Horizontal Borehole Research Articles

Cutting transport simulation in a horizontal borehole near tool joints

In the presented study, we investigated the influence of cuttings transport in a channel with variable cross-section area to determine the pressure drop and obtain preliminary data on tool joint effect on solids deposition. The influence of boundary conditions at the channel’s inlet on flow regimes was analysed. The average particle concentration over the cross-section significantly reduced due to increasing the velocity in the narrowing area on the tool joint. Behind the tool joint, particles deposition appeared due to the reverse flow, which manifested itself as a local maximum in concentration distribution along the channel. The impact of a tool joint on the pressure drop in a single-phase flow has been more pronounced if compared to one in a two-phase flow at the given conditions.

Intensification of gas recovery from coal seams applying the method of hydrodynamic impact in horizontal boreholes

The techniques of hydrodynamic impact on the coal seam from horizontal boreholes drilled from mine workings with the aim of increased gas recovery are considered. A method for monitoring the parameters of a coal seam hydraulic fracturing and the actual results obtained in the conditions of Kuzbass coal mines are outlined.

A novel analytical model of initial fracture pressure for horizontal staged fracturing in fractured reservoir

AbstractThe horizontal well‐staged fracturing technology has been widely used in many oilfields to enhance well production by creating fracture network. Accurate prediction of fracture initiation is significant for successful hydraulic fracturing operation. However, hydraulic fracture initiation in fractured reservoir is highly tricky due to its rich natural fractures and artificial fractures previously created during staged fracturing. These fractures are thought to be major factors in conducting efficient staged fracturing in fractured reservoir. Therefore, in this paper, induced stress from artificial fractures has been analyzed by using two‐dimensional analytical model. Besides, based on the assumption that rock in the formation is composed by rock matrix and fracture plane, considering tensile and shear failure type, failure criterion with multiple natural fracture planes has been developed. By combining impacts from natural fractures and artificial fractures, we establish an analytical model to calculate initial fracture pressure for horizontal staged fracturing in fractured reservoir. These results demonstrate that induced stress is associated with artificial fractures’ location, height, and number. This induced stress leads to increment of initial fracture pressure, restricting fracture initiation. Due to natural fractures, there are three types of initiation, which are tensile failure along rock matrix, tensile failure along natural fracture, and shear failure along natural fracture plane. When rock failure is related to natural fracture, initial fracture pressure decreases and is variable with natural fracture plane occurrence. Especially, with large number of natural fractures, only one initiation type exists, which is tensile failure along natural fracture. Additionally, for horizontal borehole, influencing extent of fracture planes is associated with wellbore trajectory. The case study proves the applicability of this analytical model, meaning it can be used to guide horizontal staged fracturing in oilfield.

Simulation of gamma-ray distribution in rocks for determining the registration characteristics of measurement unit of horizontal borehole positional control system

Theoretical aspects of gamma-ray distribution in rock are described in the article. Information concerning the values of natural gamma radiation of rocks provides for the control of the drilling tool’s orientation during horizontal drilling. A simulation of gamma-ray distribution in rock, based upon the example of western Siberian fields, has been made in a COMSOL Multiphysics interactive environment. Test results for a prototype, two-probe device for horizontal borehole positional control are contained in this document. The maximum distance for the registration of changes in the natural gamma radiation of rocks was determined. The attenuation coefficient for the natural gamma radiation of rocks introduced by the influence of a nonmagnetic drill collar was calculated. Practical advice for improvements in the registration characteristics of the prototype device is provided.

A novel anti-air-leakage method and an organic polymer material for improving methane drainage performance

Coal mine methane (CMM) drainage is an important method to curb the occurrence of gas disasters and to reduce the amount of greenhouse gas emission, and to make full use of the methane resource. High-grade sealing boreholes is the key factor to achieve efficient drainage of methane. In order to improve the efficiency of CMM drainage as well as to seal in-seam horizontal boreholes and fractures around these boreholes effectively, a dynamic anti-air-leakage method called the dynamic fractures filling technology was presented, and an organic polymer material called the PM sealing material matching with this technology was developed in this paper. Firstly, the destruction of coal body around boreholes because of a roadway and a borehole excavation was analyzed from the view of stresses, and the range of fractures zone caused by roadway excavation was determined, then the sealing technology and sealing principle of the novel technology were analyzed systematically. Moreover, the development process of the PM sealing material was introduced, and the influence of preparation parameters on material properties was investigated. Besides, taking the coal seam in the 39,021 Transportation Lane in the Zhongmacun Mine(China) as the test site of this study, the attenuation law of the methane concentration in drainage boreholes under different sealing depths was investigated via adopting the conventional sealing technology. Finally, the methane-drainage effect of the novel sealing technology and the conventional technology were compared. The results indicate that: (1) Both a roadway excavation and a borehole excavation will cause the redistribution of stresses of the original coal mass, and the coal structure around a drainage borehole has been destroyed in different degrees due to stresses overlapping effect. Moreover, the dynamic developing fractures zone is the main channel of air-leakage during drainage process. (2) The novel technology uses the conventional sealing system and the dynamic sealing system to seal drainage boreholes and dynamic development of fractures around them. (3) When the water-to-material ratio is 7:1 and the treatment temperature is 180°C for 30min, the PM material has the best performance. (4) Comparing with the conventional sealing technology, the novel technology has the obvious characteristics of short sealing depth, long efficient continuous drainage time and high drainage amount of methane. and its advantages of methane-drainage effects and economic benefits are remarkable.

Advancing CMM drainage, quality, and utilization with horizontal directional-drilled boreholes in Duanshi Coal Mine, southern Qinshui Basin, China

The Duanshi coal mine is a private, small-size producer and one of >1000 Chinese underground coal mines that pre-drain CMM prior to mining to prevent explosions, capture, and utilize fugitive gas. The coal mine opened in 2008 and employed in-seam auger-drilled boreholes to capture CMM and use as required by the China Emission Standard to curb CH4 emission, which is one of the most potent GHG. As drilling technology advanced, the coal mine used in-seam horizontal directional drilling in 2017 to boost capture of higher quality CMM for utilization but was hampered by technical and geological challenges. Deformation of Pennsylvanian–Permian coal-bearing rocks in the southern Qinshui Basin restricted horizontal directional drilling in the coal mine. The target No. 3 coal seam of the Permian Shanxi Formation is tightly folded into an asymmetrical, northward plunging syncline in the mine area, which requires designing innovative strategies for implementing drilling. Also, the “tight reservoir” (impermeable) property of the No. 3 coal makes recovery of high-volume and -quality CMM difficult. These technical and geological conditions were revealed as causes of the increasing trend in China's total CH4 emission, which is 1.1 ± 0.4 Tg CH4/yr from 2010 to 2015, specifically attributed to underground coal mining (Miller et al., 2019). The increase indicates that CMM drainage and utilization regulations have not curbed growing CH4 emission from underground coal mines. We conclude that our study of advancing CMM drainage technology and use in the Duanshi coal mine could serve as an example for China's small and medium mine operators of underground coal mines that suffer similar technical and geological challenges.

Reasonable minimum sealing length of in-seam boreholes for coal mine methane drainage: a case study

ABSTRACT Coal mine methane (CMM) drainage is an effective method to curb the occurrence of coal and gas outburst and gas explosion, to reduce the amount of greenhouse gas emission, and to make full use of the clean energy(methane). Sealing in-seam horizontal boreholes plays a vital role in CMM drainage. Reasonable sealing length is an important factor to achieve high extraction efficiency and reduce the cost of construction. To determine the minimum reasonable sealing length of drainage boreholes, the radius of the fracture zone was obtained based on the ideal elastoplastic model, and this radius was taken as the minimum sealing length. The influencing factors of the minimum sealing length were analyzed and engineering verification test was taken in Zhongmacun mine in Henan province, China. The research results show that the minimum reasonable sealing length should be longer for a larger radius of the coal roadway and the coal seam with a deeper buried depth, while the minimum reasonable sealing length can be shortened with the increase of uniaxial compressive strength and the supporting stress of the coal roadway. The 11.07 m of sealing length calculated by theories was basically consistent with the 12 m of sealing length verified by the method of drilling cuttings. The reasonableness of sealing length (12 m) was verified by the test results of methane concentration and flux within 40 days. All those outcomes can provide references for other mines to determine the minimum reasonable sealing length of in-seam boreholes for CMM drainage.

Validating Predicted Fracture Corridors by Statistical Comparison With Well Data

SummaryA challenge in oil–reservoir studies is evaluating the ability of geomechanical, statistical, and geophysical methods to predict discrete geological features. This problem arises frequently with fracture corridors, which are discrete, tabular subvertical fracture clusters. Fracture corridors can be inferred from well data such as horizontal–borehole–image logs. Unfortunately, well data, and especially borehole–image logs, are sparse, and predictive methods are needed to fill in the gap between wells. One way to evaluate such methods is to compare predicted and inferred fracture corridors statistically, using chi–squared and contingency tables.In this article, we propose a modified contingency table to validate fracture–corridor–prediction techniques. We introduce two important modifications to capture special aspects of fracture corridors. The first modification is the incorporation of exclusion zones where no fracture corridors can exist, and the second modification is taking into consideration the fuzzy nature of fracture–corridor indicators from wells such as circulation losses. An indicator is fuzzy when it has more than one possible interpretation. The reliability of an indicator is the probability that it correctly suggests a fracture corridor. The indicators with reliability of unity are hard indicators, and “soft” and “fuzzy” indicators are those with reliability that is less than unity.A structural grid is overlaid on the reservoir top in an oil field. Each cell of the grid is examined for the presence and reliability of inferred fracture corridors and exclusion zones and the confidence level of predicted fracture corridors. The results are summarized in a contingency table and are used to calculate chi–squared and conditional probability of having an actual fracture corridor given a predicted fracture corridor.Three actual case studies are included to demonstrate how single or joint predictive methods can be statistically evaluated and how conditional probabilities are calculated using the modified contingency tables. The first example tests seismic faults as indicators of fracture corridors. The other examples test fracture corridors predicted by a simple geomechanical method.

An Adaptable Water-Based Mud System for Multiple Applications While Drilling

Due to significant variations of the subsurface geology from the surface to the top of reservoir and requirement of different fluid characteristics for drilling various hole there is a need to use various mud systems. These may include a simple spud mud for surface hole section, an inhibitive drilling fluid for reactive shale section, a salt water-based mud for salt diapirs and salt formations, and a highly lubricating mud for deviated hole sections with high dogleg severity.To optimize each of these separate and distinct scenarios, there is a need to change the mud system while drilling to overcome the technical challenges associated with these formations and wellbore profiles. The change over from one mud system to another is typically done between casing points while constructing the well to overcome specific drilling challenges associated with next whole section.There is significant time and effort required to clean the mud circulation system adequately before a mud change over in order to avoid any contamination of the new mud system.This is especially true when displacing a waterbased mud by an oil-based mud or an oil-based mud by a water-based mud.If this is not done properly, contamination of the new mud by the old mud could be a source of major problems due to partial or complete loss of functional ability of the new mud system. An adaptable drilling mud system that can easily be transformed from a spud mud system to an inhibitive, or a high lubricating or a salt water mud can provide the industry a versatile fluid system with multiple hole section applications.This removes much of the NPT associated with mud changeover, reduces the mud cost as compared to mixing a totally new mud system and eliminates concerns regarding mud contamination as well as any disposal or recycling cost for the replaced system. This paper describes a volcanic ash-based drilling mud that can be used as a spud mud to drill the surface hole, can easily be converted to an inhibitive mud system to drill reactive shale sections of a borehole, a salt water-based mud to drill the salt sections and also a high lubricating water-based drilling mud to reduce torque and drag problems in deviated and horizontal boreholes. The flexible and easily convertible nature of the base volcanic ash-based drilling mud has potential to reduce total drilling cost significantly as it eliminates a significant portion of non-productive drilling time associated with mud changeover, cleaning of mud circulation system, new mud preparation, incorporation of new mud in the circulation system and displacement of the old mud from the borehole by the new mud, etc.

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.

Coastal cliff rock mass weakening of Chalk and the impact of salt water

The relationship between salt water and the strength of Chalk forming the coastal cliffs of northwest Europe was investigated. Uniaxial compressive strength (UCS) tests on core samples from three horizontal boreholes drilled at the base of Chalk cliffs in East Sussex, UK, showed the weakest Chalk to be near the cliff face. The UCS nearly doubled over the length of the 9 m deep boreholes. The UCS results were close to values expected for Chalk of this intact dry density for samples farthest from the cliff face. High chloride concentrations (salt water) of up to 69 000 mg/l were found to be associated with the lowest UCS values closest to the cliff face. Lower chloride concentrations, with values of 1850 mg/l or less and often with a non-marine origin, were found in the stronger core samples. It can be concluded that the Chalk coastal cliffs in the areas tested became weaker towards the cliff face. This can, in part, be linked to salt water weakening although it is likely that other factors are also contributing to this phenomenon.

Impact of lithofacies variations and structural changes on natural fracture distributions

Understanding and predicting the impact of lithofacies changes and structural effects on fracture distributions is vitally important to optimize a drilling location and orientation. To evaluate and model fracture intensity of the Late Ordovician-Silurian-Early Devonian Hunton Group carbonates in Oklahoma, natural fractures were studied at different scales using borehole images, three outcrops (two horizontally bedded outcrops and one anticline outcrop), and seismic data. Natural fractures identified from eight horizontal well borehole images include conductive (open), partially open, mineralized (closed), and induced fractures. Four fracture sets were identified from borehole images and from the two horizontally bedded outcrops. A 3D fracture intensity model was populated, from the fracture intensity logs at the boreholes, and compared with a 3D lithofacies model. Principal component analysis from lithology logs produced input to a self-organizing map to classify and cluster electrofacies. Thin sections and borehole images corroborate the electrofacies around the wellbores, whereas 3D seismic data were used as constraints to build a 3D lithofacies model. A 3D lithofacies model resulted from the extrapolation of the lithofacies from the well scale to the regional seismic scale. In this study area, lithofacies and structure are interrelated and control fracture distributions. Lithofacies is the primary control, whereas structure is the secondary control. Three lithofacies (wackestone, mudstone, and mud-dominated wackestone) were identified. A positive relationship between the fracture intensity and the presence of wackestone was observed at well locations and in the mapped subsurface area. The other two lithofacies do not exhibit high fracture abundance. Structural effects influence fracture distributions near faults and positive curvature areas in the subsurface measured on the 3D seismic data. For the Hunton Anticline outcrop exposure, there was a positive linear relationship between fracture intensity and changes in curvature for the mudstone and mud-dominated wackestone and an exponential relationship for the wackestone textures. The integration of lithology and structure from multidisciplinary, multiscalar data (i.e., outcrops, image logs, and 3D seismic) helps to identify and predict the fractured zones in the Hunton carbonates and can be used for horizontal well planning as well as stimulation programs. More importantly, this study proposes a generic model to predict the variability of fractures at different scales of curvatures combined with lithology changes and can be used for other carbonate reservoirs.

A record of changes in the Gran Sasso groundwater before, during and after the 2016 Amatrice earthquake, central Italy

We performed continuous recordings (May 2015 – January 2017) of hydraulic pressure and electrical conductivity of groundwater in the 190 m-long horizontal S13 borehole drilled next to the deep underground laboratories of Gran Sasso (LNGS-INFN), located in the core of the Gran Sasso carbonate aquifer (central Italy) at a distance of about 39 km south-eastward from the 24 August 2016 Amatrice earthquake (6.0 Mw) epicenter. Using a 3-channel, 24-bit ADC we achieved a sampling rate of groundwater physical properties up to 50 Hz for each channel. We focused on the analysis of data recorded before, during and after the Amatrice earthquake, describing and discussing in detail the evidence for significant hydraulic pressure and electrical conductivity anomalies recorded before the main shock. We identified unambiguous signals in the hydraulic pressure data starting on 19 August, i.e. five days before the 24 August mainshock. A more careful analysis allowed us to detect the inception of a weak change up to 40 days before the Amatrice earthquake and a significant variation in the electrical conductivity data about 60 days before. The data revealed highly dynamic aquifer behaviour associated with the uprising of geogas probably related to the preparation stage of the Amatrice earthquake.

Determination of Long Horizontal Borehole Height in Roofs and Its Application to Gas Drainage

Accurately determining the height of the gas-guiding fracture zone in the overlying strata of the goaf is the key to find the height of the long horizontal borehole in the roof. In order to determine the height, in this study we chose the 6306 working face of Tangkou Coal Mine in China as a research example and used both the theoretical model and discrete element method (DEM) numerical simulation to find the height of the gas-guiding fracture zone and applied the height to drill a long horizontal borehole in the roof of the 6303 working face. Furthermore, the borehole was utilized to deep into the roof for coalbed methane drainage and the results were compared with conventional gas drainage measures from other aspects. The height of the gas-guiding fracture zone was found to be 48.57 m in theoretical model based on the bulk coefficient and the void ratio and to be 51.19 m in the DEM numerical simulation according to the temporal and spatial variation characteristics of porosity. Taking both the results of theoretical analysis and numerical simulation into consideration, we determined that gas-guiding fracture zone is 49.88 m high and applied it to drill a long horizontal borehole deep into the roof in the 6303 working face field. Compared with conventional gas drainage measures, we found that the long horizontal borehole has the high stability, high efficiency and strong adaptability for methane drainage.

An improvement sealing method for gas pressure measurement by drilling a reducing‐nipple borehole

AbstractGas pressure measurement is one of the crucial steps for CBM resource extraction. However, the previous sealing methods are not applicable for horizontal boreholes owing to the autoshrinkage effect of cement mortar. The autoshrinkage tests of cement specimens with different water‐cement ratios were performed here, and then the gas flow model in an eccentric ring gap was established to analyze the autoshrinkage effect on the hole sealing. Furthermore, the selections of the initial sealing position and the grouting pressure were also discussed through the numerical method. The results show that the shrinkage amount would be decreased with the water‐cement ratio rising in the early setting stage, and the gas leakage amount of the eccentric model is larger than that of the concentric ring model. The initial sealing position should be kept away from the plastic zone around tunnel, which is related to the lithology of surrounding rock, the buried depth and the size of tunnel section. The diffusion radius of cement mortar would be enlarged with an increasing grouting pressure. While the grouting pressure reaching the threshold, the increment of diffusion radius would gradually descent. Eventually, a pressurized sealing method by drilling the reducing‐nipple borehole was successfully applied for the gas pressure measurement of the horizontal borehole in 7# coal seam, Liulong coal mine.

Hydrogeophysical characterization and monitoring of the hyporheic and riparian zones: The Vermigliana Creek case study

The hyporheic and riparian zones are critical domains in a river ecosystem since they mediate the interactions between surface water and groundwater. These domains are generally strongly heterogeneous and difficult to access; yet their characterization and monitoring still rely mostly on hard-to-perform invasive surveys that provide only point information. These well-known issues, however, can be overcome thanks to the application of minimally invasive methods. In this paper, we present the results of the hydrogeophysical characterization of the Vermigliana Creek's hyporheic and riparian zones, performed at an experimental site in the Adige catchment, northern Italy, by means of electrical resistivity tomography (ERT), distributed temperature sensing (DTS), and hydrological modeling. A major advancement is given by the placement of electrodes and of an optical fiber in horizontal boreholes at some depth below the river bed, put in place via directional drilling. The results of this static and dynamic (time-lapse) geophysical characterization identify the presence of two subdomains (the sub-riverbed and the left and right banks) and define the water flow and solute dynamics. The ERT information is then used, together with other hydrological data, to build a 3D subsurface hydrological model (driven mainly by the watercourse stage variations) that is calibrated against local piezometric information. A solute transport model is then developed to reproduce the variations observed in the dynamic geophysical monitoring. The results show good agreement between ERT data and the model outcome. In addition, the transport model is also consistent with the temperature data derived from DTS, even though some slight discrepancies show that the heat capacity of the solid matrix and heat conduction cannot be totally neglected.

Insights Into Complex Subdecimeter Fracturing Processes Occurring During a Water Injection Experiment at Depth in Äspö Hard Rock Laboratory, Sweden

AbstractWe investigate the source characteristics of picoseismicity (Mw < −2) recorded during a hydraulic fracturing in situ experiment performed in the underground Äspö Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28‐m‐long, horizontal borehole located at 410‐m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high‐frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000 Hz. Here we study the high‐frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small‐scale seismic events with moment magnitudes MW < −3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double‐difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well‐characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation.

Determination of the resistance of the cylindrical­tubular drill for trenchless laying of underground communications

The determination of the total resistance to penetration of the annular drill into the soil is based on the concept of changing the elastic state of the soil during its compaction, which is defined by the compression modulus of soil deformation. This parameter comprises all the physical and mechanical properties of each type of soil and makes it possible to specify the laws of the normal pressure of penetration resistance acting on the surface of the conical and cylindrical parts of the working body. The proposed theoretical models of processes occurring during penetration of the annular drill into the soil give s an opportunity to determine the influence of the parameters on the resistance force for each working procedure, depending on the physical and mechanical properties of the soil. It has been found that the maximum length of the annular drill is determined for the conditions of soil movement (unplugged condition), which, for example, with a cylinder diameter of 28 mm, is 0.87 m, 1.04 m and 1.16 m respectively for sandy clay, semi-solid loam and tough clay. It is now clear that a 2-fold increase in the internal diameter leads to an increase in the core length of 1.75 times. It has been determined that the two-cone drill does not facilitate passage of soil through itself and it causes soil plugging as well as formation of soil plugs on the frontal planes, which leads to an increase in drag force. Therefore, to provide unplugged conditions during pipe jacking, the drill with a single external cone should be used. The obtained results of the work can be used to substantiate the rational parameters of the working equipment for creating a horizontal borehole in different types of soils.

Hydraulic fracturing initiation and near-wellbore nonplanar propagation from horizontal perforated boreholes in tight formation

Tight formations, which are rich in reserves in China, have a high economic development value. Perforated fracturing would need to be used widely to exploit these formations because of reservoir characteristics such as low permeability, low porosity and high density, resulting in high fracture pressure, limited fracture propagation, and complex fracture geometry. To address these issues, the mechanism of fracture propagation under different construction conditions must be understood and described. Therefore, the effect of different perforation parameters (shot density, shot length, shot diameter, shot phase, and shot interval spacing) and different horizontal stress contrasts on fracture geometry, initiation, and propagation from perforated horizontal wells in tight formations is studied based on numerous triaxial hydraulic fracturing simulation experiments. Three types of fracture geometry are observed in these experiments: A single flat fracture can be created by an initiated perforation or more initiated perforations with good connectivity, spiral-shaped fractures are generated by a large perforation diameter or high perforation density, and multiple-parallel fractures are induced by a high perforation phase. Moreover, each fracture type has certain pressure behaviors corresponding to a fracture curve shape. The perforations are likely to initiate when the angle between the perforation and maximum horizontal stress is small, and the initiation position is most likely at the base of the perforation. It is difficult to predict the initiation order of perforations; however, the first initiated perforation can be ensured by the profile of the relief pressure. In addition, a higher horizontal stress contrast value results in more initiated perforations, lower fracture pressure, and shorter breakage time. Ultimately, a large perforation diameter, high perforation density, and perforation phase of 60° should be chosen for fracturing at low fracture pressure and simple fracture geometry in tight formations.

Permeability Enhancement and Fracture Development of Hydraulic In Situ Experiments in the Äspö Hard Rock Laboratory, Sweden

A new advanced protocol of progressively increased cyclic injection and pulsed injection design for hydraulic fracturing experiments was implemented at 410 m depth in the Aspo Hard Rock Laboratory in Sweden. A monitoring array was installed around the tested horizontal borehole to detect the acoustic emissions and micro-seismic events during the fracturing process. The aim is to identify optimized injection schemes to reduce the seismicity related to the fracturing processes. The cyclic stimulation scheme of loading and unloading the fracturing net pressure leads to a lower accompanied seismicity if compared to the conventional hydraulic fracturing with constant flow rates. The related permeability of the tested rock interval can be increased, but this increase is less pronounced than that of the conventional treatments and, especially, if compared to the last of the re-fracturing series with high seismicity increase. Despite these limitations, in field applications with expected high risk of unwanted seismic events, this advanced protocol can be a feasible option to reduce this risk.