Abstract
It is well established within the Industry that injection of (produced) water almost always takes place under fracturing conditions. Particularly when large volumes of very contaminated water are injected-either for voidage replacement or disposal-large fractures may be induced over time.
Unfortunately, not much work has been carried out to date to provide methodologies for predicting and measurement of the size of waterflood-induced fractures. This contrasts to the vast amount of work that has been done for stimulation (e.g. propped) fractures.
Injection Fall-Off (IFO) test analysis offers a cheap way to infer the dimensions of induced fractures from welltests. This paper presents a new methodology for IFO test analysis of fractured waterflood wells. This methodology derives the dimensions of induced fractures, and the extent to which these are contained to the target injection layer. Furthermore, the paper focuses on the application of this methodology to a waterflood offshore Sakhalin in the Russian far East.
The methodology is based on an exact solution to the fully transient elliptical fluid flow equation around a closing fracture with changing conductivity, face skin, and multiple reservoir mobility zones. It also captures the case that during closure the fracture generally shrinks from adjacent geological layers. It is demonstrated that the analyses based on the storage and linear flow regimes can be integrated into one analysis in order to reduce error bounds.
The method is applied to a number of examples in a water-flood offshore Sakhalin. Here, start-up of injection wells was accompanied by regular IFO testing in order to monitor fracture growth over time. The interpreted fracture dimensions were compared with predicted dimensions using a recently developed in-house waterflood fracture simulator. The fracture lengths as interpreted from IFO test analysis appeared to be systematically lower than the predicted ones, and a number of explanations for this difference are presented in the paper.
