The oil and gas industry widely relies on reservoir knowledge for successful exploration and production. It is, therefore, significant to have a comprehensive understanding of the reservoir in order to optimize its performance in the future. Horizontal drilling, artificial fracturing, and passive seismic monitoring enhanced the extraction of oil and gas from tight sands and shales. While shales are considered to be superior reservoirs of oil and gas, they were initially considered to be the source of hydrocarbons. According to statistics, the North American shale harbors a natural gas supply of over 100 years (on footing of the current rate of consumption).Due to the perceived abundance in gas supply, the dependence on international sources of energy drastically went down leading to the abandonment of gas-rich shale plays (Wang and Carr 2174).This attributes to the inadequate knowledge of basic stratigraphic and physiochemical properties of shales. Therefore, reservoir characterization is the only pathway to gaining this knowledge and understanding. It is, however, evident that no discipline can offer a complete understanding of reservoir characteristics hence raising the need for expertise integration. Today, the study of reservoir characterization features the collaborative efforts of geology, petrophysics, petroleum engineering, geophysics, geochemistry, computer science, geostatistics, and biostratigraphy (Wang and Carr 2174).This paper focuses on reservoir characterization and its application in Marcellus shale.
The Marcellus Shale
The Marcellus Shale is said to be the largest unconventional shale gas reservoir in the U.S., covering approximately 500,000km² of the Appalachian basin. Its organic-rich intervals and the average gross thickness are 34ft and 80ft respectively (Wang and Carr 2173). A significant step has been made towards the exploration and development of unconventional shale gas in the past decade worldwide. This has been enhanced by the technologies of hydraulic fracture stimulation and horizontal drilling which have enabled economic extraction of gas from low porosity reservoirs. To facilitate the optimization of horizontal well designs as well as stimulation strategies, geologic analysis of lithofacies, a basic reservoir property, is used.
Before looking at reservoir characterization, it is important to note that there are two types of reservoirs; conventional and unconventional. Conventional gas reservoirs are deposits driven by buoyancy and they occur at discrete accumulations in stratigraphic, structural, or strati-structural traps. On the other hand, unconventional wells are “regionally pervasive accumulations through large geographic areas mostly independent of structural, stratigraphic, or strati-structural traps” (Chopra et al. 8). The following are characteristics of shale gas reservoirs.
Depositional Environment. These are environments where deposition occurs, namely:marine and non-marine environments. The clay content and the composition of brittle minerals like quartz and carbonates in marine shales are quite high. Brittle shales fracture easily during hydraulic stimulation. Similarly, the composition of clay content is high in non-marine shales but more ductile compared to marine shales. Unlike marine shales, non-marine shales do not fracture easily during stimulation.Therefore, the knowledge on reservoir mineralogy helps to determine the feasibility of hydraulic stimulation based on the induced fractures (Chopra et al. 9).
Depth. Gas is produced from source rocks in forms of biogenic gas or thermogenic gas. Biogenic gas is formed from the anaerobic processes of micro-organisms burial in the early diagenetic stage. Thermogenic gas results from the breakdown of kerogen at deeper depths and high temperatures, therefore, biogenic gas normally occurs at depths of 1000/1100m and below. Regions shallower than 1000m have low gas concentration and lower pressure while regions greater than the depth of 5000m have low permeability hence higher costs of drilling and production (Chopra et al. 11).
Thermal Maturity.High heat is needed to convert organic matter to hydrocarbons. Thermal maturity is the degree to which a formation has been exposed to. Vitrine reflection is significant in indicating whether the rock has produced hydrocarbons and determining whether it is a potential source rock. Higher thermal maturity is characterized by nanopores, which contribute to more shale porosity (Chopra et al.13).
Total Organic Carbon (TOC) Content. Organic matter such as plants and micro-organism fossils are a great source of carbon, hydrogen, and oxygen atoms, which are responsible for the generation of natural gas and oil (Chopra et al. 14). Reservoir organic richness is, therefore, important in determining potential source rocks (Chopra et al. 14).
Permeability. Shale gas is stored in different forms; as free gas in inter-granular porosity and natural fractures, as gas dissolved in kerogen, or as gas adsorbed onto kerogen. Permeability can be achieved naturally in the fracture system or induced by hydraulic stimulation (Chopra 15).
Gas in Place. Gas in place is determined by four elements; temperature, pressure, net organically rich shale thickness, and gas-filled porosity. High gas concentration is synonymous with high pressure; therefore, pressure is determined by the quantity of gas in place. Normally, temperature gradient of 1°F per foot is used while the porosity is achieved from cores and logs. Logs are used to analyze shale intervals acquired from seismic interpretation in order to determine organic-rich intervals. The results are then used to calculate a net-to-gross thickness of shale (Chopra et al.16).
The above characterization provides a quantitative geologic framework for the exploration and development of the Marcellus shale in order to optimize its performance.
Reservoir Characterization Techniques
Measuring Properties at Different Scales. There are three main stages of field development, namely: exploration, appraisal, and production (Course Material). The first phase of exploration uses geologic data of the area including the basin structure, evolution, and stratigraphy (Course Material). An analysis of 2D OR 3D seismic reflection is then performed to produce a regional-scale image of the targeted area (Course Material). Conventional well logs are drilled once a potential shale play is determined, in order to establish characteristics of the well bore. This stage is helpful in determining the possible accumulations of hydrocarbons to allow drilling of more wells or shooting more 3D seismic for the appraisal phase (Course Material). Successful appraisal stage offers more knowledge on the reservoir. An evaluation of outcrop analog is then done to determine depositional environment and stratigraphy, which helps to generate a 3D model of stratigraphic continuity and connectivity of the shale reservoir (Course Material). This procedure also provides useful data on well pressure and the rate of initial potential flow, which enhances reservoir characterization (Course Material). The quantified data is used to build a 3D reservoir model to optimize the stimulation hence enhanced reservoir performance (Course Material).
Computers. Computers are quite useful in the collection and manipulation of seismic data to enhance processes like the generation of well logs and images, evaluation of numerical variables, the development of mathematical formulas, and the development of maps (Course Material). They provide visual 3D images and facilitate analysis of huge amounts of data for exploration and production (Course Material).
Seismic Reflection and Subsurface Imaging. Seismic-reflection and subsurface imaging technology has been extensively used in the exploration of hydrocarbons and to determine reservoir characterization (Course Material). The technique of seismic-reflection or shooting is used to provide imaging of the subsurface. While the image does not offer full geological information of the subsurface, it is enough to enable the imaging of large or medium stratigraphy and structures (Course Material). This technique uses the knowledge that energy source rocks like dynamite generate sound waves that are reflected off an interface of two rocks that have different acoustic characterization (Course Material). The reflected energy then travels back to the earth surface. Electronic receivers known as geophones are used to record the reflected energy hence providing seismic reflection imaging that indicates subsurface features (Course Material). Source trucks are used on land to produce the energy required to push waves down the earth’s surface. Seismic shooting is also employed in marine environments to determine reservoir structure and stratigraphy. This technique features 2D, 3D, 4D imaging. Other processes featured under this technique include spectral decomposition, seismic inversion, croswell seismic investigation, and multi-component seismic investigation (Course Material).
Logging and Sampling. A well is drilled on land or in marine to acquire the fluids of the subsurface rocks for sampling. A light mixture of water and mud is constantly pumped down the hole to enhance lubrication during drilling (Course Material). The slurry mixture is also used to grab the pieces of the hole, known as “cuttings.” A mud logger analyzes the cuttings, measures the reservoir fluids, determines the lithology and bags the cuttings for future (Course Material). However, since well sampling is costly and time consuming, conventional wireline logs are instead used in the characterization of the subsurface rock and fluid. Diverse tools for measuring various properties are connected to the drill string released down the well steadily. For instance, gamma ray logs indicate lithology while density logs provide data on the density of the formations. Neutron logs are used to establish the concentration of hydrogen atoms in the pores (Course Material). Other types of specialized logs used are unconventional logs, diameter logs, and nuclear resonance logs (Course Material).
Chopra, Satinder, Sharma, Ritesh, and Marfurt, Kurt. “Shale Gas Reservoir Characterization Workflows”. Search and Discovery. www.searchanddiscovery.com/documents/2014/41266chopra/ndx_chopra.pdf. 27 Jan, 2014.
Wang, Guochang and Carr, Timothy. “Organic-rich Marcellus Shale Lithofacies Modelling and Distribution Pattern Analysis in the Appalachian Basin”. AAPG Bulletin. pp. 2173-2201. DOI: 101306/05141312135. Dec, 2013.
Course Material. File 254086169 (sent by client).