Research on Adaptation Techniques of Early Polymer Injection Reservoir in Bohai Oilfield during the Middle and Later Stage
Supervisor:lu xiang guo
The reservoirs of the Bohai offshore oilfield in China have the characteristics of low bonding strength,high oil viscosity,high permeability and severe heterogeneity.Excess water production easily caused rock structure destruction and formed the macroscopic throats or high permeability channels in these reservoirs,especially near the well bore.Since LD10-1 oil field developing by water flooding in 2005,Cr3+ polymer gel as profile control and flooding were carried out firstly in A23 well in 2006,and quantity of injection well gradually increased to 8.By 2007,LD10-1 oil field have entered mid and late development of polymer flooding,and water content approached 80%,development effects became worse year by year.Thus,the problem should be resolved using the comprehensive adjustment for the reservoir.Field test data indicated that inversion degree of liquid injection profile has been very serious,and middle and low permeable formation had poor fluid absorption ability.Meanwhile,narrow space of offshore oil and gas platforms restricted the number and volume of maturation tank space,leading to insufficient polymer dissolution;and the high salinity of sea water or output sewage formulated polymer solution that increased the difficulty of the polymer dissolution.A lot of incompletely dissolved polymer directly injected into well bore,and screen completion resulted in polymer particles detaining in the well bore,which caused the artificially high pressure of wellhead and the difficulties of polymer injection.Finally these factors influenced the development effects of polymer injection.To solve the problems existed in LD.10-1 oil field development,directed by theory of polymer chemistry,polymer physics and modern reservoir engineering,based on instrument analysis,physics simulation and numerical simulation,focused on reservoirs geology and fluid of LD10-1 oil field,this paper investigated the acceleration dissolution of HPAM,prepared blocking agent applied in the large pore paths or high permeability channel,synthesized a novel polymer microsphere as profile control oil displacement agent and optimized injection process parameters.The following several aspects of achievements and innovations have been obtained through this research:(1)Investigated a novel approach to accelerate the polyacrylamide(HPAM)dissolution by adding wetting agent,and evaluated its effects.A large quantity of single-well polymer injection,offshore oil and gas platforms with narrow space and low load-bearing has limited the dissolution device of scale.To meet the requirements of polymer injection amount,dissolution time should be shortened.In addition,solvent water of platforms had high salinity(9000mg/L),especially the concentration of calcium and magnesium ion were higher(1000mg/L),which increased the difficulty of polymer dissolution and prolonged the curing time.Thus,polymer dissolution had been the chief problem to enlarge application scale and improve development effects for offshore oilfield.In this paper,we investigated that wetting agent promoted polymer stretching and increased macromolecular clew size of polymer,due to hydrogen-bond interaction formed between the co-solvent and polymer decreasing the glass transition temperature and complexing with calcium and magnesium ions,which had contribution to acceleration dissolution,improving rheological property and seepage characteristics.The results showed that dissolving time of polymer power reduced to 50min,and enhancing ratio of polymer solution viscosity reached 5%with the agent I of 544mg/L and agent Ⅱ of 2459mg/L.(2)Prepared and evaluated a novel strong gel that could be used to significantly plug the large pore paths or the high permeability channels.Offshore reservoir rock of low bonding strength,intensive strength of single well injection-production and long-term water flooding and polymer flooding destroyed the rock structure,promoted the forming of the large pore paths and the high permeability channels,aggravated the reservoir heterogeneity,reduced the swept volume of water and polymer flooding.Finally,these factors influenced development effects of water flooding and polymer flooding.We developed a novel fluid diversion agent called CPP gel,which had low initial viscosity,high gel strength and good stability.CPP hard-gel was a type of polyacrylamide gel modified by flexible macromolecule and rigid macromolecule with a semi-IPN microstructure.This increased the structural uniformity and narrowed down mesh sizes of networks.Moreover,improving the hydrogen bonding interaction between molecular chains enhanced the mechanical properties of the gel in a great degree and increased the gel strength in porous media.The results showed that CPP gel had obvious improvement in increasing viscosity,viscoelasticity and plugging performance afteroptimized composition.The viscosity,yield stress and residual resistance factor of the CPP gel improved with increasing the water salinity,firstly improved and then reduced with increasing of temperature.On this basis,the gelation mechanism of the CPP gel in the pore was explored.The result showed that CPP gels mainly formed comb-like molecules during the early reaction period,the side chains of macromolecule were crosslinked,and then interpenetrating network structure was formed with delay of reaction.In addition,increasing of the water salinity and temperature promoted crosslinking reaction of polymer.In relatively low permeability pores,the corresponding cross-linking was probability generated among the side chains of same macromolecules.In relatively high permeability pores,intramolecular crosslinking was gradually converted to intermolecular crosslinking,which increased seepage resistance of the CPP gel.(3)Synthesized a novel modified microsphere and characterized its properties.LD10-1 oil field had undergone polymer injection development for many years,and liquid profile reversed gravely that influenced the recovery efficiency of polymer flooding(only about 28%).Thus,further measures to improve oil recovery should be taken.Polymer microspheres are type of selective profiling agent and have performance of narrow diameter distribution,plugging large pore and no blocking small pore,which could contribute to delay the behavior of liquid profile reversing.In recent years,research and application of polymer microspheres have received serious attention from the scientific and technical personnel,which achieved good effects of decreasing water cut and increasing oil production in offshore oil displacement experiment.To meet the requirement of subsequent development in LD10-1 oil field,we synthesized a novel modified microsphere by hydrophobic association monomer ALE through the water-in-oil emulsion method.The microspheres had the performance of swelling rate control and improvement viscoelasticity.On this base,this paper used swelling rate,viscoelasticity and seepage characteristic as evaluation indexes,characterized the microscopic structure and macroscopic properties,optimized the microsphere formulation and synthetic process,researched influencing factors and mechanism of slow swelling properties and viscoelasticity,and explored the ways to improve the plugging performance of micropheres.The results showed that hydrophobic associated action and intermolecular hydrogen bonding interaction between macromolecule chains of microspheres reduced the mesh size of network,increased the crosslink density,controlled the swelling speed and swelling ratio,and increased the viscoelasticity.These factors contributed to the improvement of deep migration properties and plugging performance.With increasing the ALE content from 0%to 0.22%,the swelling rate and swelling ratio of microspheres gradually reduced,the yield stress and creep compliance increased gradually,and residual resistance coefficient increased accordingly.(4)Comprehensively adjusted technology parameters,optimized the physical simulation and numerical modeling.After LD10-1 oil field have experienced more than ten years development of polymer injection,at present the main problems to be resolved included:firstly,acceleration dissolution of polymer power and artificially high injection pressure;secondly,adjustment of liquid profile(plugging large channel and decreasing the starting pressure of middle and low permeability);thirdly,development of novel oil displacement agent and evaluation its adaptability;fourthly,optimization the technology parameters of oil displacing agent.This paper researched measure of further enhanced oil recovery after polymer flooding and optimized injection process parameters.The results showed that technical-economic effect could be achieved through the measure of plugging large pore in water injection well and oil production well,broken down of medium and low permeable layer and alternating injection of weak gel or microspheres,increase rate of oil recovery exceeded 5%.