Classical trajectory calculations, using an ab initio Ag–O potential, are presented of O2 scattering from Ag(111) in a broad energy range. These calculations can explain the results of various O2/Ag(111) scattering experiments performed at different energies. For O2 scattering around 1 eV, the experimentally observed angular broadening in the scattered signal can be explained as an onset of surface rainbows. The degree of dissociation observed in scattering around 200 eV (incoming angle with respect to surface normal (θi=70°) and around 2 keV θi=85°) can be explained by a collision-induced dissociation. Since O2/Ag(111) is a very corrugated system, we also studied the effect of the parallel velocity on the scattering. With constant normal velocity, the ‘‘largest corrugation’’ (most energy loss and dissociation) occurs for θi between 30° and 60°. For grazing incidence (θi≊85°), the scattering shows the characteristics of scattering from a flat surface: energy losses only decrease the normal component of the incident velocity and energy losses and fractions of dissociated molecules are independent of the parallel component. However, the molecule gets more rotationally excited than in the case of scattering from a flat surface with the same normal incident velocity. Finally, we investigated the relative amounts of vibrational and rotational excitation leading to dissociation. For a flat surface, the dissociation turns out to be mostly due to vibrational excitation, but for a corrugated surface, also rotational excitation largely contributes to dissociation.

This paper was selected for presentation by an SPE Program Committee following rewew of mforma!lon contained m an abstract sutmvtted by the author(s). Contents of the paper, aspresented, have not~Bn reviewed by the .%eiety of Petroleum Engineers and are subjecf 10 correction by the author(s). The material, as presented does not necassarify reflect any posifion of the Scmety of Petroleum Engineers, its officers, or members Papers, presented at Petroleum &gineers ?%sswn !0 copy IS restricted 10 an abstract of "ot mom than 300 SPE meetln s are su c to publication rev!ew by Editorial Committees of the %ciely of words Illustrations may not be copied The abstract should contain conspicuous '1? ackncwle en?ant of where and by wfwm the paper IS presen{ed Write Librarian, SPE, P O Sox 8338 6, Richardson, TX 75063-3636, U.S.A fax 01-214.952.9435Abstract This paper discusses a number of aspects in relation to simulating unfiltered produced water re-injection (PWRI) under fracturing conditions.A numerical model has been developed that fully couples the reservoir engineering and tlacture mechanics aspects of the problem, and includes features such as finite, non-uniform fracture conductivity, fracture growth, filtercake build-up on the fracture face, formation impairment around the fracture, and backstresses resulting from pore pressure inflation and formation cooling.An essential difference with simulation of conventional waterflood fracturing is that owing to fracture fill-up with injected solids the fracture conductivity cannot be assumed infinite any more. This relates to the important PWRI issue of where the injected solids go. Using our model, we show that the pressure drop over a finite conductivity fracture can lead to a significant increase in fracture volume without necessarily leading to a significantly higher injection pressure. Thus, a picture emerges in which the fracture conductivity 'adjusts' itself in order to accommodate injected solids. This picture allows the computation of well injectivity as a function of total injected water volume, solids loading, etc. This concept can also be used to qualitatively explain the PWRI field observation that injectivity appears to be partially or fully reversible as a function of water quality, A field example from the Middle East is presented. The effect of parameters such as water quality, formation stiffness, and filtercake permeability on well injectivity and fracture size is discussed. It is shown that for most practical applications, an approximate analytical formulation for the computation of well injectivity and fracture size provides good results.

We conducted fracture propagation experiments on blocks of cement paste, cement paste with sand, and a tight outcrop sandstone rock. A novel acoustic monitoring system was developed for measuring the fracture profile and radius during the tests.Results of laboratory tests on cement agreed with numerical simulations based on elastic rock deformation and linear elastic fracture mechanics. Tests at confining stresses of 20 and 23 MPa gave higher pressures than predicted.

No 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.