he carbonate reservoir has tight lithology and is operated at high temperature in S block. In order to improve the oil recovery, it is necessary to carry on the acid fracturing treatment to the target reservoir. According to the characteristics of the target reservoir, the acid fluid for acid fracturing was optimized and evaluated. Through the porosity and permeability experiment, the porosity of S block was 0.1%~2.0%, and the permeability was below 0.01 mD, which belonged to ultra⁃low permeability reservoir. According to the uniaxial compression test, the compressive strength of the reservoir was high, ranging from 40 to 70 MPa. X⁃ray diffraction showed that dolomite and calcite were the main minerals in the reservoir ( 80%~90% in total), with a small amount of quartz and clay (mass fraction was about 10%). The main acid was 20% hydrochloric acid, the thickener was 0.3% xanthan gum, and the dissolution rate of rock powder was about 90% when hydrochloric acid acted alone, and the main acid solution was hydrochloric acid. Taking into account that the reservoir lithology of carbonate rock in S block was dense and the formation temperature was 100~140 ℃, the viscosity of modified xanthan gum was about 10 mPa·s at 140 ℃. The modified xanthan gum was selected as high temperature resistant thickener.

At present, the polymer injection target of Lamadian oilfield has been shifted to the second type reservoir. Compared with the second type A reservoir,the second type B reservoir has serious heterogeneity and low permeability.For this reason,the study on the characteristics of polymer flooding seepage in the second type B reservoir is carried out.The results show that with the increase of core permeability,the iso⁃permeability point of polymer flooding relative permeability curve moves to the right and the span of two phases increases.Increasing the polymer molecular weight and the injection concentration can make the right end of the curve shift to the right.The polymer flooding resistance coefficient and residual resistance coefficient increase with the increasing of polymer solution concentration and molecular weight,while decrease with the increase of core permeability.There is a good correspondence between the polymer flooding resistance coefficient and the shunt rate.When the resistance coefficient reaches the peak,the shunt rate will reach the peak,and then the shunt rate of the high permeability layer will rise.The higher the molecular weight of the polymer, the higher the concentration of the solution,the greater the difference in permeability level,and the earlier the diversion rate of high permeability layer rises.