Editor’s Note: This is the fourth article in our Tuesday series of “Unconventional Oil Exploration’
Tight Oil Trending
In recent years, tight oil has made a significant contribution to the North American oil supply, a trend that is expected to continue. According to a recent article published on the U.S. Energy Information Association (EIA) website, U.S. tight oil production averaged an estimated 3.22 million barrels per day (MMbbl/d) in the fourth quarter of 2013, most of which came from the Eagle Ford in South Texas (36% of total U.S. tight oil production), and the Bakken Shale in North Dakota and Montana (28% of total U.S. tight oil production). The United States produces 91% of all North American tight oil, with the remaining 9% coming from Canada. Right now most tight oil comes from North America but Russia, China, and Argentina will likely become significant producers as well.
What is Tight Oil?
Tight oil is found within shale and sedimentary rock formations with very low permeability where fluids can’t flow freely. Even though tight oil is the same as oil extracted by conventional means, it’s classified as unconventional because it can’t be extracted using conventional methods. The most common technologies for tight oil production are hydraulic fracturing (fracking) and horizontal drilling. Fracking involves pumping large volumes of water, sand, and chemicals into underground rock at high pressure to free the trapped oil. Horizontal drilling is a technique that makes more rock surface area accessible, increasing the amount of oil that is released. Once a wellbore is created, conventional technologies can be used for production.
Reservoir Evaluation Techniques
Drilling for tight oil sometimes takes place at Brownfield sites when most of the conventional, easily-accessible oil has been extracted and data is available to indicate how much tight oil may be available. Vertical wells are drilled and core sample are taken to evaluate reservoir characteristics. The data to be considered is derived from techniques that include inferring mineralogy, inferring lithology, and chemostratigraphy.
- Mineralogy: The chemical and physical properties of the reservoir can be determined by studying its mineralogy. High quartz or carbonate content, for example, indicates brittle, and therefore more frackable, shale.
- Lithology: Rock chips that are brought to the surface by circulating drilling media (most commonly mud) are evaluated for fracability and permeability.
- Chemostratigraphy: Chemostratigraphy indicates the variation of chemistry within sedimentary sequences. Chemostratigraphic markers can be used to steer the well bore in real time or confirm the well bore path.
All of these types of analysis can be done on site using portable x-ray fluorescence (XRF) analyzers or sent to an offsite lab for XRF analysis. XRF is emerging as an important tool in E&P operations because it provides fast and reliable geochemical data at the drill, in the field, and in the core lab to help petroleum geologists more accurately locate oil-bearing strata, improve mud-logging, and support geo-steering. Although XRF analyzers cannot analyze hydrocarbons, they report various major and trace elements that can be used to characterize reservoir properties that influence porosity (cements), permeability (clays, cement type), fracture population (Si content), and productivity (e.g. Mo content).
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