Stress controls on transport properties of the Mercia Mudstone Group: Importance for hydrocarbon depletion and CO2 injection

Abstract

The physical properties of the Mercia Mudstone Group (MMG) are of interest to Carbon Capture and Storage (CCS) in the UK, both in terms of the sealing capacity of certain horizons and in order to assess scenarios involving CO2 migration in the overburden above potential CCS sites. In this study, the hydromechanical properties of MMG samples from the Larne Basin, Northern Ireland, were directly measured under steady-state conditions. Test samples were found to be good seals, with hydraulic permeabilities ranging from ≈2.1x10-18 to 8.4x10-21 m 2 (2.1x10-3 to 8.5x10-6 mD). A detailed examination of the consolidation behaviour of the material yielded values for compressibility, hydraulic permeability and specific storage, as a function of effective pressure. Consolidation testing also provided preconsolidation pressure values of between 30 and 37 MPa. Test data were fitted to a linear elastic model using a two-dimension finite element approach, yielding hydraulic permeability and Young's Modulus, as a function of effective pressure. Findings suggest localisation of flow, due to small-scale heterogeneity, may play a role even in relatively large test samples. Results also highlight the impact of methodology on resulting permeabilities and the importance of using values measured at boundary conditions appropriate for the specific application. Critical state envelopes were derived from test data and used to conduct a scenario analysis, considering a range of stress paths, to examine the impacts of depletion and reinflation during CO2 injection. Initial stress conditions, stress path gradient and caprock heterogeneity were all found to be influencing factors on the potential for yield during depletion and the resulting deformation mode. The response to CO2 injection is less clear, but will be impacted by the initial caprock permeability and resulting drainage response. An awareness of these controls on caprock performance during stress path changes may aid in the selection of depleted CCS sites with geomechanically favourable characteristics for reinjection.

Investigation of the Effect of Confining Pressure on the Mechanics-Permeability Behavior of Mudstone under Triaxial Compression

“…Therefore, mudstone formations are an effective barrier to geofluid accumulation and play an important role in petroleum systems and in deep energy and waste disposals. Although not always the case, the permeability of mudstones in a well-compacted and undisturbed state is less than 1 μD (≈10 -18 m 2 ) based on laboratory measurements of intact mudstone samples [3][4][5][6][7][8][9]; the reason for this result is that mudstone is rich in fine-grained and clay sediments and usually has a mean pore radius of less than 100 nm [4,10,11], which is more than several orders of magnitude smaller than that of coarser-grained rocks such as sandstone that retains geofluids [12,13]. Since the clay grains are platy shaped, mudstone has higher compressibility than other sedimentary rocks and its permeability, which is related to porosity, decreases greatly with increasing diagenesis depth or consolidation pressure [14][15][16][17][18].…”

“…However, when CO 2 is injected into deep saline aquifers or depleted hydrocarbon fields, the fluid pressure of the reservoirs will inevitably change, which influences the stress field of the caprock. Stress changes during injection may cause deformation and damage to the caprock and even result in failure of the caprock when the stress state approaches the failure envelope [9]. Since the pore structure of caprock can be altered due to these behaviors, its permeability will also change during CO 2 injection, which can affect the sealing performance of the caprock.…”

On modelling of consolidation processes in geological materials

“…Note that in Algorithm 1 the facts that (a) the Young's modulus mainly determines the total volume of fluid expelled (line 11) and (b) the permeability mainly determines the rate at which fluid is expelled (line 12) have been used, (Figure 3). This fitting algorithm provided reasonable theoretical flow-time curves for three Mercia Mudstone Group samples, see Harrington et al (2018). However, as highlighted there and as seen in the synthetic example of Figure 3, Biot's model is unable to reproduce the timedependent behaviour of the flow versus time curve.…”

“…To minimise possible osmotic swelling of samples, a synthetic porewater solution was prepared for use as the backpressure fluid and as the permeant during all hydraulic testing. In the absence of pore-fluid composition data for the Mercia mudstone, a salt-saturated solution was made using crushed halite from close in the succession to the sampling location (Harrington et al, 2018). Each test consisted of a hydration phase, an initial hydraulic test and a consolidation phase.…”

“…This material can be described as a fine-grained mudstone to microsparstone, but it should be noted that at the microscopic scale it is highly heterogeneous. For a detailed description of the material, we refer to Harrington et al (2018).…”