Study On Seawater-mixed Acid System For Sand Reservoir Stimulation

Acidizing is one of the most commonly used measures to improve production and injection in offshore oilfields.In matrix treatment,water,acid and other additives used in the fluid preparation are all transported from the land.Due to the cost of fresh water supply,it is inconvenient and expensive to transport such large amounts of water from the land to offshore platform.Therefore,it is more viable to use local materials,seawater,instead of fresh water to prepare the acid fluid system for most of the offshore oilfields where fresh water is not adequately accessible.However,compare with carbonate matrix stimulation,hydrofluoric acid used for sandstone reservoirs acidizing will release F^-and secondary SiF₆^2-into the solution,which will combine with Mg²⁺,Ca²⁺,Na⁺,K⁺ions in seawater.Then large quantities and various types of precipitates will form when exceeding critical scaling tendency,which results in serious formation damage.In this paper,artificial simulated seawater?about 35,000 mg/L?was mixed with various acid such as hydrochloric acid,hydrofluoric acid,mud acid,multi-hydrogen acid and fluoboric acid.By means of spectrophotometry method,X ray diffraction analysis,dynamic light scattering measurement,electrochemical study,static and dynamic reaction simulation,and numerical simulation,the potential damage during the preparation of seawater-mixed acid and the reaction between acid and rock were systematically researched.Results shows that conventional hydrofluoric acid and multi-hydrogen acid precipitate to varying degrees when prepared with seawater.XRD showes that fluoride is the main type of secondary damage.The formation tendency of fluoride precipitates measured by physical and numerical simulation indicates that high concentration of hydrochloric acid can effectively inhibit the formation of fluoride precipitates in seawater-mixed hydrofluoric acid.Since the organic phosphonic acid contained in the multi-hydrogen acid may form complexes with the metal ions,precipitation mitigation capacity of multi-hydrogen acid is stronger than that of mud acid.Mixing seawater with MH before the addition of MF can make multi-hydrogen acid most stable for the competitive adsorption of F^-and MH on Mg²⁺and Ca²⁺.The damage types and scale in the reaction of seawater-mixed acid and different rock were analyzed by static and dynamic simulation.When seawater-mixed acids were used to treated carbonate,a great increase of pH results in the precipitation of calcium sulfate and fluoride.A new type of formation damage type as MgF₂ gel was found.MgF₂ gel formed during the reation beweent mud acid and sand reservoir with high content of dolomite at high temperature.Its damage to formation permeability is greater than other types of precipitate because of the high viscosity(up to 60000 mm²·s^-1).Na₂SiF₆ is the main precipitate when seawater-mixed acid reacted with quartz and aluminosilicate minerals.As products of primary reaction in acidizing,fluorosilicate precipitates are mainly determined by the dissolution rate.Therefore,Na₂SiF₆ produced by the reaction between seawater-mixed acid and clay is much higher than that of quartz.The hydrolysis rate of fluoboric acid is too slow to form precipitate at low temperature,therefore,it is suitable for the treatment of reservoir with high clay content.Chemical effects of current seawater-mixed acid were investigated,including acid composition and concentration,HCl volume,seawater dilution ratio,and inorganic salt on damage quantities during acid preparation and reaction.Fluoride precipitates including CaF₂and MgF₂ can be effectively mitigated by decreasing the free F^-concentration,increasing HCl concentration,diluting seawater and adding the inorganic electrolytes to improve the ionic strength of solution.Based on the multi-hydrogen acid system,a special and abnormal common-ion effect was discovered.The addition of calcium ions in the seawater-mixed multi-hydrogen acid solution produces a super-clear liquid system with the turbidity lower than 30 FTU.A highly effective inhibitor DTPMA was screened out to improve the solubility of calcium sulfate produced in the reation between seawater-mixed hydrochloric acid and carbonate.Although fluorosilicate precipitates is hard to dissolve,adding proper amount of hydrochloric acid or ammonium chloride can mitigate the precipitates to some degree by promoting the hydrolysis of SiF₆^-.Organophosphonic acid has a good ability to inhibit the precipitation of Na₂SiF₆ by changing the structure of the Na₂SiF₆ crystal growth surface and inducing lattice distortion.However,due to monovalent metal ions are not attached to the organophosphonic acid by chelation,the Na-O bond is vulnerable to the protonation effect under the strong acid condition.Based on the established evaluation indexes and standards of the seawater-mixed acid,fluoride,precipitation inhibitor and supporting acid were screened.On this basis,four kinds of seawater-mixed acid were proposed:preflush and postflush were 15%HCl+2%DTPMA+seawater,the main acid was selected from multiple-mud acid,multiple-fluoroboric acid and pre-chelated phosphonic acid according to the mineral composition of formation.Acid additives including corrosion inhibitor,iron ion stabilizer,etc,were screened by evaluating basic performance and acid compatibility.Core flooding tests showed that these clean seawater-mixed acid system could eliminate secondary damage during acidizing treatment and could be widely used in the stimulation of offshore oilfield with high permeability and high porosity.