ALBERT

Description: Concerns about potential climate change related to greenhouse gas emissions have spurred researchers across the world to assess the viability of geologic storage of CO 2 . In the Illinois Basin in the United States, the Cambrian Mount Simon Sandstone has been targeted as a reservoir for carbon capture and storage (CCS). In this CCS system, the Eau Claire Formation is expected to serve as the primary seal to prevent upward migration of the CO 2 plume; however, little work has been done to specifically determine how well it will function as a seal. Although the lateral extent and thickness of the Eau Claire Formation, along with its generally low permeability, certainly make it a prime candidate to serve in this capacity, the primary depositional fabric and mineralogy, which are the fundamental controls on the petrophysical charter of this unit, remain poorly constrained. Therefore, the purpose of this study is to investigate the lithologic, mineralogical, and petrophysical properties of the Eau Claire Formation in an effort to characterize its potential as a functional seal in a CCS system. Sixty-six core-derived Eau Claire Formation samples from seven wells within the Illinois Basin are described using a combination of petrography, reflectance spectroscopy, x-ray diffraction, geochemical, and petrophysical analyses. These analyses show that the Eau Claire Formation contains five different lithofacies (sandstone, clean siltstone, muddy siltstone, silty mudstone, and shale) with fine-scale heterogeneities in fabric and mineralogy that greatly influence the petrophysical properties. Porosity, permeability, and entry-pressure data suggest that some, but not all, lithofacies within the Eau Claire Formation have the capability to serve as a suitable CCS seal. Abundant authigenic minerals and dissolution textures indicate that multiple generations of past fluid-rock interactions have occurred within the Eau Claire Formation, demonstrating that much of the formation has behaved as a fluid conduit instead of as a seal. Minerals that would be potentially reactive in a CCS system (including carbonate, glauconite, and chlorite) are common in the Eau Claire Formation. Dissolution of these and other phases in the presence of carbonic acid could potentially jeopardize the sealing integrity of the unit. Although complexities in the sealing properties exist, the dynamics of the CCS system and the potential for precipitation of new minerals should allow the Eau Claire Formation to serve as an adequate seal.

Description: Pre-drill modelling of fracture pressure (FP) is an essential part of well planning, reserve estimation and evaluation of the potential for inducing seismicity as the result of fluid injection. Estimation of stress ratio or Poisson's ratio values or compaction state with depth is required in frequently used FP models. A new method to estimate FP is proposed which is based on Leak Off (LOT) and pore fluid pressure (Pp) data from offset wells and vertical stress ( S v )–depth relationships. LOT/ S v ratios observed in intervals of offset wells that are normally pressured (hydrostatic) are used to define an expected FP/ S v ratio for hydrostatic Pp conditions for all depths. Typical FP/ S v ratios for hydrostatic conditions derived using LOT data range from 0.81 to 0.89. Observed LOT values associated with Pp greater than hydrostatic (overpressured) in offset wells are used to quantify the rate of increase in FP with increasing overpressure (OP). The expected FP for hydrostatic conditions is compared with observed LOT values from depths where the pore fluid is overpressured and a relationship of increased FP, relative to the expected FP for hydrostatic conditions (residual FP (FPr)) with increasing OP is defined. The FPr:OP ratio typically ranges from 0.24 to 0.43. Fracture pressure models developed by this procedure may be used to predict FP for wells in different water depths and with Pp conditions different from those in the offset wells. The use of the model is demonstrated in three case studies taken from different geological settings: the Scotian shelf (offshore Nova Scotia), offshore Central Gulf of Mexico and the chalk interval from the Central North Sea.

Description: Compaction disequilibrium is a widely accepted cause of overpressure, especially in clay-rich, rapidly deposited sediments. Clay diagenesis has been associated with the occurrence of overpressure greater than the compaction disequilibrium overpressure. These observations have led to the expectation that overpressure will be greater than the compaction disequilibrium contribution when clay diagenesis occurs within an overpressured mudstone. Clay diagenesis have been reported in a Pliocene section of a well from the Gulf of Mexico, offshore Louisiana. Pressure and log data from that well indicate that despite clay diagenesis, the overpressure can be attributed solely to compaction disequilibrium. This paper examines the whole mudstone and clay mineralogy composition and petrophysical characteristics of the offshore Louisiana well with clay diagenesis, but without a diagenesis contribution to overpressure and contrasts that data with results from other clay diagenesis and petrophysical studies. The comparison suggests that the offshore Louisiana well was relatively smectite poor compared with wells from regions associated with a clay diagenesis contribution to overpressure. The lower smectite content resulted in a lower percentage of reacted volume that was insufficient to allow the load transfer often associated with clay diagenesis. Petrophysical features of the offshore Louisiana well and nearby wells differ from the features associated with clay diagenesis in other Gulf of Mexico wells and a limited number of international wells. Comparison of location, age, depositional package, clay mineralogy, and petrophysical features suggests that provenance may control the occurrence of Gulf of Mexico mudstones that do not experience increased overpressure as a result of clay diagenesis.

Description: The stress regime in the Illinois Basin was investigated to assess how the rock column might respond to the injection of fluids, including coproduced formation brines and supercritical CO 2 .This response is a concern because injection practices could increase pore fluid pressure and potentially induce seismicity. Data were collected to determine the magnitude and orientation of a three-component stress field: vertical stress ( S v ) and minimum ( S h ) and maximum ( S H ) horizontal stresses. The S v was evaluated with a six-layer lithostratigraphic column. A two-layer pressure–depth S v model was generated for the central part of the basin, and a single pressure gradient model was constructed for the surrounding region. In the central part of the basin, the S v gradient is 24.9 MPa/km (1.11 psi/ft) to a depth of 2134 m (7000 ft), followed by a gradient of 27.1 MPa/km (1.20 psi/ft) below 2134 m (7000 ft). For the area surrounding the deep basin, the S v gradient was 25.5 MPa/km (1.13 psi/ft). The S h was evaluated from multiple data sources, primarily hydraulic fracture records or extended leak-off tests. The S h gradient calculations ranged from 24.1 to 27.3 MPa/km (1.07 to 1.21 psi/ft). The S h values for the basal Paleozoic clastic units are lower than those for units in the overlying horizons. The S H was based on a critically stressed model yielding values between 40.0 and 82.6 MPa/km (1.77 to 3.65 psi/ft). Stress orientation data for the Illinois Basin were collected from multiple sources. The orientation of S H across the study area is relatively uniform in strike at approximately N60°E. Marked deviations in S H result from localized structural discontinuities.