Abstract: The present invention is related to a roller blind comprising: a length of fabric having a first end connected to a roller tube; a bottom rail have a top piece and a bottom piece releasably secure to one another; a connector for attaching to a second end of the length of fabric; wherein the connector comprises a first retention means adapted to releasably secured to the bottom piece of the bottom rail.

Abstract: The present disclosure relates to a roller blind idle end assembly for roller blinds comprising a splined bush, a support, a spring, two sealed ball bearings, rotor, metal strap, and an end pin. This assembly reduces friction, wear and the likelihood of failure of the idle end assembly, creating a quiet design and dissipating static electricity.

Abstract: A stop structure for a cordless curtain includes a bracket, a mounting seat, a rotation wheel, a locking piece, and a spring. The bracket has a resting portion. The mounting seat has an axle. The rotation wheel is mounted on the axle and has at least one positioning hole. The locking piece is rotatably mounted on the mounting seat and has a first end provided with a positioning projection positioned in the at least one positioning hole. A push member is biased between a second end of the locking piece and the mounting seat. When the resting portion presses the locking piece, the positioning projection is detached from the at least one positioning hole. The spring is mounted in the rotation wheel and connected with the rotation wheel and the axle.

Abstract: A ladder safety stabilizer is a telescopically adjustable device that attaches to the rung of a ladder on one end and to the building that the ladder is leaning against at the other end to prevent the feet of the ladder from sliding while a user is climbing, and to provide a second stabilization point in addition to the feet of the ladder to provide greater ladder stability, increased safety, and reduced accident risk.

Abstract: An electric driven hydraulic fracking system is disclosed. A pump configuration that includes the single VFD, the single shaft electric motor, and the single hydraulic pump that is mounted on the single pump trailer. A pump configuration includes a single VFD configuration, the single shaft electric motor, and the single shaft hydraulic pump mounted on the single pump trailer. The single VFD configuration converts the electric power at the power generation voltage level distributed from the power distribution trailer to a VFD voltage level and drives the single shaft electric motor to control the operation of the single shaft electric motor and the single hydraulic pump. The VFD voltage level is a voltage level that is required to drive the single shaft electric motor. The VFD configuration also controls operation of the auxiliary systems based on the electric power at the auxiliary voltage level.

Abstract: A hydraulic accumulator-based controllable pressure injection device includes a pressure storage system, a pressure maintaining system, a sealing system, and an injection system. The pressure storage system includes a pressure cylinder front half portion, an annular piston and a middle connecting body. The pressure maintaining system includes an accumulator, a pressure cylinder rear half portion and a rear end cover. The sealing system includes a hydraulic propulsion cylinder, a propulsion piston, a front end cover, a sealing tube, a thickened end portion, and an expansion rubber tube. The injection system includes a hydraulic valve body and an injection gun tube. The hydraulic accumulator-based controllable pressure injection device quickly applies pressure to the bottom of a borehole in a hydraulic manner, so that rock experiences tensile failure, and the hard rock excavation efficiency is improved.

Abstract: In embodiments, a drilling rig system includes a rig unit having rig equipment and a rig electrical subsystem configured for electrical communication with the rig equipment and including a rig electrical subsystem input transformer, an electrical system including a primary power supply subsystem remote from the rig unit, and an intermediate electrical subsystem.

Abstract: A releasable connection mechanism for use downhole with a tubular member includes a collet, a collet stop, and a ring housing. The collet is configured to engage the tubular member, the collet stop is axially movable with respect to the collet between a disengaged position to not engage the collet and an engaged position to engage the collet, and the ring housing is selectively axially movable with respect to the collet. In the disengaged position, the collet is configured to move with respect to the ring housing to engage the ring housing and prevent the collet from disengaging the tubular member when the collet is tensioned with respect to the tubular member. In the engaged position, the collet is configured to remain axially stationary with respect to the ring housing and disengage the tubular member when the collet is tensioned with respect to the tubular member.

Abstract: A sampling-by-pushing system includes a setting-by-pushing mechanism and a centralizing device. The setting-by-pushing mechanism is connected with the centralizing device through a connecting pipe, and is configured to be set and sealed in a borehole. The centralizing device includes a base body and a retractable support arm arranged on the base body. The support arm is configured to be in contact with the wall of the borehole so as to position the setting-by-pushing mechanism in the center of the borehole.

Abstract: A method for operation in wellbore includes contacting a downhole article comprising a cyanate ester composite with a treatment fluid comprising propylene carbonate at a temperature of about 25° C. to about 300° C.; and disintegrating the downhole article.

Abstract: The apparatus for removing the production pipe (10) in the borehole is mechanically connected by means of BHA to an electrical power supply (12), coolant supply (13), data cable (14) and control unit (15). The equipment (X) is placed in a contactless manner inside the production pipe (10) and comprises a liquid precursor supply (11) which enters the generator (5) of the plasma-forming medium connected to the nozzle system inlet (6) connected to the nozzle system (7).

Abstract: A non-explosive downhole tool for creating openings in tubulars and or earthen formations includes a carrier holding a non-explosive material, such as thermate, a head connected with the carrier and having a port to eject a product of the ignited material from the head and a communication path extending from the material to the port and a moveable member in a closed position blocking the communication path and in an open position opening the communication path.

Abstract: An apparatus and system for accessing a flow system (such as a subsea tree) in a subsea oil and gas production system, and method of use. The apparatus comprises a body defining a conduit therethrough and a first connector for connecting the body to the flow system. A second connector is configured for connecting the body to an intervention apparatus, such as an injection or sampling equipment. In use, the conduit provides an intervention path from the intervention apparatus to the flow system. Aspects of the invention relate to combined injection and sampling units, and have particular application to well scale squeeze operations.

Abstract: An expansion system is capable of expanding a tubular that has no shoe or plug. The tubular is expanded by a cone coupled to a mandrel, which is energized by a jack actuator to effect a plurality of strokes. The jack actuator is supported by a support member. The jack actuator releasably engages and locks to an operational pipe at predetermined length intervals corresponding to the length of each actuator stroke. The actuator jack is reset and moved to a new locking location between strokes. The operational pipe holds the expandable tubular in place during expansion. After expansion, the cone, mandrel, actuator jack, and operational pipe are free of the expandable tubular and can be pulled out of the wellbore. The expansion system can be used for applications of patches having a small size in wellbores where milling debris may be detrimental.

Abstract: The present disclosure provides a fracturing bridge plug, and belongs to the technical field of downhole plugging devices of oil and gas fields, which solves technical problems that the existing packer rubber is axially compressed, a bridge plug is relatively long, and a dissolving or drilling effect is not ideal. The fracturing bridge plug comprises a cone, an upper cone, a seal assembly, a lower cone, a slip, a locking ring and a guide shoe, a matching surface of the upper cone and the seal assembly is a first ramp, one end, far away from the lower cone, of the first ramp is a large end while one end, close to the lower cone, thereof is a small end, and the diameter of the first ramp from the large end to the small end is gradually reduced.

Abstract: A valve assembly includes a valve body and an opening extending through the valve body along a longitudinal axis. The valve assembly also includes a valve member arranged within the opening, the valve member being movable, along the longitudinal axis, between an open position and a closed position. The valve assembly includes an upper seal positioned to seal against at least a first portion of the valve body. The valve assembly includes a lower seal positioned to seal against at least a second portion of the valve body, wherein both the upper seal and the lower seal travel with the valve member as the valve member moves between the open position and a closed position.

Abstract: Methods and apparatus for utilizing a valve with a pump rotor passage with a downhole production string, the pump rotor being on a rotatable rod with a bobbin moving along the rod between a position for opening the passage to fluid flow, when the bobbin is not seated on a shuttle seat, and a position for closing the passage to fluid flow, when the bobbin is seated on the shuttle seat. The pump rotor and rod are removable through the passage while leaving the pump stator in place upstream of the valve.

Abstract: Provided are systems and methods incorporating design features that may be beneficial over current available well safety valves, such as a sleeved receptacle for a piston, a constrained piston rod, and a trapped flapper valve. A safety valve may comprise: an outer housing having a central bore extending axially through the outer housing, wherein the outer housing comprises an outer wall having a receptacle formed in the outer wall; a sleeve disposed in the receptacle; a piston disposed in the sleeve, wherein the piston is actuated to travel in the sleeve; a spring positioned to apply a biasing force to the piston; and a valve member disposed in the outer housing, wherein the valve member is operable to selectively restrict flow through the safety valve in response to movement of the piston.

Abstract: A downhole valve has a valve body having at least one port and slidably receiving in a longitudinal bore thereof at least a first sliding sleeve for opening and closing the at least one port. The first sliding sleeve has a circumferential actuation groove. An actuation assembly is extendable into the first sliding sleeve and has an actuation housing axially movably receiving thereon a compressible sealing element and a slip assembly. The slip assembly has one or more slips radially outwardly extendable under a hydraulic pressure for engaging the circumferential actuation groove of the first sliding sleeve for opening the at least one port, and the actuation assembly is longitudinally extendable to position a portion thereof on an inner side of the one or more slips for supporting the one or more slips at a radially outwardly extended configuration.

Abstract: A downhole valve has a valve body having at least one port and slidably receiving in a longitudinal bore thereof at least a first sliding sleeve for opening and closing the at least one port. The first sliding sleeve has a circumferential actuation groove. An actuation assembly is extendable into the first sliding sleeve and has an actuation housing axially movably receiving thereon a compressible sealing element and a slip assembly. The slip assembly has one or more slips radially outwardly extendable under a hydraulic pressure for engaging the circumferential actuation groove of the first sliding sleeve for opening the at least one port, and the actuation assembly is longitudinally extendable to position a portion thereof on an inner side of the one or more slips for supporting the one or more slips at a radially outwardly extended configuration.

Abstract: A downhole valve has a valve body having at least one port and slidably receiving in a longitudinal bore thereof at least a first sliding sleeve for opening and closing the at least one port. The first sliding sleeve has a circumferential actuation groove. An actuation assembly is extendable into the first sliding sleeve and has an actuation housing axially movably receiving thereon a compressible sealing element and a slip assembly. The slip assembly has one or more slips radially outwardly extendable under a hydraulic pressure for engaging the circumferential actuation groove of the first sliding sleeve for opening the at least one port, and the actuation assembly is longitudinally extendable to position a portion thereof on an inner side of the one or more slips for supporting the one or more slips at a radially outwardly extended configuration.

Abstract: A method of decontaminating naturally occurring radioactive material (NORM) from downhole equipment may include injecting a NORM dissolver into an isolated region of a wellbore in which NORM-contaminated production equipment is located; and removing the NORM contaminants from the production equipment. The method may also include recommencing production of hydrocarbons following the decontamination.

Abstract: A submersible vehicle is provided which includes a housing, an optical fiber coupled with the housing and communicatively coupled with a data acquisition system, and a propulsion system. The propulsion system is configured to propel the submersible vehicle in a fluid at a velocity. The optical fiber is configured to be released at a release rate equal to or greater than the velocity of the submersible vehicle.

Abstract: A well treatment method can include pumping a treatment fluid through a tubular in a well, deploying plugging devices into the tubular, and blocking flow of the treatment fluid through perforations formed through the tubular. At least one characteristic of the treatment fluid or the plugging devices is varied, thereby in the blocking flow step selectively blocking the flow of the treatment fluid through the perforations in a predetermined sequence. Another well treatment method can include varying at least one characteristic of perforations along an interval, thereby selectively blocking flow of a treatment fluid through the perforations in a predetermined sequence. Another well treatment method can include varying a CS ratio along an interval, thereby selectively blocking flow of a treatment fluid through perforations in a predetermined sequence.

Abstract: A valve system for use in a wellbore, comprising a side pocket mandrel having a main bore for alignment with a wellbore tubular and a laterally offset side pocket bore separated from the main bore by an internal wall, the laterally offset side pocket bore comprising a first receptacle for a first device and a second receptacle for a second device, the internal wall having an opening through which each of the first device and the second device is independently retrievable. There is also provided a side pocket mandrel, a hydrocarbon well and a method of operating a hydrocarbon well.

Abstract: According to one or more embodiments presently described, petroleum may be recovered from a sub-surface reservoir by a method that may include passing one or more exothermic reaction components into an aqueous zone of the sub-surface reservoir in a geological formation. The reservoir may comprise a petroleum zone including petroleum and an aqueous zone including water. The petroleum zone and the aqueous zone may be separated, at least partially, by a tar zone. The method may further include reacting the one or more exothermic reaction components to increase the temperature, pressure, or both, in the aqueous zone to reduce the viscosity of the tar zone such that pressure in the aqueous zone may drive extraction of petroleum from the petroleum zone to the surface.

Abstract: According to one or more embodiments presently described, petroleum may be recovered from a sub-surface reservoir by a method that may include reducing the break-down pressure of a geological formation from an original break-down pressure to a reduced break-down pressure by exothermically reacting one or more reaction components in a carrier fluid adjacent to or within the geological formation, and forming fractures in the geological formation by hydraulic fracturing the geological formation.

Abstract: A method and a system for recovering oil from currently inaccessible oil containing geological units by activating the deep biosphere microbial seed bank. Nutrient and thermal enhancement of microorganisms in oil containing geological units allows for stimulation of inactive and/or dormant microorganisms such that they proliferate and produce gas. The oil viscosity that is decreased by heat, along with the gas pressure produced by activated microbes which allows previously inaccessible oil to flow toward production wells.

Abstract: Sliding sleeve assemblies may include one or more sliding sleeve tools to stimulate one or more zones in a wellbore. The one or more sliding sleeve tools may be actuated based on an actuation sensor. A property sensor may be disposed adjacent to a sliding sleeve tool to collect data indicative of a wellbore property associated with one or more different zones of a fracture or the actuation sleeve. The property sensor may transmit data to the surface or to other property sensors associated with downhole tools. Configuring or disposing one or more property sensors to a downhole tool may provide real-time feedback regarding the rate of production for a particular zone or area downhole.

Abstract: To improve or optimize a wellbore treatment operation, a target net treating pressure may be determined and the constant net treating pressure maintained to effectively enhance formation breakdown and fracture complexity as well as provide a reduction in wear and tear of equipment and completion time. The target net treating pressure may be based on one or more treatment parameters and these parameters may be adjusted during the wellbore treatment operation to maintain a constant net treating pressure at or about the target net treating pressure. The injection rate or pressure of a treatment fluid may be adjusted to maintain the constant net treating pressure. Measurements associated with the wellbore treatment operation may be compared to an operational constraint and adjustments to the wellbore treatment operation may be made based on the comparison. The wellbore treatment operation may be terminated based on a parameter falling below or exceeding a threshold.

Abstract: Methods, systems, and storage media are presented for operating a drill string within a well. Exemplary implementations may: detect, using one or more sensors, data representing an analog control command sequence; analyze the analog control command sequence; determine that the control command sequence represents a command to change an operational mode of the drill string from a first operational mode to a second operational mode; and transmit a signal to an actuator of the drill string, the signal representing an electronic command to activate the actuator in such a way as to change the drill string from the first operational mode to the second operational mode. In some embodiments, the analog control sequence may comprise a predetermined sequence of rotational inputs, vibrational inputs, pressure inputs, or a combination thereof.

Abstract: Apparatus, systems, and methods for ranging operate to use a wireline active ranging system to initially determine a relative distance and relative direction from a first well (e.g., ranging well) to a second well (e.g., target well) and an EM azimuthal logging tool to maintain or adjust the distance from the target well while drilling the ranging well. Additional apparatus, systems, and methods are disclosed.

Abstract: A system for stimulating oil or gas production from a wellbore includes a hydraulic fracturing pump unit having one or more hydraulic fracturing pumps driven by one or more electrical fracturing motors, a variable frequency drive (VFD) controlling the electrical fracturing motors, a fracturing pump blower unit driven by a blower motor, and a fracturing pump lubrication unit having a lubrication pump driven by a lubrication motor and a cooling fan driven by a cooling motor. The system may further include a blender unit and a hydration unit. A system control unit may control the operational parameters of the system.

Abstract: Embodiments of the subject technology provide for receiving real-time drilling data comprising different drilling parameters measured during a drilling operation. The subject technology calculates a kick detection parameter based at least in part on the different drilling parameters. The subject technology detects an occurrence of a kick during the drilling operation when the kick detection parameter deviates from a trend formed by previously calculated kick detection parameters. Further, the subject technology activates an alarm during the drilling operation in response to detected occurrence of the kick to facilitate preventing a blowout.

Abstract: According to one aspect, data identifying a component is received, wherein the component is part of a system associated with a wellhead. A location at which the component is positioned relative to one or more other components is identified. The useful remaining operational life of the component is predicted based on at least an operational parameter specific to the location, and the operational history of the component or one or more components equivalent thereto. According to another aspect, a model representing at least a portion of a proposed system associated with a wellhead is generated, the model comprising a plurality of objects, each of which has a proposed location and represents an existing component. The useful remaining operational life for each object is predicted based on an operational parameter at the corresponding proposed location, and data associated with the respective operational history of the existing component.

Abstract: Conventional practice for determining a life expectancy of a drilling tool has been based on simple static bending moment evaluation and/or conservative past experience life limits. This archaic practice has often led to premature scrapping of the tools and has proven to be overly conservative and cost-ineffective. Introduced herein is a BHA condition monitoring technique that combines both field data and advanced models in one system. The introduced technique is based on a combination of system and component level models to monitor and evaluate the current health and life of BHA components. The introduced technique can apply to all directional drilling BHAs, including mud motors and rotary steerable systems, and can be used at different levels of the tool's life cycle to improve efficiency, reduce downhole failure incidents, and maximize assets' utilization.

Abstract: Impedance is used to determine the performance of paraffin inhibitors in oil containing paraffin. The method and system can use a specially designed impedance cell having a cell constant of less than 1 cm?1. Further, the method can include obtaining at least impedance measurements above the wax appearance temperature (WAT) for an oil sample treated with a paraffin inhibitor and an oil sample not treated, and impedance measurements below the WAT for the treated oil sample and the untreated oil sample. Thereafter, the impedance measurements are correlated to determine paraffin inhibitor performance.

Abstract: A system for optimizing a tubular running operation (TRO) in which a running string is disposed in a wellbore includes sensors located at the wellsite, plus one or more processors, displays, and user-input devices, which may be at the wellsite or elsewhere. The sensors are configured to measure miming string parameters including the running strings position within the well, the miming and rotation rates, and loads acting at the top of the miming string. Based on this information, the processor(s) perform torque-and-drag analysis (TDA) in the top-down direction to estimate downhole loads on user-selected miming string components, accounting for measurement and modelling uncertainties. Based on the estimated loads, the processor(s) calculate one or more damage indicators, which are communicated to the user via the display(s), enabling the user to take corrective action to prevent or manage damage to the running string.

Abstract: Systems and methods of controlling the performance of client nodes served by an electric power utility are provided. In one exemplary embodiment, a method performed by a first network node that is operable to control via a client node an electric motor configured to operate a pump jack comprises sending, to the client node, an indication to change an amount of electric power consumed by the motor to operate the pump jack based on at least one of a value of a first parameter associated with operation of the motor and a value of a second parameter associated with operation of the pump jack so as to reduce an average amount of electric power consumed during a certain time period by the motor in operating the pump jack.

Abstract: The disclosure provides a well system environment ranging system including a well shoe device with a ranging source. In one aspect the ranging source is a magnetic source that can be a permanent magnet, and electromagnet, or a smart electromagnet. A method is also provided wherein a ranging receiver can be deployed to determine the relative location of the well shoe device to the ranging receiver. The well shoe device and the ranging receiver can be deployed in adjacent wellbores. In one aspect, multiple well shoe devices including a magnetic source can be inserted into one or more wellbores where the magnetic sources can have different specified magnetic field intensity so the well shoe positions can be determined by one or more ranging receivers.

Abstract: A stratigraphic analysis of a subject wellbore may include determining a heat map curve indicative of misfit values between stratigraphic vertical depth obtained using logged data from an offset wellbore and measured depth over a depth range of the wellbore. A first likelihood that the target measured depth is equal to or within a tolerance of stratigraphic vertical depth respectively for each of a sets of sequences of stratigraphic vertical depth may be calculated. Also, a probability weighted average and an uncertainty that the target measured depth is equal to or within a tolerance of stratigraphic vertical depth may be calculated. A continuous probability function over a range of measured depth that the target measured depth is equal to or within a tolerance of stratigraphic vertical depth may also be calculated. The first likelihood, the probability weighted average, the uncertainty, and the continuous probability function may be used to make and evaluate drilling decisions of the wellbore.

Abstract: Systems and methods corresponding with improved wellfield management are provided. In some examples, the system may initiate a step drawdown test of well efficiency to determine a cause of a decreased discharge rate of the wellfield site. The system may alter pumping of the wellfield site based on the step drawdown test, wherein a command signal is transmitted via a SCADA system to the wellfield site.

Abstract: A downhole acoustic transmitter has a piezoelectric transducer, an enclosure in which the piezoelectric transducer is housed, a transducer preload means which applies a selected compressive force against the transducer such that a mechanical preload is applied to the transducer, and an acoustic tuning element which has a first end coupled to the transducer preload means or the transducer, and an open second end. The acoustic tuning element is not coupled to anything but the transducer preload means or transducer, so the transducer preload means effectively has a second open end and thus can maintain the same preload compressive force on the transducer even when the transmitter is subjected to tension and compressive forces during operation.

Abstract: A method for acquiring an opening timing of natural fracture under an in-slit temporary plugging condition and a device thereof are provided. The method includes steps of: acquiring physical parameters of stratum according to site geological data, and measuring a slit length L of a hydraulic fracture; dividing the hydraulic fracture into N unit bodies of equal length and numbering them sequentially; and dividing a total calculation time t into in time nodes with equal interval; calculating a width of each unit body in the hydraulic fracture at the initial time; calculating a fluid pressure in the hydraulic fracture at the k-th time node; calculating a closed pressure at an entrance of the natural fracture on an upper side and a lower side of the hydraulic fracture at the k-th time node; and determining whether the natural fracture is opened by a determining criteria based on the above calculation results.

Abstract: Disclosed are systems and methods for receiving, by at least one processor, calibration information comprising a first power spectral density (PSD) value and a first sand concentration value at a particular frequency, collecting, by at least one processor, data in a liquid flow at the particular frequency at a first depth in a downhole environment, determining, by the at least one processor, a second power spectral density (PSD) value for the data in the liquid flow at the particular frequency, comparing, by the at least one processor, the second PSD value with the first PSD value and determining that sand is present in the liquid flow when the second PSD value is greater than the first PSD value by a predefined threshold, and quantifying, by the at least one processor, a second sand concentration value of the sand present in the liquid flow based on the first sand concentration value.

Abstract: A downhole fluid analysis system includes an optical sensor that includes a light source configured to emit light, a light detector, and an optical tip optically coupled to the light source and the light detector. At least a portion of the light emitted from the light source travels through the optical tip and returns to the detector, wherein the optical tip has a bi-conical shape. The system further includes a piezoelectric helm resonator, in which the piezoelectric helm resonator generates a resonance response in response to an applied current, and an electromagnetic spectroscopy sensor positioned symmetrically with respect to the piezoelectric helm resonator in at least one direction. In some embodiments, the optical tip includes a first conical portion and a second conical portion.

Abstract: A technique facilitates fluid analysis in situ at a downhole location. According to an embodiment, a sample of fluid, e.g. oil, is obtained from a reservoir at the downhole location in a borehole. A downhole sampling system is used to determine contamination of the sample of fluid and to determine other selected characteristics of the sample. The data obtained is then processed to determine a true phase transition property of a native fluid in the sample of fluid. The sample analysis may be performed at selected stations along the borehole during, for example, a well cleanup operation. The immediate analysis of fluid samples downhole facilitates rapid development of an understanding of fluid characteristics in the reservoir, thus enabling an improved oil recovery strategy.

Abstract: A downhole fluid analysis system includes an optical sensor comprising, which includes a light source configured to emit light comprising a plurality of wavelengths, a light detector, and an optical tip through which at least a portion of the light travels and returns to the detector, wherein the incident angle of the light causes total internal reflection within the optical tip. The system further includes a piezoelectric helm resonator that generates a resonance response in response to an applied current, and an electromagnetic spectroscopy sensor positioned symmetrically with respect to the piezoelectric helm resonator in at least one direction. The light may be reflected in the optical tip at one or more reflection points, and each reflection point may generate an evanescent wave in a medium surrounding the optical tip. The light may be internally reflected in the optical tip at a plurality of reflection points.

Abstract: Disclosed are a vehicle body utilized in a rockbreaker, a rockbreaker, and a rock breaking method. A rock breaking operation area is configured to pass through the vehicle body and in a direction perpendicular to and above the ground. An operation area of a rockbreaker arm is arranged within a vehicle range.

Abstract: A high wall mining method has the steps of: providing a drilling machine with a GPS/heading locator; providing a tablet or computer accessible to cloud-based apps; downloading a software program from the cloud for high wall mining