Combined use of fly ash and silica to prevent the long-term strength retrogression of oil well cement set and cured at HPHT conditions  

作　　者：Guo-Dong Cheng Xue-Yu Pang Jin-Sheng Sun Zheng-Song Qiu Chuang-Chuang Wang Jian-Kun Qin Ning Li 

机构地区：[1]School of Petroleum Engineering,China University of Petroleum(East China),Qingdao,266580,Shandong,China [2]National Key Laboratory of Deep Oil and Gas,China University of Petroleum(East China),Qingdao,266580,Shandong,China [3]Oil and Gas Engineering Research Institute(Tarim Oilfield Company),China National Petroleum Corporation,Tarim,841000,Xinjiang,China

出　　处：《Petroleum Science》2024年第2期1122-1134,共13页石油科学（英文版）

基　　金：National Natural Science Foundation of China(No.51974352 and No.52288101);China University of Petroleum(East China)(No.2018000025 and No.2019000011)。

摘　　要：The long-term strength retrogression of silica-enriched oil well cement poses a significant threat to wellbore integrity in deep and ultra-deep wells, which is a major obstacle for deep petroleum and geothermal energy development. Previous attempts to address this problem has been unsatisfactory because they can only reduce the strength decline rate. This study presents a new solution to this problem by incorporating fly ash to the traditional silica-cement systems. The influences of fly ash and silica on the strength retrogression behavior of oil well cement systems directly set and cured under the condition of 200°C and 50 MPa are investigated. Test results indicate that the slurries containing only silica or fly ash experience severe strength retrogression from 2 to 30 d curing, while the slurries containing both fly ash and silica experience strength enhancement from 2 to 90 d. The strength test results are corroborated by further evidences from permeability tests as well as microstructure analysis of set cement. Composition of set cement evaluated by quantitative X-ray diffraction analyses with partial or no known crystal structure(PONKCS) method and thermogravimetry analyses revealed that the conversion of amorphous C-(A)-S-H to crystalline phases is the primary cause of long-term strength retrogression.The addition of fly ash can reduce the initial amount of C-(A)-S-H in the set cement, and its combined use with silica can prevent the crystallization of C-(A)-S-H, which is believed to be the working mechanism of this new admixture in improving long-term strength stability of oil well cement systems.

关 键 词：Fly ash Long-term strength retrogression High temperature Quantitative X-ray diffraction(QXRD) Partial or no known crystal structure(PONKCS) 

分 类 号：TE256[石油与天然气工程—油气井工程]