| Offshore oil field in China is rich of heavy oil resources and has great potential for enhanced oil recovery.Offshore oil field recovery enhanced for each additional 1%is equivalent to discovery of a billion tons of geological reserves of large oil fields.Due to the loosening of the reservoirs in the offshore heavy oil field,the high viscosity and high density of the crude oil,the complexity of the injected water source,the thickness of the oil layer and the well spacing,especially the influence of ocean engineering conditions such as the narrow space of the platform,many mature technologies in the land field cannot be directly copied to the offshore heavy oil field development.Since the innovative application of polymer flooding technology(2003)to the present,we formed series knowledge of offshore oil field chemical flooding production,achieved cumulative incremental oil more than 515 million cubic meters.At present,SZ36-1 oil field has been built as the world’s largest offshore polymer flooding oil field.Despite to the above success,we also found many problems which restrict the application of chemical flooding technology in offshore oilfield,such as the poorly injected effect of parts of wells in the main chemical flooding oil field,deterioration of mixed sewage water quality,the difficulty of injection water to meet the standard,high injection pressure,high proportion of under injection wells,insufficient injection of water wells,short period of validity of water injection,and obvious conflict between the different horizons.Based on the systematic analysis of the geological characteristics of reservoirs,this paper establishes the method of compatibility evaluation and the method of water quality determination of polymer-contained sewage.This method is used to compare the differences of scaling mechanism between polymer-contained sewage and ordinary sewage.This paper summarizes the damage mechanism of polymer-contained sewage reinjection wells and summarizes the difference of the water absorption rules between the polymer-contained sewage reinjection wells and the conventional injection wells.The plug type and the clogging range of the injection wells are analyzed.The technical measures of reservoir protection and the technique of plug-free-plus-injection are put forward,and some applications of field test are carried out.A study for the difference of scaling mechanisms between polymer-contained sewage and general sewage in SZ36-1 oilfield indicates that both polymer-contained sewage and general sewage occur to formation the calcium carbonate scale.The calcium carbonate scale in the general sewage shows high autotype degree,cubic morphology and the grain size of which is greater than 20μm.The polymer in the polymer-contained sewage match and bond with the Ca2+by the-COO-,which controlled the multilateral crystal of calcium carbonate transforms to sphere nanocrystalline(<5μm),the scaling crystal adhere each other,and crosslink with the suspended solids,formation fines,remnant polymer etc.,by which generating composite clogs with bulky size(>50μm).After injected into the reservoir,the polymer-contained sewage can block the large or medium pore throat of reservoir,and generate inner filter cake and end face filter cake,so that which increases the depth and complexity degree of formation damage.The heterogeneity is very strong in-layer,between layers and on the plane of the reservoir in SZ36-1 oilfield,which resulted in the main water entry being blocked and non-main water entry changing into main water entry periodically between the different oil sets of the polymer-contained sewage reinjection well.It is different from the rule in the normal water injection well that the main water entry remains all along;At last,resulting in the oil sets with lower permeability adsorbing little water or none water and even the injection well becoming under-injected.As the polymer-contained sewage is rich in emulsified oil and with strong stability,there are some problems such as test result with great error,time wasting etc.for the recommended method of water quality measuring in the current industrial standard and enterprise standard,so the method of water quality measuring has been optimized and improved,which with much higher test accuracy and efficiency.It has been found that the water quality indexes such as oil content,suspended solids content,total iron and sulfate reducing bacteria etc.Both exceeded the standard.According to the principle of reservoir protection,the prevention effect of ground pre-scaling,combined with ultrasonic action to control the plugged reservoirs was evaluated by laboratory experiment,and the damage rate was reduced by more than 30%.In addition,this paper proposed the reservoir protection approach of reducing the produced polymer flocculating mechanical impurities to yield compound mismatches,and a novel sewage disposal agent was developed.During the field experiment period,the water quality was improved significantly.Also,this paper put forward some pieces of suggestion,such as reduction of water injectivity,optimization of dosing position spacing and classification regulation of water quality,which provided a solid technical support for treating produced fluid from offshore polymer flooding oil fields.Aimed at the plugged under-injected well,that near well 1～2 meter range is the most serious plugging zone was cleared.The underground plugging material,mainly consisting of organic polymer,oil,calcium carbonate scale,corrosion product and formation fines,formed in the long-term accumulation of composite plug in the underground.A high efficiency block remover system was firstly proposed to penetrate,swell,disperse and peel off complex plug.Then polymer degradation,organic matter dissolution and acid dissolution of inorganic block were employed.Finally,high pressure water injection in a short time was utilized,which could result in micro-fracturing.All the above contributed to effective damage removal and stimulation of polymer-contained sewage reinjection wells. |
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