A Study On Retention Of Gelled Acid Thickener In Calcite

Gelled acid is the most widely used treatment fluid in acid fracturing,however,the adsorption and blocking of thickener in the pore of reservoir rock,can cause serious damage of reservoir permeability.In order to enhance the flow conductivity obtained by acid fracturing,it is necessary to carry out a systematic study on the mechanism of reservoir retention damage.In this paper,calcite,the principal component of limestone,was used as retention medium,then static adsorption and dynamic retention mechanism of polyacrylamide in single mineral were studied.So that a careful analysis about thicker retention in rocks with complex mineral composition was carried out,based on which,it is reasonable to try to use the real core to relieve the detention.Making use of the adsorption amount obtained by UV,the relationship of adsorption amount and factors,including the concentration of polyacrylamide,liquid-solid ratio,calcite particle size,temperature,type of background ions and its concentration,were all analysed.It is indicated that the adsorption equilibrium affects the adsorption capacity of the system.Varied experimental methods such as Fourier Transform Infrared Spectrometer(FTIR Spectrometer),Zeta potential test were performed to explore the possible adsorption force of polyacrylamide-calcite adsorption system respectivelly,it is found that after urea urea was added into the system,adsorption amount can be decreased to 60%,so we confirm the main force of this system is hydrogen bonding.Moreover,the experimental data fitted well to monolayer adsorption model of Langmuir,that further confirmed the hydrogen adsorption sites and adsorption interface polyacrylamide together determine the adsorption law of this system.To make clear the retention retention mechanism of polyacrylamide,the change law of damage degree and the reduction of the permeability with the displacement flow rate,discharge flow rate and the flowback time were analyzed.It is found in calcite there is a top strength limit of polyacrylamide plugging.Based on the analysis of the floc morphology and the pore structure of the calcite,the laws of polyacrylamide plugging were discussed.Integrated survey of the static adsorption mechanism and floc blocking conditions of polyacrylamide in calcite was performed,then the process that retention and damage rate change with the concentration of polyacrylamide was revealed into three typical stages.It is delivered that in the case of a certain pore diameter and the size of the thickener floc,for the composition of permeability damage,adsorption is minor(less than 20%)while blocking takes a chief part(no less than 50%).Pore structure of untreated cores and damaged cores by mercury porosimeter,And it confirmed that polyacrylamide retention will also cause rock aperture and blockage.Based on the analysis of the relationship between the retention of single mineral and core mineral,the feasibility of urea and fluorocarbon surfactants to relief the retention of polyacrylamide was demonstrated.And then decreasing damage experiment was performed using real limestone core with high calcite content.It is showed that urea could relieve the damage of more than 10%,while damage reduction rate is minor by fluorocarbon but obvious effect can be achieved in a short time(30min).