| The output and reserve-production ratio of conventional reservoirs show a increasingly decreasing tendency, and unconventional oil and gas resources is considered to be the most promising energy supplement with the global demand for oil and gas resources on the increase, so the exploration and development of low permeability reservoir become more and more attention. In the past decade of years, the proportion of crude oil reserves of low permeability reservoirs in the proved oilfield has been increasing year by year. In addition, due to medium to high permeable reservoirs continuously mining,and remaining reserves decreasing,also resulted in the proportion of crude oil reserves in low permeability reservoirs in the total oil resources on rapid increase. Low permeability and super-low permeability reservoirs is one of main potential yield increase oil field in the future, Research indepth on the related work,it is far-reaching significance of the long-term and sustainable growth of oil exploitation specially in the low permeability. However, fracturing is one key link and important content of reserves difficult to exploitat, and how to improve the low permeability recovery,becomes a major problem to be solved.This article is mainly to study the characteristics of ultra-low permeability reservoir:(1) poor reservoir property, low permeability,low original porosity, complex pore structure and poorer sorting etc increase percolation resistance of crude oil in the reservoir.which lead to that an effective production pressure difference wasn’t fromed, and oil and gas well stimulation effect was caused by failure;(2) Sand-mud layer interact,high cementation content mainly is the clay with high content and serious heterogeneity, binding water saturation is higher;(3) Capillary force have obvious influence on seepage with non-darcy flow characteristics, start-up pressure increase with the reduction of permeability;(4) The power communication in reservoir is poor, and the oil discharge area in single well control is small, and in the fracturing operation, fracturing fluid easily cause damage to ultra-low permeability reservoir after coming into the ground. Once being hurted, the effect on reservoir will be permanent. So, the performance of fracturing fluid is one key of the many factors influence the success or failure on fracturing and construction.During fracturing operation process, the damage to oil and gas formation includes two aspects:the damage caused by the uncompatibility of fracturing fluid and strata rock in the formation; the damage to the support fracture diverting capacity caused by bad additives, propping agent in fracturing fluids. Thickener is the most basic additive in water-based fracturing fluid, and its function is to improve viscosity, lower filter liquid loss, suspension and carry propping agent, and is a main component to influence on the performance of fracturing fluid.This paper mainly study several thickener types of water-based fracturing fluid:the performance of plant gum and its derivatives, biopolymer, synthetic polymer and clean fracturing fluid was optimized and other comprehensive properties was evaluated.Under the precondition of ensuring the overall performance of fracturing fluid, reservoir protection effect of different water-based fracturing fluid types was comparatively evaluated,and the main factors of the damage of different fracturing fluid types to reservoir was studied,and the basis for maximum improving the capacity of fracture diverting, reducing fracture fluid comprehensive damage rate was provided.The research of laboratory experiment was mainly as follows:1) The performance of gel fracturing fluid adding organic borate cross-linker into guanidine gum and hydroxypropyl guanidine gum was evaluation and compared,and the added amount of each types of thickener was determinated according to sand suspension properties and the residue content in gel breaking in each type of fracturing fluid. This paper also studies species and added amount of other additives used in four types of water-based of fracturing fluid demulsification cleanup additive, such as fungicide, clay inhibitors, demulsification cleanup additiveand so on, preventing and reducing the reservoir pollution caused by the water-based fracturing fluid.2) This paper studies that all kinds of additives influence on the rheological property of VES clean fracturing fluid, the addition of oil-soluble filtrate reducer greatly reduces HTHP filter loss, and adding20%kerosene simulate breaking, which prove that it do have good gel breaking effect, and avoiding the general polymer not breaking completely and polluting reservoir,and prove that VES clean fracturing fluid has very good reservoir protection effect.3) This contrast experimental study on the rheology of five different types of fracturing fluid was carried out, and the main difference of various properties was determined by thickeners’types,of which biological polymer HXC, XC showed excellent resistance to high temperature and shearing capacity, and it also has high viscoelastic. Other polymer fracturing fluid all have superiorfluid loss properties,and especially HXC fracturing fluid, except that VES clean fracturing is more fluid loss,and need to add oil-soluble filtrate reducer. The performance of friction reduction of fracturing fluid is also evaluated,and HXC has a good friction reduction.The compatibility experimental results shows that glue breaking of fracturing fluid has good compatibility with formation water, avoiding bad damage to the formation.According to the particularity of the ultra-low permeability reservoir, the comprehensive damage caused by five kinds of fracturing fluid to fracture reservoirs was studied.The damage caused by fracturing fluid to reservoir matrix and to flow conductivity of propping agent was respectively studied.(1) The damage caused by the filtrate, residue, concentrated on glue solution of different types of fracturing fluid to reservoir matrix:(1) the damage experiments of the filtrate of fracturing fluid adding different additives show that different clay inhibitors affect on permeability recovery, needing to add clay inhibitors KCl generally used in pilot area, in addition to add1%of organic clay stabilizer polyamine UHIB to enhance stability of the clay, which makes permeability recovery value increase10%.In addition, effects of demulsification cleanup additive also were compared and evaluated,which show that demulsification cleanup additive not only reduces the oil and water emulsion ability, but also improves gel breaking flowback capacity;(2) The damage to the core caused by gel breaking of different types of fracturing fluid was studied and analyzed in depth. Besides, the content of residue in gel breaking of different types of fracturing fluid is easy to damage to the core section forming filter cake, the particle size of residues and the pore diameter of reservoir rock exist a certain relationship, when the particle size of residue is larger easy to form filter cake, smaller inclined to blocking throat, causing a deeper pollution;(3)In order to eliminate damage caused to the formation by the fracturing fluid breaking not thoroughly, except that adding too much gel breaking agent, but the damage caused by glue solution concentration plugging in the core surface was evaluated and analyzed, polymer fracturing fluid concentration was easy to cause larger damage, but VES clean fracturing fluid in the core surface was not easily form filter cake, and meeting crude oil was easily natural breaking, leading to the damage for the core is more small.(2)The damage to the propping agent caused by fracturing fluid was mainly as follows:the residue and fracturing fluid breaking not thoroughly leading to glue solution concentration caused that propping agent was injuried.①the damage mechanism by gel breaking to flow conductivity of propping agent was analyzed, mainly caused by the content and particle size distribution of residue in the gel breaking. The more content and the larger particle size of residue is in the gel breaking, the easier blocking between the gap of crack beween propping agent; The less content and the smaller particle size of residue (less2/3of pore particles’ diameter),the lesser damage extent to the core;②The influence of fracturing fluid glue solution on diverting capacity of haydite are connected with the viscosity of fracturing fluid and adsorption capacity which are stonger, and easily blocked porosity of supporting agent, yield efficiency greatly which was reduced.In a word,... |
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