Vertical Wells Research Articles

Impacts of wellbore orientation with respect to bedding inclination and injection rate on laboratory hydraulic fracturing characteristics of Lushan shale

A series of hydraulic fracturing experiments were conducted using Lushan shale cores (50 mm in diameter and 100 mm in length) to unravel the formation mechanism of fracture networks under the combined effects of wellbore type (vertical or horizontal well), wellbore orientations relative to the bedding plane (0°, 30°,45°, 60°, and 90°), bedding inclinations (0°, 30°,45°, 60°, and 90°), and injection rates (0.03, 0.3, 3, and 30 mL/min). We monitored acoustic emission and surface displacements during the test and conducted post failure inspection via a fluorescent tracer and a 3D laser scanner. We show that adjusting the maximum principal stress perpendicular to the wellbore is beneficial for reducing the breakdown pressure. Inclined bedding tends to initiate along the bedding plane with relatively low fracture initiation pressure and breakdown pressure. Deploying the wellbore inclined to bedding inclination and/or adopting a low injection rate is beneficial to reduce the shale breakdown strength and create a complex fracture network. However, this will correspondingly increase the fracture surface roughness, which may impede the effective transportation of proppant. These findings can provide an important reference for field hydraulic fracturing.

Well Testing Analysis Method and Practice for Low Permeability Gas Reservoirs

With the gradual maturity of well testing interpretation theory, some scholars have conducted research on vertical well testing models, horizontal well testing models, and inclined well testing models for fractured and vuggy reservoirs, and plotted typical well testing curves. However, during the drilling process, the wellbore is usually connected to both the fractures and the reservoir matrix, resulting in a dual permeability situation where the fractures and matrix simultaneously supply fluid to the wellbore. Based on this, some scholars have established a horizontal well dual hole dual permeability testing model, a vertical well three hole dual permeability testing model, and a horizontal well three hole dual permeability testing model, making the results of well testing interpretation more reasonable and reliable. For the well testing analysis of inclined wells in fractured and vuggy reservoirs, the existing inclined well models only consider the three hole single permeability situation where the fractures are connected to the wellbore. There is little research on the three hole dual permeability well testing model where the inclined well matrix and fractures supply the wellbore simultaneously. Therefore, based on previous research, this article establishes and solves a well testing model for inclined wells with three pores and dual permeability, using the effective wellbore diameter and the principle of Duhame superposition, taking into account the bottom hole pressure effect, and draws sample curves for the analysis of inclined wells in fractured and vuggy reservoirs. This provides a reference for the analysis of inclined wells in fractured and vuggy reservoirs.

Effects of Reservoir Boundary Conditions, Drainage Shape, and Well Location on Productivity of a Vertical Well

This paper gives a review of steady state and pseudosteady state productivity equations for an unfractured fully penetrating vertical well in a permeability anisotropic reservoir. This paper also studies the effects of drainage area, reservoir boundary conditions, drainage shape, and well location on productivity. The production performances of an unfractured vertical well in a circular reservoir, a sector fault reservoir and a rectangular reservoir are studied and compared. Mechanical skin factor is included in the productivity equations. This paper examines the steady state and pseudosteady state production performance of oil wells with constant flow rates in different drainage shapes, a library of productivity equations is introduced, several combinations of closed and/or constant pressure boundary conditions are considered at lateral reservoir boundaries. The equations introduced in this paper can be used to determine the economical feasibility of a drilling an unfractured fully penetrating vertical well. It is concluded that, drainage area and reservoir boundary conditions have significant effects on productivity of a well, and productivity is a weak function of drainage shape and well location.

Numerical Simulation of Hole Cleaning of a Horizontal Wellbore Model with Breakout Enlargement Section

Horizontal wells are more likely than vertical wells to have enlarged wellbore sections due to borehole instability. However, there is scarce research on borehole cleaning of horizontal wells with enlarged wellbore sections. In this paper, we establish a horizontal wellbore model with a breakout enlargement section using field borehole diameter data. We used the three-dimensional computational fluid dynamics (CFD) method and the Realizable k-ε turbulence model with the Euler–Euler approach to simulate the effects of the drilling fluid circulation return speed and the spinning speed of the drill pipe on the cutting movement of conventional horizontal wells and horizontal wells with a breakout enlargement section. The simulation results demonstrate that increasing the drilling fluid circulation return speed and the spinning speed of the drill pipe does not significantly improve the hole cleaning impact for horizontal wells with a breakout enlargement section. We analyzed the effects of the enlargement ratio, ellipticity, and principal axis orientation on the borehole cleaning effect of horizontal wells with a breakout enlargement section. The results show that the cleaning impact is better when the enlargement ratio is lower; moreover, the ellipticity is larger and the principal axis orientation is perpendicular to the gravity direction. This paper fills a gap in the existing theory of hole cleaning in horizontal wells and provides a theoretical basis for improving the hole cleaning effect in actual drilling processes.

Research on a New Method for Intermittent Production of Horizontal Wells in Low-Permeability Gas Field

With the development of drilling technology in recent years, an increasing number of horizontal wells have been widely used in low-permeability gas fields. Although horizontal wells are much more productive than vertical wells, fluid accumulation can occur when the formation energy drops and the gas flow rate in the wellbore is not sufficient to remove the loaded fluid from the wellbore. Intermittent production is a good method for preventing liquid loading, but thus far, there is no difference between the open and closed working system of horizontal wells and that of vertical wells, and there is a certain misunderstanding here. In this work, experiments were conducted on the opening process and closing process of a horizontal well, and it was found that the loaded fluid in the horizontal wellbore is a large source of water. It enters the vertical section during the opening process, thereby raising the liquid level and storing the fluid loaded in the vertical section during the closing process, which is difficult to unload. Combined with the experimental results, the production dynamics of a horizontal well with intermittent production was analyzed, and the well opening process and well closing process were divided into four stages. On this basis, a new multifrequency well opening method for intermittent production of horizontal wells was proposed to unload the liquid in horizontal wellbore. The field application case shows that this method can effectively eliminate the drawbacks caused by using conventional methods and increase the average gas production in one cycle by 46%.

Triaxial Deformation of the Goldwyer Gas Shale at In Situ Stress Conditions—Part II: Viscoelastic Creep/Relaxation and Frictional Failure

To understand the geomechanical implications of long-term creep (time-dependent deformation) response of gas shale, short-duration creep was recorded from laboratory triaxial tests on ten Goldwyer gas shale samples in the onshore Canning Basin at in situ stress conditions under constant differential axial stress. A simple power-law function captures primary creep behaviour involving elastic compliance constant B and time-dependent factor n. Experimental creep data revealed larger axial creep strain in clay and organic-rich rocks, than those dominated by carbonates. Anisotropic nature of creep was observed depending upon the direction of constant axial stress application (perpendicular or parallel to the bedding plane). Upon the application of linear viscoelastic theory on laboratory creep fitting coefficients, differential horizontal stress accumulation over a geological time scale was estimated from the viscoelastic stress relaxation concept. Further, this model was used to derive lithology-dependent least principal stress (Shmin) magnitude at depth for two vertical wells intersecting the Goldwyer gas shale formations. This newly proposed Shmin model was found to have a profound influence on designing hydraulic fracture simulation. Further, pore size distribution and specific surface area value SN2 were derived from low-pressure gas adsorption experiments. These physical properties along with weak mineral components were linked with creep constitutive parameters to understand the physical mechanisms of creep. A strong correlation was noted between SN2 and creep parameters n and B. Finally, an attempt was made to investigate how gas shale composition and failure frictional properties can influence shear fracturing.

Pressure-Transient Behavior of Vertical Wells Considering Dynamic Water Hammer and Dynamic Induced Fracture: Theory and Case Studies.

Water injection can result in the creation of induced fracture by connecting natural fractures. The induced fracture penetrates the entire reservoir, leading to the interconnection of injection and production wells and ultimately resulting in water breakthrough and the abandonment of production wells. During well work, the induced fracture exhibits dynamic behavior characterized by extension and closure, known as the dynamic-induced fracture phenomena. During the shut-in process, fracture closure phenomena are often accompanied by water hammer phenomena, which can be detected bottom hole pressure data. However, numerical simulation methods are difficult to describe their dynamic processes. Therefore, we urgently need a mathematical model to fill this gap. In this work, we developed a waterflooding-induced dynamic fracture (WIDF) model. Dynamic water hammer phenomena, multi-dynamic closure phenomena, and fracture storage phenomena are introduced into the WIDF model to describe the induced fracture dynamic behavior. Field cases demonstrate that the WIDF model can improve the accuracy of interpreted parameters and correct incorrect model selection. Green function and Newman product method are used to characterize single-phase water flows within tight reservoirs. Then, the induced fracture is discrete into N segments through the boundary element method. Discrete-induced fractures are divided into n parts. Based on the induced fracture bending property, conductivity variation is independent in each part. The feature line method is used to solve the pressure response of the dynamic water hammer. The Duhamel principle is applied to couple the storage effects and reservoir pressure response. The Laplace transform method transforms the model into Laplace space so that it can be solved easily, and the Stehfest numerical inversion method transforms the model into real space to obtain the final solution. Our results show that the dynamic water hammer flow (DWH) regime with an oscillation curve and the multi-dynamic closure flow (MDCF) regime with peaks exist on the type curve. Resistance coefficient (f) and inertia coefficient (I) control the DWH regime. Fracture storage coefficient (Cf), dynamic fracture closure rates (Delpatm), and induced fracture closure half-lengths (xdfm) control the MDCF regime. It is worth noting that the peak position can be adjusted by the Delpatm and xdfm parameters, achieving a more accurate match to the field cases. The Cf is identified, which corrected for the amplified wellbore storage coefficient. Obtained permeability is also in a reasonable range of permeability for tight reservoirs. In summary, a series of mathematical methods are used to develop and solve an injection well model to identify the dynamic behavior of the induced fracture. Numerical simulation methods are used to verify the accuracy of the WIDF model. Two field cases from X Oilfield demonstrate the practicability of the WIDF model. Dynamic identification of the induced fracture would help engineers develop appropriate injection schemes to prevent water breakthroughs.

Numerical simulation of deflagration fracturing in shale gas reservoirs considering the effect of stress wave impact and gas drive

Methane deflagration fracturing is a new reservoir stimulation method that serves the efficient development of shale gas reservoirs. However, the propagation law of deflagration fractures is still unclear. In this paper, a numerical model considering the effect of stress wave impact and gas drive of deflagration fracturing was established based on the continuum–discontinuum element method (CDEM). The correctness of the numerical model was verified by comparing it with a laboratory experiment, the steady and unsteady analytical solutions of gas flow, and the approximate solution of fracture propagation. Then, numerical simulations of methane deflagration fracturing in vertical wells and horizontal wells under different factors were carried out to analyze the fracture mechanism. The results indicate that deflagration fracturing in vertical wells can break through the stress concentration around the borehole; the initial radial fractures are formed under the action of stress wave impact and then propagate substantially under the driving action of high-pressure gas. The in-situ stress difference affects the deflagration fracture propagation and makes the half-fracture length in the direction of maximum principal stress larger than that in the direction of minimum principal stress. The more significant the stress difference is, the more noticeable this deviation will be. When the deflagration peak pressure is high, the reservoir burst degree is large, which is conducive to enlarging the stimulation range of deflagration fracturing. Staged deflagration fracturing in horizontal wells can form 5–8 obvious fractures perpendicular to the horizontal borehole in each explosion section. A large cluster spacing and explosion section length are conducive to expanding the stimulation scope. Moreover, the propagation of deflagration fractures will be induced by the natural fractures, and the natural fracture with a considerable length or a slight angle between the dip angle and the propagation direction of deflagration fractures is more likely to be activated.

A Transient Productivity Prediction Model for Horizontal Wells Coupled with Oil and Gas Two-Phase Seepage and Wellbore Flow

Capacity prediction is the basis for the optimization of oil and gas well production work systems and parameter optimization design. Horizontal wells are becoming increasingly popular for oil and gas extraction. However, the seepage law of reservoirs produced with horizontal wells is more complicated than that of reservoirs produced with vertical wells, especially when the bottom hole flowing pressure or formation pressure is less than the saturation pressure of crude oil in the reservoir. Oil and gas two-phase seepage can occur in a part or all areas of the wellbore and reservoir. Because the oil and gas two-phase seepage characteristics of reservoir oil well production will be reduced—possibly greatly reduced—the formation seepage law is complex. Thus, it is very important to better predict the horizontal well capacity. To address this, a method and process of establishing a transient calculation model of two-phase flow in horizontal wells are introduced in detail from three aspects: fluid physical properties, reservoir oil and gas two-phase seepage, and the coupling model of the inflow performance and flow in the wellbore. The model is found to be reliable through verification with production data from five wells in two oilfields. The established model simplifies the reservoir model, does not involve very complex meshing, and only simulates one well. Therefore, the calculation speed will be faster than that of other reservoir numerical simulation methods under the same conditions.

Flow field and erosion characteristics of the valve head of a vertical well inclinometer

During the drilling, the flow field characteristics of the vertical well inclinometer are complex. In a harsh underground environment, the valve head is prone to erode and wear. To overcome these disadvantages, the three-dimensional (3D) model of the valve head of a vertical well inclinometer was developed, and the flow field and erosion characteristics were simulated by using a numerical program. Results show that the maximum velocity and the maximum turbulence effect occur at the throttling inlet through which the drilling fluid passes the guide sleeve. By analyzing the particle trajectory of the discrete phase model, it is found that the particles above the valve head surface have obvious accumulation and rebound phenomena and that the erosion effect appears to some extent on the valve head surface. Analysis of the shear stress on the surface of the valve head shows that the stress concentration occurs in the valve head directly impacted by the drilling fluid and in the groove of the flow drilling fluid, respectively. The feasibility and accuracy of the simulation are verified by comparing the theoretical results with that of the field test. This investigation can be used to explain the failure of the valve head of the vertical well inclinometer and provide the design and optimization of the valve head with scientific support.

Analysis Reveals Depth of Vaca Muerta’s Potential

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 205573, “Vaca Muerta: An Emerging Shale Petroleum Reservoir,” by Rahimah B. A. Karim and Roberto Aguilera, SPE, University of Calgary. The paper has not been peer reviewed. _ The objective of the complete paper is to present geological and reservoir characterization, drilling and production strategies, historical performance, and economics of the Vaca Muerta reservoir. The word “petroleum” as used in this paper includes oil, natural gas, and natural-gas liquids. The authors conclude that oil and gas potential in the Vaca Muerta shale is significant and rivals the potential of shales widely developed in the United States and Canada. Reservoir Background In 2008, exploration activities began in the Loma Campana (LC) field, leading to Vaca Muerta’s discovery in 2010. The LC area was selected as the first factory-mode development because of its pre-existing infrastructure and accessibility. The urgency of boosting hydrocarbon production in Argentina has driven multiple companies to invest substantially in unconventional resources. The complete paper details the shale’s geological setting and reservoir characterization. Reservoir heterogeneity and its effect on productivity are described for selected development areas, including La Amarga Chica (LAC), the central area, and LC. Vaca Muerta is an attractive target for shale development because of a few factors. One is its thermal maturity that increases from east to west. This has resulted in multiple types of hydrocarbon windows, from oil to dry-gas windows. Furthermore, it is laterally extensive and has formation thickness of up to 500 m. Total organic carbon (TOC) also is high (2–10%), with a mineralogy content of less than 30% clay. At the basin level, the average matrix porosity of Vaca Muerta ranges from 4 to 14%, with a narrower range at the field or block scale. The formation shows similarities with other well-known shale plays with similar porosity volumes and pore types, such as the Haynesville (between 8 and 16%) but higher than the Eagle Ford (between 8 and 10%). Significant formation thickness drove the application of vertical wells during the first factory drilling in Vaca Muerta. It allowed placement of four to six hydraulic fractures controlled by the lithologies and vertical heterogeneity. In 2015, however, a major change from vertical to horizontal well development occurred as a result of better understanding of the formation complexity and optimal landing zones. In the past few years, operators have increased lateral length from 1000 to 2500 m to improve productivity. Most operations use multiwell pad drilling. The pad is a four-well line with 10-m spacing on the surface and a minimum of 300-m spacing in the reservoir. This well spacing is wide by US standards but is optimal for Vaca Muerta to minimize well interference in the reservoir.

Gas recovery from silty hydrate reservoirs by using vertical and horizontal well patterns in the South China Sea: Effect of well spacing and its optimization

Gas recovery from silty hydrate reservoirs by using vertical and horizontal well patterns in the South China Sea: Effect of well spacing and its optimization

Waste to Energy: Review on the Development of Land Fill Gas for Power Generation in Sub-Saharan Africa

This study focused on the development of land fill gas for power generation in Sub-Saharan Africa. In rapidly expanding cities in developing and emerging nations, it has been observed that municipal solid waste (MSW) has increased dramatically, raising public concern about the effects on the environment and public health. In Sub-Saharan Africa today, the garbage of people within this region especially in Nigeria is carelessly disposed. Environmental pollution and its effects on people's quality of life have become more sensitive topics among residents and decision-makers in Sub-Saharan Africa. Additionally, municipal solid waste management (MSWM) is becoming a more important topic on the local political agenda. Local decision-makers routinely debate whether to invest in waste-to-energy technologies as part of their effort to modernize waste management systems. Waste-to-Energy technologies are being promoted more and more as an alluring solution to a number of problems, including the urgent issue of waste disposal. These issues include inadequate power production, a shortage of landfill space, and greenhouse gas emissions from improper waste management. As an alternative to waste burning and composting, landfilling is one of the municipal solid waste (MSW) disposal techniques that are most frequently used. Due to its financial benefits, the sanitary landfill method is still often employed in various nations for the final disposal of solid waste. Landfill gas (LFG) is mostly produced by the anaerobic breakdown of the biodegradable component of municipal solid waste (MSW), specifically kitchen and yard trash, which is disposed of in landfills. Due to the anaerobic breakdown of the organic portion of solid waste, landfill gas is continuously produced. As a result, if an extraction system is not constructed in a landfill, there will be an overpressure that will force the biogas to be released into the atmosphere, which will have an adverse effect on the environment. Methane and carbon dioxide make up the majority of the gases that make up landfill gas, which is a mixture of other gases. Many landfill sites include an operational gas collection system that draws gas from both horizontal and vertical gas wells using a blower. The gas from the landfill was thought to only include carbon dioxide and methane. Methane's typical volume composition is 49%, hence it was believed that carbon dioxide would have a volume composition of 51%. Reviewing the information gathered by numerous studies regarding the volume of waste being dumped in landfills reveals that the waste produced in sub-Saharan Africa is sufficient to power the area with electricity. It was discovered that the quantity of electricity generated will fluctuate over time based on the flow rate of landfill gas. It will initially rise until a peak is attained. A million tons of landfill waste typically emits 434,000 cubic feet of LFG each day, which is sufficient to generate 0.80 MW of power. About 70% of LFG projects use internal combustion engines, gas turbines, and micro-turbines to produce power.

The role of free‐strain assumption in axisymmetric electro‐osmosis‐enhanced preloading consolidation of unsaturated soil

AbstractThe axisymmetric, equal‐strain consolidation theory of soil is widely employed in engineering due to the user‐friendliness of its solution. However, the radially averaged excess pore pressure (EPP) is unable to reveal the coupling effect of the radial electric field when the soil is consolidated by electro‐osmotic and surcharge preloading. In this paper, the axisymmetric consolidation analytical models with free strain and equal strain assumptions are developed to investigate the importance of the free strain assumption in the consolidation of unsaturated soil by electro‐osmosis‐aided preloading. The finite Hankel transform, decoupling technique, and Laplace transform are then utilised to derive the semi‐analytical solutions. Finally, the comparison of EPP, surface settlement, and average degree of consolidation under the two assumptions reveals that the free‐strain solution accurately shows the distribution of EPP at any point in the soils and the development of surface settlement at different radii, thus helping to analyse the electro‐osmotic consolidation characteristics. Moreover, it was discovered that electro‐osmosis combined with preloading allows for a more uniform consolidation of unsaturated soil. The difference in the average degree of consolidation under both assumptions is insignificant, while the installation distance of the cathode and anode vertical wells has a significant effect.

Full implicit simulator of hydrate (FISH) and analysis on hydrate dissociation in porous media in the cubic hydrate simulator

Natural gas hydrate is a promising strategic energy resource in marine and permafrost sediments. To precisely characterize the coupled Thermal-Hydraulic-Chemical (THC) process in hydrate reservoirs, a bona fide mathematical model with improved key parameters is needed. We developed a Full Implicit Simulator of Hydrate (FISH), a three-phase, four-component mathematical model, to simulate gas recovery from hydrate reservoirs. The experiment of methane hydrate formation and dissociation under depressurization via a single vertical well was executed in the Cubic Hydrate Simulator (CHS). FISH was employed to reproduce the experiment of hydrate dissociation in quartz sands. This study successfully obtains unified parameters that are suitable for the entire hydrate dissociation process. Simulation results of gas and aqueous production profiles and the temperature distribution agreed well with that in the experiment. The Mean Absolute Percentage Error (MAPE) of the simulated volume of gas produced was 2.845%. The mass conservation in the pressure vessel was confirmed by both experimental data and numerical simulation results. This numerical simulator can be applied to predict the behavior of hydrate-bearing-sediments in the laboratory or to evaluate the gas production potential from the marine or permafrost hydrate reservoirs.

Permeability anisotropy impact on wormhole propagation in openhole and limited-entry completions: A 3D numerical study

The permeability in carbonate reservoirs is not the same in all directions (i.e., anisotropic), a property seldom considered in acid stimulation. The study hypothesizes that permeability anisotropy impacts acid treatment efficiency significantly. The investigation included the openhole and limited-entry completions in a vertical and horizontal wellbores configuration. A model based on the two-scale continuum approach was constructed in three-dimensional space (3D), resembling the big block experiment with the wellbore at the center. Permeability anisotropy significantly impacts acid efficiency in terms of the pore volume to breakthrough (PVBT) and optimum injection rate. For instance, simulations of acid placement in an openhole horizontal well with 10 anisotropy ratio resulted in PVBT of 0.15 compared to 0.35 in a vertical well. Limited entry completions result in more extended wormholes where the permeability anisotropy positively impacted wormhole length as compared to isotropic permeability. Nevertheless, the permeability anisotropy negatively impacted wormhole coverage along the wellbore in most scenarios. The simulations showed that a spear-like wormhole propagates from a limited entry horizontal wellbore in anisotropic formation, which previous studies assumed to be spherical or ellipsoidal. This implies that the skin factors models should be revised based on the new findings. This is the first study to assess the permeability antitropy impact on the stimulation of openhole and perforated wells in a 3D space.

Production Capacity Variations of Horizontal Wells in Tight Reservoirs Controlled by the Structural Characteristics of Composite Sand Bodies: Fuyu Formation in the Qian’an Area of the Songliao Basin as an Example

In order to improve the combined exploitation efficiency of horizontal and vertical wells, and given the fact that the complex and varied spatial structure of sand bodies in the Fuyu oil layer in the Qian’an area of Songliao Basin leads to significant differences in production characteristics of horizontal wells, the sand body types and internal spatial structure are finely dissected according to the theory of configuration analysis, and the internal spatial structure is divided into three configuration styles: spatial clipping type, overlapping type, and separation type. Then, by comparing the productivity characteristics of horizontal wells with different configurations of sand bodies, combined with the analysis of fluid flow law under horizontal well volume fracturing, a main fracture–fracture network–matrix coupled fluid flow model in a tight reservoir based on composite sand body configuration is established. Combined with the actual volume fracturing the horizontal well area, the productivity curves of each cluster in the horizontal section after numerical simulation of volume fracturing of typical horizontal well groups are extracted, which are divided into four types: high-yield stable type, high-yield two-stage type, high-yield rapid-decline type, and low-yield rapid-decline type, and the coupling relationship between the productivity characteristics of each cluster in the horizontal well volume fracturing and sand body configuration style is established, which provides a theoretical basis for the adjustment of different sand body development methods in subsequent oilfields.

Prediction of Sanding Likelihood Intervals Using Different Approaches

Sand production is undesirable matters, occurring in wells that are producing from sand reservoirs. It causes many problems such as erosion and grains accumulation in downhole and surface equipment’s, and formation subsidence. Important stage in sanding problem solution is a prediction of likelihood sand production intervals. In present paper, a vertical well X1 that is producing from Asmari reservoir in Y field at southern Iraq was selected for study. Asmari reservoir was classified to six units: A, B1, B2, B3, B4, and C. B zones consisted from sandstone with others rock types. Eight approaches were used for prediction sanding onset intervals by dealing with X1 well as open hole completion. Utilized eight prediction methods are compressional sonic wave (CSW), unconfined compressive strength (UCS), total porosity (PHIT), shear modulus to bulk compressibility (G/Cb), B-Index, Schlumberger index (S- Index), combined index (Ec-Index) and critical drawdown pressure (CDDP). All these methods performed based on 2462 measured points of CSW, sonic shear wave log (SSW), and density log (DL). Sand production likelihood intervals was selected by determination of cutoff values of adopted methods. Sand is possible to occur if interval has values lower than cutoff values of G/Cb, UCS, B-Index, S-Index, Ec, and CDDP and greater than cutoff values of CSW, and PHIT. Obtained cutoff values of eight approaches were 800 x 109 psi2, 36 Mpa, 0.2, 80 us/ft, 10000 Mpa, 108 Mpa, and 2700 Mpa, of G/Cb, UCS, PHIT, CSW, B-Index, S-Index, and Ec respectively. As well as sand production is possible to occur of bottomhole flowing pressure lower than calculated CDDP. Some Intervals had high CDDP that referred to abnormal pressure zones consisted from shale. Determination of sand onset intervals is a key for selecting best methods for controlling.

Numerical Simulation of Optimized Step-Wise Depressurization for Enhanced Natural Gas Hydrate Production in the Nankai Trough of Japan

The utilization of natural gas hydrates as an alternative energy source has garnered significant attention due to their proven potential. Despite the successful offshore natural gas hydrate production tests, commercial exploitation has not been achieved. This study aims to enhance the understanding of gas production behavior through simulations from a single vertical well in the Nankai Trough and assess the effectiveness of the step-wise depressurization method for gas production using TOUGH + HYDRATE. The simulation results showed that the effective permeability for the water phase decreased as the hydrates were decomposed, and the invasion of the pore water from the underburden eliminated this effect. Compared with the direct depressurization method, the step-wise depressurization method significantly increased the cumulative gas production by more than 10% and mitigated the rapid generation of gas and water production during the moment of depressurization. The results also indicated that the depressurization gradient was more sensitive to the cumulative gas production than the maintenance time of depressurization. In view of the gas and water production characteristics coupled with the challenges in carrying out the step-wise depressurization method, it is suggested that a depressurization gradient of 1 MPa and a maintenance time of 1 day should be employed.

Evaluation of Reconstruction Potential for Low-Production Vertical Wells of CBM in the Southern Qinshui Basin

Production practice has shown that not all low-production coalbed methane (CBM) wells can be reconstructed into high-production wells through secondary stimulation, so it is necessary and timely to establish an evaluation index system, form an evaluation method, and evaluate the reconstruction potential of low-production wells. Based on the development practice of CBM in the southern Qinshui Basin, this paper analyzes the influencing factors of low production in vertical wells from the aspects of coal and rock reservoir conditions, drilling and completion engineering, and drainage engineering. It is proposed that the evaluation of the reconstruction potential of low-production wells should focus on the quality of CBM resources, the difficulty of CBM desorption and diffusion, and the degree of damage to coal reservoirs caused by the initial reservoir stimulation. Twelve parameters, including gas content, gas saturation, reservoir pressure gradient, critical desorption–reservoir pressure ratio, and permeability, were systematically selected as evaluation indicators, and the grading reference values for each evaluation indicator were comparatively given. Then, a multi-factor comprehensive evaluation method for the reconstruction potential of low-production wells based on gray correlation analysis method was established. The reconstruction potential of low-production wells was divided into three levels: high, medium, and low. When reconstructing low-production wells, it is recommended to prioritize the low-production wells with high reconstruction potential, followed by those with medium reconstruction potential, while low-production wells with low reconstruction potential are not recommended for reconstruction. Finally, the evaluation method was used to evaluate the reconstruction potential of five low-production wells in a CBM block, and suggestions for the reconstruction order and reconstruction potential levels for each well were given.