Application of New Fall-Off Test Interpretation Methodology to Fractured Water Injection Wells Offshore Sakhalin

Abstract

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 waterflood 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. 1. Introduction Injection Fall-Off (IFO) test analysis offers one of the cheapest ways to determine the dimensions of induced fractures. Unfortunately, hardly any work has been carried out to date in order to provide a methodology for interpreting the pressure transient data of fractured water injection wells. This contrasts to the vast amount of work that has been carried out in the area of pressure transient analysis for wells with propped fractures. Both pressure transient tests during hydraulic fracture stimulation (called "minifrac tests") (e.g.1), and pressure transient tests during production after stimulation (that is, build-up tests) (e.g.2–5) have been studied extensively. The theories as developed in Refs.1–5 by now are well-accepted "textbook" methodologies. This paper deals with the subject of pressure fall-off analysis on fractured water injection wells. In this area, the situation is entirely different from the one above, in the sense that until recently 6–7, there existed no practical methodology dedicated to pressure fall-off analysis on fractured water injectors. The very limited interest in fall-off test analysis on fractured water injectors may be well related to the fact that most operators have been traditionally unaware that their water injectors are fractured. Only in recent years, this situation has started to change. Unfortunately, one of the consequences of the lack of a dedicated method of analysis is that fall-off tests on injectors are generally interpreted in the wrong way, even if one realises that they are fractured. Typically, such interpretations lead to wellbore storage coefficients that can be up to orders of magnitude too high, and to fracture lengths only based on analysis of the linear formation flow period (e.g.8). The objective of our study is to fill the gap as described above, i.e. to provide a dedicated interpretation methodology for fall-off tests on fractured water injectors, and illustrate the use of this on a specific set of field examples. In the current paper, we focus on the application of this method to fractured injection wells in a waterflood offshore Sakhalin Island in the Russian far East. In this field, start-up of injection wells was accompanied by regular IFO testing in order to monitor the induced fracture growth over time. This is of importance both from the point of view of well injectivity, and of induced fracture containment to the appropriate injection zone.