No abstract

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.

Summary Accurate modeling of an ASP flood requires detailed representation of geochemistry and, if natural acids are present, the saponification process. Geochemistry and saponification affect the propagation of the injected chemicals and the amount of generated natural soaps. These in turn determine the chemical phase behavior and, hence, the effectiveness of the ASP process. In this paper, it is shown that by coupling a multipurpose reservoir simulator (MPRS) with PHREEQC (Parkhurst and Appelo 1999; Charlton and Parkhurst 2008), a robust and flexible tool is developed to model ASP floods. PHREEQC is used as the chemical-reaction engine, which determines the equilibrium state of the chemical processes modeled. The MPRS models the impact of the chemicals on the flow properties, solves the flow equations, and transports the chemicals. The validity of the approach is confirmed by benchmarking the results with the ASP module of the UTCHEM simulator (Delshad et al. 2000). Moreover, ASP corefloods have been matched with the new tool. The functionality of the model also has been tested on a 2D sector model. The advantages of using PHREEQC as the chemical engine include its rich database of chemical species and its flexibility in changing the chemical processes to be modeled. Therefore, the coupling procedure presented in this paper can also be extended to other chemical enhanced-oil-recovery (EOR) methods.

The displacement of a wetting fluid by a nonwetting fluid in a porous medium, i.e., drainage, was studied by means of experiments and simulations. A network model was formulated, capable of describing the fluid saturation of the porous medium and the pressure during the displacement. The results of experiments with several micro flow models are reported, using two fluid systems and varying displacement rates. These measurements allow interpretation in terms of the contributions of the capillary and viscous pressures. The results of the simulations are in agreement with the experiments, with the exception of one case at a high displacement rate and characterized by a high viscosity ratio.

Summary An essential difference between conventional waterflood fracturing and unfiltered produced water reinjection (PWRI) under fracturing conditions is that in the latter case, the fracture conductivity cannot be assumed infinite anymore as a result of fracture fillup with injected solids. This relates to the important PWRI issue of where the injected solids go. Using numerical model calculations, 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 filter cake 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. Introduction Field experience of produced water reinjection (PWRI) under fracturing conditions (for example, Refs. 1-3) has shown that well injectivity depends a.o. on injection water quality and temperature. One key uncertainty in well injectivity relates to the issue of where injected solids go.1 PWRI field experience shows that several fracture volumes of solids can be injected without significant loss of injectivity.1–3 To date, no explanation exists for this observation. This paper discusses several aspects of simulating unfiltered PWRI under fracturing conditions. A numerical model has been developed that fully couples the reservoir engineering and fracture mechanics aspects of the problem, and includes features such as finite, nonuniform fracture conductivity, fracture growth, external filter cake buildup on the fracture face, impairment of the formation around the fracture by deep penetration of oil and solids, and backstresses resulting from pore pressure inflation, and formation cooling. An essential difference with conventional simulators for waterflood-induced fracturing is that owing to fracture fillup with injected solids the fracture conductivity cannot be assumed to be infinite any more. Using our model, we are able to show that the pressure drop over a finite conductivity fracture can lead to "ballooning," i.e., a significant width increase. As a result, the fracture volume can become several times larger. The results also show that fracture ballooning does not necessarily lead to a significant injection pressure increase. 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. A field example from the Middle East is presented, where highly contaminated produced water was successfully injected at rates around 6000 m3/d. The computed results (wellhead injection pressure (WHP) and fracture size as a function of injection rate) appear to be in good agreement with initial measurements. The model has also been used for long-term (20 year) predictions of fracture size and well injectivity. The sensitivity of the results on a number of relevant parameters (injection water temperature and quality, degree of formation impairment, formation Young's modulus) has been studied. It appears that well injectivity strongly depends on formation Young's modulus and external filter cake permeability (skin). Well injectivity also depends on the assumed permeability profile within the fracture. Fracture size strongly depends on external filter cake permeability. Although Young's modulus may be estimated from laboratory measurements, representative values for external filter cake permeability can only be obtained from in-situ injection tests. A simple method is proposed to extrapolate results from water injection trials into the long-term future. Description of Numerical Model gives a short description of the numerical model, and compares the results for infinite conductivity "square" fractures with a recent radial model for water injection above fracturing pressure.4 Field Example: Simulation and Sensitivities presents and discusses results for finite conductivity fractures. These results have been computed as part of a PWRI study for a field in the Middle East. Analytical Model of Fracture With Tip Plug discusses an analytical approximation for fractures with low-permeability tip plugs. Finally, conclusions are presented in the last section. Description of Numerical Model This model is an extension of Koning's model for waterflood-induced fracturing. 5 The fracture is assumed to fully penetrate a permeable layer which is bounded by impermeable layers on top and bottom. The fracture is surrounded by four elliptically shaped zones (see Fig. 1 for a plan view): an impaired zone where oil and/or solids have penetrated (to be discussed below), a cooled (cold water) zone, a zone flooded by injected water which has warmed up, and an oil zone. Note that if produced water is injected into a separate (waterbearing) disposal formation, the "oil zone" becomes a warm water zone. Each zone is characterized by its own temperature, water (oil) viscosities, and relative permeabilities. The extent of the zone boundaries is calculated from the injected volume, and from the heat capacities of the water and formation rock by the method of Ref. 5. The fracture face is covered by an external filter cake consisting of injected oil and solids that have not penetrated into the formation. Eventually, the fracture may be filled with solids (oil) that have not penetrated into the formation, leading to a finite fracture conductivity. Note that the damage zone, external filter cake and finite fracture conductivity were not included in Koning's model.5