Semi-analytical modeling for water injection well in tight reservoir considering the variation of waterflood - Induced fracture properties – Case studies in Changqing Oilfield, China

Abstract

It is well acknowledged that water injection may induce formation fracturing in tight reservoir, and the injection well may experience shrinking fracture length (SFL) and variable fracture conductivity (VFC) during fracture-falloff period, which shows the bi-storage behavior. However, available pressure-transient analysis (PTA) models hardly consider these effects (SFL, VFC and bi-storage behavior) on pressure-transient response, which can lead to erroneous results. This paper aims at presenting an innovative approach to model the water injection wells considering the dynamic characteristics of waterflood-induced fracture (WIF).We first modeled fluid flow between reservoir and the fracture which characterized by fracture-storage coefficient and fracture-face skin factor, and flow between wellbore and the fracture which represented by wellbore-storage coefficient and choked-fracture skin factor. In addition, the effects of SFL and VFC were included by finite difference method. A semi-analytical solution was then derived to model injection well with waterflood-induced fracture (IWWIF). Finally, this methodology has been successfully applied in Changqing oilfield, China.There are six flow regimes for the IWWIF model in the tight reservoir, including wellbore-storage regime, transitional-flow regime, fracture-storage regime, the second transitional regime, linear flow regime and bilinear flow regime. Compared to current models, the bi-storage phenomenon is discovered and validated as two straight lines with unit slope in the pressure derivate curve of type plot. The WIF that closely links with the wellbore can be regarded as an “expanding wellbore”, which illustrates the reason that the “wellbore-storage coefficients” are increased by up to several orders of magnitude in a great many of water injection wells. The VFC behavior could cause the upward of pressure-derivative curve, which is generally recognized as the characteristic of closed boundary condition while the variable fracture-storage effect may occur because of SFL. In addition, the relation between matched fracture-storage coefficient and fracture length is given, which provides us another way to obtain the probable length of WIF. Finally, this new model has been successfully used to evaluate well performance and reservoir characteristics based on bottom-hole pressure (BHP) history of five field cases in Changqing oilfield, China.