can minimize the toxicity of the corrosion inhibitor and render it good solubility and dispersity. Laboratory evaluation of the corrosion inhibitor using high temperature high pressure dynamic corrosion evaluation method showed that a fluid containing 15%HCl has corrosion rate less than 60 g/(m2·h) at 200℃ if the fluid was treated with 5.5% corrosion inhibitor. If the concentration of the HCl was increased to 20%, the corrosion rate was less than 70 g/(m2·h). The 15% HCl acid fluid has good compatibility with other additives. Electrochemical corrosion test showed that an acid treated with the corrosion inhibitor has electric current density of 6.07×10-6 A/cm2. The corrosion inhibition efficiency of the corrosion inhibitor decreases with increase in the amount of the main additive and the synergists used, and increases with the increase of the acid concentration and the amount of the thickening agent used. Electrochemical evaluation of the corrosion inhibitor showed that addition of the corrosion inhibitor reduced the electric current density of the acid fluid; when added into the acid to a certain concentration, the corrosion inhibition efficiency will not be further improved.
Abstract: Acid fracturing is a key measure to the efficient development of oil and gas from carbonate reservoirs. The properties and compatibility of the working fluid used for acid fracturing with the reservoir formations are prerequisite to the succuss of the stimulation job. Acid fluids presently in use cannot well satisfy the requirements of acid fracturing deeply buried high temperature reservoirs, and a new high temperature gelled acid for acid fracturing carbonate formations was thus developed to fill this gap. Performance evaluation and field application of the gelled acid showed that the gelling agent used in the new gelled acid (0.8%) can reduce flow friction by 68.5%. Test on N80 specimen with the corrosion inhibitor used and the gelled acid at 180℃ showed that the rate of corrosion inhibition can be 96.70% and 98.03%, respectively. After being sheared at 180℃ and 170 s-1 for 70 min, the gelled acid still retained a viscosity of 23.0 mPa·s. Aged for 4 h at 180℃, the gelled acid was still homogeneous, showing no precipitation, residue and stratification etc. The rate of reaction of the new gelled acid with rock samples was half the rate of reaction of the conventional gelled acids presently in use with the same rock samples. These properties of the new gelled acid are beneficial to the mitigation of operational difficulties and risks. Using this new gelled acid, deep penetration of the acid fluid into the reservoirs can be achieved, thereby forming longer acid-itched fractures and realizing in-depth stimulation and production enhancement. Early success has been obtained with the new gelled acid in fracturing high temperature carbonate reservoirs; it satisfied the needs of fracturing carbonate reservoirs with temperatures as high as 180℃. The gelled acid was planned to be further applied in more fracturing jobs.
Competent Authorities：China National Petroleum Corporation Ltd
Sponsored by：CNPC Bohai Drilling Engineering Co. LtdPetroChina Huabei Oilfield Company
Address： Editorial Department of Drilling Fluid and Completion Fluid, Bohai Drilling Engineering Institute, Yanshan South Road, Renqiu City, Hebei Province