Thursday, September 26, 2019

Tight Oil Chapter 4

Success in extracting crude oil and natural gas from shale reservoirs depends largely on the hydraulic fracturing process (Speight, 2016b) that requires an understanding of the mechanical properties of the subject and confining formaitons. In hydraulic-fracturing design, Young's modulus is a criterion used to determine the most-appropriate fracturing fluid and other design considerations. Young's modulus provides an indication of the fracture conductivity that can be expected under the width and embedment considerations. Without adequate fracture conductivity, production from the hydraulic fracture will be minimal or nonexistent (Akrad et al., 2011). 

Typical of the crude oil from tight formations (tight oil- tight light oil and tight shale oil have been suggested as alternate terms) is the Bakken crude oil which is a light highly volatile crude oil. Briefly, Bakken crude oil is a light sweet (low-sulfur) crude oil that has a relatively high proportion of volatile constituents. The production of the oil also yields a significant amount of volatile gases (including propane and butane) and low-boiling liquids (such as pentane and natural gasoline), which are often referred to collectively as (low boiling or light) naphtha. 

By definition, natural gasoline (sometime also referred to as gas condensate) is a mixture of low-boiling liquid hydrocarbons isolate from crude oil and natural gas wells suitable for blending with light naphtha (light naphtha) can become extremely explosive, even at relatively low ambient-temperatures. 

Some of these gases may be burned off (flared) at the field well-head, but others remain in the liquid products extracted from the well (Speight, 2014a).

Bakken crude oil is considered to be a low-sulfur (sweet) crude oil and there have been increasing observations of elevated levels of hydrogen sulfide (H2S) in the oil. Hydrogen sulfide is a toxic, highly flammable, corrosive, explosive gas (hydrogen sulfide) and there have been increasing observations of elevated levels of hydrogen sulfide in Bakken oil. Thus, the liquids stream produced from the Bakken formation will include the crude oil, the low-boiling liquids, and gases that were not flared, along with the materials and byproducts of the hydraulic-fracturing process. 

Tuesday, September 24, 2019

Tight Oil Chapter 3

The most notable tight oil plays in North America include the Bakken shale, the Niobrara formation, the Barnett shale, the Eagle Ford shale, and the Miocene Monterey play of California's San Joaquin Basin in the United States, and the Cardium play in Alberta. In many of these tight formations, the existence of large quantities of oil has been known for decades and efforts to commercially produce those resources have occured sporadically with typically disappointing results. However, starting in the mid-2000s, advancements in well-drilling and stimulation technologies combined with high oil prices have turned tight oil resources into one of the most actively explored and produced targets in North America. 

Furthermore, of the tight oil plays, perhaps the best understood is the Bakken which straddles the border between Canada and the United States in North Dakota, Montana, and Saskatchewan. Much of what is known about the exploitation of tight oil resources comes from industry experiences in the Bakken and the prediction of future tight oil resource development described in this study are largely based on that knowledge. The Bakken tight oil play historically  includes three zones, or members, within the Bakken Formation. 

The upper and lower members of the Bakken are organic-rich shales which serve as oil source rocks, while the rocks of the middle member may be siltstone formations, sandstone formations, or carbonate formations that are also typically characterized by low permeability and high oil content. Since 2008 the Three Forks Formation, another tight oil-rich formation which directly underlies the lower Bakken shale, has also yielded highly productive oil wells. Drilling, completion, and stimulation strategies for wells in the Three Forks Formation are similar to those in the Bakken and the light, sweet crude oil that is produced from both plays has been geochemically determined to be essentially identical. Generally, the Three Forks Formation is considered to be part of the Bakken play, though the authors of published works will sometimes refer to it as the Bakken-Three Forks play.

Other known tight formations (on a worldwide basis) include the R'Mah Formation in Syria, the Sargelu Formation in the northern Persian Gulf region, the Athel Formation in Oman, the Bazhenov formaiton and Achimov Formation in West Siberia, Russia, the Coober Pedy in Australia, the Chicontepex formation in Mexico, and the Vaca Muerta field in Argentina (US EIA, 2011, 2013). However, tight oil formations are heterogeneous and vary widely over relatively short distances. Thus, even in a single horizontal drill hole, the amount of oil recovered may vary as may recovery within a field or even between adjacent wells. This makes evaluation of shale plays and decisions regarding the profitability of wells on a particular lease difficult and a tight reservoir which contains only crude oil (without natural gas as the pressurizing agent) cannot be economically produced (US EIA, 2011, 2013).

Tight Oil Chapter 2

The challenges associated with the production of crude oil from shale formation are a function of the compositional complexity and the varied geological formations where they are found. These oils are light, but they often contain high proportions of waxy constituents and, for the most part, reside in oil-wet formations. These phenomena create some of the predominant difficulties associated with crude oil extraction from the shale formations and include (1) scale formation, (2) salt deposition, (3) paraffin wax deposits, (4) destabilized asphaltene constituents, (5) equipment corrosion, and (6) bacteria growth. Thus, multicomponent chemical additives are added to the stimulation fluid to control these problems. 

 While crude oil from tight shale formations is characterized by a low content of asphaltene constituents and low-sulfur content, there can be a significant proportion of wax constituents in the oil. These constituents may exhibit a broad distribution of the molecular weight. For example, paraffin carbon chains of C10-C60 have been found and some tight crude oil may even have hydrocarbon carbon chains (wax) up to C72. While this may be a relief from recovery of high-asphaltene heavy oils, the joy is short-lived and the deposition of waxy constituents can cause as many problems as asphaltene incompatibility. To control deposition and plugging in formations due to paraffin, a variety of wax dispersants are available for use. In upstream applications, the paraffin wax dispersants are applied as part of multifunctional additive packages where, for convenience, asphaltene stability and corrosion control can also be addressed simultaneously. 

 Scale deposits of calcite , carbonates, and silicates must also be controlled during production or plugging problem arise. A wide range of scale additivies is available. These additives can be highly effective when selected appropriately. Depending the nature of the well and the operational conditions, a specific chemistry is recommended or blends of products are used to address scale deposition.

Monday, September 16, 2019

Tight Oil

In addition, oil from tight sandstone and from shale formations is another type of crude oil which varies from a gas-condensate type liquid to a highly volatile liquid.

Tight oil refers to the oil preserved in tight sandstone or tight carbonate rocks with low matrix permeability- in these reservoirs, the individual wells generally have no natural productivity or their natural productivity is lower than the lower limit of industrial oil flow, but industrial oil production can be obtained under certain economic conditions and technical measures. Such measures include acid fracturing, multistage fracturing , horizontal wells, and multilateral wells. 

The term light tight oil is also used to describe oil from shale reservoirs and tight reservoirs because the crude oil produced from these formations is light crude oil. The term light crude oil refers to low-density petroleum that flows freely at room temperature and these light oils have a higher proportion of light hydrocarbon fractions resulting in higher API gravities (between 37 and 42 degrees) (Speight, 2014a). However, the crude oil contained in shale reservoirs and in tight reservoirs will not flow to the wellbore without assistance from advanced drilling (such as horizontal drilling) and fracturing (hydraulic fracturing) techniques. 

There has been a tendency to refer to this oil as shale oil. This terminolgy is incorrect insofar as it is confusing and the use of such terminology should be discouraged as illogical since shale oil has been the name given to the distillate produced from oil shale by thermal decomposition. 

There has been the recent (and logical) suggestion that shale oil can be referred to as kerogen oil (IEA, 2013).

Monday, September 9, 2019

Tight Gas

In respect of the low permeability of these reservoirs, the gas must be developed via special techniques including stimulation by hydraulic fracturing (or fracking) in order to be produced commercially.

 Conventional  gas typically is found in reservoirs with permeability >1 mD and can be extracted via traditional techniques. A large proportion of the gas produced globally to date is conventional, and is relatively easy and inexpensive to extract. In contrast, unconventional gas is found in reservoirs with relatively low permeability (<1 mD) and hence cannot be extracted via conventional methods. However, there several types of unconventional gas resources that are currently under production but the three most common types are (1) shale gas, (2) tight gas, and (3) coalbed methane although methane hydrates are often included with these gases under the general umbrella of unconventional gas.

Generally, shale gas is a natural gas contained in predominantly fine, low-permeable sedimentary rocks, in consolidated clay-sized particles, at the scale of nanometers. Gas shale formations are organic-rich formations that are both source rock and reservoir. 

The expected value of permeability to gas flow is in the range of micro- to nanodarcy. The gas retained in such deposits is in the form of adsorbed material on rock, trapped in pore spaces and as an interbedding material with shales. Although the shale gas is usually very clean, it is hard to recover from deposits because of the structural complexity and low hydrodynamic conductivity of shales.

Shale gas is part of a continuum of unconventional gas that progresses from tight gas sand formations, tight gas shale formations to coalbed methane in which horizontal drilling and fracture stimulation technology  can enhance the natural fractures and recover gas from rocks with low permeability. Gas can be found in the pores and fractures of shales and also bound to the matrix, by a process known as adsorption, where the gas molecules adhre to the surfaces within the shale. During enhanced fracture stimulation drilling technology, fluid is pumped into the ground to make the reservoir more permeable, then the fractures are propped open by small particles, and can enable the released gas to flow at commercial rates.  By drilling multilateral horizontal wells followed by hydraulic fracturing, a greater rock volume can be accessed.

More specifically, shale gas is natural gas that is produced from a type of sedimentary rock derived from clastic sources often including mudstones or siltstones, which is known as shale. Clastic sedimentary rocks are composed of fragments (clasts) of preexisting rocks that have been eroded, transported, deposited, and lithified into new rocks. Shales contain organic material which was lain down along with the rock fragments. 

In areas where conventional resource plays are located, shales can be found in the underlying rock strata and can be the source of the hydrocarbons that have migrated upwards into the reservoir rock. Furthermore, a tight gas reservoir is commonly defined as is a rock with matrix porosity of 10% or less and permeability of 0.1 mD or less , exclusive of fracture permeability. 

Shale gas resource plays differ from conventional gas plays in that the shale acts as both the source for the gas and alsto the zone (also known as the reservoir) in which the gas is trapped. The very low permeability of the rock causes the rock to trap the gas and prevent it from migrating toward the surface. The gas can be held in natural fractures or pore spaces, or can be adsorped onto organic material. With the advancement of drilling and completion technology, this gas can be successfully exploited and extracted commercially as has been proven in various basins in North America.

Aside from permeability, the key properties of shales, when considering gas potential, are total organic carbon (TOC) and thermal maturity. The total organic content is the total amount of organic material (kerogen) present in the rock, expressed as a percentage by weight. Generally, the higher the total organic content, the better the potential for hydrocarbon generation. The thermal maturity of the rock is a measure of the degree to which organic matter contained in the rock has been heated over time and potentially converted into liquid and/or gaseous hydrocarbons. Thermal maturity is measured using vitrinite reflectance (Ro).

Because of the special techniques required for extraction, shale gas can be more expensive than conventional gas to extract. On the other hand, the inplace gas resource can be very large given the significant lateral extent and thickness of many shale formations. However, only a small portion of the total world resources of shale gas is theoretically producible and even less likely to be producible in a commercially viable manner. 

Sunday, September 8, 2019

Tight Gas andd Tight Oil

The terms tight oil and tight gas refer to crude oil (primarily light sweet crude oil) and natural gas, respectively, that are contained in formations such as shale or tight sandstone, where the low permeability of the formation makes it difficult for producers to extract the crude oil or natural gas except by unconventional techniques such as horizontal drilling and hydraulic fracturing. The terms unconventional oil and unconventional gas are umbrella terms for crude oil and natural gas that are produced by methods that do not meet the criteria for conventional production. Thus, the terms tight oil and tight gas refer to natural gas trapped in organic-rich rocks dominated by shale while tight gas trapped in in sandstone or limestone formations that exhibit very low permeability and such formations may also contain condenstate. Given the low permeability of these reservoirs, the gas must be developed via special drilling and production techniques including fracture stimulation (hydraulic fracturing) in order to be produced commercially (Gordon, 2012).

Unlike conventional mineral formations containing natural gas and crude oil reserves, shale and other tight formations have low permeability, which naturally limits the flow of natural gas and crude oil. In such formations, the natural gas and crude oil are held in largely unconnected pores and natural fractures. Hydraulic fracturing is the method commonly used to connect these pores and allow the gas to flow. The process of producing natural gas and crude oil from tight deposits involves many steps in addition to hydraulic fracturing, all of which involve potential environmental impacts (Speight, 2016b). 

Hydraulic fracturing is often misused as an umbrella term to include all of the steps involved in gas and oil production from shale formations and tight formations. These steps include road and well-pad construction, drilling the well, casing, perforating, hydraulic fracturing, completion, production, abandonnment, and reclamation. 

Tight sandstone formations and shale formations are heterogeneous and vary widely over relatively short distances. Thus, even in a single horizontal drill hole, the amount of gas or oil recovered may vary, as may recovery within a field or even between adjacent wells. This makes evaluation of tight plays (a play is a group of fields sharing geological similarities where the reservoir and the trap control the distribution of oil and gas). Because of the variability of the reservoirs- even reservoirs within a play- is different, decisions regarding the profitability of wells on a particular lease are difficult. Furthermore, the production of crude oil from tight formations requires that at least 15-20% v/v of the reservoir pore space is occupied by natural gas to provide the necessary reservoir energy to drive the oil toward the borehole; tight reservoirs which contain only oil cannot be economically produced (US EIA, 2013) 

In tight shale reservoirs and other tight reservoirs, there are areas known as sweet spots which are preferential targets for drilling and releasing the gas and oil. In these areas, the permeability of the formation is significantly higher than the typical permeability of the majority of the formations. The occurence of a sweet spot and the higher permeability may often result from open natural fractures, formed in the reservoir by natural stresses, which results in the creation of a dense pattern of fractures. Such fractures may have reclosed, filled in with other materials, or may still be open. However, a well that can be connected through hydraulic fracturing to open natural fracture systems can have a significant flow potential. 

Gas Hydrate

Methane hydrates is a resource in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice (Kvenvolden, 1995).. 

Natural gas hydrates are solids that form from a combination of water and one or more hydrocarbon or non-hydrocarbon gases. In physical appearance, gas hydrates resemble packed snow or ice. In a gas hydrate, the gas molecules (such as methane, hence the methane hydrates) are trapped within a cage-like crystal structure composed of water molecules. Gas hydrates are stable only under spesific conditions of pressure and temperature. Under the appropriate pressure, they can exist at temperatures significantly above the freezing point of water.  The maximum temperature at which gas hydrate can exist depends on pressure and gas composition. For example, methane plus water at 600 psia forms hydrate at 5 degree C, while at the same pressure, methane with 1% v/v propane forms a gas hydrate at 9.4 degree C. Hydrate stability can also be influenced by other factors, such as salinity (Edmonds et al, 1996).

Thursday, September 5, 2019

Fractured Reservoirs

  • Fractured reservoirs are reservoirs in which production and recovery is influenced to a greater or lesser extent by fractures. They can be subdivided into four different types (cf. Nelson , 2001; Allan & Qing Sun , 2003)
  • The variability in fracture network interconnectedness, and in the architecture and properties of the matrix, are the basic reasons that fractured reservoirs show a large variety of behaviors during hydrocarbon production. These large uncertainties make the appraisal, development and management of fractured reservoirs difficult. Failure to asses uncertainties properly leads to missed opportunities and low hydrocarbon recovery.
  • The special nature of fractured reservoirs lies in the interaction between, the (relatively) high pore volume , low permeability matrix (the storage domain) and the low pore volume, high permeability fracture system (the flow domain). This interaction is a function of matrix architecture and fracture network geometry, but also the mechanisms and physical processes that control the transfer of hydrocarbons from the matrix to the fracture network. The initial and developing stress state and the presence or absence of an aquifer also influence performance.