Basins are bowl-shaped structures with higher edges than the bottoms. A basin is a type of depression formed on the Earth's crust and are formed due to forces acting above Earth's surface, like erosional activity or due to the forces acting beneath the surface of the Earth like seismic tremors. Basins may take millions of years for their formation and may range hundreds of kilometres.
Basins can be of many types depending on the location and containment. River basins demarcate a river system; Oceanic basins are located below sea level, Groundwater basins refer to water acquirers, Sedimentary basins are filled with sediments of hydrocarbon or minerals.
Tectonics is the standard mode witnessed in the development of a basin. After a basin is formed, typically it is over time filled up with the sediments shed from the surroundings bringing additional subsidence, causing the basin to be more deeper while making more accommodation space.
Types of Basin:
Basins are mainly classified into three broad categories including drainage basin, sedimentary basin and structural basin.
A drainage basin is a depressed or an inclining region from which spillover gathers and moves into a channel, stream, or lake. The flow of any stream or river is collected into the drainage basin. These basins may vary in sizes ranging from few square kilometres to millions of square kilometres. Closed basins have no outlet, and any stream entering into it cannot exit.
The structural basin is formed in an area where the formations dip towards the centre of the structure. A structural basin is similar to the stack of progressive smaller bowls fitted into the next larger bowl. Each bowl represents independent rock formation or layer, maintaining concentric arrangement. The size range of these basins may range from a few kilometres to hundreds of kilometres.
Sedimentary basins are formed due to subsidence of Earth's crust resulting from the tectonic activity. A depression is formed on the surface of Earth where sediments from the surroundings get accumulated. Extra subsidence can be noticed when sediments accommodate the depressed region creating an overload. Their shapes may range from circular to oval or elongated. Irregular shapes can also be found. The size of these basins may range from tens to hundreds of kilometres in diameter. There are more than 600 sedimentary basins on Earth with over 150 basins containing petroleum resources.
How are Basins Formed?
Basins are formed at the plate margins that are convergent, divergent and transform boundaries. Let us have a look at the type of basins that are formed at each plate boundary.
When two plates move away from each other creating a horizontal stretch by ridge push or trench pull mechanism, the lower hot portion of the lithosphere moves slowly away from the mainland compared to the relatively upper cooler lithosphere. The crust's brittle nature due to cold temperature tends to create a fracture or fault resulting Earth's surface to subside. The subsidence creates an accommodation space for the sediments to deposit.
Another case of divergent boundary creating a basin is observed where lithospheric plates stretch, resulting in oceanic basins' formation with central ridges.
When two lithospheric or oceanic plates converge, a collision takes place. A denser plate subducts under the less dense plate, creating a foreland basin, for example, Molasse Basin in Germany.
When oceanic plate subducts under the continental plate, Trench slope basins and accretionary wedges are formed due to the thrusting movement of deep-sea sediments scrapping off from the subducting oceanic plate.
Back arc basins are formed on the continental magmatic or intra-oceanic island due to the subducting trench and slab rollback. Another form of basins known as Retro-arc-foreland basins is formed in the fold and thrust belt creating a continental crust juxtaposed against young magmatic arc.
When the two plates pass one other parallelly, the strike-slip fault comes into existence. Basins are formed at vertical fault plane curves when the curves move apart from each other. A negative flower structure is formed when the plates get to pull apart. An excellent example of this case is the Los Angeles basin.
Define Upstream In the oil & gas sector, all the activities which are related to exploration and production of oil and natural gas are termed as Upstream activities. The operations of Oil & Gas Industry include searching for subsurface O&G deposits to drill wells for production and then transporting the produced material to refineries to convert it to a usable form and finally make it available for end-users. The whole Oil & Gas process can be classified into three main categories which are known as Upstream, Midstream and Downstream. Image Source © Kalkine Group Upstream Process Upstream is referred to as “E&P” which means Exploration & Production. Exploration involves all the activities that are involved in search of specific subsurface areas where a petroleum reservoir is located. Whereas production involves all the processes involved in bringing the found subsurface deposit of oil & natural gas to the surface using drilling techniques. The upstream operations consist of Acquisition, Exploration, Appraisal, Development, Production and Abandonment Image Source © Kalkine Group Acquisition The acquisition consists of various activities starting from the purchase of license blocks and searching for O&G deposits using multiple sophisticated techniques. Acquisition on acreage can be made by open tendering either by direct negotiation or by the farm-in process into an existing block. Exploration After the acquisition of the O&G block, various extensive surveys like gravity, magnetic, seismic are conducted to delineate the subsurface deposits of hydrocarbon. The primary step in the exploration phase is Basin Assessment which is carried out to study the area of interest. An exploratory well is drilled to assess the geological details of that area. The companies use advance modelling techniques to prepare realistic basin models and to study the basin evolution. Based on available geological and geophysical survey data and results of the analysis made on that data, a decision is made either to move further with the lease or not. The fundamental objective of this phase is to evaluate the risk and to evaluate the volume of hydrocarbons present in the reservoir. Appraisal The main objective of appraisal stage is to assess the potential and extent of reservoir better. After completion of the exploratory phase, the reservoir engineers and geologists start planning appraisal activities. More wells are drilled to reduce the uncertainty of the evaluated volume of hydrocarbon. Expected Reservoir performance is evaluated and the production forecast for the field is prepared. Development After the completion of the Appraisal phase, the economic presence of hydrocarbon is confirmed or denied. If the field contains commercial deposits of hydrocarbon, a Field Development Plan (FDP) is prepared for the phased development of the field. The objective is to deplete the reservoir in most technically significant, safest end economical manner to get optimum Return on Investment (ROI). Appropriate locations for drilling various wells are drawn based on economics & survey results to start the production in a full-fledged manner. A proper assessment for LNG site is also conducted, which requires the construction of large scale export-import, storage and transportation facilities. Production After the completion of FDP, the field is developed, multiple wells are drilled, and production starts. The field starts producing from wells in a full-fledged manner. It is the first stage in the life-cycle of a field when the extracted hydrocarbon gives the first revenue from selling the O&G. When the revenue exceeds the investment that the company made during the initial phases, the company starts getting profit. This stage can last up to 45 years, depending on the potential of field and easiness to explore the reserves. After due course of time, the rate of production starts deteriorating due to the decline in reservoir pressure. Special EOR (Enhanced Oil Recovery) techniques like steam injection, water flooding is used to regain again the declined pressure of the reservoir which is commonly known as "Maintenance" phase to enhance the production rate. Finally, after producing the commercial hydrocarbons, the well is shut and abandoned. Abandonment The gradual decrease in production rates due to the decline in reservoir pressure leads to non-economic recovery of reserves from the reservoir. When the field reaches its economic limit, the company decides to stop production from that field and abandon the operations from there. The producing wells are plugged & abandoned (P&A). The objective is to protect the future commercial zones and near freshwater zones from contamination. Decommissioning of previously installed production facilities is done as they are no longer economical. The land under the facility is reconditioned, and environmental restoration is carried out.
Horizontal Well An oil and gas well with one or more of its sections drilled at an angle of eighty degrees or more from the vertical, for any specific objective, is known as Horizontal Well. It is drilled through a multi-directional drilling technique and used in places where conventionally drilled vertical wells cannot be drilled or otherwise, do not serve the purpose due to the complex shape of the reservoir. Vertical and Horizontal Wells Cross-sectional View (Source: Kalkine Group Image) The combination with hydraulic fracturing technique can lead to incredible results with greater success stories like the exploration of Marcellus shale in Appalachian basin and Bakken formation in North Dakota. How does it work? Drilling of Horizontal Wells in the mining industry has become more prominent after the successful track record set in the exploration of hydrocarbons during 2010 in terms of economics and efficiency, especially in the U.S. It was not possible to drill round-cornered wells on earth with conventional drilling techniques. In conventional drilling, the rotary torque to rotate the drill bit is provided by rotary-table present on the surface. The whole drill string rotates in this type of conventional drilling. On the flip side, it was not possible to drill horizontal wells by a conventional method as the rotation of the whole drill-string was not possible in this case. So, there was a need to rotate the drill-bit autonomously, without rotating the drill-string and a technique to steer the drill bit, to take a look underground. A big thank you to thrilling technology. Mud-motors and Rotary Steerable Systems (RSS) tools came into existence as a ray of hope. A mud-motor does not require rotary torque from the rotary table present on the surface. It gets driven by the supply of drilling fluid known as mud. Hydrostatic pressure build by mud rotates the shafts present in mud-motor which ultimately rotates the drill bit. RSS tools point the drill bit in the desired direction while operating from the surface along with Measurement while Drilling (MWD) tools which give the information about direction and inclination of the bit. A Revolution- Horizontal Well & Hydraulic Fracturing Kalkine Group Image The process of horizontal drilling involves drilling of a conventional vertical well to a certain depth and then bending the trajectory of the well to make it nearly horizontal. Usually, horizontally drilled footage overpasses the vertically drilled footage in these types of wells. Horizontal well allows more of the segments of a borehole to remain in contact with the paying zone or producing formation, increasing the yield of oil and gas. As per EIA accounts, hydraulically fractured horizontal wells contributed 69% of total O&G wells drilled in the U.S. in 2016. The “Shale Revolution” of the U.S. is driven by the combination of hydraulic fracturing and horizontal drilling which significantly increased the production of oil and natural gas from tight oil formations that accounts for about 36% of the total U.S. crude oil production. These deposits were inaccessible to traditional vertical wells due to limited contact of the wellbore in low porous and low permeable producing formation. Why drill Horizontal Wells? Even the per foot cost of drilling a horizontal well when combined with hydraulic fracturing is around three times more than a conventional vertical well. Still, they are preferred due to higher efficiency and production yields. Enlisted below are some of the significant reasons for drilling a horizontal well: Kalkine Group Image To reach inaccessible reservoirs: There are cases when the oil and gas reservoir is located just below the monument, park or a city’s prime location from where drilling a vertical well is nearly impossible. In this case, recovery from the reservoir can be made using a horizontal well drilling set-up. Produce more from a single well: This strategy has been used to reduce the surface impression of drilling activity. Recovery can be made from a broader zone using a single drilling pad by dodging the drilling of multiple wells, thereby reducing the drilling cost. Pay zone extension: As compared to vertical wells, horizontal well provides better contact between a borehole and producing formation. Increase in the extent of pay zone will lead to more fluids flowing into the well, increasing the productivity. Increased productivity in Fractured Reservoir: Production yield in a fractured reservoir can be attained by drilling a well in a manner that it can encounter the maximum number of fractures to increase the flow of fluid into the borehole. A limited number of fractures can be intersected using a vertical well. A relief well is a well that is drilled to control the sudden explosion of O&G in a well. A wild well is killed by intersecting it near its source to pump mud and water from directional relief well.
What are unconventional resources? An unconventional resource is an umbrella term used in petroleum industry energy resources tapped in reservoirs other than conventional ones. The porosity and permeability of unconventional reservoirs are less porous and permeable and require advanced techniques of exploration. Generally, the extraction of these resources is challenging on both technical and economic grounds. Still, gradual technological upliftment and intense global energy demands are paving the way to explore these resources. Some examples of unconventional resources are Coal Bed Methane (CBM), shale gas, shale oil, gas hydrates and tight gas sands. Birth of Unconventional Resources Three primary reasons played a key role in the popularity of unconventional resources in recent years. The major one being the abundant presence of unconventional resources and low degree of conventional resources recovery. Global unconventional oil resources are equivalent to conventional oil resources, whereas global unconventional gas resources are far more than conventional gas resources. EIA recently released its latest International Energy Outlook (IEO), which expects energy demand to grow by nearly half worldwide, between 2012 and 2040. The demand in developing countries will jump by almost 71 per cent while the rise will be ~18 per cent for developed nations. Kalkine Group Image The advancement in technology, including cost-effective methods of exploration and industrial automation, is another factor contributing to the popularity of unconventional resources. The introduction of advanced technology has converted economically & technically challenged reserves into commercially viable reserves. Horizontal drilling and hydrofracturing are some of the important breakthroughs; the petroleum industry has got. Distribution of Unconventional Resource Around 84 per cent of recoverable unconventional resources are situated in regions like Russia, Asia Pacific, North America, and Latin America. Most of the unconventional oil and gas resources are accumulated in passive continental margins and account for around 49 per cent of the world's undiscovered resources. On the other hand, foreland basins and continental rift basins, both individually account for approximately 20 per cent of the world's total unconventional resources. Finally, the craton basins contain the world's 11 per cent of the world's undiscovered unconventional resources. Unconventional Resource Potential The global resource of tight oil, heavy oil, natural bitumen, and oil shale oil is about 412×109 t combined, and the global resource extent of tight gas, coalbed methane and shale gas are 921.9×1012 m3. North America, South America and Asia are the global leaders having maximum recoverable resources of unconventional oil. On the other hand, Asia Pacific, Russia, and North America are the global leaders of recoverable unconventional gas. Extraction of Unconventional Resources One of the most important breakthroughs in the extraction of Unconventional resource is the introduction of Hydraulic Fracturing technique. It produces fractures in oil and gas-bearing formations with less porosity and permeability, stimulating the flow within the reservoir and increase the recoverable volume. Fractures are created by pumping an enormous quantity of pressurized fluids that mainly contain water and proppants along with some acidizing agents. After the completion of the injection process, the hydrocarbon present in the reservoir is activated by the internal pressure of rock formations, and it starts flowing towards the surface through the wellbore. Kalkine Group Image The other crucial technique is horizontal drilling which is used to drill a well horizontally. It increases the contact between reservoir and borehole, creating a better production interface in low porous and low permeable reservoirs, which is impossible to achieve in traditional vertical wells. Both horizontal drilling and hydraulic fracturing and, have opened new avenues for oil and gas development, with an emphasis on natural gas reservoirs, for example, coalbed, shale, and tight sands.