Abstract
We report the results from a series of chalk flow-through-compaction experiments performed at three effective stresses (0.5 MPa, 3.5 MPa and 12.3 MPa) and two temperatures (92℃ and and 130℃). 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 MgCl2 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 (approx. 2 days), and a creep phase that lasts for more than 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 of the core. The observed effluent concentration shows a reduction in Mg2+, while the Ca2+ concentration is increased. This, together with SEM-EDS analysis, indicates that magnesium-bearing mineral phases are precipitated leading to dissolution of calcite, an observation . 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. Typically. H, higher stress and temperature lead to increased concentration differences of Mg2+ and Ca2+ concentration changes.. The observed strain can be partitioned additively into a mechanical and chemical driven component.
Highlights
The study of how the physicochemical interplay between fluids and rocks alters the mechanical behavior of porous materials has enhanced the understanding of long-term creep behavior of crustal rocks (e.g., [1,2,3]), and has found industrial applications in e.g., CO2 sequestration, ore deposits, hydrology, pharmaceutical industries [4] as well as hydrocarbon migration, petroleum production, and reservoir engineering.Stress and Temperature Affect Chalk DeformationThe mechanical integrity of reservoir chalks during seawater injection has been of significant interest to the scientific and industrial communities since the seabed subsidence and reservoir deformation was discovered in the Ekofisk field in the 1980s [5]
At Ekofisk, over-pressure depletion led to reservoir compaction which induced overburden deformation and seafloor subsidence which led to detrimental effects on the production equipment and to the platforms resting on the seafloor
Seawater injection has until now been a great financial success because, in addition to maintain a pressure gradient and to reduce the effective stress, the seawater imbibes from fractures into water-wet chalk matrix where it displaces the oil, an effect leading to increased oil production [11, 12]
Summary
The study of how the physicochemical interplay between fluids and rocks alters the mechanical behavior of porous materials has enhanced the understanding of long-term creep behavior of crustal rocks (e.g., [1,2,3]), and has found industrial applications in e.g., CO2 sequestration, ore deposits, hydrology, pharmaceutical industries [4] as well as hydrocarbon migration, petroleum production, and reservoir engineering.Stress and Temperature Affect Chalk DeformationThe mechanical integrity of reservoir chalks during seawater injection has been of significant interest to the scientific and industrial communities since the seabed subsidence and reservoir deformation was discovered in the Ekofisk (chalk) field in the 1980s [5]. At Ekofisk, over-pressure depletion led to reservoir compaction which induced overburden deformation and seafloor subsidence which led to detrimental effects on the production equipment and to the platforms resting on the seafloor. In the secondary production phase, recognized by seawater injection, pressure support was provided to maintain the pressure gradient through the oil-field and to reduce the effective stresses and corresponding compaction to pre-production times. Seawater injection has until now been a great financial success because, in addition to maintain a pressure gradient and to reduce the effective stress, the seawater imbibes from fractures into water-wet chalk matrix where it displaces the oil, an effect leading to increased oil production [11, 12]. At Ekofisk, for example, since the seawater injection started in the mid-1980s it took several years before the reservoir pressure and stress conditions were increased to pre-production level. The prevailing compaction and seafloor subsidence after repressurization indicate that stress and pressure effects drive deformation, and the chemical nature of the pore fluid
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