Mohammad Hossain Bhuiyan

Laboratory tests were performed on the Draupne shale formation, which may serve as a seal over CO₂ storage sites. Different techniques were used to assess the integrity and mechanical properties of the shale, with the main objective of investigating whether exposure to CO₂ would in any manner alter these properties. The laboratory methods used encompass traditional triaxial tests; however, with fluid substitution prior to increasing axial stress to failure. These tests were conducted on smaller cylindrical plugs than standard, taking advantage of the finer grained nature of the shale. Another set of experiments used the low-frequency technique, whereby small amplitude, cyclic axial strains are applied on the specimen, allowing a direct measurement of stiffness. Long exposure, with change of fluid from brine to CO₂, allowed for quantifying small changes in stiffness, thanks to the many repeated cycles of non-destructive testing. In a final experimental technique, the punch test, shear strength of the same material was obtained by cutting a central disk from a larger intact shale disk, while measuring the shear force needed to perform the cut.

Reservoir formation waters typically contain scaling ions which can precipitate and form mineral deposits. Such mineral deposition can be accelerated electrochemically, whereby the application of potential between two electrodes results in oxygen reduction and water electrolysis. Both processes change the local pH near the electrodes and affect the surface deposition of pH-sensitive minerals. In the context of the plugging and abandonment of wells, electrochemically enhanced deposition could offer a cost-effective alternative to the established methods that rely on setting cement plugs. In this paper, we tested the scale electro-deposition ability of six different formation waters from selected reservoirs along the Norwegian continental shelf using two experimental setups, one containing CO₂ and one without CO₂. As the electrochemical deposition of scaling minerals relies on local pH changes near the cathode, geochemical modelling was performed to predict oversaturation with respect to the different mineral phases at different pH values. In a CO₂-free environment, the formation waters are mainly oversaturated with portlandite at pH > 12. When CO₂ was introduced to the system, the formation waters were oversaturated with calcite. The presence of mineral phases was confirmed by powder X-ray diffraction (XRD) analyses of the mineral deposits obtained in the laboratory experiments. The geochemical-modelling results indicate several oversaturated Mg-bearing minerals (e.g., brucite, dolomite, aragonite) in the formation waters but these, according to XRD results, were absent in the deposits, which is likely due to the significant domination of calcium-scaling ions in the solution. The amount of deposit was found to be proportional to the concentration of calcium present in the formation waters. Formation waters with a high concentration of Ca ions and a high conductivity yielded more precipitate.

Carbon capture and storage (CCS) by geological sequestration comprises a permeable formation (reservoir) for CO₂ storage topped by an impermeable formation (caprock). Time-lapse (4D) seismic is used to map CO₂ movement in the subsurface: CO₂ migration into the caprock might change its properties and thus impact its integrity. Simultaneous forced-oscillation and pulse-transmission measurements are combined to quantify Young’s modulus and Poisson’s ratio as well as P- and S-wave velocity changes in the absence and in the presence of CO₂ at constant seismic and ultrasonic frequencies. This combination is the laboratory proxy to 4D seismic because rock properties are monitored over time. It also improves the understanding of frequency-dependent (dispersive) properties needed for comparing in-situ and laboratory measurements. To verify our method, Draupne Shale is monitored during three consecutive fluid exposure phases. This shale appears to be resilient to CO₂ exposure as its integrity is neither compromised by notable Young’s modulus and Poisson’s ratio nor P- and S-wave velocity changes. No significant changes in Young’s modulus and Poisson’s ratio seismic dispersion are observed. This absence of notable changes in rock properties is attributed to Draupne being a calcite-poor shale resilient to acidic CO₂-bearing brine that may be a suitable candidate for CCS.

Utilization of natural shale formations for the creation of annular barriers in oil and gas wells is currently discussed as a mean of simplifying cumbersome plugging and abandonment procedures. Shales that are likely to form annular barriers are shales with high content of swelling clays and relatively low content of cementation material (e.g., quartz, carbonates). Shales with large content of quartz and low content of swelling clays will be rather brittle and not easily deformable. In this paper we ask the question whether and to what extent it is possible to modify the mechanical properties of relatively brittle shales by chemically removing some cementation material. To answer this question, we have leached out carbonates from Pierre I shale matrix using hydrochloric acid and we have compared mechanical properties of shale before and after leaching. We have also followed leaching dynamics using X-ray tomography. The results show that removal of around 4–5 wt% of cementation material results in 43% reduction in Pierre I shale shear strength compared to the non-etched shale exposed to sodium chloride solution for the same time. The etching rate was shown to be strongly affected by the volume of fluid staying in direct contact with the shale sample.

In carbon storage activities, and in shale oil and gas extraction (SOGE) with carbon dioxide (CO₂) as stimulation fluid, CO₂ comes into contact with shale rock and its pore fluid. As a reactive fluid, the injected CO₂ displays a large potential to modify the shale’s chemical, physical, and mechanical properties, which need to be well studied and documented. The state of the art on shale–CO₂ interactions published in several review articles does not exhaust all aspects of these interactions, such as changes in the mechanical, petrophysical, or petrochemical properties of shales. This review paper presents a characterization of shale rocks and reviews their possible interaction mechanisms with different phases of CO₂. The effects of these interactions on petrophysical, chemical and mechanical properties are highlighted. In addition, a novel experimental approach is presented, developed and used by our team to investigate mechanical properties by exposing shale to different saturation fluids under controlled temperatures and pressures, without modifying the test exposure conditions prior to mechanical and acoustic measurements. This paper also underlines the major knowledge gaps that need to be filled in order to improve the safety and efficiency of SOGE and CO₂ storage.

Wells are essential in oil and gas production and construction of them is one of the main cost drivers for field development. It is normally needed to drill and construct new wells from existing fields during most of the production time. In order to reduce costs one can re-use parts of existing wells when they are no longer efficient. This is done in offshore fields also when there is limitation for new wells due to capacity of the subsea template. Through tubing drilling is a method to drill a side track through the wellbore tubulars. However, this will normally result in a smaller and less effective well completion. Removing parts of the casing section and drill a larger size sidetrack is an option to provide a new full-size wellbore. Removing the 9 5/8” casing through the settled particle in the annulus can be challenging. The wellbore annulus is normally filled with old drilling fluid, displacing fluid and/or cement slurry. The solid particles of these annular fluids are settled during years of shut-in and make it difficult to move the casing sections. There are several techniques for pulling the casing section, but there is a lack of knowledge of some of the key mechanism causing the resistance in these operations. In order to study and address the dominating effects in these operations, down-scaled laboratory tests are performed. The experiments reported here are performed by pulling steel pipes through the settled barite in the annulus. The pipes used in the tests are down-scaled from typical casing sizes with and without collars. The barite slurry compacted inside the annulus have different hydrostatic and pore pressures. When the pipe is pulled the required mechanical force is measured. Results show that the single most significant factor causing resistance when pulling the tubulars is the collars outside the pipe. Furthermore, it is identified that the pore pressure improves the mobility of the settled particle around the collar. In total these results provide improved understanding on the dominating factors during pulling pipes from a packed annulus.

Gassmann's fluid substitution theory is commonly used to predict seismic velocity change upon change in saturation, and is hence essential for 4-D seismic and AVO studies. This paper addresses the basics assumptions of the Gassmann theory, in order to see how well they are fulfilled in controlled laboratory experiments. Our focus is to investigate the sensitivity of shear modulus to fluid saturation, and the predictability of Gassmann's fluid substitution theory for P-wave modulus. Ultrasonic P- and S-wave velocities in dry and saturated (3.5 wt per cent NaCl) unconsolidated clean sands (Ottawa and Columbia) were measured in an oedometer test system (uniaxial strain conditions) over a range of 0.5–10 MPa external vertical stress. This study shows shear modulus hardening upon brine saturation, which is consistent with previous data found in the literature. Analysis of the data shows that most of the hardening of the ultrasonic shear modulus may be explained by Biot dispersion. Isotropic Gassmann's fluid substitution is found to underestimate the P-wave modulus upon fluid saturation. However, adding the Biot dispersion effect improves the prediction. More work is required to obtain good measurements of parameters influencing dispersion, such as tortuosity, which is very ambiguous and challenging to measure accurately.