Formation damage mechanism and control strategy of the compound function of drilling fluid and fracturing fluid in shale reservoirs

For the analysis of the formation damage caused by the compound function of drilling fluid and fracturing fluid,the prediction method for dynamic invasion depth of drilling fluid is developed considering the fracture extension due to shale minerals erosion by oil-based drilling fluid.With the evaluation for the damage of natural and hydraulic fractures caused by mechanical properties weakening of shale fracture surface,fracture closure and rock powder blocking,the formation damage pattern is proposed with consideration of the compound effect of drilling fluid and fracturing fluid.The formation damage mechanism during drilling and completion process in shale reservoir is revealed,and the protection measures are raised.The drilling fluid can deeply invade into the shale formation through natural and induced fractures,erode shale minerals and weaken the mechanical properties of shale during the drilling process.In the process of hydraulic fracturing,the compound effect of drilling fluid and fracturing fluid further weakens the mechanical properties of shale,results in fracture closure and rock powder shedding,and thus induces stress-sensitive damage and solid blocking damage of natural/hydraulic fractures.The damage can yield significant conductivity decrease of fractures,and restrict the high and stable production of shale oil and gas wells.The measures of anti-collapse and anti-blocking to accelerate the drilling of reservoir section,forming chemical membrane to prevent the weakening of the mechanical properties of shale fracture surface,strengthening the plugging of shale fracture and reducing the invasion range of drilling fluid,optimizing fracturing fluid system to protect fracture conductivity are put forward for reservoir protection.

Rock fracture density characterization using measurement while drilling(MWD) techniques

Accurate determination of rock mass properties is a critical part of open-pit mine planning activities to enable more prescriptive blast designs to achieve improved loading and hauling and downstream process efficiency. Better and more accurate blast practice that delivers enhanced outcomes(better fragmentation, improved diggability, less dilution, etc.) is a critical and fundamental element of being able to achieve an effective Mine-Mill approach at a mining operation. Based on previous work, it has been demonstrated that an accurate representation of the rock mass properties can be obtained from the analysis of variations in blasthole drill performance as derived from measurement while drilling(MWD) systems when using tricone bits. This paper further investigates how monitored rate of penetration,pulldown force, rotary torque, rotary speed and bailing air pressure responses can be used to determine the presence of open and partially open fractures having varying dip angles. Based on a correlation of geophysically measured fracture logs and monitored drill performance variables in the same blastholes, the results show that the latter responses can accurately determine open versus closed fractures. The results also identified that variations in rate of penetration and rotary torque show the most sensitivity in the presence of open fractures that intersect a vertical blasthole at near orthogonal angles.

Vertebral Bone Drilling (Puncture) Attenuates the Acute Pain Due to Vertebral Compression Fractures

Purpose: The Investigational Vertebroplasty Efficacy and Safety Trial (INVEST), a randomized blinded controlled study of Vertebroplasty, demonstrated similar improvements in pain between blinded Vertebroplasty and sham-Vertebroplasty groups. The result from the RCT study suggested that the observed efficacy of the Vertebroplasty procedure, instead of representing the cement-mediated reduction in pain, may relate to the vertebral bone drilling per se. The aim of this study was to demonstrate the effectiveness of pain relief of vertebral bone drilling at the site of painful osteoporotic vertebral compression fractures in the acute phase. Materials and Methods: Twenty-six patients with painful osteoporotic compression fractures underwent the vertebral bone drilling. We assessed primary outcome measures in the NRS pain score and RDQ score at day 0 and 3 following the drilling. Comparisons were made by using Wilcoxon signed rank test. Results: The mean baseline NRS and RDQ score, and the mean NRS and RDQ score at day 3 were 7.3 ± 1.2, 15.7 ± 4.2, 4.6 ± 1.4, 7.3 ± 2.2, respectively. Among the patients, we detected significant improvements in NRS pain score and RDQ score at day 3 following the drilling compared with day 0 (P < 0.001). Conclusion: Vertebral bone drilling at the site of painful vertebral compression fractures alleviated the intractable pain due to osteoporotic vertebral compression fractures.

Vertebral Bone Drilling (Puncture) Attenuates the Intractable Pain Due to Vertebral Fractures without Collapse

Purpose: Osteoporotic vertebral fractures with no sign of vertebral collapse on initial radiographs, which is so-called occult vertebral fractures (VFs), exist. Occult VFs have a high rate of missed diagnosis, and the treatment of these fractures has rarely been discussed in the literature. We evaluated the effects of vertebral bone drilling for the pain due to occults VFs. Materials and Methods: Eighteen patients with painful osteoporotic occult VFs underwent the vertebral bone drilling. We evaluated the clinical outcome by comparing numerical rating scale (NRS) and activity of daily life (ADL) values between before and after the vertebral bone drilling. Comparisons were made by using Wilcoxon signed rank test. Results: The mean baseline NRS and ADL score, and the mean NRS and ALD score after the bone drilling were 8.4 ± 0.8, 2.2 ± 0.6, 2.4 ± 1.0, 4.6 ± 0.5, respectively. Among the patients, we detected significant improvements in NRS pain score and ADL score after the drilling compared with baseline score (p < 0.0002). Conclusion: Vertebral bone drilling at the site of painful vertebral compression fractures alleviated the intractable pain due to osteoporotic occult VFs.

Fracture development during disposal of hazardous drilling cuttings by deep underground injection: A review

Drilling fluids with complicated compositions are becoming more common as the oil and gas industry develops. The production of hazardous cuttings is increasing, which not only stifles the oil and gas industry’s development but also poses a severe environmental threat. Deep underground re-injection is a cost-effective and efficient method for dealing with hazardous cuttings. Numerous experiments and numerical studies on cuttings re-injection have been conducted in the past thirty years. However, there is still a divergence of views on the fracture development in the process of cuttings re-injection. A comprehensive review of existing studies is necessary to help researchers advance this technology. This paper provides a review of the fundamental studies on fracture behaviors during the deep underground re-injection of drilling cuttings. The limitations of the existing studies are also discussed to inspire new research endeavors.

A Gel-Based Solidification Technology for Large Fracture Plugging

Fault fractures usually have large openings and considerable extension. Accordingly, cross-linked gel materials aregenerally considered more suitable plugging agents than water-based gels because the latter often undergo contaminationvia formation water, which prevents them from being effective over long times. Hence, in this study, aset of oil-based composite gels based on waste grease and epoxy resin has been developed. These materials havebeen observed to possess high compressive strength and resistance to the aforementioned contamination, therebyleading to notable increase in plugging success rate. The compressive strength, thickening time, and resistance toformation water pollution of these gels have been evaluated indoors. The results show that the compressivestrength of the gel can reach 11 MPa;additionally, the related gelation time can be controlled to be more than3 h, thereby providing a safe construction time;Invasion of formation water has a small effect on the gel strengthand does not shorten the thickening time. All considered performance indicators of the oil-based gel confirm itssuitability as a plugging agent for fault fractures.

Acoustic Borehole Images for Fracture Extraction and Analysis in Second Pre-pilot Drillhole of CCSD

Ultrasonic imaging logging provides continuous and oriented images of structures vs. depth. In the Chinese Continental Scientific Drilling (CCSD) Project, acoustic borehole images were recorded in the second pre-pilot drillhole which penetrates the metamorphic rocks. This paper focuses on fracture evaluation of the drillhole with these images. Both least square fit and a modified Hough transform are used for fracture extraction, and 269 fractures were mapped in the interval from 69.5 to 1 020 m. Most fractures dip steeply, with an average angle of 54°. Fracture dip directions are dominantly in the range of 220°-280° above the depth of 267 m, but 80°-120° in the lower zones. These observations may indicate the differences in structural movements or in-situ stress fields between the upper and lower zones in the drillhole.

Fracture sealing performance of granular lost circulation materials at elevated temperature:A theoretical and coupled CFD-DEM simulation study

Lost circulation is a common downhole problem of drilling in geothermal and high-temperature,high-pressure(HTHP)formations.Lost circulation material(LCM)is a regular preventive and remedial measure for lost circulation.However,conventional LCMs seem ineffective in high-temperature formations.This may be due to the changes in the mechanical properties of LCMs and their sealing performance under high-temperature conditions.To understand how high temperature affects the fracture sealing performance of LCMs,we developed a coupled computational fluid dynamics-discrete element method(CFD-DEM)model to simulate the behavior of granular LCMs in fractures.We summarized the literature on the effects of high temperature on the mechanical properties of LCMs and the rheological properties of drilling fluid.We conducted sensitivity analyses to investigate how changing LCM slurry properties affected the fracture sealing efficiency at increasing temperatures.The results show that high temperature reduces the size,strength,and friction coefficient of LCMs as well as the drilling fluid viscosity.Smaller,softer,and less frictional LCM particles have lower bridging probability and slower bridging initiation.Smaller particles tend to form dual-particle bridges rather than single-particle bridges.These result in a deeper,tighter,but unstable sealing zone.Reduced drilling fluid viscosity leads to faster and shallower sealing zones.

An Analysis on the Transmission Shaft Fracture of Screw Drill

The transmission shaft of the underground screw drill fractured when milling-shoe and grinding bridge plug was applied to the coiled tubes in a horizontal well of Sichuan province, but the position of this transmission shaft fracture did not occur at the minor-diameter retracting position and reducing position that were easy to fracture. An analysis of the transverse planes of the fracture found that the cause of the transmission shaft fracture surface was that the defects of the initial surface were propagating to cracks and gave rise to the fracture under torque load. To specifically know well the strength of the transmission shaft under damaging fracture, a statics analysis was conducted on the transmission shaft through ANSYS finite element simulation software, and the finite element models under no cracks, different-depth circumferential cracks, and similar situation of the transverse planes of fracture were established respectively. An analysis of the crack-free finite element model found that the fracture of the transmission shaft was really not caused by the self-structure of the transmission shaft; an analysis of circumferential crack finite element model found that strong stress concentration would appear in the tip of cracks, and the value of the stress would increase along with the increase of the circumferential crack depth, the stress of the entire crack top tended to fluctuate like waves, and also the strength of the transmission shaft was greatly impacted by the presence of cracks; an analysis of the similar crack finite element model found that stress concentration would appear in the tip of cracks, and the initial cracks always started to propagate from the tip of the external surface of the transmission shaft and would propagate inward until the propagating areas of two cracks overlapped, and finally reached the position of transient interruption, and then the transmission shaft fractured completely and the fracture strength was onlv 1/5 of that under no cracks.

Development and applications of solids-free oil-in-water drilling fluids

The increasing application of near balanced drilling technology to low-pressure and depleted fractured reservoirs requires the use of low-density drilling fluids to avoid formation damage. Solidsfree oil-in-water （O/W） emulsion drilling fluid is one type of low-density drilling fluid suitable for depleted fractured reservoirs. In this paper, the solids-free O/W drilling fluid was developed and has been successfully used in the Bozhong 28-1 oil and gas field, by which lost circulation, a severe problem occurred previously when drilling into fractured reservoir beds, was controlled, thereby minimizing formation damage. The O/W emulsion drilling fluid was prepared by adding 20% （by volume） No. 5 mineral oil （with high flash point, as dispersed phase） into seawater （as continuous phase）. Surfactant HTO-1 （as a primary emulsifier） and non-ionic surfactant HTO-2 （as a secondary emulsifier） were added into the drilling fluid system to stabilize the emulsion; and YJD polymer was also added to seawater to improve the viscosity of the continuous phase （seawater）. The drilling fluid was characterized by high flash point, good thermal stability and high stability to crude oil contamination.

Fracture evolution and pressure relief gas drainage from distant protected coal seams under an extremely thick key stratum

When an extremely thick rock bed exists above a protected coal seam in the bending zone given the condition of a mining protective seam, this extremely thick rock bed controls the movement of the entire overlying stratum. This extremely thick rock bed, called a ＂main key stratum＂, will not subside nor break for a long time, causing lower fractures and bed separations not to close and gas can migrate to the bed separation areas along the fractures. These bed separations become gas enrichment areas. By analyzing the rule of fracture evolution and gas migration under the main key stratum after the deep protective coal seam has been mined, we propose a new gas drainage method which uses bore holes, drilled through rock and coal seams at great depths for draining pressure relief gas. In this method, the bores are located at a high level suction roadway （we can also drill them in the drilling field located high in an air gateway）. Given the practice in the Haizi mine, the gas drainage rate can reach 73% in the middie coal group, with a gas drainage radius over 100 m.

Three-dimensional numerical simulation of methane drainage by high-level drill holes in a lower protective coal seam with a “U” type face

Different drill-hole positions may produce different drainage results in low protective coal seams.To investigate this possibility,a 3D stope model is established,which covers three kinds of drill holes.The FLUENT computational fluid mechanics software is used to solve the mass,momentum and species conservation equations of the model.The spatial distributions of oxygen and methane was obtained by calculations and the drainage results of different drill-hole positions were compared.The results show that,from top to bottom,methane dilution by oxygen weakens gradually from the intake to the return side,and methane tends to float;methane and oxygen distribute horizontally.The high-level crossing holes contribute to better methane drainage and a greater level of control.Around these holes,the methane density decreases dramatically and a "half circle"distribution is formed.The methane density decreases on the whole,but a proportion of the methane moves back to deep into the goaf.The research findings provide theoretical grounds for methane drainage.

Characterization and prevention of formation damage for fractured carbonate reservoir formations with low permeability

Stress sensitivity and water blocking in fractured carbonate reservoir formations with low permeability were determined as the main potential damage mechanisms during drilling and completion operations in the ancient buried hill Ordovician reservoirs in the Tarim Basin. Geological structure, lithology, porosity, permeability and mineral components all affect the potential for formation damage. The experimental results showed that the permeability loss was 83.8%-98.6% caused by stress sensitivity, and was 27.9%-48.1% caused by water blocking. Based on the experimental results, several main conclusions concerning stress sensitivity can be drawn as follows： the lower the core permeability and the smaller the core fracture width, the higher the stress sensitivity. Also, stress sensitivity results in lag effect for both permeability recovery and fracture closure. Aimed at the mechanisms of formation damage, a modified low-damage mixed metal hydroxide （MMH） drilling fluid system was developed, which was mainly composed of low-fluorescence shale control agent, filtration control agent, lowfluorescence lubricant and surfactant. The results of experimental evaluation and field test showed that the newly-developed drilling fluid and engineering techniques provided could dramatically increase the return permeability （over 85%） of core samples. This drilling fluid had such advantages as good rheological and lubricating properties, high temperature stability, and low filtration rate （API filtration less than 5 ml after aging at 120 ℃ for 4 hours）. Therefore, fractured carbonate formations with low permeability could be protected effectively when drilling with the newly-developed drilling fluid. Meanwhile, field test showed that both penetration rate and bore stability were improved and the soaking time of the drilling fluid with formation was sharply shortened, indicating that the modified MMH drilling fluid could meet the requirements of drilling engineering and geology.

Controlling mine pressure by subjecting high-level hard rock strata to ground fracturing

When mining extra-thick coal seams,the main cause of strong ground pressure are the high-level thick and hard strata,but as yet there is no active and effective control technology.This paper proposes the method of subjecting hard roofs to ground fracturing,and physical simulation is used to study the control effect of ground fracturing on the strata structure and energy release.The results show that ground fracturing changes the structural characteristics of the strata and reduces the energy release intensity and the spatial extent of overburden movement,thereby exerting significant control on the ground pressure.The Datong mining area in China is selected as the engineering background.An engineering test was conducted on site by ground horizontal well fracturing,and a 20-m-thick hard rock layer located 110 m vertically above the coal seam was targeted as the fracturing layer.On-site microseismic monitoring shows that the crack propagation length is up to 216 m and the height is up to 50 m.On-site mine pressure monitoring shows that(1)the roadway deformation is reduced to 100 mm,(2)the periodic weighting characteristics of the hydraulic supports are not obvious,and(3)the ground pressure in the working face is controlled significantly,thereby showing that the ground fracturing is successful.Ground fracturing changed the breaking characteristics of the high-level hard strata,thereby helping to ameliorate the stress concentration in the stope and providing an effective control approach for hard rock.

Estimation of Fracture Geometry Parameters and Characterization of Rock Mass Structure for the Beishan Area,China

The accurate estimation of fracture geometry parameters and the characterization of rock mass structure are two important topics in the geological disposal system of high-level radioactive waste(HLW).The Beishan area,as the current preselected area for China’s HLW disposal,has three subareas considered to be the key survey area at the stage of site selection.In this paper,a comprehensive survey method conducted on the outcrop is developed to estimate fracture geometry parameters.Results show that fracture occurrence obeys a Fisher distribution,fracture trace length obeys a normal distribution,and the distribution of spacing obeys a negative exponential distribution.An evaluation index,Rock Mass Structure Rating(RMSR),is proposed to characterize rock mass structure for the three subareas.The results show that the Xinchang area is more suitable to act as China’s HLW disposal repository site.At the same time,the index can also be applied to characterize surface rock mass structure and rock mass integrity at the site selection phase of HLW disposal.

Deep gold mineralization features of Jiaojia metallogenic belt,Jiaodong gold Province:Based on the breakthrough of 3000 m exploration drilling

Recently,continuous breakthroughs have been made about deep gold prospecting in the Jiaodong gold province area of China.Approximately 5000 t of cumulative gold resources have been explored in Jiaodong,which has thus become an internationally noteworthy gold ore cluster.The gold exploration depth has been increased to about 2000 m from the previous<1000 m.To further explore the mineralization potential of the Jiaodong area at a depth of about 3000 m,the Shandong Institute of Geological Sciences has drilled an exploratory drillhole named“Deep drillhole ZK01”to a depth of 3266 m.Hence,as reported herein,the mineralization characteristics of the Jiaojia metallogenic belt have been successfully documented.ZK01 is,to date,the deepest borehole with an gold intersect in China,and constitutes a significant advance in deep gold prospecting in China.The findings of this study further indicate that the depth interval of 2000 m to 4000 m below the ground surface in the Wuyi Village area incorporates 912 t of inferred gold resources,while the depth interval of 2000 m to 4000 m below the surface across the Jiaodong area possesses about 4000 t of inferred gold resources.The Jiaojia Fault Belt tends to gently dip downward,having dip angles of about 25°and about 20°at vertical depths of 2000 m and 2850 m,respectively.The deep part of the Jiaojia metallogenic belt differs from the shallow and moderately deep parts about fracturing,alteration,mineralization,and tectonic type.The deep zones can generally be categorized from inside outward as cataclastic granite,granitic cataclasite,weakly beresitized granitic cataclasite,beresitized cataclasite,and gouge.These zones exhibit a gradual transitional relation or occur alternately and repeatedly.The mineralization degree of the pyritized cataclastic granite-type ore in the deep part of the Jiaojia metallogenic belt is closely related to the degree of pyrite vein development;that is,the higher the pyrite content,the wider the veins and the higher the gold grade.Compared to the shallow gold ores,the deep-seated gold ores have higher fineness and contain joseite,tetradymite,and native bismuth,suggesting that the deep gold mineralization temperature is higher and that mantle-sourced material may have contributed to this mineralization.ZK01 has also revealed that the deep-seated ore bodies in the Jiaojia metallogenic belt are principally situated above the main fracture plane(gouge)and hosted within the Linglong Granite,contradicting previous findings indicating that the moderately shallow gold ore bodies are usually hosted in the contact zone between the Linglong Granite and Jiaodong Group or meta-gabbro.These new discoveries are particularly significant because they can help correct mineralization prospecting models,determine favorable positions for deep prospecting,and improve metallogenic prediction and resource potential evaluation.

Formation mechanisms of fracture plugging zone and optimization of plugging particles

As formation mechanisms of plugging zone and criteria for fracture plugging remain unclear,plugging experiments and methods testing granular material mechanical properties are used to study the formation process of the plugging zone in fractured formations,analyze composition and ratios of different sizes of particles in the plugging zone,and reveal the essence and driving energy of the formation and damage of the plugging zone.New criteria for selecting lost circulation materials are proposed.The research results show that the formation of the plugging zone has undergone a process from inertial flow,elastic flow,to quasi-static flow.The plugging zone is composed of fracture mouth plugging particles,bridging particles and filling particles,and the proportion of the three types of particles is an important basis for designing drilling fluid loss control formula.The essence of the construction of the plugging zone is non-equilibrium Jamming phase transition.The response of the plugging zone particle system to pressure is driven by entropy force;the greater the entropy,the more stable the plugging zone.Lost circulation control formula optimized according to the new criteria has better plugging effect than the formula made according to conventional plugging rules and effectively improves the pressure-bearing capacity of the plugging zone.The research results provide a theoretical and technical basis for the lost circulation control of fractured formations.

Calculation of safe drilling mud density window for shale formation by considering chemo-poro-mechanical coupling effect

It is difficult to define safe drilling mud density window for shale sections.To solve this problem,the general Biot effective stress principle developed by Heidug and Wong was modified.The Weibull statistical model was used to characterize the hydration strainrelated strength damage.Considering drilling fluid sealing barrier on shale,a calculation method of safe drilling mud density has been established for shale formation under drilling fluid sealing-inhibition-reverse osmosis effect,combined with a flow-diffusion coupling model.The influence of drilling fluid sealing and inhibiting parameters on safe drilling mud density window was analyzed.The study shows that enhancing drilling fluid sealing performance can reduce the pore pressure transmission and solute diffusion;the inhibiting performance of drilling fluid,especially inhibition to strength damage,is crucial for the wellbore collapse pressure of shale section with significant hydration property.The improvement of drilling fluid sealing and inhibition performance can lower collapse pressure and enhance fracturing pressure,and thus making the safe drilling fluid density window wider and the collapse period of wellbore longer.If there is osmosis flow in shale,induced osmosis flow can make the gap between collapse pressure and fracturing pressure wider,and the stronger the sealing ability of drilling fluid,the wider the gap will be.The safe drilling mud density window calculation method can analyze the relationships between collapse pressure,fracturing pressure and drilling fluid anti collapse performance,and can be used to optimize drilling fluid performance.

Mechanism and experiment of substituting high drainage roadway with directional long drilling group to extract pressure-relief gas

By establishing the numerical model in the vertical plane and the similar model in the horizontal plane of gas flow in goaf, the influence of high drainage roadway or drilling on the gas seepage field was analyzed, and the extraction mechanism was clarified. On this basis, the academic thought of directional long drilling group instead of high drainage roadway was put forward. And then using complex function theory, the permeation mechanical model of drilling group with circle distribution in the mining-induced fracture zone was established to explore the coupling relationship between the drilling quantity, extraction volume and the equivalent extraction rate of single drilling. Finally, combined with the concrete geological production conditions, the main parameters of directional long drilling group were determined. The distance between the drilling group center and the air-return roadway is 24 m, the height is 18 m, and the three drillings are in an approximate equilateral triangle distribution with a space of 8 m. The equivalent extraction square is 4.15 m2. It is shown that the effect of directional long drilling group is evident. The gas content in the upper comer is controlled below 0.95%, the content in the tail roadway is kept below the alarm value, and the content is over 50% in the drill, realizing the secure and effective extraction of coal and gas.

Lost circulation material for abnormally high temperature and pressure fractured-vuggy carbonate reservoirs in Tazhong block, Tarim Basin, NW China

To effectively solve the problem of lost circulation and well kick frequently occurring during the drilling of abnormally high temperature and pressure fractured-vuggy reservoirs in the Tazhong block, a rigid particle material, GZD, with high temperature tolerance, high rigidity(> 8 MPa) and low abrasiveness has been selected based on geological characteristics of the theft zones in the reservoirs. Through static pressure sealing experiments, its dosage when used alone and when used in combination with lignin fiber, elastic material SQD-98 and calcium carbonate were optimized, and the formula of a new type(SXM-I) of compound lost circulation material with high temperature tolerance and high strength was formed. Its performance was evaluated by compatibility test, static sealing experiment and sand bed plugging experiment. The test results show that it has good compatibility with drilling fluid used commonly and is able to plug fractures and vugs, the sealed fractures are able to withstand the static pressure of more than 9 MPa and the cumulative leakage is 13.4 mL. The mud filtrate invasion depth is only 2.5 cm in 30 min when the sand bed is made of particles with sizes between 10 mesh and 20 mesh. Overall, with good sealing property and high temperature and high pressure tolerance, the lost circulation material provides strong technical support for the safety drilling in the block.