| Panguliang chang-6reservoir in changqing oilfield with reservoir temperature75℃, formation water salinity88420mg/L, water type of CaCl2and no fringe and bottom water is elastic dissolved gas drive. It possesses the characteristic of low permeability, abnormal low pressure and low abundance; this means it is necessary for fracturing at the initial production. So natural micro-fractures and non-natural fractures are presented in the reservoir.Three technological problems including injectivity, filtration and stability of the blocking agent are existence during water shutoff in the reservoir. Sufficient studies shown two systems of pre-cross-linked polymer water shutoff agent and HPAM-UF compound water shutoff agent are adaptation to the fractured reservoir. The achievements though a large number of indoor experiments of this paper were as follows:(1) The formulations of water shutoff agent of pre-cross-linked partially hydrolyzed polyacrylamide blocking system suitable for the fractured reservoir were as follows:the concentration of partially hydrolyzed polyacrylamide arranged from3500mg/L to4000mg/L which was manufactured with diluted formation water with the salinity of22105mg/L, the concentration of pre-cross-linked agent arranged from50mg/L to60mg/L, the volume percentage of secondary cross-linker agent arranged from1.0%to1.5%.(2) The formulations of water shutoff agent of HPAM-UF compounded blocking system for the fractured reservoir were as follows:the volume percentage of UF arranged from40.0%to50%, the concentration of partially hydrolyzed polyacrylamide arranged from500mg/L to1000mg/L. the concentration of curing agent400mg/L, the concentration of delayed curing agent arranged from10000mg/Lto15000mg/L.(3) Based on difficulties of degrading high-strength UF water blocking agent, an environment-friendly water block remover was developed. Solution was always clarified and the samples of UF water block agent were smaller and suspended in the solution until they disappeared completely during the degrading.(4) Evaluation static performance of anti-temperature, anti-salinity and anti-shearing of the two blocking agent system, evaluation dynamic performance by simulation fracture model. The results shown as follow:both of them were good performance of anti-temperature, anti-salinity and anti-shearing; the results of core following experiments shown as follow:the pre-cross-linked partially hydrolyzed polyacrylamide water shutoff agent system with the resistance factor arranged from10.17to13.80, the percentage of shutoff rate98.93%, the gradient of breakthrough pressure25.52MPa/m; the HPAM-UF compounded water blocking agent with the resistance factor arranged from1.13to2.25, the percentage of shutoff rate98.99%, the gradient of breakthrough pressure accessed to30.50MPa/m.Both of them shown excellent performance in water shutoff in fractured system.(5)Analysis of microscopic structure of pre-cross-linked polymer system by scanning electron microscope, the structure was laminar of brine system for the molecular chain of partially hydrolyzed polyacrylamide, it shown the structure of water shutoff agent; existence of "hollow" and "pore" in polymer provided the conditions of dehydration, so that the free water in system was extrusion. But this phenomenon could be circumvent though adding pre-cross-linker into system, because pre-cross-linker and polymer chains could form cross-linked structure which increasing the steric hindrance of the macromolecular chains and the stability of system.(6) Study on the cross-linking density between pre-cross-linker and partially hydrolyzed polyacrylamide, at the given reservoir condition the reason of dehydration of system is excessive cross-linking. The value of cross-linking density in the vicinity of the critical cross-linking density, the stability of water shutoff agent system is best. |
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