In various subsurface resource development or fluid piping problems, subsurface fluid-filled fractures often appear. Fracture location determination has always been critical in related fields. Acoustic wave reflection at the junction and boundary in the pipeline can carry information about the property of the system. By using the accompanying acoustic wave combined with the water hammer effect, the location of subsurface fractures can be estimated. A numerical fluid flow model for instantaneous shut-in is presented based on water hammer effect. Fluid penetration effects, wellbore storage effect and fluid inertial effect are considered. The fracture location estimation method called cepstral predominant peak (CPP) analysis is first proposed. By cepstral, we means the inverse Fourier transform of the logarithm of the estimated signal spectrum. Also the relationship between instantaneous shut-in pressure and cepstrum response is investigated in detail. To improve the robustness, CPP analysis based on Kaiser windowed cepstrum was used to identify the impulse period of fracture. Compared with original cepstrum, Kaiser windowed cepstrum has the better performance for CPP analysis. The proposed flow model is impactful as it can provide pressure data with known fracture locations. The data can be used to optimize and examine the performance of CPP analysis. A field experiment is conducted to validate the analysis about the acoustic wave in a pipeline system with fractures. The actual instantaneous shut-in pressure for an oil well is obtained. The experiment result shows the CPP analysis can get the fracture location efficiently and accurately, which can provide insights for engineering.

This paper studies the influence of large-size cave on pressure transient characteristics in fracture-caved carbonate gas reservoirs (FCCGR). With the rapid increase of energy demand, the exploration and development of unconventional oil and gas becomes more and more important. In recent years, many FCCGR have been discovered in western China and contribute significantly to Chinese gas reserves. However, with the presence of large-size cave, FCCGR have complex pore structures and strong heterogeneity. Traditional pressure transient analysis models cannot describe the gas flow accurately. This paper develops a novel pressure transient analysis model for FCCGR by coupling the fluctuating pressure and minor energy loss. Based on the solutions, the typical curves are plotted to analyze the pressure transient characteristics. It is found that the flow process can be subdivided into six stages, including the following: (I) wellbore storage, (II) first transition stage, (III) cave storage, (IV) second transition stage, (V) interporosity flow, and (VI) radial flow. The findings indicate that a concave is added, and the wellbore storage occurs earlier due to the existence of cave. Then, the influences of key parameters are studied. The pressure propagation coefficient and cave volume factor influence the stages I, II, III, and IV. When pressure propagation coefficient increases, the wellbore storage becomes larger and cave storage becomes smaller. The first concave moves to upper right. When cave volume factor increases, the wellbore storage occurs earlier and the curves move left in stage I. Interporosity flow factor and storage ratio influence the location and depth of the second concave. Finally, a field gas well is interpreted by using the proposed model, which verifies the reliability and correctness of the model. The findings of this study can help to better understand the influence of large-size cave on pressure transient characteristics. In addition, it can help engineers invert the cave volume, which is of great significance for the development in FCCGR.

The inter-porosity transfer is one of the decisive factors for gas development in fractured-porous gas reservoirs. In this article, we establish an analytical solution for the inter-porosity transfer rate and producing degree of matrix. Then, we study the law of inter-porosity transfer based on the solution. Through the Stehfest inversion transform, the typical curves of inter-porosity transfer rate and producing degree of matrix are plotted. It is found that the producing degree of matrix is close to zero in the initial period. Then, the inter-porosity transfer rate begins to increase, and the producing degree of matrix becomes larger. In the late period, the producing degree of matrix remains constant. In addition, the differences between the quasi-steady state model and the three kinds of unsteady state models are compared. It is found that the inter-porosity transfer occurs earlier in unsteady models. However, when pressure propagates to the external boundary, the transfer rate is equal between quasi-steady and unsteady models. It is also found that the inter-porosity transfer rate is slightly different in the three unsteady models, whereas in the spherical model it is largest at the intermediate period. Next, we discuss the influence of key parameters. The results reveal that gas reservoir radius, storage ratio, and inter-porosity flow factor can play an essential role in inter-porosity transfer. The findings of this study can improve our understanding of gas flow between fractures and matrix. Besides, it helps field engineers better understand the variation law of gas productivity in fractured-porous gas reservoirs, which can provide the scientific basis for making a development scheme.