Borehole Pressure Research Articles

Response of Borehole Strain Observation Data to Air Pressure Based on EEMD

In this paper, based on the empirical mode decomposition (EEMD) method, considering the frequency dependent characteristics of the pressure response, the borehole strain observation data of Daixian station in Shanxi Province and the pressure observation curve of synchronous observation are decomposed, and the eigenmode function (IMF) sequences of different frequency bands are obtained. Then, on the same narrow frequency band of IMF, the one-dimensional regression analysis of borehole strain observation value and pressure change is carried out respectively, and the frequency dependent pressure response coefficient is obtained, which realizes the refined pressure correction of borehole observation data. The results show that this method can effectively deduct the influence of air pressure, and the frequency distribution characteristics of air pressure response are clear.

A new method of monitoring the stability of boreholes for methane drainage from coal seams

Methane drainage plays a significant role in the utilisation of clean energy, reducing greenhouse effects, and hazards arising from methane-induced disasters during mining. Drainage boreholes have been applied widely, while stability monitoring is needed to guarantee the effect of methane drainage. At present, there is no effective method and device with which to measure the deformation and failure of drainage boreholes. In view of this problem, the mechanical conditions and failure modes of borehole deformation were analysed. A set of simulation and testing devices and systems were designed to explore a new method for monitoring the deformation and failure of a borehole. Moreover, similar simulation experiments were carried out. From the results, the functional relationship between the degree of damage to a borehole and relative pressure on the sensor in the device was established. Meanwhile, the critical value of the degree of destruction of a borehole was revealed. Moreover, the experimental results were also analysed and verified with by the numerical simulation results from RFPA. This study provided a new engineering application for borehole deformation measurement.

A case study of the stability of a non-typical bleeder entry system at a U.S. longwall mine

Longwall abutment loads are influenced by several factors, including depth of cover, pillar sizes, panel dimensions, geological setting, mining height, proximity to gob, intersection type, and size of the gob. How does proximity to the gob affect pillar loading and entry condition? Does the gob influence depend on whether the abutment load is a forward, side, or rear loading? Do non-typical bleeder entry systems follow the traditional front and side abutment loading and extent concepts? If not, will an improved understanding of the combined abutment extent warrant a change in pillar design or standing support in bleeder entries? This paper details observations made in the non-typical bleeder entries of a moderate depth longwall panel—specifically, data collected from borehole pressure cells and roof extensometers, observations of the conditions of the entries, and numerical modeling of the bleeder entries during longwall extraction. The primary focus was on the extent and magnitude of the abutment loading experienced due to the extraction of the longwall panels. Due to the layout of the longwall panels and bleeder entries, the borehole pressure cells (BPCs) and roof extensometers did not show much change due to the advancing of the first longwall. However, they did show a noticeable increase due to the second longwall advancement, with a maximum of about 4 MPa of pressure increase and 5 mm of roof deformation. The observations of the conditions showed little to no change from before the first longwall panel extraction began to when the second longwall panel had been advanced more than 915 m. Localized pillar spalling was observed on the corners of the pillars closest to the longwall gob as well as an increase in water in the entries. In addition to the observations and instrumentation, numerical modeling was performed to validate modeling procedures against the monitoring results and evaluate the bleeder design. ITASCA Consulting Group’s FLAC3D numerical modeling software was used to evaluate the bleeder entries. The results of the models indicated only a minor increase in load during the extraction of the longwall panels. These models showed a much greater increase in stress due to the development of the gateroad and bleeder entries--about 80% development and 20% longwall extraction. The FLAC3D model showed very good correlation between modeled and expected gateroad loading during panel extraction. The front and side abutment extent modeled was very similar to observations from this and previous panels.

A New Width Measurement Method of the Stress Relief Zone on Roadway Surrounding Rocks

Determining the width of the stress relief zone on roadway surrounding rocks is the premise to optimize drilling borehole effect and increase gas extraction efficiency. In this study, a new width measurement method of the stress relief zone on the roadway surrounding rocks was proposed, which determined the width according to gas pressure attenuation speeds in roadway boreholes at different depths. Then, the variation curve of the gas pressure in boreholes at different depths with the time was gained through a field test. On this basis, laws of the gas pressure attenuation and the gas transmission and loss in boreholes at different depths were explored through a numerical simulation based on COMSOL Multiphysics, thus concluding the stress on roadway surrounding rocks, the distribution of plastic zones, and the stress-permeability relation. The scientificity of the proposed method was illustrated theoretically. Finally, the proposed method was verified by the field test data and numerical simulation results of the gas extraction at different sealing depths. Research results demonstrate that the pressure in boreholes attenuates in the logarithmic function pattern. The attenuation speed decreases with the increase of the drilling depth. The width of the stress relief zone on roadway surrounding rocks in the studied area was determined to be about 11 m according to the proposed method. Both the numerical simulation and the field test of the gas extraction efficiency prove the feasibility and validity of the proposed method in determining the sealing depth of the borehole for the gas extraction. Research conclusions are of important significance to enrich width measurement methods of the stress relief zone on roadway surrounding rocks and to optimize sealing parameters of underground boreholes for gas extraction.

Microseismicity caused by injection of water in a gas-saturated reservoir

Microseismic events along preexisting zones of weakness occur in a reservoir due to pore pressure buildup and fracturing during fluid injection. We have used a multiphase fluid-flow numerical simulator to model water injection in a gas reservoir. Previous studies generally consider a single fluid, in which the relative permeabilities and capillary pressure play no role. We analyze the effects of partial saturation on the injection process. On the basis of a spatial distribution of weak stress zones and a threshold pore pressure, the simulator models fluid transport in the formation and allows us to obtain the spatiotemporal distribution of the microseismic events. We consider uniform and fractal distributions of the pore pressure at which microearthquakes are triggered. We analyze the influence of the initial water saturation and the presence of preexisting natural fractures, as well as the effect of updating the rock properties after the microseismic events occur. Moreover, we perform simulations in a low-permeability reservoir in which the borehole pressure increment generates a system of fractures that propagate into the reservoir. The importance of considering two-phase fluid flow resides in the fact that partial saturation greatly affects the trigger time evolution. This is mainly due to the difference in compressibility of the two phases.

A fracture initiation model for carbon dioxide fracturing considering the bottom hole pressure and temperature condition

The properties of CO2 under different conditions significantly affect the breakdown pressure during CO2 fracturing. In this study, the fracture initiation model is combined with real bottom-hole temperature-pressure condition by considering CO2 borehole pressurization rate. The results show that the breakdown pressure can be reduced by 26.2%–35.5%. With a constant pump rate of 0.5 m3/min, bottom-hole pressure exceeds the breakdown pressure at approximately 408s, meanwhile, bottom-hole temperature decreases by 4.38% and borehole pressurization rate expands by 2.4 times. As the pump rate increases, the pressurization rate reaches its peak at a pump rate of 6.0 m3/min, and the downtrend is delayed with inlet pressure increasing. Case study shows that the breakdown pressure predicted by the new model agrees well with in-situ data, and the range of optimal inlet pump rate is 1.9–2.5 m3/min under the in-situ inlet condition (T0 = 258 K and p0 = 6 MPa). Furthermore, the range of optimal inlet pump rate expands with inlet pressure increasing and shrinks with inlet temperature increasing. This new initiation pressure model can provide more insights for CO2 fracturing mechanisms and design.

Investigation of longwall headgate stress distribution with an emphasis on pillar behavior

This study uses an extensive suite of borehole pressure cell (BPC) data from a US longwall mine to better characterize the global stress redistribution process during mining. A new methodology employing the Bieniawksi’s strength gradient equation was used for converting the BPC measurements into rock pressure. It was observed that the Bieniawksi gradient equation underestimates coal strength for points located deeper into the pillar. Comparisons with previous empirical and analytical equations are also presented for different stages of longwall loading. Very low front abutment load in comparison to the side abutment load was observed, along with a low abutment angle for this mine. This abutment angle value was verified from stress measurements made within the solid coal face ahead of the advancing panel. Some other atypical trends were noted during data analyses, such as a monotonic drop in stress from the panel edge within the coal face; these trends were explained using an elastic FLAC3D model. Finally, the load distribution in a pillar is presented in three-dimensions using interpolated bar graphs for different locations of the longwall face.

A Theoretical Model for Hydraulic Fracturing through Two Symmetric Radial Perforations Emanating from A Borehole

The production enhancement of oil, gas, or geothermal reservoirs through hydraulic fracturing requires an in-depth study on the fracture initiation and propagation from the borehole. According to the linear elastic fracture mechanics, a theoretical model is developed to calculate the stress intensity factors of two symmetric radial cracks emanating from a pressurized borehole. The maximum tangential stress criterion under the mix-mode condition is developed to investigate the hydraulic fracture initiation. The critical water pressure and critical initiation angle predicted by the theoretical model match closely the experimental results reported in the literature. The influence of the stress anisotropy coefficient, the perforation angle and length, the borehole radius, the ratio between the water pressures in the fracture and the borehole, and Biot’s coefficient are investigated. Moreover, the effects of the injected high water pressure (i.e., larger than the critical water pressure) on the fracture initiation angle are studied to further understand the characteristics of hydraulic fracture initiation. The results indicate that the perforation angle and length, the borehole radius, and the stress anisotropy coefficient have a relatively strong influence on the critical water pressure and critical initiation angle. During high-pressure water injection, the fracture initiation angle decreases as the ratio between the water pressure in the fracture and the borehole and Biot’s ratio increase. The theoretical model provides a comprehensive understanding of the fracture twist, the mixed-mode fracture propagation feature, and the hydraulic fracturing optimization.

Overpressure at the Macondo Well and its impact on the Deepwater Horizon blowout

At the Macondo well, the overpressure (fluid pressure greater than hydrostatic) in the main reservoir is nearly identical to that within a stratigraphically equivalent sandstone at the Galapagos development 21 miles (34 km) to the south; we interpret that the reservoirs share a permeable, laterally extensive, and hydraulically connected aquifer. At Macondo, pore pressure approximately parallels the overburden stress to a depth of 17,640 ft (5,377 m) subsea and thereafter decreases abruptly by 1,200 psi (8.3 MPa) over 370 ft (113 m) as the main sandstone reservoir is approached. In contrast, at Galapagos, pore pressure increases with the overburden stress for the entire well depth. The pore pressure regression at Macondo was responsible for a reduction in the least principal stress. This, in combination with the extreme pore pressures within overlying strata, drastically narrowed the range of safe operational borehole pressures. These geologic phenomena produced challenging conditions for drilling, prevented successful temporary abandonment of the well, and contributed to the well’s failure.

Cement sheath sealing integrity evaluation under cyclic loading using large-scale sealing evaluation equipment for complex subsurface settings

Cement sheaths should provide zonal isolation and structural support through the entire service lifetime of wells. However, even if high-quality primary cementing was achieved, variations in pressure, for example, those due to staged fracturing in shale gas wells, are likely to lead to sealing integrity failure of the cement sheath. This work presents large-scale sealing evaluation equipment to assess the failure mechanisms of cement sheaths under cyclic loading. The equipment includes a borehole system, pressure system, and control and data acquisition system for automatic loading and unloading, gas channeling monitoring and pressure change and gas output recording. The pressure limit of the equipment is 120 MPa. A finite element model of the borehole system and sealing equipment was built to study the stress and strain under internal pressure. The failure mechanisms of the sealing integrity of the cement sheath were obtained by combining the test results with the finite element calculations. At a high internal pressure, plastic deformation was produced during loading due to the residual strain that arose after unloading of the cement sheath. Therefore, incompatible deformation occurred, and the tensile stress increased at the interfaces of the cement sheath. Under repeated loading and unloading, the plastic deformation increased, and the residual strain and interface tensile stress also increased. When the interface tensile stress exceeded the bonding strength, a microannulus formed between the cement sheath and casing. Based on these results, guidelines for the assessment of the long-term sealing integrity of a shale gas well cement sheath and for the selection of mechanical properties of the cement systems are given to mitigate the risk of zonal isolation failure.

Effect of Borehole Pressurization Scheme on Breakdown Pressure

Effect of Borehole Pressurization Scheme on Breakdown Pressure

Study on the Dynamic Prediction Method of ECD in Horizontal Well Drilling

Well pressure and ECD control is one of the most important things in horizontal well drilling. This paper establish horizontal borehole pressure and ECD management considered seaside, temperature, drill pipe eccentric, drill pipe rotation and cutting bed though research horizontal well drilling. Sensitive factors like drill fluid density, drill fluid viscosity and so on has been found though analyzed the sensitive of ECD to drilling parameter. ECD real-time correction with different condition has been proposed though dimensionless comparison of factors. The first correction based on measured export parameters on the ground and use pump pressure to verify the correction. The second correction use trial method after compared pressure curve measured in down hole with model curve, verify the correction by pattern contrast. Precise ECD data can obtained use real-time correction mentioned to correct ECD management, ensure drilling safety in narrow density window.

Challenges of mining the first right-handed longwall panel in a new reserve block in Pittsburgh Seam

In this paper, the mining experience and challenges for the first right-handed longwall panel in the Pittsburgh Seam are introduced. The longwall headgate T-junction experienced very high face convergence (up to 61 cm), accompanied by roof sag, floor heave, and rib loading. The headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded. Different underground instruments, such as a roof scope, de-gas drill, tell-tale, laser meter, and borehole pressure Cell (BPC), were employed to explore the roof geology and to monitor the entry convergence and the stress changes in the pillar. In addition, the impact of other geologic factors, such as large overburden depth, laminated sandstone roof geology, soft floor, and large headgate equipment, were also analyzed. Subsequently, geotechnical solutions were provided to avoid or mitigate the impact of these challenging geologic factors.

Experimental and numerical investigations of dynamic failure process in rock under blast loading

For study the dynamic failure processes between adjacent boreholes with decoupling charge, the single-hole and double-hole blast loading experiments are set up with granitic samples. High-speed cameras (HS) and digital image correlation (DIC) methods are adopted to obtain strain field and crack development process. According to experimental results, the distribution of damage is symmetric around center hole under single-hole loading, shock wave falls off rapidly with increasing distance from the bore hole. Under double-hole loading, the damage and plastic strain are accumulated under second loading. The second loading makes the superposition of stress waves and the radial cracks formed between two holes. Meanwhile, the bimodal strain-time curve is found along vertical direction, and indicates the no damage is generated by elastic deformation is formed. According to the numerical results, the dynamic responses under two loads are agreed well with experimental results. The borehole pressure is much high when decoupling factors (K) is 1, and then linearly decreased with K increase. The horizontal cracks run through specimen as K = 2 ∼ 5, and the vertical cracks linearly decrease with K increases.

Zaczyny cementowe do uszczelniania otworów wiertniczych o wysokich temperaturach i ciśnieniach złożowych

Zaczyny cementowe do uszczelniania otworów wiertniczych o wysokich temperaturach i ciśnieniach złożowych

A system for measuring borehole diametric deformation based on mechanical contact and micro-optical imaging

Quantifying diametric deformations of boreholes is vital in rock engineering due to its applications in the determination of in-situ stresses using the stress relief methods. New methods are desired to overcome the harsh environment featuring high temperature and high pressure in deep boreholes. Therefore, this study develops an apparatus using rigid probes combined with microscopic camera to measure borehole diametric deformation. The structural design of the rigid probe and package of the measuring device avoid the advert impact of environmental factors. The usage of multi-probes and high resolution micro-optical imaging technique enables simultaneous collection of multi borehole diametric deformation data. Furthermore, using these data, a new method of borehole shape measurement is proposed at high speed compared with traditional methods that measure one diametric deformation at a time. The feasibility of the system is verified by conducting laboratory tests. This system can be readily used in the in-situ stress measurement based on borehole shape analysis.

Exploring Rock–Explosive Interaction Through Cross Blasthole Pressure Measurements

The detonation of the explosive in a blasthole generates pressure to the tune of several giga-pascals that in turn drives the breakage of rockmass in several complex ways. However, nature and the interactions of such pressure with the surrounding rockmass are not explicitly known. This is primarily due to the destructive and rapid nature of the process that hinders measurements in or around the blasthole. Accordingly, such tests are quite cumbersome, complex and costly. The instrumentation has to have standard specifications with several mega-samples per second monitoring capacity. Also, the sensors deployed for the purpose should have a steep rise time because of the fact that the event is transitory in nature. This may be one of the reasons that the focus of researchers has been on determination of energies involved in the process by indirect means. There have been attempts to understand the blasthole pressure and its interactions with the nearby rockmass but due to expensive nature of testing, the progress is not as anticipated. This study attempted the cross blasthole pressure measurements by placing pressure sensors at a distance from the blasthole. The paper presents some examples of cross borehole pressures measured in varied type of rock masses with a good range of acoustic impedance of the rockmass and its joint spacing. Attempt has been made to explain the pressure traces with the aim that further studies in this domain will lead to better understanding of rock–explosive interaction. The method is quite in infancy and it is expected that future studies shall aid in standardisation of the method thus developed.

An analysis of drilling fluid pumping pressure for the Maxi-HDD crossing project

For horizontal directional drilling crossing project, pumping pressure is a key factor that influences the drilling efficiency and the stability of borehole wall. There are nevertheless many theoretical difficulties in calculating and analyzing the dynamic law of pumping pressure. This paper aims to resolve these problems based on construction features and rheological theory. The power-law fluid model is adopted to describe rheological properties of drilling fluid. In order to prevent blowout from occurring in the drilling process, the maximum allowable mud pressure at different reaming stages is calculated by numerical method under condition of complicated geological structure. The borehole pressure must be greater than minimum required mud pressure which is achieved by hydrodynamic calculation method, only meet this condition can it transport the cuttings to the ground surface. Moreover, the pressure loss in the drill string, including loss in drill rod and loss through bit nozzles, is also calculated by newly derived equations. Finally, combining with practical engineering field data, the appropriate range of pumping pressure at different reaming stages is obtained through the calculation results of three types of pressure, which can help engineers minimize the occurrence of accidents and enhance the drilling efficiency.

Investigation of land subsidence with the combination of distributed fiber optic sensing techniques and microstructure analysis of soils

Land subsidence is a global phenomenon caused by a variety of factors, and there are still many challenges in land subsidence monitoring. The results of an investigation of land subsidence using the combination of Distributed Fiber Optic Sensing (DFOS) techniques and microstructure analysis of soil are here presented. Two DFOS techniques: Brillouin optical time domain reflectometry (BOTDR) and fiber Bragg grating (FBG), were developed in two boreholes (namely GM03 and SZ01) in Su-Xi-Chang area, China, to monitor movement of strata crossed by the borehole and pore pressure in some specific strata. Scanning electron microscope (SEM) and Mercury intrusion porosimetry (MIP) were employed to quantify and characterize the microstructures of undisturbed soils from the boreholes. Based on the BOTDR monitoring results, the annual levels of land subsidence in GM03 and SZ01 are ~1.2 mm and ~2.9 mm, respectively and the movement history of strata were clearly tracked. For both boreholes, the main contributors to subsidence are two aquitards (Ad2 and Ad3) that exist adjacent to the second aquifer (Af2). The results obtained by FBG-based sensors indicate good agreement with BOTDR and it confirms that the deformation of aquifer is significantly controlled by pore pressure. SEM images were analyzed with Pore/Particle and Crack Analysis System (PCAS). Three statistical parameters including average form factor (ffavg), fractal dimension (Df) and area probability distribution index (b), were used to describe the geometric characteristics of pores. It is found that larger average form factor, larger fractal dimension and higher proportion of large pores in soil microstructures favors land subsidence. MIP results confirm that area probability distribution index can be used for the estimation of strata compression potential in land subsidence investigations.

Cryogenic Fracturing of Wellbores Under True Triaxial-Confining Stresses: Experimental Investigation

Summary A laboratory study of cryogenic fracturing was performed to test its ability to improve oil/gas recovery from low-permeability reservoirs. Our objective is to develop well-stimulation technologies using cryogenic fluids [e.g., liquid nitrogen (LN)] to increase permeability in a large reservoir volume surrounding wells. The new technology has the potential to reduce formation damage caused by current stimulation methods and minimize or eliminate water usage. The concept of cryogenic fracturing is that a sharp thermal gradient (thermal shock) created at the surfaces of formation rocks by applying cryogenic fluid can cause strong local tensile stress and start fractures. We developed a laboratory system for cryogenic fracturing under true-triaxial loading, with LN-delivery/control and -measurement systems. The loading system simulates confining stresses by independently loading each axis up to approximately 5,000 psi on 8×8×8-in. cubes. Temperature in boreholes and at block surfaces and fluid pressure in boreholes were continuously monitored. Acoustic and pressure-decay measurements were obtained before and at various stages of stimulations. Cubic blocks (8 × 8×8-in.) of Niobrara shale, concrete, and sandstones were tested, and stress levels and anisotropies varied. Three schemes were considered: gas fracturing without cryo-stimulation, gas fracturing after low-pressure cryogen flow-through, and gas fracturing after high-pressure cryogen flow-through. Results from pressure-decay tests show that LN stimulation clearly increases permeability, and repeated stimulations further increase the permeability. Acoustic velocities and amplitudes decreased significantly after cryo-stimulation, indicating fracture creation. In the gas fracturing without the stimulation, breakdown (complete fracturing) occurs suddenly without any initial leaking, and major fracture planes form along the plane containing principal-stress and intermediate-stress directions, as expected theoretically. However, in the gas fracturing after cryogenic stimulations, breakdown occurred gradually and with massive leaking because of thermal fractures created during stimulation. In addition, the major fracture direction does not necessarily follow the plane containing the principal-stress direction, especially at low confining-stress levels. In tests, we observed that cryogenic stimulation seems to disrupt the internal stress field. The increase in borehole temperature after stimulation affects the permeability of the specimen. When a stimulated specimen is still cold, it maintains high permeability because fractures remain open and local thermal tension is maintained near the borehole. When the rock warms back, fractures close and permeability decreases. In these tests, we have not used proppants. Overall, fractures are clearly generated by low- and high-pressure thermal shocks. The added pressure of the high-pressure thermal shocks helps to further propagate cryogenic fractures generated by thermal shock. Breakdown pressure is significantly lowered by LN stimulation, with observed breakdown-pressure reductions up to approximately 40%.