Abstract

Three ASP (Alkaline, surfactant and polymer) flooding projects and one multicomponent foam flooding project have been carried out in Daqing Oilfield. The increases of oil recovery rates are all above 20% (OOIP). One of the major troubles that happened during the three ASP flooding projects and the multicomponent foam flooding project was silicate scale deposition in the tubings of production wells which caused frequent downhole operation failure. To prevent scale deposition in production wells, a scale inhibitor, which works well in brines of pH values as high as 12 was developed. The scale inhibitor is effective both on carbonate scale and on the mixed scale of carbonate and silicate. The scale inhibitor was tested on 6 production wells in two ASP flooding project zones. The pull-out period of the tested production wells were elongated from 1 month to 9 month during the test.

Introduction

Three ASP (Alkaline, surfactant and polymer) flooding projects and one multicomponent foam flooding project have been carried out in Daqing Oilfield. The increases of oil recovery rates are all above 20% (OOIP). As shown in Figure 1, one major trouble that happened during the three ASP flooding projects and the multicomponent foam flooding project was silicate scale deposition in the tubings of production wells which caused frequent downhole operation failure. In the process of ASP flooding, NaOH reacts with the reservoir rock resulting in the dissolution of some rock materials. The flooding liquid carries the dissolved materials to the production wells. The blending of produced liquid coming from different layers, decrease of pressure, loss of the dissolved gas and decrease of temperature result in precipitation of silicates and its deposition on the tubing, surface pipeline and pumps etc. The silicate deposition has caused downhole sieve pipe plugging, tubing plugging and thus break of the beam pump rod. Frequent operation failures of the production wells have been observed. To prevent scale deposition in production wells, a scale inhibitor, which works well in brines of pH values as high as 12 was developed. The scale inhibitor is effective both on carbonate scale and on the mixed scale of carbonate and silicate. The scale inhibitor was tested on 6 production wells in two ASP flooding project zones. The pull-out period of the tested production wells were elongated from 1 month to 9 month during the test.

Silicate scale analysis

Four scale samples taken from the tubings of three ASP flooding production wells were analyzed. In the first step, the scale samples were heated in a furnace to 700°C. Then the compositions of the inorganic residues were analyzed. The residue compositions of the scale samples are listed in Table 1. As shown in Table 1, the contents of SiO2 in the inorganic residues of the three scale samples are all above 70%, which means that the three scale samples are all silicate scales.

Formulation of silicate scale inhibitor and its performance

Two artificial water samples, namely water A and water B, were used in selection and formulation of silicate scale inhibitor. The formulations of the two water samples are listed in Table 2. The two water samples were pumped by two metering pumps at an equal rate up to 20 MPa and mixed in a mixture before they were injected into a capillary tube set in a water bath operating at 50°C. Three days later, the capillary tube was taken out of the water bath and the amount of scale deposited inside the tube was determined. After a series of tests, one silicate scale inhibitor formulation was determined. The silicate scale inhibiting performance of the newly developed silicate scale inhibitor is shown in Figure 2. As shown in Figure 2, the silicate scale inhibiting rate of the newly developed silicate scale inhibitor can reach 90% at a dosage of 10ppm.

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