A Systematic Approach to Fault Sealing Capacity Evaluation in Underground Gas Storage: A Case Study from China

Abstract

Fault stability refers to the risk level of reactivation of the pre-existing fault in the stress field. Fault reactivation within the oilfield is mainly caused by the increase of fluid pressure in the fault zone. The quantitative evaluation index of the fault stability is the critical fluid pressure (that is, additional fluid pressure) required for fault reactivation under the current pore fluid pressure. When the formation pore pressure reaches the critical value, the corresponding fault part will be in the critical stress state. The sliding of the fault in the critical stress state will easily cause oil and gas leakage and casing damage at the edge of the fault. Therefore, it is of great significance to study fault stability for oilfield production. Ground stress is a key parameter for fault stability evaluation. There are many methods to calculate the geomechanics including hydraulic method, acoustic emission method, and the use of the logging data, among which the hydraulic fracturing method can be used to obtain the most accurate horizontal minimum principal stress. This paper calculates the continuous geomechanics by using the logging data. There are many methods available for evaluating fault stability, among which fault sealing analysis technology (FAST) method is most widely used. FAST can be used to not only quantitatively evaluate fault stability, but also evaluate the impact of fault cohesion on fault stability. There are many factors affecting fault stability. The relationship between the differential stress and tensile strength of the fault rock will affect the trend of the fault reactivation.The direction of the stress field also affects the fault stability greatly. The argillaceous material weakens the strength of fault rock. When a large amount of argillaceous material enters the fault zone, the fault tends to reactivate. The change of reservoir fluid pressure will also lead to the change of horizontal stress to affect the stability of the fault. In addition, the accuracy of seismic interpretation will also affect the evaluation results of fault stability. Based on the geological model framework and one-dimensional geomechanical model calibration, this paper establishes a three-dimensional geomechanical model by using the finite element simulation method to carry out four-dimensional geomechanical research to evaluate the fault stability in the development of the Donghe 1 Reservoir in Tarim basin. The research results show that the fracture sealing gradually strengthens during the development of Donghe 1 Reservoir, and the quantized critical fracture opening pressure is 67.38MPa.