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By P Joyce, D R. Cooke

The Dinkidi Cu-Au porphyry deposit is situated in the Caraballo Range of North Luzon, Philippines. Although comparatively small, Dinkidi has unusually high gold grades with drilling by Climax-Arimco Mining Corporation delineating a 110 Mt orebody at 1.2 g/t gold and 0.5 per cent copper. The magmatic-hydrothermal evolution of Dinkidi records a distinctive switch in mineralisation from an early orthoclase-dominated Au-rich assemblage, typical of alkaline porphyry systems, to a later quartz stockwork Au-Cu rich assemblage typical of most calc-alkaline to shoshonitic porphyry systems. The early orthoclase-dominated stage is associated with the intrusion and crystallisation of a variably-textured clinopyroxene-syenite dyke that contains high-grade (5 - 20 g/t) gold mineralisation. We consider this dyke to be a Cu-Au porphyry analogue to the pegmatites found in granitic systems. Dinkidi is hosted within the Early Miocene Didipio Igneous Complex. This alkaline igneous complex is comprised of early unmineralised gabbros (pyroxene-plagioclase cumulates), clinopyroxenites (pyroxene cummulates), diorites and monzodiorites that have been intruded by a large, weakly mineralised monzonite pluton. The Dinkidi deposit is located at the southern margin of the monzonite pluton. Mineralisation is hosted within a composite monzonite stock. Within the stock, an 800 metres by ~200 metres (plan section) elongate monzonite stock (Tunja Monzonite) predates and hosts the bulk of the Au-Cu mineralisation. This has been intruded by the thin (10 to 30 m wide), highly mineralised variably-textured magnetite- and clinopyroxene- phyric syenite pegmatite (the Balut Dyke). The youngest phases of the composite stock are leucocratic quartz-monzonite porphyry (Quan Porphyry) that grades texturally, in its core, into a post-mineralisation crystal-crowded quartz-syenite (Bufu Felsite). The late-stage Bufu Felsite is inferred to be the source of much of the Cu-Au mineralisation. The Didipio Igneous Complex has intruded a sequence of Late Oligocene alkaline andesitic lavas and volcaniclastics and overlying potassic dacitic lavas. Whole rock geochemical analyses indicate that the intrusions and the potassic dacite lavas in the surrounding region are co-magmatic. Five main stages of hydrothermal activity are recorded at Dinkidi. An unmineralised K-silicate (biotite-magnetite) alteration assemblage formed during the intrusion of the Tunja Monzonite stock. The diorites surrounding the stock display intense selectively pervasive biotite and magnetite alteration and are cut by an extensive network of orthoclase veins and monzonite dykes. The orthoclase-dominated assemblages of Stage Two carry substantial high-grade Au mineralisation and are associated with the intrusion of the Balut Dyke. The emplacement of this unusual dyke is associated with emplacement of pegmatitic clinopyroxene-orthoclase-magnetite¦sulphide dykes, localised orthoclase flooding along the margins of the dyke, and subsequent emplacement of an extensive orthoclase-actinolite-bornite¦chalcopyrite-fluorite stock-work, and development of widespread selectively pervasive actinolite alteration. The main stage of Au-Cu mineralisation at Dinkidi is intimately associated with the intrusion and initial crystallisation of the leuco-monzonite porphyry (Quan Porphyry) and emplacement of a quartz stockwork followed by carbonate-sulphide deposition and intense vein-related sericite and carbonate alteration assemblages. The quartz stockwork formed synchronous with the deposition of the æBugoy QuartzoliteÆ, a very coarse grained mosaic of quartz-orthoclase-perthite along the walls and top of the crystallising intrusion. Stage 3 was terminated by crystallisation of the distinctively vuggy Bufu quartz-syenite phase in the core of the intrusive, with associated antiperthite veins, and the subsequent explosive brecciation of the roof of the intrusion, forming a quartz-fragment breccia from the Bugoy Quartzolite. Retrograde metasomatism during Stage Four was intimately associated with the emplacement of extensive, late-stage carbonate, silica and zeolite veins and widespread selectively pervasive carbonate alteration. The last hydrothermal stage at Dinkidi, Stage 5, is comprised of strong selectively pervasive clay alteration and is associated with fault-related brecciation of the Dinkidi stock.

Jan 1, 1999

By I M. Platten, S C. Dominy

The Clogau St DavidÆs gold mine has a recorded production of around 81 000 oz Au and it is the largest mine in the Dolgellau gold-belt and BritainÆs most prolific historical producer. The original mine was first worked for copper, however; in 1854, gold was found in waste material from the St DavidÆs section. In recent times the mine has operated intermittently between 1983 - 2007. The principal black shale-hosted gold-bearing reef (Main Reef) can be traced for about 3 km, striking approximately NE-SW and dipping steeply between 60¦ - 90¦ SE. The reef system pinches and swells, changes strike and splits into a number of smaller branches. Reef width is variable, ranging from over 6 m to a few cm and in places reducing to small irregular blobs. The reef is internally complex and composed of swarms of narrow (1 - 300 mm) subparallel veins separated by country rock sheets. The gold-bearing veins within the reef package are discontinuous; though generally possess bonanza grades running 10s - 100s oz/t Au. It has historically been shown that some gold-rich vein segments can grade up to 300 oz/t Au. The predictability of the gold-rich segments is poor, but has been shown to be related to presence of the Clogau (black shale) Formation, and/or the mechanical interactions of the reef with greenstone sills, reef branching and splitting and/or cross-fractures to form ore shoots. The mine is currently being re-evaluated as a potential small-scale selective operation.CITATION:Dominy, S C and Platten, I M, 2012. Geology, mineralisation and potential of the Clogau St DavidÆs mine, Dolgellau gold-belt, Gwynedd, Wales, in Proceedings Narrow Vein Mining 2012 , pp 101-120 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Mar 26, 2012

By C. J. McKee

A gold-silver-lead-zinc deposit at Montana Tunnels, Jefferson County, Montana is located in a diatreme of middle Eocene age (45 to 50 m.y.). Sulfide minerals within the diatreme occur as disseminations, veinlets, and locally as breccia matrix. Principal sulfide minerals are pyrite, sphalerite, and galena. The diatreme is a matrix-rich quartz latitic breccia with a fine grained rather soft matrix and heterolithic clasts. The ore deposit contains approximately 50 million tons of ore at a grade of 0.025 oz/ton Au, 0.5 oz/ton Ag, 0.25 % Pb, and 0.65 % Zn. Mining is by open pit methods. The original process flowsheet included flotation and cyanide leaching. Problems with the leaching circuit led to a change to the straight flotation process which is presently in use. To maintain a reason-ably uniform feed material, blending of ore is required for grades of four metals as well as for ore hardness and oxidation.

Jan 1, 1988

By W. K. Connor, O&apos

The Bureau of Mines is investigating the Co and Mn resource of the West-Central Arkansas District, including field investigations, mineralogical characterization, beneficiation, and hydrometallurgical tests. The deposits occur within the Ouachita Mountains and are con-trolled structurally, occurring along the axes of anticlines making up the mountain ridges. Brecciated novaculite is cemented with a mixture of Mn oxides, primarily cryptomelane and psilome-lane. Cobalt is concentrated within a particular Mn oxide phase, lithiophorite. Chemical analyses on head samples reported up to 25 pct Mn and 0.17 pct Co. Magnetic separation yielded products with up to 41 pct Mn and 0.22 pct Co at recoveries of 95 pct Mn and 93 pct Co.

Jan 1, 1991

By B. R. Addison

In the Spring of 1991, Georgia Marble Company of Arizona (GMCOA), acquired the Andrada Marble Company located near Vail, Arizona, approximately 32 km southeast of Tucson. The Georgia Marble Company operation consists of an open pit quarry and a recently constructed, state of the art calcium carbonate milling and processing facility. The open pit quarry exposes white, crystalline, CaCO3, Escabrosa limestone that meets our customer criteria for Consumer and Industrial Filler applications. The Georgia Marble Company Arizona Division facility is designed and engineered to produce a wide range of milled calcium carbonate that includes finely ground and coarse screened products.

Jan 1, 1994

By L. G. Kirk

The Mississippi embayment ball clay region, including western Kentucky, western Tennessee, and northwestern Mississippi, accounted for more than 95% of the total U.S. ball clay production during 1985. Other producing states during the year were Texas and Maryland. The U.S. Bureau of Mines reported total domestic production for 1985 of 910,000 short tons. The United States exports approximately 20% of the annual ball clay production. Geologically, the ball clay deposits are sedimentary in origin and occupy the north- eastern flank of the Mississippi embayment. The commercial deposits have been confined to the Claiborne and Wilcox formations within mid to lower Eocene and upper Paleocene age strata. The deposits are lenticular in nature and highly variable in areal extent and thickness. They exhibit very abrupt lateral boundaries and are bounded laterally and vertically by unconsolidated sands, silts, and waste clays.

Jan 1, 1986

By Ihsan Berk Tulu, Haochen Zhao, Deniz Tuncay

The process of designing pillars and selecting support measures in underground coal mines relies highly on the accurate assessment of mining-induced loads and the load-bearing capacities of the supports. This study proposes a new method to estimate the percentage of load carried by the gob in underground coal mines using a new parameter that introduces the site-specific overburden geology.

Jun 1, 2023

By Ihsan Berk Tulu, Haochen Zhao, Deniz Tuncay

This paper proposes a new method to estimate the percentage of load carried by the gob in underground coal mines using a new parameter that introduces the site-specific overburden geology. The new parameter was defined as the total strong layer thickness (tstr) that represents the strength and stiffness of the overburden with a single value. Twelve field measurement case studies from 11 different U.S. longwall mines with different overburden geologies were used to develop the new methodology. 2D numerical models of these case studies (and additional parametric runs with different panel widths) were verified against field measurements such as surface subsidence and stress. Statistical analyses using the model results were conducted for a loading model with the new tstr parameter as one of the variables. The new methodology was found to have a coefficient of determination (R2) of 85% when compared to the field measurements and the parametric models. The method was then tested for practical implementation for pillar design. The percentages of load carried by the gob as estimated by the new method were implemented into the LaModel program, that is commonly used in the coal industry. The results obtained from LaModel were compared to the available field stress measurements, and the new method implemented into LaModel was found to give successful results.

Jul 18, 2022

By C. F. Chung, F. P. Agterberg

"Prediction of a variable sulphur content in Cape Breton seams is important for planning the production of a highgrade metallurgical coal. Two extrapolation methods (Kriging and a version of Universal Kriging) to predict sulphur content in blocks of coal before mining are presented and applied to the New Lingan Mine reserves in the Harbour seam. The uncertainty of the predicted values is expressed by using standard deviations. Some statistical results for the Phalen seam, which occurs about 400 ft below the Harbour seam, are presented for comparison. The two mathematical models consist of two sets of equations which have been used in computer algorithms. Some limitations of the present approach and possibilities for further statistical work are indicated."

Jan 1, 1973

By A. Henk, C. M. Aruffo, K. Fischer

"Geomechanical reservoir modeling offers a valuable tool to predict tectonic stresses, in particular the local perturbations in orientation and/or magnitude which occur at lithological boundaries and faults. The paper presents a workflow for building and calibrating such geomechanical models. The numerical simulations are based on the Finite Element method and can range from field-scale models to smaller, highly detailed submodels of specific fault blocks. The approach considers the reservoir-specific characteristics with respect to subsurface geometry (lithostratigraphic boundaries, faults), mechanical properties and ambient stress field. During the subsequent calibration stage calculated stresses are compared to stress data actually observed in borehole data. Poorly constrained input parameters are iteratively modified within reasonable limits until a satisfactory fit between calculated and measured stresses is achieved. This validated model can then be used for stress predictions between wells and in undrilled parts of the reservoir, respectively. The workflow is applied to two case studies - a large gas field in Northern Germany and a CO2 sequestration project in Australia – to test and illustrate its practical value. INTRODUCTIONDetailed knowledge of the tectonic stress field is crucial for the optimal exploration and use not only of conventional and unconventional hydrocarbon reservoirs, but also for deep geothermal reservoirs, carbon capture and storage (CCS) projects as well as underground mining and nuclear waste repositories. Any reliable stress prediction is hindered by the observation that the magnitude and orientation of the stress field can vary significantly – not only in time, but also in space. Such spatial stress variations comprise all scales, i.e., from the scale of lithospheric plates down to the grain-scale. On the reservoir- and fault blockscale, the local stress field can be influenced by faults and different mechanical properties of lithologies (Sassi & Faure, 1997). The resulting variations in stress magnitude and/or stress orientation are commonly referred to as stress perturbations and give rise to a unique, reservoir-specific stress pattern (Yale, 2003). Thereby, local stress fields can deviate by up to 90° in orientation and several tenth of Megapascal (MPa) in magnitude from regional trends."

Jan 1, 2015

By J. D. Aubertin

Rock blasting is a complex engineering process that requires in-depth knowledge of operations and geomechanical conditions. Acknowledged design precepts are based on cratering behavior associated with stiff hard rock masses. Other cratering mechanisms may however arise under certain geomechanical conditions such as extremely fractured rock masses, or with soft rocks such as rock salt and potash. The burden dimension also plays an important role in the prevalent cratering mechanisms of a given blasting condition. The present article investigates the variability of cratering profiles and geometrical trends based on characteristic geomechanical differentiation and considering the influence of burden. The differences in observable mechanisms are examined through small and large-scale single hole blast (SHB) tests. SHB tests involve a series of individual shots blasted parallel or sub-parallel to an open face. The burden dimension is varied to obtain burden dependent characteristic trends. The breakout shape is represented by a crater model in the form of a power law equation capable of describing prismatic, concave and convex cratering. The formulation comprises two reference index parameters that can describe the geometrical and geomechanical conditions. SHB test campaigns conducted in rock salt, limestone, massive amphibolite, pinolite, and grout blocks are compiled in quantitative manner to demonstrate the influence of geomechanics on cratering profiles. The observed trends are corroborated through numerical modelling carried on Finite Element Model (FEM) LS Dyna to investigate the prevalence of specific cratering mechanisms. A series of SHB tests at large and small burden values are also used to showcase the influence of geometry on the prevalent cratering mechanism and the resulting crater

Jan 1, 2024

By D. J. Hutchinson, M. Lato, M. Diederichs

Light Detection and Ranging (LiDAR)-based applications for geomechanical purposes have recently proven to be a highly reliable and accurate form of remote analysis and documentation. The ability to remotely collect, process and evaluate rockmasses for structural geomechanical applications has benefits ranging from safety to efficiency, and provides a permanent record that can be assessed before and after an excavation has been completed. With the collaboration of Queen’s University and Rio Tinto, the Iron Ore Company of Canada (IOC) mines in Labrador City, Newfoundland and Labrador, evaluated the practical application of the I-Site 4400 LiDAR equipment, in conjunction with the Vulcan geotechnical module for geomechanical purposes. The project provided a comparative analysis between conventional hand-mapping and digital analysis. The analysis indicates that digital mapping could be employed as an active tool within the general geotechnical workflow, where results would be used in pit wall and rockmass support designs. As well, the field work enabled the generation of effective operational guidelines and protocols to maximize scanning efficiency.

Jan 1, 2009

By Shenghui Chu, Zhibiao Guo, Dongshan Yang, Yuanxin Zhao, Xiaohui Kuai, Jingwei Gao

The technology of automatically formed roadway without pillars (AFRP) via roof cutting and pressure releasing (RCPR) is a reformatory method for gob-side entry retaining, which automatically creates a roadway by utilizing the fragmenting and expanding properties of rock strata and the mining pressure, to realize a novel technique for coal mining without coal pillars. The Anju Coal Mine is the first kilometer-deep mine where this technology is applied. To conduct a geomechanical model experiment, this research uses the 5307 working face in this mine as its backdrop. The experimental results show that this technology can efficiently decrease the pressure of the AFRP surrounding rock. The roof maximum stress of the AFRP is 21% lower than that of the belt roadway, while the side abutment pressure of the former is 13.4% lower than that of the latter. The comparison of model experiment data and field monitoring data shows that the roof and floor approaching deformations on the roof-cutting side and solid coal side of AFRP differ by 8.8% and 9.6%, respectively. The model experiment results and field monitoring data show good agreement, and the deformation of the AFRP surrounding rock can be successfully managed by this technique. The study’s findings can serve as a guide for using this technology in kilometer-deep coal mines.

Jul 31, 2023

By Zhibiao Guo, Jinglin You, Dongshan Yang, Yuanxin Zhao, Yuhui Li, Jingwei Gao

By utilizing collapsed gangue to create gob ribs, roof cutting and roadway maintenance (RCRM) technology enables coal pillar-free mining, presenting a novel approach. Due to their inherent structure, faults, which are common geological formations, can lead to various hazards, including fault sliding and significant deformation of roadways. In this study, a selfdesigned underground engineering simulation test device is utilized to imitate the cross-fault mining of the 11,101 working face in the Qipanjing Coal Mine (East). The objective is to examine the behavior of the surrounding rock and stress evolution. Generally, the experimental findings demonstrate that compared to the return air roadway, the RCRM roadway experiences a 3.3% decrease in maximum coal rib stress. This suggests that the implementation of this technique can effectively alleviate the stress on the surrounding rock of the roadway. The maximum coal rib stress, lateral pressure of the gob rib, and maximum deformation of the roof near the fault of the RCRM roadway are 11.3%, 25.6%, and 86.1% greater than those of the conventional geological conditions, respectively. The roadway deformation is negatively impacted by the fault structure. Based on experimental data and field measurements, a discrepancy in the maximum roof deformation between the test and field monitoring results under typical geological conditions is 8%. This indicates a favorable alignment between the test results and the real-life scenario. In conclusion, the proposed targeted control technique is implemented in the field by integrating the mining invariant equation and gangue volume expansion equation. This not only validates the effectiveness of the technology but also offers valuable insights for the future utilization of RCRM technology under comparable circumstances.

Feb 20, 2024

By R O’Sullivan, D Taylor, J G. McLellan, B Miller

The Mount Isa copper deposit is located adjacent to the city of Mount Isa, Queensland, and lies within a series of Palaeoproterozoic to Mesoproterozoic Superbasins. The deposit is hosted almost entirely within the Urquhart Shale (US), a unit of the Mount Isa Group sediments and part of the Isa Superbasin. The copper mineralisation is separated into two distinct orebodies: the X41 mine (1100 and 1900 orebodies) and the Enterprise mine (3000 and 3500 orebodies). The orebodies are situated in the hanging wall of the major controlling structures, the Paroo Fault, which has juxtaposed older basement Eastern Creek Volcanics against the younger Mount Isa Group sediments cf 1655 ± 4 Ma. Conjecture remains over the exact timing of this thrust event. However, it is clear that inflections on the Paroo Fault have a close association with the orebodies and are an important structural feature in the mineralisation of the Mount Isa copper deposit.The life-of-mine is reducing steadily and new geomechanical modelling techniques were employed to target structurally controlled mineralised zones in three dimensions. Following a structural review of the mine using underground mapping, structural data and core shed observations, a conceptual 3D mine model was constructed utilising existing data modelled in MineSight®. This conceptual model included all the main lithological units and structures. The conceptual model aimed to test the effects of deformation on the main lithological and structural components of the system and attempted to highlight areas of anomalous stress and strain that may indicate a higher likelihood of fluid focusing and mineralisation potential. This project was focused on testing the latter stages of a D3 event (NE–SW compression) and D4 event (SE–NW compression), which are considered to be responsible for the current location of the mineralisation.The fully coupled 3D finite element analysis models resulted in highlighting anomalous patterns of stress and strain and subsequent fluid flow as a result of the two deformation events. The NE–SW D3 simulation provided the best correlation with the known distribution of mineralisation. This event also indicated several areas on the Paroo Fault and within the US that have potential to host further areas of mineralisation. The SE–NW D4 simulation provided less of a correlation with known mineralised zones. However, this event may have been responsible for post mineralisation re-mobilisation of copper on a relatively small scale. Areas of maximum shear strain, shear stress, volumetric strain and fluid flow correlated well with areas of minimum principal stress and provided the best overall match with known mineralised zones. Furthermore, several additional highlighted areas provide a high potential for further discovery of copper mineralisation. Recent exploration drilling north of the Enterprise mine 3000 orebody, utilising geomechanical modelling as part of the targeting criteria, intersected copper mineralisation, providing further validation of the modelling results.CITATION:McLellan, J G, O’Sullivan, R, Miller, B and Taylor, D, 2014. Geomechanical modelling of the Mount Isa copper deposit – predicting mineralisation, in Proceedings Ninth International Mining Geology Conference 2014 , pp 197–206 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 18, 2014

By Alper Kirmaci, Taghi Sherizadeh, Gunnar Wurst, Paul Brooks, Dogukan Guner, Samuel Nowak, Mina Esmaeelpour, Kutay Karadeniz

As mines continue to deepen and become more expansive, active monitoring of larger volumes of rock mass will become more critical to calibrate numerical simulations and to ensure the safety of underground workers. Monitoring larger volumes of rock mass requires low-cost sensors that are simple in construction and installation. In this study, a novel hybrid optical fiber cable (HOFC) designed for use in distributed optical fiber sensing (DOFS) via grouted boreholes was deployed to monitor a bulk mining operation in an underground metal mine. The HOFC was successfully used to monitor approximately 2.7 × 10 m3 of rock mass above excavations surrounding a pillar removal area in which six large pillars were removed simultaneously. A total of six measurement boreholes (maximum depth of 22 m) were used to measure strain along the optical fiber during the pillar removal operation using the HOFC, allowing for 70 individual strain measurement points, which were constructed for under one U.S. dollar each. Monitoring of the excavation area took place over a 44-day period after pillar removal. Extensional strains were noted in the areas closest to the removed pillars, while areas of compression were noted directly above the remaining pillar in the area. The results of the case study demonstrate that a low-cost optical fiber strain sensing network can be rapidly installed in a large excavation area and can provide highly sensitive strain measurements in a manner that would be cost prohibitive using other methods.

Oct 1, 2024

By Taghi Sherizadeh, Samuel Nowak, Kutay Karadeniz, Paul Brooks, Alper Kirmaci, Mina Esmaeelpour, Dogukan Guner, Gunnar Wurst

As mines continue to deepen and become more expansive, active monitoring of larger volumes of rock mass will become more critical to calibrate numerical simulations and to ensure the safety of underground workers. Monitoring larger volumes of rock mass requires low-cost sensors which are simple in construction and installation. In this study, a novel hybrid optical fiber cable (HOFC) designed for use in distributed optical fiber sensing (DOFS) via grouted boreholes was employed to monitor a bulk mining operation in an underground metal mine. The HOFC was successfully used to monitor approximately 2.7 × 103 m3 of rock mass above excavations surrounding a pillar removal area in which six large pillars were removed simultaneously. A total of six measurement boreholes (maximum depth of 22 m) were used to measure strain along the optical fiber during the pillar removal operation using the HOFC, allowing for 70 individual strain measurement points, which were constructed for under one US dollar each. Monitoring of the excavation area took place over a 44-day period after pillar removal. Extensional strains were noted in the areas closest to the removed pillars, while areas of compression were noted directly above the remaining pillar in the area. The results of the case study demonstrate that a low-cost optical fiber strain sensing network can be rapidly installed in a large excavation area and can provide highly sensitive strain measurements in a manner that would be cost-prohibitive via other methods.

May 13, 2024

By Hamidreza Soltanzadeh

The injection of CO2 into a reservoir or aquifer can potentially result in deformation, induced fracturing and fault reactivation in the injection zone and/or the rocks that surround it. This paper reviews the data collection and geomechanical analyses performed to assess these potential hazards for the Heartland Area Redwater Project (HARP), in which CO2 sequestration is being considered in a carbonate (Leduc Formation) reef. A three-dimensional mechanical earth model (3D MEM) was developed using available geophysical log data from several wells. The geomechanical response to CO2 injection was studied using a finite element numerical model based on the 3D MEM and pore pressure distributions predicted using a reservoir simulator. The modeling results suggest that the potential for induced fracturing and fault reactivation is low and the maximum predicted ground surface heave is less than 2 cm. Sensitivity analyses conducted with a 2D geomechanical model that was extracted from the 3D model confirm that this conclusion holds true even when a reasonable measure of uncertainty in rock mechanical properties, ambient pore pressures and horizontal stress magnitudes is accounted for.

Jun 1, 2012

By Tiziana Apuani

The study refers to an area of about 70 km2, extended in the Italian Central Alps along the the San Giacomo Valley where different civil and mining works are present The regional geological setting is related to the Pennidic Nappe arrangement, characterized by the emplacement of sub-horizontal gneissic bodies resulting from the Mesoalpine isoclinalic folding of crystalline basements, emplaced throw East, and separated by matasedimentary cover units. The investigated geological rocks, belong to the ?Tambò?Unit, overlapped by its meta-sedimentary cover and by the ?Suretta? Unit. The tectonic contact gently dips to the E?NE. The valley, furrowed by the Liro Torrent, follows a N?S striking tectonic lineament, almost parallel to the nappe contact. A number of more then 70 sites, distributed in the studied area, were chosen and detailed structural and geomechanical field surveys were performed to characterize the rock masses, its intact rock and discontinuities. The procedure identifies the number of joint sets, their representative orientation, the set spacing, the type of movement, the amount of dilatation, the degree of alteration, the roughness coefficient, the presence and nature of infill. Rock mass quality indexes, such as the Rock Mass Rating (Bieniawski, 1989) or the Geological Strength Index (Hoek et al. 2002) have been evaluated in each surveyed site. The work is a contribute in assessing how the geometrical and geomechanical properties of the rock masses can be regarded as regionalized variables. The spatial distribution of these properties is a function of the geological and structural phenomena at which the rock masses have been subjected. The presented geostatistical analysis regards the spatial variation of the orientation of the most representative joint set.

May 1, 2009

By M. Hozayen, Q. Ni, E. S. Radwan, P. Nawrocki, A. Jebbouri

"A major geomechanical study was conducted for one of the UAE onshore oil fields with the purpose of studying reservoir rock properties and stress distribution, based on which a 3D coupled reservoir geomechanical model was built. This model was later used for wellbore stability analysis and for analyzing the effect of fractures on future reservoir production performance, including fracture deformation and permeability evolution occurring due to reservoir depletion and resulting changes in local stress field. The subject of this study was a complex fractured Upper Cretaceous carbonate reservoir characterized by high degree of heterogeneity, both vertically and laterally. Wellbore instability related drilling problems were encountered while drilling the wells in this field. The problems were mainly in drilling the horizontal wells, including breakouts and mud losses. Experimental results of laboratory tests used in this study are summarized in Part I of this paper whereas wellbore stability analysis and formulation of a3D geomechanical model that was built for that purpose is summarized herein.A 3D geomechanical reservoir model was developed, which included natural fractures. The mechanical properties of the reservoir rock, such as Young’s modulus, Poisson’s ratio, UCS, friction angle and tensile strength were populated based on the correlations established in Part I of this paper and the developed model was then used to simulate the 3D mechanical-property distribution and the evolution of the in situ stresses in the field due to depletion from 1983 to present-day. Based on the full field model, mud weight cubes were developed to predict the safe mud weight windows in different drilling directions. In addition, well stability prediction for two planned wells was performed using a 3D near-borehole model. Firstly, the computed stress state and mechanical properties simulated in the full field model were extracted along the well trajectory of an existing horizontal well which was drilled recently and then, a3D near-borehole model was constructed along the well trajectory. The wellbore stability analysis along the well trajectory, including stress state, strain distribution and displacements, was performed to identify the instability events along the well trajectory. It has been seen that the presence of natural fractures is closely related to the recorded borehole instability incidents. The computed plastic strains which indicate the wellbore failure were consistent with the recorded wellbore instability events in the FMI image logs, what provide further verification of the developed model. Then, this 3D near-borehole model was applied to two planned horizontal wells and the wellbore stability analysis for the two wells was carried out.The results showed that drilling vertically in the reservoir is safer than drilling horizontally. However, the drilling mud weight window in the vertical direction is narrow, being as narrow as 18.7 pcf or 0.299 sg locally. Drilling horizontally is less stable towards the maximum far field stress direction than towards the minimum far field stress direction. The mud weight cubes were used to identify the optimal well placement and well trajectory. For planned well trajectories, this technology can also be used to determine the optimal mud weight."

Jan 1, 2015