Fault slip induced by hydraulic fracturing and risk assessment of casing deformation in the Sichuan Basin
FAN Yu , HUANG Rui, ZENG Bo , CHEN Zhaowei , ZHOU Xiaojing , XIANG Degui , SONG Yi
1 Shale Gas Research Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu 610051, China 2 CNPC Engineering Technology R&D Company Limited, Beijing 102206, China
Serious casing deformation occurred during hydraulic fracturing in Changning-Weiyuan national shale gas demon stration area, Sichuan Basin, which has seriously impacted on fracturing and subsequent production. In order to clarify the cause of casing deformation occurring during hydraulic fracturing and to explore how to mitigate casing deformation, we take the H pad located in the Changning-Weiyuan shale gas area as the research object, observing the distribution of casing deformation, identifying the orientation and dip of faults, establishing the geomechanical model, and assessing the slip probability of faults under hydraulic fracturing operations. The correlational analysis between the positions of the casing deformation and ant tracking faults and microseismic events shows that the fault slip induced by hydraulic fracturing might be the cause of casing deformation. The multi-arm caliper logging shows that the casing deformation is S-shaped shear deformation, which is consistent with the shear characteristics of a fault slip. Based on the geological structure of this block, the fault model was established by taking the faults interpreted by ant tracking as the main body and the fault interpreted by microseismic events as the supplement. The data of conventional logging, image logging and mini- fracturing were analyzed to obtain the in-situ stress and pore pressure, and to establish the geomechanical model. The Mohr-Coulomb criterion was used to analyze the activation states of these faults, and the QRA method (Quantitative Risk Analysis) was used to analyze the slip risk of faults and the sensitivity of each factor on fault slip. The results show that most of the faults in this pad are in a critical stress state under the in-situ conditions, and the well-oriented faults can easily be activated under current pumping pressure. The slip probability of the faults which intersect the casing deformation section is up to 65% under a pressure increase of 17 MPa, which shows the casing deformation can be caused by fault slip induced by hydraulic fracturing. In this case, the pore pressure gradient and friction coefficient have the greatest influence on the results. The prediction results of the model will be more accurate if the pore pressure and friction coefficient are estimated more precisely. Based on the risk assessment results, the slip risk of faults can be calculated, which can provide reference for optimizing well trajectory design to mitigate casing deformation. This method may provide an effective method for minimizing casing deformation problems .
