Arne Stavland

Delayed gelation in corefloods using Cr(III)-malonate as a crosslinker

Abstract Flow experiments have been conducted using a hydrolysed polyacrylamide (HPAM) or an acrylamide-AMPS copolymer crosslinked with chromium malonate. Retention of the chemicals prior to gelation has been studied at different temperatures and injection rates. The retention of the polymer was independent of the crosslinker concentration. The two polymers gave different results; the acrylamide-AMPS copolymer showed very low retention; the HPAM gave quite high retention values. At low temperatures, malonate with a ratio 1:3 (CrMa 3 ) and 1:4 (CrMa 4 ) protected the chromium against precipitation in Berea cores. At high temperatures, chromium precipitation was observed; retention increased with decreasing injection rate. An excess of malonate has some protecting effects on retention. Reducing the initial pH from 7 to 5 lowered the retention. In quartz sand and Bentheim sandstone the retention of CrMa 3 was very low even at high temperatures; no precipitation was observed before gelation. After a shut-in period, gel formed at pH less than 7 in the sand and the Bentheim cores. However, no gel formed in the Berea cores.

Silicate Gel for In-depth Placement - Gelation Kinetics and Pre-flush Design

Sodium silicate gel has historically been used in the oil industry for near wellbore water shut-off. Relatively recent application of Sodium silicate gel for in-depth water diversion have generated some interest. Its main advantage is that its mobility is water-like before it gels. For in-depth diversion purposes, the gelation kinetics need to be appropriately modeled for better prediction of gel placement. This paper makes a review of different gel kinetics models found in the literature. To our knowledge, the models presented in the literature are fit-for-purpose, i.e. they are based on correlations that are fitted to the lab data. Although they describe the lab data well, it is challenging to use them to predict field scale operations, where there are significant temperature, pH, and salinity gradients throughout the reservoir. In this paper, we present an improved silicate gel model. Our model takes into account two important rate step in the formation of silica gel from a sodium silicate solution: the nucleation rate of monosilisic acid to form critical nucleus of nanosized colloids and an aggregation rate of the nano-colloids to form a pore blocking gel. It is important to allow for nano sized colloids as these are small enough to be transported a significant distance from the well before they aggregate into larger clusters that can block the pores. The model explains well the experimental observations where the gelation time is sensitive to pH, temperature, silicate concentration and brine composition. We also investigate the preflush volume and concentration that is needed to minimize the indirect rock-brine interaction that can alter the designed gelation time. Results from this simulation shows that the Cation Exchange Capacity (CEC), Mineral distribution and Temperature profile are critical design criteria for the preflush volume and concentration.

When Size Matters - Polymer Injectivity in Chalk Matrix

This paper investigates the injectivity of polymers in chalk matrix by experimental methods. Different polymers with variable molecular weight and structure was injected into a chalk core plug while monitoring the differential pressure. Based on the experimental results in this study, we found that it was possible to inject polymers with low molecular weight and low viscosity through chalk matrix core plugs, without causing major plugging of the rock.

Integration of IOR Research Projects through Generic Case Studies

The research project portfolio of The National IOR Centre of Norway includes core scale, mineral-fluid reactions at micron-/nano-scale, pore scale, upscaling and environmental impact, tracer technology, reservoir simulation tools and field scale evaluation and history matching. The complexity of each subtopic and the fact that a multitude of data, scales and disciplines is involved may be an obstacle in proper integration of the research results. For the same reasons, exploiting synergies between the various IOR research projects may be a difficult task. At the same time, a collaborative setup like The National IOR Centre of Norway should enable integrated case studies across scales and disciplines. In this paper, we investigate the relationships between the different IOR research projects within The National IOR Centre of Norway. An important objective of the presented work is to facilitate integration and motivate research that falls between the typical disciplines and projects involved in an IOR case study. To make the relationships between projects more evident, the projects are described in terms of input and output related to testing, measuring, simulating, monitoring, predicting, and optimizing fluid flow in a reservoir. The ultimate goal of the integrated IOR research is to provide a framework for monitoring, evaluating and understanding the effects of an IOR method tested in a field pilot. The presented work links simulation and history matching of fluid flow, geomechanics and geochemical effects to lab measurements, pore scale and core scale modeling, tracer characteristics, production data and 4D seismic. As part of the process, two generic case studies are defined, one for a chalk reservoir and one for a sandstone reservoir. The reservoir characteristics are chosen to be representative for fields on the Norwegian Continental Shelf. Two selected IOR methods are discussed; smart water injection and polymer injection. The paper is a result of a collaborative effort involving researchers from both academia, research institutions and the oil industry.

Associative Polymers as Enhanced Oil Recovery Agents in Oil-wet Formations - A Laboratory Approach

Summary Associative polymers recently tested for their EOR potential in water-wet systems displayed a good potential for reducing residual oil saturation in polymer-flooded cores. In this work, an oil-wet porous medium was used to investigate these observations. A low molecular weight associative polymer was tested as a displacing agent and its ability to increase oil recovery on chemically treated oil-wet Berea cores was evaluated. Linear coreflood experiments were performed using filtered associative polymer solution as the EOR agent at standard pressure and 60°C temperature. Results from the polymer floods conducted at an established waterflood residual oil saturation (Sorw) yielded increased oil recoveries, i.e., reduced residual oil saturations, Sor, in the formation. The observed incremental oil production was a function of the injected associative polymer treatment volume; Sor decreased with increased injected associative polymer volume. It should be noted that at laboratory conditions it is often hard to establish and also distinguish a 100% water-cut; in other words, true residual oil saturation, Sorw, is often difficult to be established during water injection. Oil production profile can be discussed based on fractional flow theory, which defines the true Sorw at 100% water-cut. Whenever the produced water-cut is not precisely 100%, oil saturation in the formation is higher than the true Sorw; polymer injection with an improved mobility ratio compared to the water injection one results in an additional oil production, which could be misinterpreted as a reduction in the residual oil saturation, i.e., enhance oil production. Although this accelerated oil production is an attractive possibility (mobility control), it is not an EOR process. Our results are in agreement with previously reported observations in water-wet media related to the EOR nature of the injected associative polymer as opposed to the traditional mobility control of other, either synthetic or organic, polymers. The same results showed that the polymer mobility reduction is highly affected by the injected polymer velocity at the lower spectrum of velocity values and a correlation for the velocity dependent mobility reduction was developed. Finally, during the injection of the associative polymer, a column of oil-polymer emulsion was formed gradually in the separator which caused some difficulties and introduced uncertainties in the separator’s fluids level readings, and thus eventually in the fluids saturation evaluation. Resistivity data obtained in real time were used to correct for the overestimated values of oil production during polymer injection attributed to the formation of the oil/water emulsion.

A model for non-Newtonian flow in porous media at different flow regimes

Polymeric liquids are of great practical importance for porous media flow as they can be used to improve the sweep of water in the reservoir and therefore improve the recovery of oil. Due to the non-Newtonian behavior of these liquids, they are extremely challenging to model. In this paper, we present a model that is capable of describing the most commonly observed flow regimes in porous media: (i) Newtonian, (ii) Shear thinning, (iii) Shear thickening, and (iv) Mechanical degradation. The novel feature of our model is that the time constants for the shear thinning and shear thickening behavior are related to variations in reservoir properties and conditions, thus making it possible to translate lab results to larger scale without introducing new fitting parameters. Furthermore, we present a way to estimate polymer mechanical degradation in porous media. In our model, the polymer degradation rate is linked to the effective pore radius (using a Kozeny-Carman type equation), shear stress, and polymer molecular weight, Mw. The degradation results in a lower Mw, while the polymer volumetric concentration is unaffected. The model is applied to a series of laboratory core flood experiments conducted with partially hydrolyzed polyacrylamide, HPAM, of different initial Mw ranging from 5 to 20 MDa in seawater, and core permeability varied from 137 to 2019 mD. The flow rate is varied approximately three orders of magnitude and covers the shear thinning, shear thickening, and degradation flow regimes. We show that our model is able to reproduce experimental rate-dependent flow resistance, as well as viscosity of effluent samples. An important aspect supporting the use of the model as a predictive tool is that all the simulations with a given brine have made use of a single set of input parameters to describe the observed shear thickening and degradation behavior. Simulation of a second experimental series using low salinity brine required a separate set of input parameters for the shear thickening and shear degradation. The onset of shear thickening was not affected while shear thickening was reduced and degradation appeared to be slower.

A Model for Non-Newtonian Flow in Porous Media at Different Flow Regimes

The EOR potential of polymer flooding is well documented in the scientific literature. However, it has remained a challenge to create good simulation tools that can be used for predictive purposes. A main limitation with the current models is the insufficient description of the transition between the different flow regimes that characterize the polymer rheology. Typically, Newtonian behaviour is observed at low shear rates, followed by shear-thinning, shear-thickening and shear-degradation regimes at increasing shear rates. Furthermore this is complicated by the fact that the apparent viscosity of the polymer is influenced by a combination of factors, such as adsorption, brine salinity, polymer concentration and molecular weight. In this work we present a core scale simulation model that is capable of describing all the aforementioned flow regimes. The novel feature of the proposed model is the inclusion of an equation to describe polymer (mechanical) degradation. The polymer degradation rate is linked to the effective pore radius (via permeability through a Kozeny-Carman type equation), wall shear stress, and polymer molecular weight, Mw. The degradation results in a lower Mw, while the polymer volumetric concentration is unaffected. The change in Mw over a time step is found using an implicit chord method at the end of each transport time step, and the solution is then used to update the effective polymer properties. The main flow field is computed using a standard sequential algorithm, where a linear pressure equation is solved first, followed by an implicit saturation equation formulated in a fractional flow approach. The model is applied to a series of laboratory experiments. Our model explains the core data very well, taking into account that several experimental factors have been varied such as synthetic polymer types, core length and permeability.