Formation Evaluation for a Sustainable Future: Summary and Review of Aspen Technology’s White Paper

 

     Formation evaluation is standard for oil and gas exploration and development. Much can be gained by studying and characterizing the attributes of rock formations that can be used to optimize the exploration and production of hydrocarbons. Formation evaluation is also a necessary component of carbon storage, geothermal exploration and development, the economical production of brines, the exploration for naturally occurring hydrogen and helium, and the underground storage of oil, natural gas, hydrogen, and other fluids, for siting of offshore wind turbines, and for subsurface disposal of waste and wastewater in geologic basins. Below is a graphic demonstrating the dual challenge of meeting growing energy demand and meeting sustainability goals.

 

 

     The major method of formation evaluation involves the acquisition and interpretation of wireline logging data. The goal is to characterize the properties of reservoirs and to evaluate traps, seals, temperatures, pressures, and formation fluids. That goal is subordinate to the goal of the project. In oil and gas projects the goal is often to evaluate the ability of the formation to produce economic quantities of hydrocarbons. Thus, one might say formation evaluation is a means to screen formations in prospective areas for economic hydrocarbon production potential. It can also be used to screen formations in prospective areas for storage of hydrocarbons, CO2, hydrogen, wastewater, and other fluids. The goal is to find the most suitable reservoir for each purpose. Other methods of formation evaluation include retrieving rock cores for laboratory analysis, drill stem testing, downhole imaging, drilling mud properties, logging of mud gases liberated from the formation, desorption analysis of cores, and fiber optically conveyed tools for analysis.

 

     The paper first defines what formation evaluation is and why it is crucial: “Formation evaluation/petrophysical analysis is performed to assess the volume and types of fluids and minerals present in a formation and to determine the production potential of a reservoir. It analyzes subsurface formation characteristics, such as lithology, porosity, permeability, and saturation, and is used to establish the presence of reservoir rock, evaluate reservoirs for potential hydrocarbons or other resources, and estimate the volume of those resources. It is also used to evaluate the mechanical properties of the formations of interest.” Formation evaluation is crucial in every step of subsurface projects from planning to exploration to development to production to long-term monitoring.

 

 

Formation Evaluation for Carbon Sequestration

 

  They list the challenges of Carbon Capture and Sequestration (CCS) as 1) cost – each project must be evaluated for cost and formation evaluation is a key part of that overall evaluation; 2) environmental and safety concerns – this mainly involves the potential for CO2 leaking. Due to the nature of CO2 molecules, it has more of a potential to leak than other formation fluids and so formation conditions like effective seals are very important; 3) uncertainty and risk – here they give the example of saline aquifers that may not have a great amount of data compared to oil and gas reservoirs; 4) public acceptance – well-designed regulatory frameworks will likely be a key to project success and public acceptance. They note that formation evaluation is “a multidisciplinary, collaborative process between geologists, geophysicists, geochemists, reservoir engineers and other experts, to ensure the safe and effective storage of CO2 in the subsurface.” Some of the geologic goals for CCS are to identify suitable storage sites, to determine how much CO2 each formation and site can accommodate, and to monitor the changes through time during the years of CO2 injection. Wireline logging and seismic are two of the main methods to monitor changes through time. For example, pulsed neutron capture logging can indicate the patterns of migration of CO2 through time. Formation evaluation can also be used to help define the geochemical properties of the formation fluids and how they change through time. It can be used to predict chemical reactions between CO2 and the rock. It can be used to evaluate the potential for mineral trapping where CO2 reacts with rock to form stable carbonates that sequester it as a component of the rock rather than as a fluid in the pores of the rock. It can be used to better understand fluid flow dynamics and how they change over time. It can be used to evaluate geomechanical factors such as seal integrity, stresses, and the presence and orientation of fractures and faults.

 

Formation Evaluation for Geothermal Development

     Geothermal development involves similar challenges as CCS: cost, environmental and safety concerns, risk and uncertainty, and public acceptance. Higher drilling costs can be associated with geothermal drilling due to the heat itself which increases the potential for corrosion of casing metals. The higher pressures often encountered can contribute to the same issues. The potential for induced seismicity is an environmental and safety concern and mitigating that potential requires careful monitoring. Each geothermal prospect often presents its own challenges due to the different characteristics of the formations and the hydrothermal systems within them. Newer methods such as Enhanced Geothermal Systems (EGS) where a hydrothermal system is created through hydraulically fracturing rock is very dependent on accurate formation evaluation. The sheer amount of fluids being moved from the subsurface to the surface and back can mean the possibility of surface leaks that could harm the environment and affect public acceptance. Formation evaluation can help improve drilling performance for each project, determine geothermal capacity, and the potential for decreased local formation temperatures through time as the fluids produce power.

     As in CCS, formation evaluation can be used to better understand the geological, geochemical, and geomechanical aspects of the geothermal project. Cores, images, wireline logs, and drilling mud properties are all utilized. The thermal properties of the target rock, including how well it transfers heat are another important factor in formation evaluation for geothermal. Changes in water chemistry over time can be very important in geothermal. Wellbore integrity is another issue with geothermal that formation evaluation can help by informing cementing design, drilling pressure limits, and wellbore stability.

     Aspen lists four areas where their digital analysis can benefit CCS, geothermal, and other subsurface projects: database, vendor independence, scripting, and integration. Developing a user-friendly borehole database that can be queried and compared can be beneficial. They mention accurate unit conversion and datum handling as being essential to the processing and co-visualization of data. Vendor independence refers to different data types used by different service companies. Oil & gas projects have similar issues. These are not too difficult to overcome. Scripting can provide the ability to customize processing and interpretation of data: “Languages such as Python, Tcl and C++ or dedicated platforms such as MATLAB, coupled with user configurable menus, offer the greatest possible range of customization options.” Integration refers to the ability to integrate data from different sources and different types of data into a seamless system.

     Below they offer a workflow that proceeds from formation evaluation and seismic interpretation to geological modeling to flow simulation to uncertainty assessment.

     Overall, this was a useful white paper although it does not go into much detail. Aspen is a digital technology company so they are promoting their own perspective in the process which is mostly providing digital platforms for data analysis and integration.

 

References:

Formation Evaluation for a Sustainable Future. Richard Pelling and Aurore Plougoulen. Aspen Technology. 2024. Formation Evaluation for a Sustainable Future (aspentech.com)