| The staged fracturing of horizontal well takes long construction period even up to 30 days, which causes early fractured formation to soak in the fracturing fluid for a long time.The long retention time for fracturing fluid in reservoir probably brings great harm to reservoir, having negative effect on the reservoir reform.Therefore, choosing three typical low permeability reservoir with HQ chang6, HS chang8 and ZB chang6 as research area, based on analyzing the geological features of reservoir in research area and relying on the staged fracturing flowback time of horizontal well in research area, this article not only evaluates the conventional harm and the harm in long-term retention of fracturing fluid by using reservoir core about the fracturing fluid system of PPQ, guar gum and Free-polymer water but also acquires the following achievements by doing experiment research and analyzing the influential factors on the results of assessment for harm to fracturing fluid.Firstly, the fracturing fluid systems used in research area all do serious harm to low permeability reservoir and the harm in long-term retention is even worse than that in general. In detail, PPQ is rather harmful to reservoir both in regular and long-term retention. Compared with PPQ, there is less harm in guar gum. The damage of Free-polymer water for reservoir of conventional hurt was smaller while the long-term retention is more severe.Secondly, Permeability of the reservoir rock in the study area largely affects the extent of damage. In addition, there exists an obvious difference for the result coming from Conventional evaluation and long-term retention evaluation method. PPQ, guar gum fracturing fluid systems containing residues for low permeability reservoir cores damage rate is less than average reservoir permeability,under the conventional harm evaluation; however, this case is just in an opposite direction with the long-term retention evaluation. In contrary, for Free-polymer water fracturing liquid system, the rate of harm in the core of low permeability reservoir is more than that in common permeability of the reservoir under the conventional harm evaluation while the harm to the core of low permeability reservoir is less than that to common permeability of the reservoir in the long-term retention.Thirdly,residue leads to more severe damage on high permeability cores reservoirs than low permeability reservoir. Low permeability reservoir core damage is given priority to with water locking damage. With the increase of the retention time, water locking damage increases significantly while residue damage increase amplitude is relatively small.Fourthly, with the increase of retention time, three systems of fracturing fluids all make an increased damage to reservoir core and the increase amplitude decreases continually.Fifthly, Regardless of the conventional damage or long-term retention, injection amount and the return of fracturing fluid in the process of experimental displacement all have obvious influence on damage rate,Under the same conditions, larger fracturing fluid volume and less backflow volume increase the rate of core damage. Therefore, this thesis recommends that the volume of fracturing fluid injection and flowback should be operated according to the following divided permeability, namely, when Ka≤0.1 m D, fracturing fluid injection volume is recommended for 0.3 ~ 0.5PV, fracturing fluid return displacement 3 ~ 5PV; when 0.1m D≤Ka≤0.3m D, fracturing fluid injection volume is 0.5~0.8PV, fracturing fluid displacement back 5 ~ 8PV; when Ka≥0.3m D, fracturing fluid injection volume 0.8 ~ 1PV, fracturing fluid return displacement 8 ~ 10 PV.The research achievement of this article provides the basis for the design of reservoir fracturing in horizontal wells and the improvement of fracturing fluid. At the same time, it lays a foundation for establishing the evaluation method of fracturing fluid with long term retention. |
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