Matthew Josh

The Role of Specific Surface Area and Cation Exchange Capacity in Determining Shale Rock Properties

Surface area, surface charge and the exchangeability of cations have long been known as factors in determining the physical and mechanical properties of shales. Such properties are related to the fine grain size of the various clay minerals and also the particle shapes, edge-face microstructural arrangements and are impacted by the likes of salinity and depositional environment. In general, the siltier a given clay or shale, the lower their cation exchange capacity (CEC) and specific surface area (SSA). CEC and SSA have direct impacts on mechanical and flow properties. Wellbore instability can be caused by the development of osmotic pressures resulting from differences between pore fluid composition and drilling fluid composition and the resultant time dependent effects based on permeability. Such instability can be mitigated by the use of oil-based muds or for example high KCl water-based muds. Electrical and dielectric properties are also dependent on mineral surface charge and related directly to the CEC of a given shale, especially properties such as resistivity (or conductivity) and permittivity (dielectric constant). This paper will detail SSA and CEC values of shales from multiple basins worldwide and also from individual boreholes to demonstrate their role for shale rock properties.

Wideband Electrical/dielectric Measurements From Millihertz to Gigahertz Frequencies

No single system is available to ground truth the full frequency range of geophysical and petrophysical electromagnetic measurements. We have combined a number of instruments and different measurement principles to obtain truly wide band (mHz to GHz) spectra of conductivity and permittivity of earth materials. We have designed several new cells and fixtures, and implemented a number of novel procedures for measurement and calibration. Several of our cells operate at overburden pressures (35 MPa). Our guarded parallel plate cells can be connected to more than one measurement system to cover the cross-over region adequately. The four electrode cells can be configured for time domain and electrokinetic measurements. To aid in calibration we have also implemented resonant methods giving highly accurate dielectric measurements at spot frequencies that compliment swept frequency data.

Theoretical Modeling of Dielectric Properties of Artificial Shales

Accurately modeling the anisotropic dielectric properties of shales is important for the interpretation of dielectric data acquired from shales as source rocks and unconventional reservoirs. We have developed a multiphase incremental model for the frequency dependent anisotropic dielectric properties of sedimentary rocks and presented an approach based on the developed model to simulate the measured anisotropic dielectric behaviors of artificial shales. The new model was built based on the theoretical basis of differential effective medium models for any number of mineral grain components aligned in any direction and was shown to be independent of the mixing order. The model incorporates the measured orientation distribution function of the clay particles to determine the shale dielectric anisotropy, and the frequency dependent dielectric behaviors of the wet clay minerals are obtained by inverting the dielectric properties of the artificial sample composed of clay and the same brine as in other artificial shales. The modeling technique combined important polarization mechanisms in the intermediate frequency range and was shown to give satisfactory fit to the measured frequency dependent anisotropic relative permittivity and conductivity of the artificial shales with varying silt contents by using a reasonable aspect ratio and constant dielectric parameters for the silt grains.

Organic matter network in post mature Marcellus Shale: Effects on petrophysical properties

Shale samples of the Marcellus Shale from a well drilled in northeastern Pennsylvania were used to study diagenetic effects on the mineral and organic matter and their impact on petrophysical response. We analyzed an interval of high gamma ray and anomalously low electrical resistivity from a high thermal maturity (mean maximum vitrinite reflectance > 4%) part of the shalehgas play. A suite of microanalytical techniques was used to study features of the shale down to the nanoscale and assess the level of thermal alteration of the mineral and organic phases. The samples are organic rich, with total organic carbon contents of 3–7 wt. %; the vast majority of the organic matter was identified as highly porous pyrobitumen. Matrix porosity is also present, especially within the clay aggregates and at the interface between rigid clasts and clay minerals. Mineral- and organic-based thermal maturity indices suggest that during burial the sediment had been exposed to temperatures as high as 285°C (545°F). Under these conditions, the residual, migrated organic matter assumed a partially crystalline habit as confirmed by the identification of turbostratic structures via electron microscopy imaging. Experimental dielectric measurements on organic matter–rich samples confirm that the anomalous electrical properties observed in the wire-line logs can be ascribed to the presence of an electrically conductive interconnected network of partially graphitized organic matter. The preservation of porosity suggests that this organic network can contribute not only to the electrical properties but also to the gas flow properties within the Marcellus Shale.

Experimental Characterization of Dielectric Properties in Fluid Saturated Artificial Shales

High dielectric contrast between water and hydrocarbons provides a useful method for distinguishing between producible layers of reservoir rocks and surrounding media. Dielectric response at high frequencies is related to the moisture content of rocks. Correlations between the dielectric permittivity and specific surface area can be used for the estimation of elastic and geomechanical properties of rocks. Knowledge of dielectric loss-factor and relaxation frequency in shales is critical for the design of techniques for effective hydrocarbon extraction and production from unconventional reservoirs. Although applicability of dielectric measurements is intriguing, the data interpretation is very challenging due to many factors influencing the dielectric response. For instance, dielectric permittivity is determined by mineralogical composition of solid fraction, volumetric content and composition of saturating fluid, rock microstructure and geometrical features of its solid components and pore space, temperature, and pressure. In this experimental study, we investigate the frequency dependent dielectric properties of artificial shale rocks prepared from silt-clay mixtures via mechanical compaction. Samples are prepared with various clay contents and pore fluids of different salinity and cation compositions. Measurements of dielectric properties are conducted in two orientations to investigate the dielectric anisotropy as the samples acquire strongly oriented microstructures during the compaction process.

Insights of dielectric measurements from cuttings recovered along the deepest offshore well in the world (Nankai trough accretionary prism): IODP expedition 338, site C0002F

A total of 109 cuttings were recovered during the IODP expedition 338 in site C0002F down to 2005 mbsf. A special dielectric end-load probe was designed and used for the first time at sea on this sample collection to measure dielectric and electrical conductivity from 10 kHz to 6 GHz. The whole dataset was compared to specific surface area (SSA), mineralogy from XRD measurements and resistivity log while drilling acquired during the expedition to understand the relationship between fluid, clays and lithologies. The dielectric results revealed to be very powerful to: (i) understand the clay composition and content; (ii) re-calibrate cutting depths; (iii) detect unit boundaries and (iv) detect conductive and not-conductive fault systems

Geomechanics and Physics Related to Shale Strength, Stiffness and Anisotropy

For many years, the presence of large amounts of shales in many sedimentary basins worldwide has been regarded as a double edged sword. On the positive side, they form seals to hydrocarbon traps in the subsurface and in addition are regarded as potentially useful for the storage of CO2 and nuclear wastes. All three positives are enhanced by their often areally extensive nature. However, these large volumes of shaly material also have a downside in that exploration wells often have to drill through highly unstable shale formations; such instability can be caused by high pore pressures, drilling fluid-rock interactions coupled with the generally weak nature of many overburden shales. In recent times, understanding shale behaviour became more critical with the advent of shales as reservoirs, although it should be noted that these shales are not at all similar to typical overburden shales encountered for example in the North Sea or Gulf of Mexico. However, both types of shales are of interest in terms of geomechanical properties, overburden shales for seal integrity and gas shales for hydraulic fracturing. In addition, the coupled geomechanics and rock physics response is also of interest as overburden shales have shown anomalous responses in 4D seismic operations, while gas shales require an understanding of the mechanics and physics of partial saturation and presence/type of organic matter. This contribution will investigate the impacts of some critical factors governing shale geomechanical properties, outline the impact of changing stress on rock physics response through evaluation of velocity and its anisotropy and finally will look at the relationship between composition, dielectric properties and shale strength.

A Multi-Disciplinary Workflow for Characterising Shale Seals

Evaluation of the various rock properties of shales has become more prevalent in recent years although our understanding of these properties and the links between them is still relatively embryonic. While thick shale sequences can form sealing units above hydrocarbon traps, intra-reservoir shales can form baffles to flow in both petroleum and groundwater contexts. High field and low field nuclear magnetic resonance were used to evaluate wettability of shales. Preserved shales show mineral dependent variations in surface affinity for oil versus water. Hydrophilic shales have a higher cation exchange capacity (e.g. shales rich in illitic and/or smectite), whereas kaolinitic mudrocks are potentially hydrophobic and can be wetted preferentially by oil, sometimes retaining oil on the mineral surfaces after further exposure to brines. Porosity and cation exchange capacity correlate well with strength properties and dielectric constant measurements on intact shales and pastes made from powdered shales show strong relationships between high frequency electrical properties, mineralogy, cation exchange capacity and mechanical strength. Calibrating wireline logs with laboratory measurements and the development of physics-based models allows the prediction of rock properties and extrapolation to the borehole scale.

Empirical Correlations to Shale Strength

SUMMARY A suite of preserved shales from basins widely separated in space and time underwent multi-stage triaxial tests in the laboratory to evaluate static and dynamic mechanical properties. Coupled with these tests was a workflow for extensive characterisation of physical and petrophysical properties, including porosity, bulk density, specific surface area, electrical properties and microstructural evaluation. Good correlations have been found between shale strength (cohesion and unconfined compressive strength) and some physical properties and there are encouraging relationships between dielectric properties and both static and dynamic mechanical properties. Poor relationships were found to friction coefficient and also between properties previously thought good for predicting shale strength, such as P-wave velocity.

Seal Effectiveness Prediction Using a BP Proprietary Toolkit

Outline of talk - Business importance of seal effectiveness - Leakage processes and evidence for them - Characterising mudrock seals - Data uncertainties and limitations - Introduction to BP proprietary toolkit- BP Petroleum Prediction Toolkit o Seal Risk basic o Seal Risk Advanced - Example of usage - Conclusions