Ezio Mesini

Wettability effects on oil‐water‐configurations in porous media: A nuclear magnetic resonance relaxation study

1H spin‐lattice relaxation and other physical properties are compared to study the oil‐water‐surface interplay in suitably chosen and prepared natural porous media of different wettability. Whereas the water‐oil arrangement is well established for both strongly water wet and strongly oil wet porous media, the problem is still open for intermediate wettability surfaces. Relaxation behavior reflects the pore space geometry and shows wettability effects on sweeping efficiency and on immiscible liquid arrangement in well‐defined equilibrium situations. A simple picture at the pore‐length scale is verified for strongly water wet samples, whereas a more complex picture arises for intermediate wettability samples.

Chemical Equilibrium Models: Their Use in Simulating the Injection of Incompatible Waters

One of the problems encountered in waterflooding projects is scale formation caused by chemical incompatibility between potential injection waters and reservoir brine. Chemical compatibility evaluation through laboratory experiments on cores at reservoir conditions is of limited value because only first-contact phenomena are reproduced. A numerical model is presented that couples a reservoir-fluid-flow/thermal-equilibrium simulator with a chemical-equilibrium computer code. This model, AGIPS, enables us to calculate the evolution in time of the amount of scale formed at any point in the reservoir and inside the wells when changes occur in the temperature of the injected water and when the injection water mixes with reservoir brine. Moreover, the model calculates temperature and pressure profiles in the reservoir, together with their evolution in time, taking into account the permeability reduction caused by scale formation. Results are presented for the chemical-equilibrium code validation by matching experimental data on scale formation in mixtures of incompatible waters. An example is also given of the use of AGIPS in simulating a five-spot waterflood where incompatible water is injected.

Problems in identifying multimodal distributions of relaxation times for NMR in porous media

Abstract NMR data for some water-saturated sandstones show distributions of relaxation times covering ranges of a thousand or more. Pore size distributions have been associated with distributions of relaxation times and can also cover wide ranges. Several dozen good sets of NMR relaxation data for water in porous sandstones have been analyzed in terms of continuous distributions of exponential components. About a dozen of these are for sandstones having significant relaxation time components over ranges of factors of a thousand. In all cases adequately good fits to the data could be obtained with distributions of relaxation times that were monomodal when plotted as functions of log-time (or log-rate). Thus, it appears that bimodality (or multimodality) for the logarithmic plots is not demanded by these particular sets of data, although these distributions plotted linearly are not monomodal. On the other hand, many multimodal distributions can always be found giving adequate fits to the data, since excessively sharp detail is not resolvable. Many programs using regularization methods to prevent excessive detail in computed distributions tend to give undershoot at sides of peaks, and noise tends to give not-quite-periodic oscillations. Lack of adequate range and density of either data points or computed points can lead to multimodal computed solutions. Some resolution expressions are used to indicate what level of detail in a computed distribution is meaningful for a given data set.

Diffusion-weighted spatial information from1H relaxation in restricted geometries

SummaryNMR relaxation of water1H confined in restricted geometries, whatever is the nature of the system (porous media saturated by water as well as biological tissues), exhibits common characteristics. Artificial microporous media saturated by water have been chosen as model systems to study the longitudinal and transverse relaxation of1H magnetization of water molecules diffusing in restricted geometries. These systems are very stable, easy to prepare, with well-characterized pore size distribution and connections, and with highly homogeneous surface properties. The response was compared with that from more complex natural porous media. Scanning Electron Microscopy techniques demonstrated spatial characteristics and surface properties of the samples. The information content of longitudinal relaxation curves associated with spatial structure and due to restricted diffusion is shown in these samples. The effect on transverse relaxation of self-diffusion in the presence of spatially varying magnetic fields due to susceptibility differences is shown. A simple linear relationship has been found in all samples between the transverse relaxation rate and the interpulse delay in CPMG experiments, in spite of the variety of pore shapes and sizes. In general, one can say that relaxation curves beardiffusion-weighted information on the pore space framework. The role of the investigated relaxation mechanisms is important also in the response of biological tissues, including in the presence of MR Imaging contrast agents inducing microscopic magnetic-field gradients.

A proton relaxation study of immiscible liquid arrangement in microporous structures

Abstract The relaxation behaviour of immiscible hydrogenated liquids in well characterized microporous porcelain structures after a well defined stepwise preparation was studied to obtain information on the contact with surfaces and on the reciprocal arrangement in the pore space. Water-wet samples were fully saturated by brine, then brought to irreducible water saturation by oil and finally to residual oil saturation by brine. This step-wise preparation was repeated using deuterium oxide instead of brine. The observed changes of the 1 H spin-lattice relaxation curves reflected each step of preparation. The quantitative analysis of the curves gave evidence that no contact occurred between surfaces and oil because of a layer of water on the surface, except when only oil was present. On the contrary, when the wettability properties of the surfaces were changed, our data indicated that a contact between oil and walls occurred. So, the traditional picture of water-oil arrangement at the pore-length scale was verified for water-wet samples, whereas a more complex picture arose for oil-wet samples. While on one hand this study on model systems can help in explaining the information content of relaxation curves in more general conditions and systems, on the other hand it yields useful indications on some solid-liquid and liquid-liquid interaction effects in the displacement processes by water or by oil in microporous geometries.

Specific Surface and Fluid Transport in Sandstones Through NMR Studies

Recent nuclear magnetic resonance (NMR) studies in water-saturated porous media showed that magnetic resonance relaxation of {sup 1}H nuclei is a powerful tool for studying the interplay between geometry and fluid transport. Proper combinations of spin-lattice relaxation lifetime, T{sub 1}, and porosity allow permeability to be predicted. T{sub 1}, as defined here, provides a bridge between structural and transport properties because it can be viewed as a dynamically weighted (by diffusion) version of the specific surface. In this paper, the authors probe this role of T{sub 1} for a suite of clean sandstone samples in which, besides permeability and porosity, specific surface by mercury porosimetry and the formation resistivity factor (FRF) also have been measured. The authors studied the correlations among these properties and found that the ability of T{sub 1} to estimate permeability is a result of its linear dependence on the PV-to-surface ratio, V{sub p}/S.

Magnetic resonance lifetimes as a bridge between transport and structural properties of natural porous media

Abstract Results are reported of an NMR spin-lattice relaxation study on a suite of sandstone cores fully saturated with brine. Relaxation curves of 1 H nuclei of water filling the pores, obtained by Inversion Recovery pulse sequence, were analyzed to extract relaxation lifetime values. These lifetimes have been recently proposed as suitable to characterize the relaxation behavior of fully saturated sandstones as well as able to predict permeability in conjuction with porosity. An improvement in the comprehension of interplay among geometry and transport properties of the pore space can be obtained from the comparison between these lifetimes and conventionally measured properties. The results obtained are in agreement with previous results on sandstones and reproduce very well results on model systems. They led to a functional dependence among permeability ( k ), porosity (φ) and average lifetimes ( T ls ) that allowed a good estimation of k from φ and T ls . The dependence of the error in the permeability estimation in terms of the variations of φ and T ls exponents in the relationship that predicts k was also discussed. The reason of the correlation between k and T ls lies in their common dependence on the surface-to-volume ratio S / V p . In this way, 1 H spin-lattice relaxation lifetimes behave as a bridge between structural and transport properties and may be viewed as a dynamically weighted version of S / V p . The discussion of these results can help in clarifying the informative content of 1 H spin-lattice relaxation curves on a broad spectrum of heterogeneous systems with high surface-to-volume ratio.

A two-phase finite-element program for displacement simulation processes in porous media

Abstract Program documentation is presented for a two-dimensional, two-phase immiscible flow model. The model is based on the Galerkin procedure and refers to an incompressible system. Capillary pressure and gravity effects are considered. Spatial discretization is obtained by linear-triangular finite elements, by which the terms of elemental matrices are expressed in analytical form, whereas a Crank-Nicolson finite-difference scheme is used for the step-by-step integration in time. A simultaneous-solution technique is used to solve for potentials and saturations. A highly efficient Gauss elimination algorithm is used to solve the set of linear algebraic equations which results from the Galerkin method. The program may be used to simulate displacement processes between two immiscible fluids into porous media, particularly to simulate water injection, for two-dimensional areal and sectional geometries or axis-symmetric geometry. The listing of the computer code written in FORTRAN 77 is included along with a test case to show the use of the program.

Magnetic resonance relaxation study of preferential wettability effects on displacement efficiency in chalk samples

Abstract In this paper evidence is given that 1 H spin-lattice relaxation can be successfully employed to study preferential-wettability surface effects on the configuration of water and oil in chalk samples, after displacement of one liquid by the other. The comparison between relaxation behavior and other physical properties when surface wettability is changed, furnishes different pictures of water and oil configurations. Moreover, the narrow distribution of pore sizes in this kind of samples permits one to interpret easily the relaxation hehavior in terms of a possible role played by the pore sizes in the displacement phenomena.

Non-Darcy Flow: Laboratory Measurements in Unconsolidated Porous Media

In recent years, the petroleum industry has shown a renovated interest in Non-Darcy flow, in order to better understand reservoir performances. Non-Darcy flow is typically observed in gas wells when the fluids converging to the wellbore attains the velocity peculiar of turbulent flow. As a consequence, pressure drop around the wellbore cannot be estimated from the classic Darcy equation, where the pressure gradient is a linear function of the flow velocity. In that case, in fact, the use of Darcy equation would lead to inaccurate production performances evaluation. In order to describe correctly this phenomenon, the well-known Forchheimer equation is normally used, where the inertial coefficient β is defined. In gas wells this coefficient is usually determined by the analysis of multi-rate pressure tests performed on site. Unfortunately, such data are not easily available (or not economical) in many cases. So, it is a common practice to use particular theoretical and empirical correlations that can be derived by exploiting experimental values of the inertial coefficient. This paper reports a laboratory study in which the inertial coefficient β can be correlated to the structure of the porous media, and, in particular, of its grain size distribution. Gas flow laboratory experiments have been performed on laboratory models of glass beads and natural sands of different sizes. Moreover, porosity and permeability, together with the Klinkenberg constant, have been determined on natural sand cores with both flat and peaked grain size distribution, and a correlation with the Forchheimer equation has been checked. Forchheimer’s number has been calculated and correlated to the superficial velocity as well. In the light of the above, specific laboratory equipment has been devised in order to rely on a wide range of flow rate under appropriate pressure gradients. Introduction When a fluid is flowing through a porous medium there are different flow regimes that may exist, depending on local fluid velocity through the porous space. These are generally known as Darcy flow, Weak Inertia flow (Pre-Darcy flow, according to Basak), Non-Darcy flow or turbulent flow. As a consequence, pressure losses in the different flow regimes may be described by different equations that are reasonably valid also for describing the flow in fractured porous media. As far as fluid flow in reservoir conditions is concerned, in regions sufficiently far from the wellbore fluid velocity is low and Darcy’s law remains valid. It is possible to reproduce such flow rates in the lab and to study the fluid behavior. On the contrary, in the region near the wellbore the high pressure gradient induces both large condensate saturation and high gas velocity which may lead to significant deviations from Darcy’s law, in both single-phase and two-phase flow. Thus, the near wellbore region plays a key role in the productivity decline of the well. At the present time physically relevant models exist which take into account these Non-Darcy (or inertial) two-phase flow effects. A better description of flowing properties may lead to improved predictions of well performances and its productivity decline. Inertial effects appear to be stronger for a gas phase flowing at high flow rates in a porous medium in the presence of a mobile liquid rather than a non-mobile one, even though this has never been supported by any conclusive experiment and reliable correlation haven’t been yet fully formulated. In recent years, the petroleum industry has shown a significant interest in Non-Darcy flow studies, especially applied to the field of fractured rocks. Hydraulic fracturing treatment is in fact necessary to put on stream low permeability hydrocarbon reservoirs at commercial flow rates. Hydraulic fracturing is the process of injecting high pressure fluid into a well to create tensile stresses in the formation exposed to the fluid pressure. If the stresses become large enough they will break down the formation and fractures are initiated. The fracturing fluid must contain a proppant so that the fractures will remain open beyond the period of pumping and crack propagation. Actually, in this case the productive well capability and the overall reserves recovery estimation can be lowered by the effect of reduction of a propped half-length to a considerably shorter