Interference test analysis with two fractured horizontal wells

Abstract

Fracturing horizontal wells is a common technique to produce hydrocarbons from low-permeability reservoirs (milli- to micro-Darcy). Recently, interest in and study of pressure response and performance (analytical and numerical) of fractured horizontal wells have increased significantly. However, despite numerous studies that show the advantages of fractured horizontal wells, understanding of their performance characteristics in low-permeability reservoirs is still limited. One of the most important questions in field applications is the optimum spacing of fractured horizontal wells. The issue of well spacing is closely related to reservoir connectivity and it is customary to run interference tests involving two wells (active and observation wells) to obtain information about the connectivity and characteristics of the reservoir between the two wells. This thesis presents a semi-analytical solution to simulate interference tests that involve two fractured horizontal wells in low-permeability reservoirs. Three mathematical models are used to generate the semi-analytical solution: (1) the finite-conductivity fracture model developed by Cinco-Ley and Meng (1988), (2) the pressure distribution model for an infinite and closed rectangular reservoir developed by Ozkan (1988), and (3) the fractured horizontal well model developed by Raghavan et al. (1997). These three models are combined into a general equation using the superposition theorem and evaluated by a semi-analytical approach. This model is used to generate pressure transient responses (pwD and dpwD/dlntD versus tD) of a pair of fractured horizontal wells and the results are evaluated to understand their interference characteristics in naturally fractured, low-permeability reservoirs. Sensitivities of results to the number of hydraulic fractures, well separation (spacing), and matrix permeability are documented to highlight the general characteristics of interference tests in low-permeability reservoirs. Flow regimes in both active and observation wells are also discussed. The results of this study show that the existence of a fractured horizontal observation well does not influence the responses of the active well for practical well spacing used in the field. The responses of the active well are merely influenced by its own properties, configuration, and reservoir properties. In contrast, the pressure transient responses of the observation well are greatly affected by the configuration of both wells, distance between wells, and reservoir properties. This thesis also provides applications of the semi-analytical model for other cases of practical interest such as interference tests in a closed rectangular reservoir, with open horizontal sections, and when a stand-alone fracture crosses both wells.