Merete Madland - Academia.edu (original) (raw)

Papers by Merete Madland

Eurock 2006: Multiphysics Coupling and Long Term Behaviour in Rock Mechanics

Eurock 2006: Multiphysics Coupling and Long Term Behaviour in Rock Mechanics, 2006

80th EAGE Conference and Exhibition 2018, 2018

This study focuses on chalk mechanics for oil and water saturated cores. Tests were carried out o... more This study focuses on chalk mechanics for oil and water saturated cores. Tests were carried out on four Kansas chalk cores initially saturated by 1.1 M NaCl-brine. Two cores were wettability altered by a mixture of Heidrun oil - heptane (60/40 volume ratio) and then aged for three weeks at 90°C. The wettability index was estimated from test series of equally treated cores. All four cores were hydrostatically loaded to 1.5 times above yield at 130°C and left to creep. During the first 15 days of creep the fluids in the pores were stagnant. It was followed by flooding of 0.219 M MgCl2-brine at two flow rates (0.01 ml/min and 0.04 ml/min) for different durations. It is observed that during loading with stagnant fluids inside the pores, the stiffness and strength are affected by wettability. The axial creep strain curves for different wettability cores overlap during the stagnant fluid creep period and during flooding of MgCl2-brine at both flow rates. We see that the creep curves, give...

Pressure decline during oil recovery from chalk reservoirs exhibit increase in effective stress a... more Pressure decline during oil recovery from chalk reservoirs exhibit increase in effective stress and this in turn leads to reservoir compaction causing seabed subsidence. Although, there is positive impact of compaction on the oil recovery, the necessity of pressure maintenance brought the introduction of water injection to chalk fields in the southern part of North Sea (eg. Ekofisk and Valhall). Sea water injection has successfully resulted in an increase in petroleum output. However, as sea water displaces oil and increases pore pressure, the reservoir continues to compact. The phenomenon is referred to as the water weakening effect on chalks. Since little was understood about the impact of aqueous chemistry on the mechanical behaviour of chalk reservoirs, there have been several many studies to better understand this phenomenon.

PhD thesis in Petroleum engineering. At this point, only the introduction is available online, no... more PhD thesis in Petroleum engineering. At this point, only the introduction is available online, not the articles.

Journal of Petroleum Science and Engineering, 2021

Seawater injection for Enhanced Oil Recovery (EOR) purposes can increase the hydrocarbon recovery... more Seawater injection for Enhanced Oil Recovery (EOR) purposes can increase the hydrocarbon recovery factor in carbonate reservoirs but are also responsible for weakening their mechanical strength. This study investigates the geomechanical behavior of oil-bearing reservoir chalk subject to reactive flow at reservoir conditions. The obtained results are compared with previous results from experiments on water-saturated outcrop chalks. Two unwashed oil-bearing reservoir chalk cores from the Eldfisk Field in the North Sea are mechanically tested in a triaxial cell. The cores are saturated with NaCl brine prior to testing, in addition to residual oil. The cores' axial and radial deformations were monitored during hydrostatic stress loading and creep under constant 50 MPa stress, at 130 • C. During the test, the cores were flooded with 0.657 M NaCl, 0.219 M MgCl 2 , 0.219 M MgCl 2 + 0.130 M CaCl 2 and synthetic seawater (SSW). The average creep strain increased from 0.03% to 0.04% per day radially and from 0.03% to 0.06% axially after changing from the NaCl brine to SSW brine. Flooding MgCl 2 brine after NaCl increased the average compaction rate from 0.05% to 0.09% per day radially and from 0.04% to 0.07% per day axially. Adding CaCl 2 to the MgCl 2 brine reduced the average compaction rate from 0.05% per day during MgCl 2 injection to 0.02% per day both radially and axially, comparable to reports from outcrop chalk experiments. The brine composition-dependent creep compaction in the core flooding experiments was explained by dissolution of primary calcite, confirmed by Ion Chromatography, and precipitation of secondary Mg-bearing minerals, seen by Scanning Electron Microscopy (SEM) analytics. Generally, the aforementioned results describing geomechanical behaviors of oil-bearing reservoir chalk cores under hydrostatic stress and thermochemical influence in this study are comparable to those from previous studies on outcrop chalk, thus supporting many years of laboratory research as applicable in the reservoir chalk context.

Transport in Porous Media, 2020

Brines containing surface-active divalent ions such as Ca2+, Mg2+ and SO42− impact the stiffness,... more Brines containing surface-active divalent ions such as Ca2+, Mg2+ and SO42− impact the stiffness, strength and time-dependent deformation of water wet Mons outcrop chalk. This study documents how stiffness and strength of wettability-altered oil and water-saturated (mixed wet) Mons chalk compare to water-saturated and water wet samples during hydrostatic loading and creep. During hydrostatic creep, the strain rate response to magnesium chloride (MgCl2) brine injection is compared for water wet and mixed wet samples. For the mixed wet samples, the oil production was estimated during compaction and non-equilibrium MgCl2 flow. The results presented here were then compared to a similar test series on Kansas outcrop chalk. The differences were interpreted in terms of difference in physical parameters such as porosity and pore size. Two Mons samples were wettability-altered and tested in parallel to two completely water wet samples from the same chalk block at hydrostatic conditions and 1...

GEOPHYSICS, 2018

Which forces are responsible for holding highly porous chalks together? We use the effective stre... more Which forces are responsible for holding highly porous chalks together? We use the effective stress to quantify the electrostatic effects around particle contacts originating from the adsorption of ions onto charged mineral surfaces. The induration of chalk indicates that it is held together by contact cement, where planar crystal contacts allow the action of short-ranged adhesive Van der Waals forces. At particle distances exceeding a few nanometers, recent studies have indicated electrostatic repulsion between water-embedded adjacent particles. The magnitude of the repelling force depends, among other parameters, upon temperature and brine composition. Our premise is that by perturbing the electrostatic forces at the particle level, we can control the mechanical behavior of chalk samples tested in triaxial cells. We report the results of an experimental series, investigating how the mechanical strength and stiffness varied among samples saturated with four different brines, tested...

E3S Web of Conferences, 2019

Wettability in chalk has been studied comprehensively to understand fluid flow mechanisms impacti... more Wettability in chalk has been studied comprehensively to understand fluid flow mechanisms impacting coreflooding experiments. Wettability becomes paramount in understanding the parameters influencing chalk-fluid interactions. The main objective of this work is to evaluate as to which degree the wettability in chalk core samples can be controlled in the laboratory. Kansas chalk samples saturated with brine (1.1 M/64284 ppm NaCl) and an oil mixture (60% - 40% by volume of Heidrun oil and heptane) were aged at a constant temperature of 90oC with aging time as the laboratory control variable. A multimodal method incorporating contact angle measurements, wettability index via USBM test, and SEM-MLA analysis was applied in evaluating wettability. A systematic approach was applied with the three different methods to quantify the degree of uncertainty linked to a) wettability estimation and b) the aging procedure to control wettability alteration of Kansas chalk. With a comprehensive suite ...

Transport in Porous Media, 2019

The injection of seawater-like brines alters stiffness, strength and time-dependent deformation r... more The injection of seawater-like brines alters stiffness, strength and time-dependent deformation rates for water-saturated chalks. This study deals with the mechanical effects and oil production upon brine injection through wettability-altered samples. The results from two test programs are presented: (a) 'Wettability determination program' and (b) 'triaxial test program'. Kansas chalk samples were saturated by a mixture of oil and water and aged over time at 90 °C. The wettability index of the altered samples was estimated using chromatographic separation tests by co-injecting sulphate ions that adsorb on the water-wet mineral surfaces and non-affine tracer. A good repeatability was observed. In the triaxial test program, unaged water-wet and aged mixed-wet samples were hydrostatically loaded to 1.5 times yield stress so stiffness and strength could be determined. The samples were kept at the same stress level over time to monitor the volumetric creep. After a stagnant flow period of 15 days, MgCl 2 brine and seawater were flushed through the samples so the oil production and ion concentration of the effluent water could be obtained. The combined observations of the bulk volume, oil volume and estimated solid volume (from effluent analyses) enabled us to calculate pore volume and thereby oil saturation with time. The mixed-wet samples were found to be stiffer and stronger than the water-wet samples, and when the stress was kept at 1.5 times yield the creep curves overlapped. During the flow-through period, the changes in ion composition are insensitive to the presence of oil, and ongoing water weakening for mixed-wet samples is the same as in the water-wet samples. Further, we found that oil was only produced during the first 2-3 pore volumes (PVs) injected. Afterwards, no oil was produced even though the chemical reactions took place and pore volume reduced.

IOR 2017 - 19th European Symposium on Improved Oil Recovery, 2017

This paper deals with exploring elastic (bulk modulus and Young’s modulus) and plastic parameters... more This paper deals with exploring elastic (bulk modulus and Young’s modulus) and plastic parameters (yield stress, creep and rebound) during deviatoric loading and time-dependent deformation. A series of experiments were carried out at Ekofisk reservoir temperature (130°C) to study the effect of four different fluids, viz., distilled water (DW), NaCl-brine, MgCl2-brine and seawater (SSW), on Mons outcrop chalk. The cores were deviatorically loaded and left to creep at a constant value of 69-73% of the axial yield stress obtained from reference tests with the same brine. Variations in the bulk modulus and Young’s modulus were observed as function of saturation fluid, although the significance of these observations require more data. SSW had the lowest yield stress followed by NaCl and MgCl2, and highest for DW, which conforms the results from earlier studies. The final creep strain was highest for SSW and was 1.3-1.5 times higher than for other brines. The core initially saturated by SSW showed the highest plastic component of the total strain inferring that the ions in SSW does play an important role in inducing permanent damage.

Geological Society, London, Special Publications, 2016

The mechanical strength, porosity and permeability of chalk are affected by chemical and mineralo... more The mechanical strength, porosity and permeability of chalk are affected by chemical and mineralogical changes induced by fluids that are chemically out of equilibrium with the host rock. Here, two high-porosity Upper Cretaceous chalk cores from Liège were tested at effective stresses beyond yield at 130°C during flooding with MgCl2and NaCl brines. Core L1 (flooded by MgCl2brine) deformed more than L2 (flooded with NaCl brine), with volumetric strains of 9.4% and 5.1%, respectively. The porosity losses estimated from strain measurements alone are 5.82% for L1 and 3.01% for L2. However, this approach does not account for dissolution and precipitation reactions. Porosity calculations that are based on strain measurements in combination with (i) the weight difference between saturated and dry cores and (ii) the solid density measurement before and after flooding show an average porosity reduction of 3.69% between the two methods for L1. This discrepancy was not observed for core L2 (wi...

Frontiers in Physics, 2016

We report the results from a series of chalk flow-through-compaction experiments performed at thr... more We report the results from a series of chalk flow-through-compaction experiments performed at three effective stresses (0.5, 3.5, and 12.3 MPa) and two temperatures (92 and 130 • C). The results show that both stress and temperature are important to both chemical alteration and mechanical deformation. The experiments were conducted on cores drilled from the same block of outcrop chalks from the Obourg quarry within the Saint Vast formation (Mons, Belgium). The pore pressure was kept at 0.7 MPa for all experiments with a continuous flow of 0.219 M MgCl 2 brine at a constant flow rate; 1 original pore volume (PV) per day. The experiments have been performed in tri-axial cells with independent control of the external stress (hydraulic pressure in the confining oil), pore pressure, temperature, and the injected flow rate. Each experiment consists of two phases; a loading phase where stress-strain dependencies are investigated (approximately 2 days), and a creep phase that lasts for 150-160 days. During creep, the axial deformation was logged, and the effluent samples were collected for ion chromatography analyses. Any difference between the injected and produced water chemistry gives insight into the rock-fluid interactions that occur during flow through the core. The observed effluent concentration shows a reduction in Mg 2+ , while the Ca 2+ concentration is increased. This, together with SEM-EDS analysis, indicates that magnesium-bearing mineral phases are precipitated leading to dissolution of calcite. This is in-line with other flow-through experiments reported earlier. The observed dissolution and precipitation are sensitive to the effective stress and test temperature. Higher stress and temperature lead to increased Mg 2+ and Ca 2+ concentration changes. The observed strain can be partitioned additively into a mechanical and chemical driven component.

Journal of Geophysical Research: Solid Earth, 2015

We report the complete chemical alteration of a Liège outcrop chalk core resulting from a 1072 fl... more We report the complete chemical alteration of a Liège outcrop chalk core resulting from a 1072 flow-through experiment performed during mechanical compaction at 130°C. Chemical rock-fluid interactions alter the volumetric strain, porosity, and permeability in a nontrivial way. The porosity reduced only from 41.32% to 40.14%, even though the plug compacted more than 25%. We present a novel analysis of the experimental data, which demonstrates that the geochemical alteration does not conserve the volume of the solids, and therefore, the strain is partitioned additively into a pore volume and solid volume component. At stresses beyond yield, the observed deformation can be explained by grain reorganization reducing the pore space between grains and solid volume changes from the rock-fluid interactions. The mechanical and chemical effects are discussed in relation to the observed permeability development. NERMOEN ET AL.

SEG Technical Program Expanded Abstracts 2015, 2015

We propose to extend the effective stress relation to incorporate the electrostatic effects at pa... more We propose to extend the effective stress relation to incorporate the electrostatic effects at particle contacts originating from the mineral surface charges. Where contact cement already keeps minerals together, electrostatic repulsion in the area outside the contact cement will decrease the measured stiffness of the rock. The surface charge induces an electrical double layer, and where double layers from two mineral particles overlap, a disjoining pressure arises in granular contacts. When brines and waters are introduced into the porous rock, this can lead to a reduction in the observed rock mechanical stiffness, i.e. to an apparent water weakening effect. Now, since the disjoining pressure is a stress-pressure effect, we choose in this paper to modify the effective stress relation (left hand side of equation 4) rather than modeling water weakening behavior by changing the rock frame elastic bulk modulus (right hand side of equation 4). We quantify the magnitude of the electrostatic stress contribution by comparing data from loading experiments with different brine chemistry to loading data for oil-filled cores, where electrostatic forces are minimized. We find that for highly porous chalks, the electrostatic stress in the proposed extended stress relation may dominate the observed stress strain behavior.

Injection of CO2 into carbonate oil reservoirs has been proposed as a possible utilization of the... more Injection of CO2 into carbonate oil reservoirs has been proposed as a possible utilization of the captured CO2 due to its capability to enhance the oil recovery. In offshore reservoirs such as Ekofisk and Valhall it has been discussed to alternate the CO2 and sea water injection (WAG) to reduce the cost and keep the beneficial effects of both the sea water (SSW) and the gas injection. Water and CO2 mix to form carbonic acids that enhance the solubility of carbonates into the pore fluid and thereby change the wettability of the chalk surface. Thus a serious concern has been raised upon the potential de-stabilization of the reservoirs during CO2 injection. In the presented work we focus on how carbonated sea water alters the mechanical integrity of carbonate rocks both to evaluate the safety of carbon storage sites and in the planning of production strategies in producing fields since enhanced compaction may have both detrimental and beneficial effects. Here we will present the result...