Thursday, October 17, 2019

Tight Gas chapter 2

By definition, shale gas is the hydrocarbon present in organic rich, fine-grained, sedimentary rocks (shale and associated lithofacies). The gas is generated and stored in situ in gas shale as both adsorbed gas(on organic matter) and free gas (in fractures or pores). As such, shale containing gas is a self-sourced reservoir. Low-permeable shale requires extensive fractrues (natural or induced) to produce commercial quantities of gas.

Shale is a very fine-grained sedimentary rock, which is easily breakable into thin, parallel layers. It is a very soft rock, but it does not disintegrate when it becomes wet. The shale formations can contain natural gas, usually when two thick, black shale deposits sandwich a thinner area of shale. Because of some of the properties of the shale deposits, the extraction of natural gas from shale formations is more difficult and perhaps more expensive than that of conventional natural gas. Shale basins are scattered across the United States. 

There are several types of unconventional gas resources that are currently produced: (1) deep natural gas - natural gas that exists in deposits very far underground, beyond "conventional" drilling depths, typically 15,000 ft or more, 

(2) shale gas- natural gas that occurs in low-permeability formations, (3) tight natural gas- natural gas that occurs in low-permeability shale formations , (4) geopressurized zones - natural underground formations that are under unusually high pressure for their depth, (5) Coalbed methane- natural gas that occurs in conjunction with coal seams, and (6) methane hydrates- natural gas that occurs at low-temperature and high-pressure regions such as the sea bed and is made up of a lattice of frozen water, which forms a cage around the methane. 

Coalbed methane is produced from wells drilled into coal seams which act as source and reservoir to the produced gas (Speight, 2013). These wells often produce water in the initial production phase, as well as natural gas. Economic coalbed methane reservoirs are normally shallow, as the coal  matrix tends to have insufficient strength to maintain porosity at depth.

Tuesday, October 15, 2019

Tight Gas

Tight gas describes natural gas that has migrated into a reservoir rock with high porosity but low permeability. This type of reservoir is not usually associated with oil and commonly require horizontal drilling and hydraulic fracturing to increase well output to cost-effective levels. In general, the same drilling and completion technology that is effective with shale gas can also be used to access and extract tight gas. 

Tight gas is the fastest growing natural gas resource in the United States and worldwide as a result of several recent development (Nehring, 2008). Advances in horizontal drilling technology allow a single well to pass through larger volumes of a shale gas reservoir and, thus, produce more gas. The development of hydraulic-fracturing technology has also improved access to shale gas deposits. This process requires injecting large volumes of water mixed with sand and fluid chemicals into the well at high pressure to fracture the rock, increasing permeability and production rates

 To extract tight gas, a production well is drilled vertically until it reaches the shale formation, at which point the wellbore turns to follow the shale horizontally. As drilling proceeds, the portion of the well within the shale is lined with steel tubing (casing). After drilling is completed,small explosive charges are detonated to create holes in the casing at intervals where hydraulic fracturing is to occur. In a hydraulic-fracturing operation, the fracturing fluid is pumped in at a carefully controlled pressure to fracture the rock out to several hundred feet from the well. Sand mixed with the fracturing fluid acts to prop these cracks open when the fluids are subsequently pumped out.  After fracturing, gas will flow into the wellbore and up to the surface, where it is collected for processing and sales.

Shale gas is natural gas produced from shale formations that typically function as both the reservoir and source rocks for the natural gas. In terms of chemical makeup, shale gas is typically a dry gas composed primarily of methane (60-95 %v/v) , but some formations do produce wet gas.The Antrim and New Albany plays have typically produced water and gas. Gas shale formations are organic-rich shale formations that were previously regarded only as source rocks and seals for gas accumulating in the strata near sandstone and carbonate reservoirs of traditional onshore gas development. 

Thursday, October 3, 2019

Tight Oil chapter 5

Oil from tight shale formation is characterized by low-asphaltene content, low-sulfur content, and a significant molecular weight distribution of the paraffinic wax content (Speight, 2014a, 2015a). Paraffin carbon chains of C10-C60 have been found, with some shale oils containing carbon chains up to C72. To control deposition and plugging in formations due to paraffins, the dispersants are commonly used. In upstream applications, these paraffin dispersants are applied as part of multifunctional additive packages where asphaltene stability and corrosion control are also addressed simultaneously (Speight, 2014). In addition, scale deposits of calcite (CaCo3), other carbonate minerals (minerals containing the carbonate ion, CO3 2-) and silicate minerals (minerals classified on the basis of the structure of the silicate group, which contains different rations of silicon and oxygen) must be controlled during production or plugging problem arise. 

A wide range of scale additives is available which 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.

Another challenge encountered with oil from tight shale formations- many of which have been identified but undeveloped - is the general lack of transportation infrastructure. Rapid distribution of the crude oil to the refineries is necessary to maintain consistent refinery throughput- a necessary aspect of refinery design. 

Finally, the properties of tight oil are highly variable. Density and other properties can show wide variation, even within the same field. The Bakken crude is light and sweet with an API of 42 degrees and a sulfur content of 0.19% w/w. Similarly, Eagle Ford is a light sweet feed, with a sulfur content of approximately 0.1% w/w and with published API gravity between 40 and 62 degrees API.

Paraffin waxes are present in tight oil and remain on the walls of railcars, tank walls, and piping. The waxes are also known to foul the preheat sections of crude heat exchanger (before they are removed in the crude desalter). Paraffin waxes that stick to piping and vessel walls can trap amines against the wall which can create localized corrosion. 

 In many refineries, blending two or more crude oils as the refinery feedstock is now standard operating procedure which allows the refiner to achieve the right balance of feedstock qualities. However, the blending of the different crue oils may cause problems if the crude oils being mixed are incompatible (Speight,2014a). When crude oils are incompatible, there is increased deposition of the asphaltene constituents (Speight,2014a) which accelerates fouling in the heat exchanger train downstream of the crude desalter.