Modeling of Multi-Stage Fractured Horizontal Wells

Date
2013-12
Authors
Yao, Shanshan
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Publisher
Faculty of Graduate Studies and Research, University of Regina
Abstract

Horizontal wells stimulated by multiple fractures unlock tight formations and shale gas reservoirs that used to be considered as uneconomic plays. However, the popularity of such techniques presents new challenges to the reservoir and fractures evaluation. Fractured horizontal wells’ pressure and production rate behaviour exhibit complex trends that are quite different from previous horizontal and fractured vertical wells. To facilitate the pressure and rate analysis, this work developed semi-analytical models under different assumptions and comprehensively described the fluid flow of a multi-stage fractured horizontal well in a bounded reservoir. The governing Partial Differential Equations (PDEs) in this work are highly nonlinear, and, therefore, analytical methods are not applicable to obtaining results of drawdown and build-up tests. The semi-analytical modeling method here shows advantages in applicability over the analytical modeling. For fractured horizontal wells with constant fracture conductivities, four kinds of fluid flow, including flow from the reservoir to fractures and to the horizontal wellbore, flow inside the fractures as well as inside the horizontal wellbore were all taken into consideration. Standard type curves for transient pressure analysis were documented. The unique pressure behaviour reveals that multiple fractures play a more important role than the horizontal wellbore in the whole system. Also, the applicability of these type curves in the transient pressure analysis was proved when compared with the method based on typical characteristic lines. The stress-sensitive hydraulic fracture conductivity was the other factor incorporated into the semi-analytical models. A series of type curves were also generated to evaluate the fractures’ stress-dependent characteristics. Stress-dependent conductivities can cause pressure and pressure derivative curves to increase rapidly, which seem to be “apparent boundary-dominated flow”. The influence of these changing conductivities strongly depends on the fractures’ properties (i.e., fracture stress-sensitive characteristic value df and initial fracture conductivity CfDi.). When the bottomhole flowing pressure remains constant, the semi-analytical model can be further used to study the fractured horizontal wells’ production rate behaviour. After the comparison with analytical solutions, it can be concluded that horizontal wells with multi-stage fractures produce as if the fractures worked individually at early times. Moreover, if considering the stress-sensitive fracture conductivities, the straight lines of reciprocal rates vs. time exhibit special slopes deviate from 1/4 and 1/2 for bilinear and linear flow, respectively. In addition to the advantages over analytical methods, the methodology and models presented are flexible and widely applicable as numerical models. Remarkable progress can be achieved if the methodology is extended to solve flow problems in complex fracture systems and dynamic matrix permeability.

Description
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in Petroleum Systems Engineering, University of Regina. xvii, 172 p
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