To aim at the contradiction between the complex force of the motor and stability of expectation,we designed the fuzzy PID controller used by the fuzzy control and PID control.It had some merits:flexible control,Adaptability,haute precision.It can control the complex systems and the high accuracy servo system.The results of simulation show that the fuzzy PID control can obtain the better effects.

Fluid-blocking and water sensibility were easily induced by workover fluid loss during the processes of workover operations in the low-permeability gas condensate reservoir formation with low formation pressure of natural micro fractures.In order to minimize the damage,the main additives suited to target formation were selected by rotating hanging plate and dynamic weight loss method,and so on.The best formula of the XJPS workover fluid was： 0.3% XC,2% JMPS-1,1% PRDS and supplementary additives.The results of laboratory tests showed that the rheological property of XJPS was satisfied to the engineering requirements and had the performance of low filtrate loss（8 mL）,high core permeability retention rate（85%）,and high cleaning-up rate.Low damage could be achieved under the circumstances of solid free and no forming dense mud cake.Compared with the previous workover fluid system,the decrease of oil and gas production and increment of water content were relatively small after the field application of XJPS.The fluid blockage and oil-gas production decrease caused by workover fluid loss were obviously relieved,and the effect of formation damage control and efficiency in workover operations were improved.

Condensate gas reservoirs with low-permeability are characterized by a tiny size of pore throats,poor permeability,high content of clay minerals and severe heterogeneity.Therefore,this type of reservoirs is susceptible to the damage of water sensitivity,water blocking,etc.caused by invasion fluids,and frequently to the damage of specific retrograde condensation.We analyzed mechanisms of formation damage systematically based on reservoir characteristics of the Baka condensate gas reservoir with low-permeability,the results showed that water blocking was the major damage to this reservoir and the damage ratio induced by water blocking ranged from 65.23% to 91.01%.On the basis of the sulphonic polymer drilling fluid previously used in this area,surfactants capable of effectively reducing the interfacial tension of oil/water contact were optimized,a new protection method characteristic of synergetic effectiveness by combining film forming with ideal packing was adopted,and consequently a drilling fluid with low damage was developed.The protection effect of this fluid on reservoirs was verified in compatibility tests and dynamic damage tests.The results showed that this fluid was of excellent rheological performance and compatibility,being capable of increasing the returned value of core permeability from 63.2% to 84.7% and of distinct protection of reservoirs.

为了实现修井液在无固相、不形成致密泥饼前提下的低滤失性能,有效预防或减轻修井液对低孔、低渗、喉道细小及裂缝较发育储层造成的液锁损害,达到修井后流体产出能力的最大化,研制出了XJPS无固相、多功能弱凝胶修井液体系.该体系具有流变参数适宜、低滤失(＜8mL)、渗透率恢复值高(＞85％)、返排效果好等特点,Q3气井应用后产气、产油量降低幅度和含水量上升幅度均较小,明显减轻了因修井液漏失到储层造成的液锁损害和产能损失,提高了储层保护效果和修井效率.

Fluid locking damage was one of the most essential damage factors in low permeability reservoirs,the level of damage was mainly related with core permeability,porosity,initial water saturation and oil-water interfacial tension of foreign fluid invasion.Based on the laboratory experimental results of water-locking damage of low permeability cores from Tarim Oilfield.The method of grey association analysis is established for predicting the fluid locking damage,and a rapid intelligent software is developed with predicting consistent ratio being over 95%.It has obvious advantage in intelligence,speediness and accuracy.It is an ideal method of predicting fluid locking damage.

The deep reservoir in the Block 3 of the center part of Junger Basin is commonly characterized by high initial water saturation,high capillary pressure,complicated structure,serous heterogeneity,high oil-gas flow resistance and natural fractures.Laboratory core tests confirmed that one of factors for severe damage of reservoir was water blocking effect caused by aqueous invasion fluids during drilling and production operations.The damage extent of permeability ranges from 49.3% to 89.2%.By adopting the liquid trap prevention method,ideal packing theory and d90 rule,a high-performance drilling fluid was developed,which was composed of polyalcohol SYP-1,efficient water blocking preventing surfactant HAR and temporary bridging agents.The protection characteristics of fluid to reservoir were verified by compatibility test,surface and interfacial tension test,dynamic damage test and phase trap test.The test results and field data showed that the fluid system could satisfy the safe and fast requirement of ultra-deep drilling operation.The synergy of HAR and SYP-1 could efficiently minimize water blocking,reduce the surface-interfacial tension and improve the displacement rate of drilling fluid.

The stress sensitivity evaluation test was made by using artificial fracture rock sample based on characteristic analysis on Ordovician buried hill low-permeability fractured carbonate reservoir in Tarim basin.Test results show that testing samples present strong stress sensitivity with 92.48 percent loss of mean permeability and 76.37 percent drop of fracture width.MMH drilling mud formula was improved according to analysis of stress sensitivity test to reservoir and the good application effect has been obtained.

Various forms of fluid-blocking are easily induced during the processes of drilling/completion and production in the Qiudong gas condensate reservoir. The main factors that cause fluid blocking include core permeability, porosity, original water saturation, and oil-water interfacial tension, as well as lithologic characteristics, types and content of cementing substances, pore structure, and properties of invasion fluids. Based on the damage mechanisms and characteristics of fluid-blocking during the drilling/completion, production, fracturing, acidizing and workover operations, a set of matching techniques were developed to minimize fluid-block damage, which mainly include fluid-block prevention, low damage, film-forming drilling fluid, as well as the techniques for keeping proper formation pressure, enhancing formation pressure and improving drainage rate. The modified drilling fluid system can improve the return permeability of test cores from averaged 54% to 81%. The newly developed cleaning-up agent consisted of combinational surfactants can improve the drainage rate of backflow acid (up to 88.9%), the newly developed solids-free work-over fluid system can improve the return permeability by 16.1%. After the application of the new techniques, the pay zone is preserved better, with the comprehensive development-investment benefits enhanced.

This paper discusses the systematic design and development of low-damage drilling fluid to protect the low-permeability gas reservoir of the Sulige block in the Ordos Basin, Inner Mongolia Autonomous Region, China. Based on investigation of the geological characteristics and the potential formation damage of the Permian formation of the reservoir, waterblocking due to invasion of drilling or completion fluids was identified one of the most severe causes of damage to gas well deliverability. By adopting the phase trap prevention method, ideal packing theory, and film-forming technology, a low-damage drilling fluid, sodium formate brine containing efficient waterblocking preventing surfactants, optimized temporary bridging agents (TBAs), and film-forming agents has been developed. The performance of the new drilling fluid was evaluated by using a variety of techniques. The results show that the fluid has good rheological properties, good strong shale-swelling inhibition, good temporary plugging effect, ultra-low filtration, and good lubricity. It can efficiently minimize waterblocking and can be used to drill horizontal wells with minimal intervention of the reservoir in the Sulige Gas Field.

<Title>APPLICATION OF A NEW IDEAL PACKING TEMPORARY PLUGGING METHOD IN TUHA LOW PERMEABILITY RESERVOIRnullThe conventional temporary plugging method is optimizing the particle size of temporary bridging agent based on the average pore throat diameter of target formation,but it is difficult to seal the pore throats with large size.According to the distribution features of pores size in Tuha Qiudong low permeability reservoir,the grain-size distribution of temporary bridging agent was determined,a novel ideal packing temporary bridging drilling fluid was developed.The performance evaluation results in lab.indicated that the temporary bridging drilling fluid has good rheological property and low filter loss.Compared with the conventional temporary bridging methods,the ideal packing temporary bridging drilling fluid could plug the pore throats with different size in the same formation effectively,and the invasion depth of the drilling fluid to reservoir was shallower,could obtain higher coefficient of permeability and lower breakthrough pressure gradient and reduce reservoir damage.null

Researchers from the China University of Petroleum have devised new completion and drilling fluids to reduce damage in Jurassic sandstones and stress-sensitivity damages caused by drilling. The new low-permeability drilling and completion fluids is made primarily of highly effective surfactants, temporary plugging agents, different classes of bridging agents and polyether glycol. This fluid solution takes into account the properties of geological structure, lithology, permeability, porosity, and mineral components of a conventional low permeability, high-pressure, dense reservoir formation in the Tarim oil field in China. The new low-permeability KCl sulfonated polymer drilling fluids have shown to be applicable to high-pressure exploratory wells. The new drilling fluids showed increased permeability ratios of cores and exhibit good rheological properties, high temporary plugging effect and low filtration.

The rheological properties of two kinds of oil-based drilling fluids with typically composition were studied at pressures up to 138 MPa and temperatures up to 204 °C using the RheoChan 7400 Rheometer. The experimental results show that the apparent viscosity, plastic viscosity and yield point decrease with the increase of temperature, and increase with the increase of pressure. The effect of pressure on the apparent viscosity, plastic viscosity and yield point is considerable at ambient temperature. However, this effect gradually reduces with the increase of temperature. The major factor influencing the rheological properties of oil-based drilling fluids is temperature instead of pressure in the deep sections of oil wells. On the basis of numerous experiments, the model for predict the apparent viscosity, plastic viscosity and yield point of oil-based drilling fluids at high temperature and pressure was established using the method of regressive analysis. It is confirmed that the calculated data are in good agreement with the measured data, and the correlation coefficients are more than 0.98. The model is convenient for use and suitable for the application in drilling operations.

The creating mud cake with ultra-low filtration with drilling fluid requires the match between the pore throat size and the temporary shielding/plugging agents(TPA) particle size.A new scientific and reasonable TPA optimizing method is developed."1/3 bridging rule" and "temporarily plugging/shielding technology" mainly based on mean formation pore size works effectively for homogenous reservoir,while is difficult to seal large size pore throats in heterogeneous reservoir.Based on "the Maximum Packing Efficiency Principle","Ideal Packing Theory" and "the D90 rule",a new graphical method to optimize plugging particle size distribution was proposed,and the associated software was developed which can quickly and accurately select and recombine TPA with different particle size in a given proportion based on formation correlative parameters,such as pore throat size data.Field tests shown that drilling fluids and well completion fluids composed of optimized TPA can create fine mud cake effectively,seal pore throats with different size in heterogeneous formation.Consequently it can prevent the solid particles and filtrate of invasive fluids from entering formation.Compared with conventional temporary bridging method,this new method has smaller invasion radius(<2cm),and higher return permeability(>83%),a lower breakthrough pressure gradient(1.05MPa/m),and better reservoir protection.

Low-permeability reservoirs are characterized by tight reservoirs and possess characteristics of high content of argillaceous cement,poor permeability,complex structure,serious heterogeneity,strong water sensitivity and tiny pore throat,prominent capillary phenomenon,big dynamic resistance for hydrocarbon flow and accompanied with natural fractures.A number of researches show that liquid-locking damage is one of the key damages in low-permeability reservoirs with a hit range from 70%to 90%.In the paper,on the basis of studying the damage mechanism and key influencing factors,a gray correlation analysis method for prediction of water block damage is established and the fast intelligent prediction software is developed(it is seldom reported).Examination of the actual data both in or out of the sample library indicates that the method has good adaptability and fault tolerance.It needs less parameters and has high coincidence rate(average above 90%).It is helpful in preventing reservoir damage and protecting original layer and improving comprehensive development benefit.

Low permeability reservoirs have characteristics such as high argillaceous cement content,high water saturation,high capillary pressure,highly water sensitive,small pore throats,poor permeability,complex structures,highly inhomogeneous,and high resistance to flow,and often are accompanied by naturally fractures.Researches had proved that water block was the most important cause for permeability impairment,with impairment percent of 70%-90%.This paper analyzed from different aspects the sources and the influential factors of water block.An amphoteric polymer sulfonate drilling fluid was developed with multiple functions,minimal damage to reservoir permeability,and ultra-low filter loss.This drilling fluid is beneficial to the development of low-permeability reservoirs.

To determine the damaging mechanism of the Jurassic reservoir in Tarim Basin,which is a low porosity,low permeability and high pressure reservoir,laboratory studies were done based on the reservoir structure,lithology,porosity-permeability nature and mineralogy.It was shown that the major damaging factors were water block(accounting for 70%~90%) and stress-sensitivity(accounting for 25%~60%).To protect the reservoir,several drilling fluid additives with specific functions were selected to formulate a drilling-completion fluid with strong encapsulation and inhibitive capacity.The application of the fluid in a high pressure deep exploration well was successful: the API fluid loss was 1.0 mL,the HTHP fluid loss was 1.8 mL,and the permeability recovery was greater than 85%.By preventing the formation of water block,the permeability impairment of the reservoir was minimized.Combined with appropriate engineering measures,this fluid can be used in drilling the consolidated sand stone formations with coal-bed layers.

Stress sensitivity and water blocking in fractured carbonate reservoir formations with low permeability were determined as the main potential damage mechanisms during drilling and completion operations in the ancient buried hill Ordovician reservoirs in the Tarim Basin. Geological structure, lithology, porosity, permeability and mineral components all affect the potential for formation damage. The experimental results showed that the permeability loss was 83.8%–98.6% caused by stress sensitivity, and was 27.9%–48.1% caused by water blocking. Based on the experimental results, several main conclusions concerning stress sensitivity can be drawn as follows: the lower the core permeability and the smaller the core fracture width, the higher the stress sensitivity. Also, stress sensitivity results in lag effect for both permeability recovery and fracture closure. Aimed at the mechanisms of formation damage, a modified low-damage mixed metal hydroxide (MMH) drilling fluid system was developed, which was mainly composed of low-fluorescence shale control agent, filtration control agent, low-fluorescence lubricant and surfactant. The results of experimental evaluation and field test showed that the newly-developed drilling fluid and engineering techniques provided could dramatically increase the return permeability (over 85%) of core samples. This drilling fluid had such advantages as good rheological and lubricating properties, high temperature stability, and low filtration rate (API filtration less than 5 ml after aging at 120 °C for 4 hours). Therefore, fractured carbonate formations with low permeability could be protected effectively when drilling with the newly-developed drilling fluid. Meanwhile, field test showed that both penetration rate and bore stability were improved and the soaking time of the drilling fluid with formation was sharply shortened, indicating that the modified MMH drilling fluid could meet the requirements of drilling engineering and geology.

The reservoirs of the Qiudong oilfield are characteristic of low permeability and slim pore throats,coupled with water sensitive clays and fractures.To minimize the impairment of the reservoir permeability caused by drilling fluid filtrate and pressure changes,some special mud additives,such as ABSN(for the prevention of water block),filming agent OCL-BJ and ideal packing temporary plugging agent were developed in laboratory.When treated with these additives in a specified concentration,a zwitterionic polymer sulfonate drilling fluid acquired the function of reservoir protection.Laboratory evaluation showed that this fluid was stable at 140 ℃ with filtrate loss being less than 5 mL,and HTHP filtrate loss less than 10 mL.Friction coefficient was less than or equal to 0.141.The filtrate of the fluid was compatible with the fluids in the reservoir rocks,and core test showed that the dynamic permeability recovery was greater than 81.0%.Also observed in the experiments were that the spurt loss was almost zero and a high quality film generated by the fluid.

Low-permeability dense reservoirs, including micro-fractured reservoirs, are commonly characterized by high content of clay substances, high original water saturation, high sensitivity to invasive fluids, high capillary pressure, complicated structure and anisotropic, high flow-resistance and micro pore throats etc,. Generally they also have lots of natural micro fractures, probably leading to stress sensibility. Main damaging factors in such reservoirs are water-sensibility and water-blocking caused by invasive fluids during drilling and production operations. Once damaged, formation permeability can rarely recovered. Numerous studies have shown that damaging extent of water-blocking ranges from 70% to 90%. Main damaging mechanisms and influencing factors of water-blocking were systematically analyzed. Also some feasible precaution or treating approaches of water-blocking were put forward. In a laboratory setting, a new multi-functional drilling fluid composed mainly of amphion polymer, sulfonation polymer, high effectively preventive water-blocking surfactants, ideal packing temporary bridging agents (TBA) and film-forming agents, etc., were developed. New low-damage drilling fluids has many advantages, such as good rheological properties, excellent effectiveness of water-blocking prevention, good temporary plugging effect, low filtration and ultra-low permeability (API filtration⩽5 mL, HTHP filtration⩽10 mL, mud cake frictional coefficient⩽0.14, permeability recovery>81%), can efficiently prevent or minimize damage, preserve natural formation and enhance comprehensive development-investment effect in TUHA Jurassic dense sandstone reservoir formation with low-permeability, the only one developing integrated condense gas field. Some references can be provided to similar reservoir formations.

The surface and interfacial tension and the wettability(relative contact angle on quartz) of more than 10 standard surfactants of 4 different types are determined experimentally and,based on the data obtained with the synergistic effects and the cost taken into consideration,a high efficiency cleaning-up agent,FOC,for reservoir blockage relieving/acidizing fluids is developed and composed of 20% fluorocardonic surfactant FSK,capable of reducing water surface tension to 25.3 mN/m at its CMC,40% cationic surfactant CTAB,capable of reducing interfacial tension with kerosene to 2.0 mN/m at its CMC,10% nonionic surfactant OP-10,capable of raising contact angle on granular quartz of 20% hydrochloric acid at concentration 1.6 g/L,and supplementary surfactant(s).FOC is compatible with various additives in various acids,possesses high surface and interfacial activity as shown by surface tension of 26.3 mN/m,and interfacial tension with kerosene of 2.1 mN/m for 20% hydrochloric acid with 200 mg/L FOC added,gives a high cleaning-up rate of 85.9%,and is good in thermal stability.Oil well 741313 at low permeability block 74 was seriously damaged by water invasion in pump maintenance and well washing jobs to an extent of 100% watercut and was treated by injecting a 0.5% FOC pad fluid,an 8% hydrochloric acid and an 8%/2% mud acid and 5% acetic acid slugs with 0.5% FOC added in turn;this treatment led to a reduction of watercut to 51.6% and a recovery of oil output 4.8 t/d.