Study Of Damage Mechanism And Drilling Fluid & Completion Fluid Technology For No.3 Coalbed Methane Reservoir In Qinduan Block

Coalbed methane (CBM,commonly known as gas) stored in the coal seam by adsor-ption status was a kind of unconventional natural gas, it was a new type of clean energy and high-quality chemical raw material,it was one of the important alternative energy in the 21st century. In order to realize industrialization development of CBM industry in China, it is urgent to carry out the research of damage mechanism and drilling fluid & completi-on fluid technology for CBM reservoir. The original equilibrium state of CBM reservoir would be destroyed in various degrees in the course of drilling and completion and CB-M reservoir was damaged, which will seriously affect desorption, diffusion, seepage and 1-ater exploitation of CBM. The pores and fractures structure of coal rock, the storage me-chanism of CBM, the characteristics of CBM production determined CBM reservoir dama-ge mechanism which has particularity. CBM reservoir protection drilling fluid & completi-on fluid technology mainly continued to use conventional oil and gas reservoir protection drilling fluid & completion fluid technology in recent years in China, compared with con-ventional oil and gas reservoir, the composition, forming reason,chemical composition, ph-ysical property, pore and fracture structure of coal rock has particularity, conventional oil and gas reservoir protection drilling fluid & completion fluid technology could not effecti-vely protect CBM reservoir. Therefore, it was important practical significance of further study of damage mechanism and drilling fluid & completion fluid technology for CBM re-servoir.This paper investigated development characteristics of No.3 coal seam in Qinduan b-lock, structure characteristics of coal, reservoir pore characteristics, reservoir fracture char-acteristics, porosity of reservoir, permeability of reservoir, temperature and pressure of re- servoir and formation water quality, and No.3 CBM reservoir in Qinduan block was the characteristics of low permeability, fracture development, low pressure. On the basis of the study of characteristics, potential damage factors of No.3 CBM rese-rvoir in Qinduan block were analyzed, the type and content of minerals in No.3 CBM reservoir in Qindu-an block determined that the reservoir was of velocity sensitivity, salt sensitivity, alkali sensitivity and acid sensitivity; multiple pores and fractures of No.3 CBM reservoir in Qinduan block was natural condition of water locking damage; physicaland mechanical property of No.3 CBM reservoir in Qinduan block with low modulus, high poisson’s rat-io, great brittleness, being easy of brittle fracture and compression and the characteristics of pore and fracture determined that the reservoir was of strong stress sensitivity.The stress sensitivity, velocity sensitivity, salt sensitivity, alkali sensitivity and acid sensitivity of No.3 CBM reservoir in Qinduan block were deeply studied by the combin-ation of permeability and desorption rate, and the damage mechanisms were discussed.(1)The research results of stress sensitivity showed that the permeability of reservoir decreased rapidly with increase of effective stress, but the permeability could not restore to original size during the process of effective pressure recovery; the desorption rate of CBM decreased with increase of effective stress, under the condition of the same effecti-ve stress, the desorption rate of CBM during the process of loading was greater than the desorption rate of CBM during the process of unloading; fracture aperture of No.3 CBM reservoir in Qinduan block decreased with increase of effective stress.(2)The research results of velocity sensitivity showed that the permeability of reserv-oir and the desorption rate of CBM decreased with increase of flow velocity, the amou-nt of pulverized coal No.3 CBM reservoir in Qinduan block increased with increase of flow velocity, at the same time, clay mineral particles migrated with increase of flow v-elocity, pulverized coal and clay mineral particles blocked seepage channels of CBM and decreased CBM desorption area. Reverse flow experimental results verified that velocity sensitivity of No.3 CBM reservoir in Qinduan block mainly was caused by migration of pulverized coal and clay mineral particles.(3)The research results of salt sensitivity showed that the permeability of reservoir and the desorption rate of CBM decreased with decrease of salinity of fluid when the s-alinity of extraneous fluid was lower than the salinity of formation water, the permeabil-ity of reservoir and the desorption rate of CBM decreased with increase of salinity of f-luid when the salinity of extraneous fluid was higher than the salinity of formation wat-er. When illite/smectite mixed layer in reservoir contacted with low salinity extraneous fluids, illite/smectite mixed layer had the significant trends of hydration and swelling, w-hich caused clay minerals and pulverized coal instability and falling off. When clay min-erals in reservoir contacted with high salinity extraneous fluids, diffuse double layer of clay minerals was compressed by high salinity fluids, which caused clay minerals and p-ulverized coal instability and falling off.(4)The research results of alkali sensitivity showed that the permeability of reservoir and the desorption rate of CBM decreased during rising of fluid pH value. Strong alkali-ne fluid reacted with the carboxyl group in coal molecule, which changed coal molecular structure and wettability of coal rock surface; electrokinetic potential of coal rock surface increased with the increase of pH value, the electrokinetic potential was larger, the grea-ter migration of pulverized coal and clay minerals particles; the dispersion of clay mine-rals in No.3 CBM reservoir in Qinduan block was induced by strong alkaline fluid, whi-ch caused minerals and pulverized coal instability and falling off; colloid or sedimentati-on generated by the reaction of strong alkaline fluid and clay minerals in CBM reservoir blocked seepage channels of CBM and decreased CBM desorption area. Thecarboxyl gro-up, phenolic hydroxyl group in coal molecule and clay minerals in No.3 CBM reservoir in Qinduan block was the main factor of alkali sensitivity.(5) The research results of acid sensitivity showed that the permeability of No.3 CBM reservoir in Qinduan block and the desorption rate of CBM increased after acidification. the permeability of reservoir and CBM desorption area increased by the acid dissolution of minerals in No.3 CBM reservoir in Qinduan block,CO2 generated by the reaction of acid and carbonate minerals displaced CBM absorbed on the surface of coal matrix, thus increased the desorption rate of CBM.Drilling fluid easily invaded No.3 CBM reservoir during drilling, the original balance state of No.3 CBM reservoir was destoryed by the invasion of drilling fluid and the reservoir was damaged, which would seriously affect desorption, diffusion and seepage of CBM. Based on the analysis of CBM reservoir damage factors caused by drilling fluid, the reservoir damage caused by drilling fluid filtrate invasion, solid phase particle invasion, polymer retention and drilling fluid invasion were studied by the method of combining permeability with desorption rate, and the damage mechanisms were discussed.(1)The damage mechanisms of No.3 CBM reservoir in Qinduan block caused by dr-illing fluid filtrate invasion mainly included:1)drilling fluid filtrate invasion caused swel-ling of coal matrix; 2)drilling fluid filtrate invasion caused salt sensitivity damage; 3)dril-ling fluid filtrate invasion caused water blocking damage; 4)sediment was generated bec-ause drilling fluid filtrate was incompatible or poor compatible with formation water.(2)Drilling fluid filtrate invasion caused No.3 CBM reservoir in Qinduan block wat-er blocking damage, drilling fluid filtrate invasion prolonged CBM desorption time and decreased permeability.The degree of water blocking damage increased with the increase of water cut. Multiple pores and fractures of No.3 CBM reservoir in Qinduan block was natural condition of water locking damage, capillary resistance was pressure condition of water locking damage.(3)Polymer influenced on crystal structure of No.3 coal molecules in Qinduan block, polymer retention in CBM reservoir influenced on the permeability of reservoir and the desorption rate of CBM. Coal matrix swelled because of multi-point adsorption polymer with coal matrix surface, fracture in No.3 CBM reservoir became narrow because of sq- ueezing, capillary resistance increased and the degree of water locking damage increased. After the multi-point adsorption polymer with coal matrix surface, gel layer blocked frac-ture and cleatin No.3 CBM reservoir, and isolated membrane inhibited CBM desorption.(4)The permeability of No.3 CBM reservoir and the desorption rate of CBM decrea-sed because of solid phase particles invasion, the width of the development fractures in reservoir was 5 μm to 10μm, the particle size of invading solid phase particle was sma-ller, the rate of change of the permeability of reservoir and the desorption rate of CBM was greater. Smaller particle size of solid phase particles invaded and bridged in the inter-nal of the fractures, the particle size of invading solid phase particle was smaller, the d-epth of invasion was deeper, the degree of reservoir damage increased, it was difficult f-or the larger particle size of solid phase particles which did not match the width of fra-cture to invade the depths of fracture, while they attached to the surface of coal or the superficial part of fractures.(5)The connectivity of pores and fractures of No.3 CBM reservoir in Qinduan block were better, while the pores and fractures were filled by solid particles, polymer and se-diment after the invasion of drilling fluid, functional group of No.3 coal molecular struc-ture changed after soaking in drilling fluid.Through optimization and development of drilling fluid tackifier, anti-water blocking agent and sealing agent, solid-free degradable sealing characteristics anti-water blocking drilling fluid system for No.3 CBM reservoir in Qinduan block horizontal well was dev-eloped, and its rheological property, temperature resistance, inorganic salt tolerance and inhibitory property was evaluated.(1)Drilling fluid tackifier JMY was optimized through the experiment of polymer in-fluence on crystal structure of coal molecule and rheological property of polymer in Qi-nshui River water, degradable natural polymer of JMY basically did not influence on cr-ystal structure of coal molecule, JMY had provided better viscosity-shearing improving capability.(2)The anti-water blocking agent of drilling fluid was optimized through screening and remixing of surfactant, its wettability was studied by the method of contact angle a-nd self-absorption, the anti-water blocking agent FSSJ composed mainly amino silicone a-lcohol, poly-alcohol, fluorocarbon surfactants. The anti-water blocking agent FSSJ could reduce surface tension, increase contactangle, decrease capillary resistance, realize "anti capillary effect", change wettability of coal and prevent drilling fluid filtrate from invadi-ng reservoir fractures.(3)The sealing agent FDJ of drilling fluid was synthesized from the polymerization of acryl-amide(AM),2-acrylam-ido-2-methylpropanesulfonic acid (AMPS), and the poly-mer was slight crosslinked by N,N dimethyleneallyamide, and the sealing performance of FDJ was evaluated. Deformable and self-adaptive sealing agent FDJ could seal fractures of CBM reservoir, decrease invasion reservoir depth of drilling fluid, it was beneficial to improve pressure bearing capability of No.3 CBM reservoir with strong stress sensitivity in Qinduan block, it could effectively solve the problem of invasion and leakage of dril-ling fluid.(4)Through optimization and development of drilling fluid tackifier, anti-water block-ing agent and sealing agent, solid-free degradable sealing characteristics anti-water blocki-ng drilling fluid system for No.3 CBM reservoir in Qinduan block horizontal well was developed, the formulation was composed of 0.2%JMY,0.8%FDJ,1.0%FSSJ, water for preparation of drilling fluid was from Qinshui River, and KCl as drilling fluid weighting material. The evaluation of drilling fluid property showed that it had better rheological property, temperature resistance, inorganic salt tolerance and inhibitory property.In order to effectively protect No.3 CBM reservoir in Qinduan block, attacking anti-water blocking completion fluid system which could improve No.3 CBM reservoir in Qi-nduan block was developed, its viscosity breaking and protection reservoir ability was e-valuated, and the end of coal core after gel breaking with attacking anti-water blocking completion fluid was microcosmically analyzed through the scanning electron microscope.(1)Attacking anti-water blocking completion fluid system which could improve No.3 CBM reservoir in Qinduan block was developed through optimization of gel breaker, co-rrosion inhibitor, anti-water blocking agent, scale inhibitor, clay stabilizer and their amou-nt, the formulation was composed of 2.0%PLOK,1.0%CO21,1.0%FSSJ,1.0%HEDP and 1.5%KCl, the viscosity of drilling fluid after gel breaking with attacking anti-water bloc-king completion fluid was close to the viscosity of water, which was beneficial to flow back of drilling fluid.(2)The results of viscosity breaking, protection reservoir ability and microscopic ana-lysis showed that the attacking anti-water blocking completion fluid had better viscosity breaking and protection reservoir ability.(3)Gel breaker PLOK with strong oxidation could effectively degrade polymer in dr-illing fluid, and the solution show acid after gel breaking, which could improve the pe-rmeability of reservoir and the desorption rate of CBM; anti-water blocking agent in co-mpletion fluid could effectively solve the problem of No.3 CBM reservoir in Qinduan b-lock water blocking damage; anti-water blocking agent, scale inhibitor and clay stabilizer could effectively prevent secondary damage caused by completion fluid.The main innovation of this paper was as follows:(1)The damage mechanisms of No.3 CBM reservoir in Qinduan block were deeply studied by the combination of permeability and desorption rate.(2)Through optimization and development of drilling fluid tackifier, anti-water block-ing agent and sealing agent, solid-free degradable sealing characteristics anti-water blocki-ng drilling fluid system was developed. Degradable natural polymer of JMY basically di-d not influence on crystal structure of coal molecule, the viscosity-shearing improving c-apability of JMY was better, it could effectively solve the problem of carrying cutting during drilling; the deformable and self-adaptive sealing agent of FDJ could seal the fra-ctures of CBM reservoir, decrease invasion reservoir depth of drilling fluid, it could eff- ectively solve the problem of invasion and leakage of drilling fluid; the anti-water block-ing agent of FSSJ could reduce surface tension, increase contact angle, decrease capillary resistance, it could effectively solve the problem of CBM reservoir water blocking dama-ge; solid-free degradablesealing characteristics anti-water blocking drilling fluid system c-ould prevent solid particles from invading CBM reservoir.(3)Attacking anti-water blocking completion fluid system which could improve No.3 CBM reservoir in Qinduan block was developed through optimization of gel breaker, co-rrosion inhibitor, anti-water blocking agent, scale inhibitor, clay stabilizer and their amou-nt. The viscosity of drilling fluid after gel breaking with attacking anti-water blocking c-ompletion fluid was close to the viscosity of water, which was beneficial to flow back of drilling fluid; the solution show acid after gel breaking, the permeability of reservoir and CBM desorption area increased by the acid dissolution of minerals in CBM reservo-ir,CO2 generated by the reaction of acid and clay minerals carbonate minerals displaced CBM absorbed on the surface of coal matrix, thus increased CBM production; anti-water blocking in completion fluid could effectively solve the problem of CBM reservoir water blocking damage; scale inhibitor and clay stabilizer in completion fluid could effectively prevent the secondary damage caused by completion fluid.