Abstract

Up to 2014, low production wells of CNPC oil fields account for 10%~30% in all the production wells, the number of old wells is huge. How to develop the low production wells effectively is one of the key research aspects under such low international oil price, and old wells re-fracturing is an important mean to improve single well recovery and reduce the cost. In this paper, we took tight oil wells for example; stress distribution of more than 100 wells was studied. Finite element analysis software COMSOL was used to simulate the in-situ stress and induced stress distribution by production effect. The equations of induced stress distribution by fracture net pressure were derived, stress distribution by production effect and induced stress distribution by fracture net pressure was coupled, and the re-fracturing stress distribution software was programmed. The calculation results show that: in-situ stress distribution will be changed by production effect as wells produce, but for tight oil formation, stress change by production effect is little because seepage flow rate is very slow. Stress change becomes large at first and then becomes small; so there is a change extreme point - optimum re-fracturing opportunity. Re-fracturing has a critical re-orientation fracture net pressure, the stress variation locates in the weak stress reversal region under the critical pressure, it is difficult for re-orientation; above the critical pressure, the stress variation lies in the strong stress reversal region, re-orientation could occur. Taking x well for example, after 10 years production, Stress change is small when fracture net pressure is under 3.5MPa, it locates in the weak stress reversal region. Stress change increases as fracture net pressure increases when net pressure is above 3.5MPa, but the increase amplitude stress change is reduced, and the re-orientation radius is 53m when fracture net pressure is 3.5MPa. Stress distribution with time at critical re-orientation fracture net pressure shows that: Stress change becomes large at first and then becomes small, the change extreme point- optimum re-fracturing time is 1-5 year, the re-orientation radius is 63-78m under optimum re-fracturing time, the stress distribution locates in the weak stress reversal region after 10 years production and re-orientation becomes difficult. The critical re-orientation fracture net pressure drops dramatically under optimum re-fracturing time, and the re-orientation radius under same fracture net pressure is obviously longer than that of non-optimum time. Furthermore, re-fracturing candidates under optimum time were screened, temporary plugging agents improving fracture net pressure were selected preferably, and 15 wells stimulation were guided based on the simulation results, 25-100Kg temporary plugging agents were added for each candidate. The treatment pressure was improved by 4.0-10.0MPa, fluid production was improved by 2.15 times and oil production was improved by 3.5 times. The re-fracturing stress distribution software can provide basis data for re-fracturing candidates screening and temporary plugging agents selection, and it is beneficial for old well re-fracturing and new technologies exploration under low oil price.

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