How do you use core analysis data to assess exploration targets?

It is critical to understand and manage subsurface environment, analyze seal integrity and capacity during exploration, accurately predict and monitor geopressure, maintain wellbore stability, and manage and overcome regional geologic complexities. Rock samples are the only actual physical samples of the reservoir as direct source of data. We have 2 types of core analysis: 1- Routine core analysis, 2- Special core analysis. Core analysis involves recovery,preservation, examination, and testing samples of rocks to obtain information on the characteristic properties of the potentially reservoir rocks. Therefore, accurate and representative core data should be used, among other applications, in formation evaluation and reservoir surveillance.

In my perspective, within oil and gas, particularly in LNG and electric markets, core analysis data is pivotal for assessing exploration targets. This involves evaluating porosity, permeability, and fluid saturations to understand reservoir quality and potential hydrocarbon recovery. Advanced techniques like NMR and CT scans offer insights into pore structure and fluid distribution. Integrating core data with seismic and log data allows for a comprehensive geological model, guiding decisions on well placement and production strategies, ensuring a targeted approach to exploration and development.

Various methods are employed for core acquisition in the oil and gas industry. Conventional coring involves drilling a borehole and extracting cylindrical rock samples using core barrels. Sidewall coring utilizes specialized tools to collect samples from the wellbore wall without interrupting drilling operations. Rotary sidewall coring employs a rotating bit to obtain samples from the borehole sidewall. Furthermore, wireline coring uses a wireline tool to retrieve core samples from specific depths within the wellbore. Each method offers unique advantages and is selected based on factors such as formation characteristics, wellbore conditions, and project objectives.

It is important to note that the core retrieval could sometimes damage the rocks and cores. This is important particularly for geomechanical tests for which we need the intact rock and not fissured or fractured rock and this sometimes very difficult to manage in the core selection. However, it's important to distinguishes between natural fractures and induced fractures during the core retrieval. Not all fissure and fractures are due to damage of rocks as they might be naturally exisitibg fractures. This could be discussed with geologist and core specialists in core shop to make sur that we differentiate the induced fractures (due to coring operations, retrieval ) from existing natural fractures.

Core analysis is an essential and an important tool for obtaining direct data concerning drilled rock formations. Geologists, reservoir engineers, and petrophysicists rely on accurate and representative core analysis data to provide key data input for building static and dynamic reservoir models. The core analysis program should be clearly defined, prior to engaging in any coring operation. As part of the Special core analysis (SCAL), we have the Scanning Electron Microscopy (SEM), which provides information on the mineralogy and the amount, size, and morphology or pore filling and pore-lining material.

Core preparation involves several essential steps to ensure accurate analysis of geological formations. First, the recovered core is labeled, photographed, and measured for length. Then, it undergoes visual inspection and is cleaned to remove drilling fluids and debris. Next, the core is cut into manageable sections and oriented to maintain its original position. Subsequently, samples are taken for various analyses, including porosity, permeability, and mineral composition. Finally, the core is preserved using methods such as vacuum sealing or storing in core trays to maintain its integrity for future studies. Each step is critical to obtaining reliable geological data for reservoir characterization and development planning.

Core analysis employs diverse methods to characterize reservoir properties and optimize production. Porosity assessment techniques include mercury injection capillary pressure to measure pore size distribution and helium porosimetry for small pore analysis. Permeability evaluation methods range from gas and liquid permeametry to nuclear magnetic resonance (NMR) and digital rock analysis. Mineralogy determination utilizes X-ray diffraction (XRD) and scanning electron microscopy (SEM). Fluid saturation analysis employs techniques like Dean-Stark extraction and nuclear magnetic resonance (NMR). Additionally, core analysis involves geomechanical tests to assess rock strength and stability.

Other important core analysis will be geomechanical core analysis and rock physics/petroelastic core analysis. these core analysis need to be done by the socialized lab and interpreted by specialists of those fields. Sometimes, the preservation of samples are needed to make sure that rock are at their intact condition so that they can be used for geomechanical and rock physics tests. Examples of geomechancical interpretations are triaxial tests, UCS tests, Brazilian tests compaction/pore collapse tests, compressibility and acoustic tests. Not to forget most of these tests need to be done at the in situ or realistic operational conditions.

Integrating core analysis with logs, models, and seismic data enhances reservoir characterization and facilitates informed decision-making in oil and gas exploration and production. Core analysis provides ground-truth data on rock properties, while logs offer continuous downhole measurements for correlation and interpretation. Geological models extrapolate subsurface features and heterogeneities, aiding in reservoir simulation and flow modeling. Seismic data provide a macroscopic view of the reservoir structure and help delineate key features. Combining these datasets allows for comprehensive interpretation, validating assumptions, and reducing uncertainty in reservoir understanding. This integrated approach optimizes drilling, completion.

Interpretations of geomechnical and rock physics tests are to be done by the specialists and experts in these fields. They needs to know very well the concepts and theories behind each tests (elasticity, failure, compaction, etc.) and perform the right interpretations taking into account the measurement conditions and limitations. They develop the relevant models and constitutive models that they will be used in the numerical and analytical models that will guide the operations later.

Results of Interpretations of rock mechanics or geomechanics tests are crucial for oil and gas development and sustainability projects. For example, interpreted elastic properties of rock or measured strength of rocks will be used in 1D and 3D models, this will guide many projects such as drilling wellbores stability, optimization oil and gas productions, caprock integrity, hydraulic fracturing models, CCS (CO2 storage) and storage project. This will help the safety of operations and optimization of developments.

Another crucial aspect of core analysis involves the summation of fluids method, which is used to determine porosity and fluid saturations. This method involves measuring the bulk volume of the sample and the volumes of fluids within the pore system, assuming the total fluid volume equates to the pore volume. However, this approach can introduce errors in fluid saturation and porosity values due to variations in pore system quality between samples. For example, in conventional core analysis, gas volume might be measured on one sample while oil and water volumes are measured on a separate sample, which can lead to inconsistencies if there's significant variation between the samples.

Core acquisitions remained very expensive. We cannot always acquire cores and when we have cores the availability is limited (to be shared between petrophysics, geomechanics and other labs depending on the priority of each tests). geomechnical tests needs intact and representive rocks and cores at specific sizes. a tight collabotaion between different disciplines are mandatory to find the best coring strategy, coring selection and coring analysis.