Shear rate determination from pore-scale flow fields

Aqueous solutions with polymer additives often used to improve the macroscopic sweep efficiency in oil recovery typically exhibit non-Newtonian rheology. In order to predict the Darcy-scale effective viscosity eff required for practical applications often, semi-empirical correlations such as the Cannella or Blake–Kozeny correlation are employed. These correlations employ an empirical constant (“C-factor”) that varies over three orders of magnitude with explicit dependency on porosity, permeability, fluid rheology and other parameters. The exact reasons for this dependency are not very well understood. The semi-empirical correlations are derived under the assumption that the porous media can be approximated by a capillary bundle for which exact analytical solutions exist. The effective viscosity eff( Darcy) as a function of flow velocity is then approximated by a cross-sectional average of the local flow field resulting in a linear relationship between shear rate and flow velocity. Only with such a linear relationship, the effective viscosity can be expressed as a function of an average flow rate instead of an average shear rate. The local flow field, however, does in general not exhibit such a linear relationship. Particularly for capillary tubes, the velocity is maximum at the center, while the shear rate is maximum at the tube wall indicating that shear rate and flow velocity are rather anti-correlated. The local flow field for a sphere pack is somewhat more compatible with a linear relationship. However, as hydrodynamic flow simulations (using Newtonian fluids for simplicity) performed directly on pore-scale resolved digital images suggest, flow fields for sandstone rock fall between the two limiting cases of capillary tubes and sphere packs and do in general not exhibit a linear relationship between shear rate and flow velocity. This indicates that some of the shortcomings of the semi-empirical correlations originate from the approximation of the shear rate by a linear relationship with the flow velocity which is not very well compatible with flow fields from direct hydrodynamic calculations. The study also indicates that flow fields in 3D rock are not very well represented by capillary tubes

Intra-abdominal dilation of artificial bowel sphincter pressure-regulating balloon: a case report

# The Author(s) 2011. This article is published with open access at Springerlink.com Introduction Fecal incontinence is a debilitating condition with considerable impact on quality of life. The artificial bowel sphincter may be used as a treatment option in severe cases in which no less invasive form of therapy is sufficient

Influence of divalent ion concentration on the adhesion behaviour of sulfonate self-assembled monolayers (SAM)

Surfactant-assisted oil recovery is usually employed for the production of residual oil after primary and secondary recovery techniques have been exhausted. The loss of injected surfactants via adsorption on to the porous media surfaces impede the efficiency of the procedure and greatly impact process economics. To address this issue, the choice of surfactant and aqueous fluid composition are varied. In particular, the divalent ion concentration is a property that potentially influences surfactant adsorption significantly. It is this aspect which has been addressed in this study. In this study, atomic force microscope (AFM). tips were functionalised with a self-assembled monolayer (SAM) of the alkanethiol, sodium 11-mercaptoundecanesulfonate (otherwise known as MUS) with the –SO3-headgroups oriented outwards towards the aqueous solutions, this is a model for the well-studied surfactant, sodium dodecyl sulfate, SDS. The adhesion forces between these “surfactant-functionalised” tips and Bandera Brown sandstone surfaces were measured in brine solutions of varying CaCl2 concentrations. Observations deviate from previous studies where adsorption of surfactant headgroups in the bulk solution is enhanced via Ca2+ bridging with the surface. Rather, measured adhesion forces decreased with increase in CaCl2 concentration. The results are reversible and also occurs in a reducing environment when the iron in the Bandera brown is reduced for iron (III) to iron (II). The observed behaviour is interpreted in terms of Ca2+ preferentially bridging between neighbouring headgroups on the tip due to the dense packing of molecules non-representative of respective bulk behaviours and acting as a barrier to adhesive contact. Thus, this supposed micellization at the tip aided by the formation of Ca2+ salt bridges between alternate headgroups more closely mimics a surfactant micelle, rather than dispersed individual monomers. The findings of this study provide insight into nanoscale anionic surfactant interactions in general and within sandstone reservoirs in particular where surfactant adsorption is driven largely by monomers rather than micelles. Investigating surfactant adsorption using microscopic techniques such as AFM allow for the determination of interactions at mineral-fluid interfaces and constitute an effective screening methodology for ascertaining enhanced oil recovery process feasibility

Baseline factors predictive of patient satisfaction with sacral neuromodulation for idiopathic fecal incontinence.

Sacral neuromodulation (SNM) is an established treatment for fecal incontinence (FI). A recent study from our group found that the relationship between patient satisfaction and clinical outcome is complex and does not match the traditional used success criteria. Therefore, the ability to predict patient satisfaction must be given priority. The aim of the present study is to identify baseline factors predictive of patient satisfaction, with SNM, for idiopathic FI. We analyzed data from patients treated with SNM for idiopathic FI in Aarhus, Denmark, and Maastricht, The Netherlands. A questionnaire considering self-reported satisfaction was mailed to these patients and compared to baseline characteristics. Logistic regression was used to determine the predictive value of baseline demographic and diagnostic variables. In total, 131 patients were included in the analysis. Patient satisfaction with the current treatment result was reported in 75 patients. Fifty-six patients were dissatisfied with SNM treatment, after median 46 months (range 11-122) with permanent implantation. Pudendal nerve terminal motor latency (PNTML) was the solely identified predictor for long-term patient satisfaction. A subgroup univariate-logistic regression analysis showed that PNTML a parts per thousand currency sign2.3 ms at the side of lead implantation was a statistically significant predictor for patient satisfaction (odds ratio (OR) 2.3, 95 % confidence interval (CI) 1.01-5.24, p = 0.048). Baseline PNTML measurement may be predictive of long-term satisfaction with SNM therapy for idiopathic FI. Further studies are needed to confirm this result

Atomic Force Microscopy (AFM) study of redox conditions in sandstones: Impact on wettability modification and mineral morphology

Laboratory core flood experiments performed to establish chemical enhanced oil recovery (cEOR) procedures often make use of rock samples that deviate from prevailing conditions within the reservoir. These samples have usually been preserved in an uncontrolled oxidising environment in contrast to reducing reservoir conditions, a discrepancy that affects rock wettability and thus oil recovery. The use of a reducing fluid is a predominant method, particularly regarding iron-bearing minerals, for restoring these samples to representative redox states. In this study, the adhesion of polar (NH2 and COOH) and non-polar (CH3) crude oil components to the pore surfaces of Bandera Brown, an outcrop of similar mineralogy to reservoir sandstones, was investigated using Atomic Force Microscopy to determine the potential of a reducing fluid of Sodium Dithionite in seawater to alter surface wettability. This novel workflow for the observation of redox condition effects illuminates the nanoscopic interaction forces at the rock/fluid interface responsible this phenomenon. The results obtained show that adhesion forces between the oil components and the Bandera Brown surface after treatment with the reducing fluid decreased in the order: NH2 (∼70 %) >COOH (∼36 %) >CH3 (∼3 %), due to diminishing affinity of the surface for the polar functional groups when the oxidation state of iron was altered from iron III to iron II. The morphology of Bandera Brown is noted to be affected as well with some dissolution of the mineral composition within cemented pores observed. The results demonstrate that redox state is indeed important for the assessment of wetting properties of surfaces as measurements performed in oxidising environments may not be representative of reservoir reducing conditions. Also, complete reduction of iron oxides on the mineral surfaces seems unlikely without altering the prevailing pore structure. These findings have relevance not only in EOR cases but can find applicability in a much wider context including remediation studies, in particular when iron is present