Low Coal Bed Methane Research Articles

Facile preparation of nitrogen-doped microporous carbon from potassium citrate/urea for effective CH4 separation and uptake

N-doped microporous carbons have attracted much attention for micromolecule gas separation and accumulation owning to their superior adsorption potential and surface polarizability. However, to prepare N-doped microporous carbon usually acquire excess corrosive KOH as activator, which limits its application in industry. Here, we propose a non-corrosion method for the manufacture of narrow N-doped microporous carbons using potassium citrate acts as activating agent, and urea acts as both a nitrogen doping agent and coactivator without any solvent. The BET surface area, micropore volume, pore size distribution and N content can be easily adjusted by changing the potassium citrate/urea ratio and activating temperature. The influence of porous structure and N content on CH4 uptake and separation was investigated, and ultra-micropore volume (<1nm) plays a dominate role in CH4 selectivity adsorption. ACK2N1 had the largest CH4 adsorption capacity of 3.00 mmol/g and CH4/N2 selectivity of 7.11 at 273.15 K and 100 KPa due to its largest ultra-micropore volume and N content. The adsorption breakthrough, regeneration experiments and adsorption thermodynamics showed that the prepared well-developed N-doped microporous carbons have very good potential for CH4 separation and enrichment from low coal bed methane. Furthermore, the pore formation mechanism of the well microporous carbon materials is proposed.

South Anze Structure and Its Control on Coalbed Methane Aggregation in the Qinshui Basin and the Mechanism of Syncline Gas Enrichment in the Qinshui Basin

The phenomenon of coalbed-methane synclinal accumulation in the Qinshui Basin has been widely reported, but it has mainly been observed in the core block of the Qinshui Syncline. The questions arise: does this phenomenon exist in the wing of the Qinshui Syncline and, if so, what is the mechanism behind it? Further study is required to answer these questions. This paper focuses on the South Anze No. 3 coal seam in the Qinshui Basin as an example. It conducts a systematic sorting of coalbed-methane geological characteristics and an analysis of the effects of structural assemblage characteristics, genetic mechanisms, and structural control on coalbed-methane accumulation. Additionally, it examines the basin structure and evolution during the critical period of the Qinshui Basin, as well as the gas geological characteristics of adjacent areas, in order to discuss the gas-rich mechanism of the syncline in the Qinshui Basin. Key insights obtained from the study include the following: (i) The whole South Anze is a nosing structure that plunges from west to east and superposes secondary folds and faults in different directions. Four deformation zones can be identified based on the characteristics of structural assemblage, including NEN-oriented compressive structures, ENE-trend shear fractures, EW-trend compressive fractures, and EW-trend compressive folds. The formation of structural assemblage in the study area is attributed to the compression in the Indosinian and Yanshanian, and the fault inversion in the Himalayan period. (ii) The ENE-trend shear fracture deformation area located in the nosing uplift is a low CBM (coalbed methane) content area due to gas diffusion during the Himalayan extension. The syncline in the combination of NEN-trend and EW-trend “ejective folds” in the west and south of the study area is a high-value area of coalbed-methane content. It is further verified that the law of syncline gas accumulation in the Qinshui Basin is also applicable to the wing of the Qinshui Syncline. (iii) Since the formation of the Qinshui Syncline, the main coal seam has been in an extensional environment below the neutral plane, resulting in the main dissipation of coalbed methane. During its geological history, surface water penetrated the aquifer above the main coal seam through two channels: the extensional area above the neutral plane of the adjacent anticline and the shear fracture. A hydrostatic pressure seal is formed in the Qinshui Syncline and the secondary syncline is superimposed upon it, which is the cause of gas enrichment in the syncline of the Qinshui Basin. (iv) Weak deformation in the syncline basin is the focus of global coalbed-methane exploration and development. The mechanism proposed in this paper can provide ideas and references for further understanding of coalbed-methane enrichment in this type of basin.

Genesis of Low CBM Production in Mid-Deep Reservoirs and Methods to Increase Regional Production: A Case Study in the Zhengzhuang Minefield, Qinshui Basin, China.

With the increase of the burial depth of the no. 3 coal seam in the Zhengzhuang minefield of Qinshui Basin, the production of surface coal bed methane (CBM) vertical wells was low. By means of theoretical analysis and numerical calculation, the causes of low production of CBM vertical wells were studied from the aspects of reservoir physical properties, development technology, stress conditions, and desorption characteristics. It was found that the high in situ stress conditions and stress state changes were the main controlling factors of the low production in the field. On this basis, the mechanism of increasing production and reservoir stimulation was explored. An L-type horizontal well was constructed alternately among the existing vertical wells on the surface to initiate a method to increase the regional production of fish-bone-shaped well groups. This method has the advantages of a large fracture extension range and a wide pressure relief area. It could also effectively connect the pre-existing fracture extension area of surface vertical wells, realizing the overall stimulation of the low-yield area and increasing the regional production. Through the optimization of the favorable stimulation area in the minefield, 8 L-type horizontal wells that adopted this method were constructed in the area with high gas content (greater than 18 m3/t), a thick coal seam (thicker than 5 m), and relatively rich groundwater in the north of the minefield. The average production of a single L-type horizontal well reached 6000 m3/d, which was about 30 times that of the surrounding vertical wells. The length of the horizontal section and the original gas content of the coal seam had a significant influence on the production of the L-type horizontal wells. This method for increasing the regional production of fish-bone-shaped well groups was an effective and feasible low-yield well stimulation technology, which provided a reference for increasing the production and efficiently developing CBM under the high-stress conditions in mid-deep high-rank coal seams.

CBM exploration: Permeability of coal owing to cleat and connected fracture

Coalbed methane (CBM) resources cannot be efficiently explored and exploited without a robust understanding of the permeability of fracture-size heterogeneities in coal. In this study, two sister coal samples were imparted with pre-developed cleat and connected fractures, and the permeability of the coal samples was measured under different conditions of controlled confining and gas pressures. Furthermore, the implications of the results for CBM exploration and exploitation were discussed. The permeability of coal with cleat development ranged from 0.001–0.01 mD, indicating ultra-low permeability coal. The gas migration in this coal changed from a linear flow to a non-linear flow, with the increase in gas pressure (&gt;1 MPa). Thus, the permeability of the coal initially increased and then decreased. However, the Klinkenberg effect does not exist in this ultralow-permeability coal. For the coal sample with connected fracture, permeability ranged from 0.1–10 mD, which is larger by hundred orders of magnitude than that of the sample with cleat. For this coal, with a decrease in gas pressure (&lt;1 MPa), the Klinkenberg effect significantly increased the permeability of the coal. With an increase in the applied confining pressure, both the Klinkenberg coefficient and permeability of the coal presented a decreasing trend. It is suggested that field fracture investigation is a prerequisite and indispensable step for successful CBM production. The coal beds that cleat network is well conductive to the connected fracture can be an improved target area for CBM production. During CBM production, a variety of flow regimes are available owing to the decrease in CBM reservoir pressure. In particular, under the low CBM reservoir pressure and low in situ geo-stress conditions, the gas migration in the CBM reservoir with connected facture development exhibits remarkable free-molecular flow. Thus, the reservoir permeability and predicted CBM production will be enhanced.

Invaded water flowback capability of high volatile bituminous coal after nitrogen displacing: from a perspective of fracture and seepage pore structural complexities

Water-based fracturing is applied for stimulating coal reservoir. However, invasion of water can result in low flowback efficiency and low coalbed methane (CBM) yield. Nitrogen injection is an effective method to enhance CBM production. In this study, nitrogen displacing water experiments using water saturated high volatile bituminous coal were conducted, during which magnetic resonance imaging (MRI) tests were applied to determine flowback capability. To research the relationship between flowback capability and complexities of fracture and seepage pore, characterising methods including micro-CT and mercury intrusion porosimetry (MIP) were used. The results show that flowback capabilities of complex structural seepage pores and fractures were low. Water is easily flooded into simple structural seepage pores and fractures, and water flowback capabilities of these simple spaces are relative higher, compared with those of the complex ones. Water phase trapping during nitrogen displacing water happens due to water imbibition and Jamin effect occurring in complex structural seepage pores and fractures. [Received: April 27, 2019; Accepted: August 7, 2020]

Invaded water flowback capability of high volatile bituminous coal after nitrogen displacing: from a perspective of fracture and seepage pore structural complexities

Water-based fracturing is applied for stimulating coal reservoir. However, invasion of water can result in low flowback efficiency and low coalbed methane (CBM) yield. Nitrogen injection is an effe...

Experimental study on enhancing coal-bed methane production by wettability alteration to gas wetness

The development of coalbed methane (CBM) has been challenging as many coal formations have very low permeabilities, which causes the low CBM production. One of the main stimulation techniques to enhance CBM production is large scale fracturing. The high leak-off and the low flow back percentage of the injected fracturing fluids result in poor CBM production. Gas wettability alteration (GWA) approach has been demonstrated successful in sandstone reservoirs. However, few microscopic study or experiments have been conducted in coal samples for GWA in order to enhance CBM production. Additionally, most of the studies in the literature focused mainly on surfactants that reduce the capillary pressure of coal through surface tension instead of wettability. In such cases, the incremental CBM recovery is limited. In this study, we conducted experiments with the fluorocarbon surfactants Perfluorooctyl Methacrylate Monomer-containing Polymethacrylate (PMP) and developed approaches to altering the wettability of coal samples to strong gas-wetness. ESEM (environment scanning electron microscope) observation was used to examine the microscopic mechanism of GWA. The changes in contact angles and spontaneous imbibition rates were used to judge the wettability alteration macroscopically. Experimental results demonstrated that the imbibed water decreased by 59.99%wt and gas-flooding water recovery was enhanced by about 20.31%wt after the wettability of the coal sample was altered from water- to gas-wetness. Furthermore, experimental results from both sandstone and coal samples were compared after gas wettability alteration. Although sandstone sample showed a greater water recovery than that of coal sample, the coal sample demonstrated a greater incremental enhanced recovery.

Problems in the development of high-rank CBM horizontal wells in the Fanzhuang–Zhengzhuang Block in the Qinshui Basin and countermeasures

Coal reservoirs in the Qinshui Basin are characterized by high thermal evolution degree, low permeability, low reservoir pressure, lower gas saturation and strong heterogeneity, so its coalbed methane (CBM) development is quite difficult. In this paper, the development practice of high-rank CBM horizontal wells in the Fanzhuang–Zhengzhuang Block was analyzed in terms of geological and engineering factors to clarify the productivity influencing factors, suitable geological conditions and potential tapping countermeasures of multi-lateral horizontal wells. It is shown that the reasons for the low development efficiency of multi-lateral horizontal wells are divided into three types. The first one is geological factor, such as encountering low CBM content areas or faults. The second is engineering factor, such as drilling fluid plugging, drilling collapse, drainage collapse and dust coal blockage. The third is the combination of both factors. It is concluded that encountering low CBM content areas and faults, collapse and blockage are the main reasons for the low production of multi-lateral horizontal wells in the Zhengzhuang–Fanzhuang Block, with the CBM content higher than 20 m3/t, the ratio of critical desorption pressure and reservoir pressure higher than 0.7 and vitrinite reflectance (Ro) higher than 3.8. The prerequisite for an open-hole horizontal well in this area to produce CBM at a high rate is that the well lies in the tensile stress zone. Finally, the countermeasures to tap the potential of some inefficient wells were put forward. First, the inefficient wells which are blocked with dust coal or collapsed in the later stage should be stimulated based on classifications. And second, it is necessary to explore new types of horizontal wells so as to deal with borehole collapse and continue the operation in the later stage by using tree-like roof horizontal wells, single-lateral horizontal wells with casing or screen completion and fish-bone horizontal wells.

Production characteristics and reservoir analysis of coalbed methane reservoirs

Six Appalachian coalbed methane (CBM) projects are reviewed and evaluated in terms of reservoir engineering techniques and CBM recoverability. These include three non-mine-related projects in the northern Appalachian basin, one non-mine-related, and two mine-related projects in the central Appalachian basin of southwestern Virginia. Generally, low production rates and low CBM reserves are characteristic of the central and northern Appalachian basin; therefore, minimizing production costs is fundamental to the economic success of a CBM project in these regions. Well deliverability is determined through permeability testing and through production data analysis using a reservoir simulator to history match historic production data. All of the non-mine-related projects utilize vertical wells with well spacings of 60 to 100 acre (0.23–0.40 km 2) that produce from multiple coalbeds at depths as much as 762 m. The targets are coalbeds of the Pocahontas and New River Formations and correlative Lee Formation. In the Nora Field of Virginia (Equitable Resources, developer), well spacing is 60 acre (0.23 m 2), and recoverability is high (30–60% of initial gas in place). Well stimulation is by foam or water as the fracture medium. The average gas in place is 675 MMscf well −1, and the expected lifetime of the wells is 30 years. Cost of water disposal is a major economic factor in CBM development in the Appalachian basin. Water disposal is accomplished by using water disposal holes (injection wells) rather than surface water discharged, which is generally not permitted. Water disposal in the central Appalachian basin involves relatively low volumes of water (30 STB day −1, declining to 1 to 2 STB day −1). In the northern Appalachian basin in Indiana County, PA (Belden and Blake, developer), vertical CBM holes penetrate six to eight Allegheny Formation coalbeds. Water and a sand proppant are used for stimulation. Production levels are low (<100 Mcf day −1 well −1). The life of a vertical CBM hole drilled in advance of mining is 3 to 5 years. This type of hole may be converted to a gob hole after mining is completed. Gob gas is recovered mainly from coalbeds above the mined coalbed and has high recoverability of as much as 10 MMscf day −1 well −1. Horizontal CBM holes produce almost pure methane and typically have 1 year of production.