Pressure Transient Analysis in Fractured Produced Water Injection Wells

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIt 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 disposallarge fractures may be induced over time. The current paper aims to provide a methodology for Injection Fall-Off (IFO) test analysis of fractured (produced) water injection wells. Some essential elements of IFO for fractured water injectors are: Closing fracture, (early) transient elliptical reservoir fluid flow, finite (changing) fracture conductivity, and fracture face skin. An exact semi-analytical solution is presented to the fully transient elliptical fluid flow equation around a closing fracture with changing finite conductivity, fracture face skin, and multiple mobility zones in the reservoir surrounding the fracture. Based on this solution, type curves of the dimensionless bottom-hole pressure as a function of dimensionless time are provided covering both the period during fracture closure and the period after fracture closure. The shape of these type curves is studied as a function of the different relevant parameters, in particular the fracture closure time, fracture compliance, wellbore storage, fracture conductivity, fracture skin, mobility ratio, and injection period. The type curves are compared in detail with pressure transient (build-up/fall-off) behaviour known from the literature, particularly with the aim to identify the well-known "storage", "linear", "bilinear" and "radial" flow regimes. Furthermore, based on the conceptual results presented in this paper, a number of different methods to determine fracture length from fall-off tests are discussed.Finally, these concepts are applied to two field examples, where the dimensions of the induced fractures are derived from the analysis of the IFO data.

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

“…By requiring that both methods yield the same result (length), a value for the Young's modulus can be estimated 11 . However, as was pointed out in [6][7] , the period prior to fracture closure will generally be characterized by a mixture of storage and (non-)linear formation flow. Therefore, in the analysis both "flow regimes" need to be captured simultaneously and cannot be separated.…”

“…The pressure profile is an exact solution to the transient diffusivity equation around a closing fracture 6 . The dimensionless transient diffusivity equation is given by…”

“…Employing a confocal elliptical coordinate system, The solution to (4) can be expressed as an infinite sum of Mathieu function products, see 6 . The solution as developed in 6 characterises the pressure profile:…”

“…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.…”

“…The paper is organised as follows. Section 2 presents the pressure transient solution for a closing and shrinking water injection fracture, including a brief recap of the main concepts presented in [6][7] , and illustrates the methodology for a few 'typical' field examples from around the world. Section 3 focuses on the details of the fall-off test campaign in the Sakhalin waterflood.…”