An In-Situ Stress Measuring Method Based On Fiber Optic

In-situ stress is one of the key parameters in petroleum engineering.However,the stress environment within the formations is complex,and the distribution of in-situ stress varies with depth.Moreover,the in-situ stress will be changed over time due to disturbances such as reservoir development.Therefore,it is necessary to perform multimetering and dynamic monitoring of in-situ stress.In recent years,fiber optic sensing technology has received widespread attention in the field of petroleum engineering,particularly in its application in horizontal wells.It has been verified that fiber optic sensing technology has the potential to achieve multimetering and dynamic monitoring of in situ stress,as demonstrated by its ability to perform multimetering of hydraulic fractures and dynamic monitoring of liquid production profiles.This paper aimed to establish a forward-looking research foundation for in-situ stress measurement based on fiber optic sensing.It is mainly based on the downhole pressure variation method.What is more,it includes theoretical and methodological aspects such as fiber optic integrated casing design,strain correction,inverse differential strain analysis insitu stress measurement,and rock mass orthotropic elastic parameters measurement.Through a physical simulation experiment,this paper has obtained the in-situ stress state and elastic parameters of an outcrop under theoretical and assumed conditions.The proposed in-situ stress measurement method has been preliminarily realized under indoor laboratory conditions.And,it provided an exploratory research foundation for the multimetering and dynamic monitoring of in-situ stress.The main contents are as follows.(1)Inverse differential strain analysis method for in-situ stress measurementThe differential strain analysis method,based on the mechanical behavior of rock micro-fractures,cannot be performed underground.In this paper,the deformation behavior and mechanical behavior pattern of micro-fractures under in-situ stress release and wellbore pressurizing conditions were analyzed.Based on the theoretical assumption of the differential strain analysis method,an inverse differential strain analysis method for in-situ stress measurement was constructed.And,it can be directly conducted underground.To verify the feasibility of the inverse differential strain analysis method,a rock mechanics model considering micro-fractures was established,and a virtual rock mass was constructed.By comparing the input parameters of in-situ stress in the virtual rock mass with the measurement results of the inverse differential strain analysis method,the applicability of the inverse differential strain analysis method was preliminarily verified.(2)Rock mass elastic parameters measurement based on downhole pressure variation method and fiber optic sensing technologyObtaining the elastic parameters of the rock in two states,i.e.,complete closed and complete opened micro-fractures,is a necessary condition for implementing the inverse differential strain analysis method.However,these elastic parameters must be obtained directly downhole.Therefore,in order to measure the elastic parameters of the rock mass,a new elastic parameter measurement method based on the rock elastic parameters and dynamic strain specificity is proposed.This method uses fiber optic strain sensing technology to measure the dynamic strain formed by the pressure variation in the casing and combines it with the O-SHADE orthogonal anisotropic elastic parameter inversion algorithm established in this paper to obtain the rock elastic parameters required by the inverse strain difference analysis method.The O-SHADE algorithm is established by combining the success-history based parameters adaptation for the differential evolution algorithm with the finite element numerical simulation method.The performance of the O-SHADE algorithm is verified through comparison with meta-heuristic algorithms and machine learning algorithms.Finally,the feasibility of the rock elastic parameter measurement method is verified through numerical simulations.(3)Design method of the fiber-optic integrated casingA prototype and design method of the fiber-optic integrated casing was developed.A calculation model of the macro-bending loss along the spiral winding fiber was derived.And,the influence of the casing geometry on the macro-bending loss along the fiber was investigated.Using the complex variable method,a two-dimensional strain distribution model of the casing well under the casing pressure was established.And,the effect of the formation elastic parameters and the casing geometric dimensions on the strain shielding effect,caused by the fiber-optic strain sensing accuracy limitation,was analyzed.This study provides a basic theoretical foundation for casing design for in-situ stress measurements.