Secondary Fracturing Research Articles

Quantifying Transient Effects in Altered-Stress Refracturing of Vertical Wells

Summary The production or injection of fluids in reservoirs results in a redistribution of stresses. In this paper, the extent of stress reorientation has been calculated for fractured production and injection wells and the results have been analyzed for their effect on refracturing operations. Rules of thumb and charts have been provided to help with candidate-well selection for refracturing on the basis of the study. For previously fractured wells, it is possible to create a secondary fracture that is perpendicular to the first. The secondary orthogonal fracture can be created only within a certain time window that, in turn, depends on the reservoir properties. Conditions leading to orthogonal secondary fractures in different kinds of reservoirs (oil, gas, and tight gas reservoirs) have been analyzed to establish some rules of thumb. The effects of the layers bounding the pay zone and of permeability heterogeneity and anisotropy on stress reorientation are also discussed. Our results allow us to quantify the phenomenon of orthogonal secondary fracturing around fractured production wells by calculating the extent of the stress-reversal region as a function of time. The results of our model are shown to agree qualitatively with field observations obtained from microseismic measurements. The model presented in the study helps to clarify the concept of refracturing and provides a quantitative estimate of the time window for refracturing as a function of dimensionless parameters. The final result demonstrates the potential of the model to increase the reservoir sweep in unconventional reservoirs for which the optimum time window for refracturing is on the order of months to years. The conclusions of this study are useful for the design of refracturing operations and for candidate-well selection.

A theoretical model of the explosive fragmentation of vesicular magma

Recent experimental work has shown that, when a vertical column of rock under large pressure is suddenly depressurized, the column can ‘explode’ in a structured and repeatable way. The observations show that a sequence of horizontal fractures forms from the top down, and the resulting blocks are lifted off and ejected. The blocks can suffer secondary internal fractures. This experiment provides a framework for understanding the way in which catastrophic explosion can occur, and is motivated by the corresponding phenomenon of magmatic explosion during Vulcanian eruptions. We build a theoretical model to describe these results, and show that it is capable of describing both the primary sequence of fracturing and the secondary intrablock fracturing. The model allows us to suggest a practical criterion for when such explosions occur: firstly, the initial confining pressure must exceed the yield stress of the rock, and, secondly, the diffusion of the gas by porous flow must be sufficiently slow that a large excess pore pressure is built up. This will be the case if the rock permeability is small enough.

Hydrogeological and hydrochemical characterization of the Voltaian Basin: the Afram Plains area, Ghana

In the Afram Plains area, groundwater is the main source of water supply for most uses. The area is underlain by aquifers of the southern Voltaian sedimentary basin, which are predominantly sandstones, mudstones, conglomerates and shale. Ordinary least squares regression analysis using 41 well-test data from aquifers in the Afram Plains portion of the Voltaian system reveals that transmissivity, T, exists in a non-linear relationship with specific capacity, Sc. The analysis reveals that T = 0.769Sc1.075 with R2 = 0.83 for aquifers in the area. The mudstone/conglomerate aquifer in the area appears to be the most variable in terms of both specific capacity and transmissivity with transmissivity ranging from 0.18 to 197.7 m2/day and 0.5 and 148.5 m3/day/m, respectively. Horizontal fractures and joints resulting from secondary fracturing appear to be the main determinants of both transmissivity and specific capacity in the area. PHREEQC modeling and mineral stability diagrams indicate that groundwater quality in the Afram Plains area is controlled by the incongruent weathering of silicate minerals in the aquifers. These processes concentrate calcium, sodium, magnesium, potassium, bicarbonate ion and quartz, leading to calcite, dolomite and aragonite supersaturation at most locations. Hierarchical cluster analysis performed on the raw chemical data reveals two main water types or facies: the calcium–sodium–chloride–bicarbonate facies, and the magnesium–potassium–sulfate–nitrate facies for the southern and northern sections of the Afram Plains area, respectively.

An experimental study of the texture of deformation bands: effects on the porosity and permeability of sandstones

We have investigated the texture and formation of deformation bands in relation to permeability and porosity. Video image analysis of the Brumunddal sandstone showed a decrease in the number of large pores in the deformed zones. The frequency of small pores is increasing in the intermediate zone, compared to the undeformed rock and the central zone of a deformation band. Triaxial compression tests were performed on Red Wildmoor sandstone with constant confining pressure (8 MPa). Axial P-and S-wave velocities measured during loading showed structural changes in development of a deformation band: Stage I and II – closure of micro-cracks and pores and tighter grain packing parallel to the maximum stress direction and simultaneously dilation perpendicular to the maximum stress direction. Stage III – both the P-and the S-wave velocities decreased, reflecting tighter grain packing and development of micro-fractures. These observations are supported by permeability measurements taken before, under and after triaxial compression, with recovering of permeability due to elastic effect and static reduction due to tighter packing and ultimately grain size reduction. NMR images of oil-saturated samples after loading to failure show: stage III – grain size reduction; stage IV – secondary fracturing; and stage V – development of a slip plane.

Late Quaternary Fold Deformation along the Northridge Hills Fault, Northridge, California: Deformation Coincident with Past Northridge Blind-Thrust Earthquakes and Other Nearby Structures?

Paleoseismic investigation of the Northridge Hills fault in the northern San Fernando Valley, California, helps assess the timing and style of near-surface late Quaternary deformation in the epicentral area of the 1994 Northridge earthquake. The Northridge Hills fault, a 15-km-long, north-dipping reverse fault, exhibits geomorphic evidence of late Quaternary surface deformation, including topographic scarps across late Quaternary fluvial terraces and aligned alluvial-fan apices on the footwall block. We excavated one 40-m-long trench and six test pits, and drilled nine boreholes across a 2-m-high scarp developed on a probable Holocene fluvial terrace adjacent to Aliso Canyon Wash. A continuous clayey gravel identified in the trench, test pits, and boreholes defines a south-facing monocline with 6 ± 1 m of vertical separation across the Northridge Hills fault. Based on pedochronology, the clayey gravel ranges in age from 6 to 30 ka. The borehole data also suggest that an unconformity developed on the Plio-Pleistocene Saugus Formation is warped into a monocline that has 13 ± 2 m of vertical separation across the fault. These preliminary data yield a dip-slip rate of 1.0 ± 0.7 mm/yr for the Northridge Hills fault. The absence of distinct scarp-derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small, incremental uplifts rather than large uplifts that generate distinct scarp relief. We postulate that such uplift could be produced via moderate-magnitude earthquakes ( M W 6¼) on the Northridge Hills fault, or secondary deformation induced by earthquakes on other faults. For instance, evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the Northridge Hills fault produced by movement on the underlying Northridge blind reverse fault or other nearby large structures. Based on our geologic investigations, the distribution of aftershocks following the 1994 earthquake, and pre- and post-1994 leveling and geodetic surveys, we interpret that the Northridge Hills fault underwent triggered slip during the 1994 earthquake.

Geometry and mechanics of secondary fracturing around small three‐dimensional faults in granitic rock

The orientations, locations, sizes, and relative abundances of secondary fractures observed along small natural faults can be accounted for by a three‐dimensional elastic model. Secondary fractures along small subvertical left‐lateral strike‐slip faults in massive granitic rock of the Sierra Nevada of California (1) consistently strike 25°±10° counterclockwise from their host faults and dip at angles greater than 80°; (2) generally are absent along the central portions of the fault traces; (3) are numerous near the ends of some fault traces but absent along others; and (4) in rare cases form echelon arrays either centered along a fault trace or just past the fault trace ends. These observations are consistent with secondary fractures that nucleated near the perimeter of an elliptical fault along a “cohesive rim” of high slip resistance and propagated in three dimensions normal to the local most tensile stress. The fracture orientations relative to the faults reflect small stress drops during slip on the faults. The observations and model together have direct implications for how faults grow and conduct fluids. Secondary fractures are likely to be larger at the ends of small strike‐slip faults rather than at their tops and bottoms. As a result, if strike‐slip faults grow in an unrestricted manner, they are more likely to be linked end‐to‐end rather than top‐to‐bottom, especially where slip is small. Hydraulic conductivity is likely to be enhanced at the linkages between faults, so highly conductive regions along linked strike‐slip faults are more likely to be vertical rather than horizontal.

Naudy based automodelling with trend enhancements

Recent years have seen a revival of interest in automatic interpretation of magnetic anomalies. This has resulted partly from a dramatic increase in the quantity and quality of aeromagnetic data, and partly from the development of the “improved Naudy” technique by Shi (1993) (referred to here by the term Naudy).Application of a reliable automatic interpretation method to large aeromagnetic datasets as part of processing allows contractors to produce preliminary maps of structure and depth on a routine basis. This can be done in much the same way as maps of first vertical derivative.In addition, the application of the Naudy technique on an individual line basis allows the interpreter to generate preliminary models rapidly for subsequent refinement by specialised modelling programs. This can reduce much of the setting up time traditionally associated with modelling long, detailed lines of data.One critical parameter that is not determinable directly from the Naudy method is the strike of the body. In parallel with the development of an effective Naudy tool in Intrepid, we have introduced a trend detection method. We use this method to provide strike control during Naudy processing. Body strikes are inferred before the Naudy scanning takes place thereby increasing the reliability of the interpreted models by adding a third dimension. Trends corresponding to shallower and deeper structures are handled independently.Simple two dimensional “dyke” models are constructed with attributes of strike, dip, width, depth and susceptibility. We show examples of the use of the Naudy method to provide a rapid, fully automatic, preliminary structural analysis and depth to basement map of a large petroleum exploration area. The map shows how secondary near-surface fracturing can be seen in the magnetic data. It also shows how pre-application of strike in an area where trends are well defined can significantly improve overall results.

Fragmentation of as-drawn pearlitic steel wires during torsion tests

During free end torsion tests of cold drawn eutectoid steel wire, primary fracture frequently causes further fragmentation of the wire. The size and orientation distribution of the pre-existing microcracks at the moment of the primary fracture, and the unloading stress waves determine the secondary fracturing behaviour. A model has been developed to predict the distribution of the position of the first secondary fracture. A very good agreement is obtained between measured and calculated results. Moreover, as a by-product of the calculation, the delamination toughness of the drawn wires has been estimated to have a value between 6.8 and 16.4 MPa m , which, on the scarce evidence available for this important material parameter, seems very plausible.

Effects of cohesive zones on small faults and implications for secondary fracturing and fault trace geometry

Fracture mechanics theory and field observations together indicate that the shear stress on many faults is non-uniform when they slip. If the shear stress were uniform, then: (a) a physically implausible singular stress concentration theoretically would develop at a fault end; and (b) a single curved ‘tail fracture’ should open up at the end of every fault trace, intersecting the fault at approximately 70 °. Tail fractures along many small faults instead range in number, commonly form behind fault trace ends, have nearly straight traces and intersect a fault at angles less than 50 °. A ‘cohesive zone’, in which the shear stress is elevated near the fault end, can eliminate the stress singularity and can account for the observed orientation, shape, and distribution of tail fractures. Cohesive zones also should cause a fault to bend. If the cohesive zone shear stress were uniform, then the distance from the fault end to the bend gives the cohesive zone length. The nearly straight traces of the tail fractures and the small bends observed near some fault ends implies that the faults slipped with low stress drops, less than 10% of the ambient fault-parallel shear stress.

Experimental investigation on the influence of temperature on the mechanical properties of reservoir rocks

This paper presents a numerical modelling approach for analyzing the coupled hydro-thermo-mechanical (HTM) behaviour of fractured hydrocarbon reservoirs. The coupled HTM approach is applied to generic models of five-spot reservoirs with different fracture patterns. The presence of fractures was shown to have important effects on cold water injectivity and sweep efficiency. Due to high fluid velocities along fractures, local areas particularly close to fracture intersections cannot be saturated with cold water. The volume of oil by-passed by the displacing water was found to be strongly dependent on fracture geometry, and was generally larger if the fracture conductivity enhancement due to fracture deformation is taken into consideration. The thermo-poro-elastic stress changes due to cold water injection were found to induce significant normal and shear displacements and conductivity changes along the fractures. Another important result of the numerical modelling is the possibility of creating secondary tensile fracturing close to existing fractures. The secondary fractures are generally oriented perpendicular to the natural fractures and are expected to help in draining the oil from the rock matrix towards the fractures.

Coupled HTM modelling of cold water injection in fractured hydrocarbon reservoirs

Abstract This paper presents a numerical modelling approach for analyzing the coupled hydro-thermo-mechanical (HTM) behaviour of fractured hydrocarbon reservoirs. The coupled HTM approach is applied to generic models of five-spot reservoirs with different fracture patterns. The presence of fractures was shown to have important effects on cold water injectivity and sweep efficiency. Due to high fluid velocities along fractures, local areas particularly close to fracture intersections cannot be saturated with cold water. The volume of oil by-passed by the displacing water was found to be strongly dependent on fracture geometry, and was generally larger if the fracture conductivity enhancement due to fracture deformation is taken into consideration. The thermo-poro-elastic stress changes due to cold water injection were found to induce significant normal and shear displacements and conductivity changes along the fractures. Another important result of the numerical modelling is the possibility of creating secondary tensile fracturing close to existing fractures. The secondary fractures are generally oriented perpendicular to the natural fractures and are expected to help in draining the oil from the rock matrix towards the fractures.

Coupled HTM modelling of cold water injection in fractured hydrocarbon reservoirs

This paper presents a numerical modelling approach for analyzing the coupled hydro-thermo-mechanical (HTM) behaviour of fractured hydrocarbon reservoirs. The coupled HTM approach is applied to generic models of five-spot reservoirs with different fracture patterns. The presence of fractures was shown to have important effects on cold water injectivity and sweep efficiency. Due to high fluid velocities along fractures, local areas particularly close to fracture intersections cannot be saturated with cold water. The volume of oil by-passed by the displacing water was found to be strongly dependent on fracture geometry, and was generally larger if the fracture conductivity enhancement due to fracture deformation is taken into consideration. The thermo-poro-elastic stress changes due to cold water injection were found to induce significant normal and shear displacements and conductivity changes along the fractures. Another important result of the numerical modelling is the possibility of creating secondary tensile fracturing close to existing fractures. The secondary fractures are generally oriented perpendicular to the natural fractures and are expected to help in draining the oil from the rock matrix towards the fractures.

Thrust-controlled, gold quartz-vein mineralisation at the Tom's Gully mine, Northern Territory, Australia

Metasedimentary and minor metavolcanic rocks of the Early Proterozoic Pine Creek Inlier rest unconformably on Late Archaean granitic basement. Three basin-wide, regional deformation events at ca.1885–1870 Ma are recognised: I) W- to NW-verging thrusts and recumbent folds (D2), II) upright, open to tight, doubly-plunging, NNE- to NNW-trending folds (D3), and III) open, upright, E-trending folds (D4). In the centre of the Pine Creek Inlier, post-tectonic granites (1835–1820 Ma) are spatially, temporally and probably genetically associated with mesothermal gold-quartz vein deposits. The Tom's Gully deposit consists of a shallowly S-dipping quartz reef in graphitic shale and siltstone within the thermal aureole of the post-tectonic (1831 ± 6 Ma) Mt Bundey pluton. Gold mineralisation comprises two(?) SSW-plunging sulphidic ore-shoots which are intimately associated with brecciation and recrystallisation of early barren quartz. Where early quartz is absent from the thrust, gold mineralisation is not developed, indicating that this secondary brittle fracturing was essential to sulphide and gold deposition. The ore-shoots plunge parallel to the trend of D3 fold axes. The reef is hosted by a D2 thrust fault with transport to the NW. D3 folds in the hangingwall and footwall decrease in amplitude toward the reef indicating that, during continued E-W compression, the thrust acted as a decollement zone. Field relationships and microstructural studies suggest that quartz and sulphide were deposited in a reactivated thrust during wrench shear along several NNE-trending faults associated with emplacement of the Mt Bundey pluton.

Fluid evolution in gold‐bearing quartz veins associated with feldspar porphyry dikes at Vinliden in the Skellefte District, northern Sweden

Abstract The Vinliden gold mineralization is located in the northwest‐em part of the Skellefte District in northern Sweden. The mineralization consists of a number of gold‐bearing quartz veinlets in a greywacke sequence. The quartz veins have been emplaced in small‐scale second order fault and shear zones within one of the major fault structures, which transects the folded supracrustal pile in the Skellefte District The Vinliden gold occurrence is spatially related to feldspar porphyry dikes which have intruded the greywackes. Alteration zones with albite or sericite surround the quartz veinlets. The mineralization also includes a sulfide assemblage dominated by arsenopyrite and pyrrhotite. Aqueous and carbonic (CO2‐CH4) fluid inclusions in the quartz veinlets indicate initial deposition of quartz and an early generation of arsenopyrite at a temperature exceeding 400 °C in association with a pressure drop from 5 to 1 kbar. Secondary fracturing opened passageways and allowed fluid pressure to adjust to hydrostatic conditions of c. 0.5 kbar, and subsequent sulfide and gold deposition occurred from a low salinity H2O‐NaCl‐CO2‐CH4 fluid at around 350–400 °C, accompanied by phase separation (boiling). In a post‐mineralizing stage, and after further fracturing of the quartz vein system, large volumes of a N2‐CH4 fluid (generated in the greywacke sediments) were introduced together with a brine with a complex CaCl2‐rich salt composition. Finally, a low to moderately saline aqueous solution filled larger cracks with calcite. The proposed fluid evolution X‐P‐T model at the Vinliden gold mineralization appears to be of regional significance and may be a distinct parameter for further comparisons in the Skellefte District as a whole. Broman, G, Bergström, U. & Lindblom, S., 1995: Fluid evolution in gold‐bearing quartz veins associated with feldspar porphyry dikes at Vinliden in the Skellefte District, northern Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 233–244. Stockholm. ISSN 1103–5897.

The cutting constant of the rock and the influence of the secondary fracturing of the rock on its value : V. Miklusova, E. lazarova & M. Labas, in: Geomechanics 93. Proc. conference, Ostrava, 1993, ed Z. Rakowski, (Balkema), 1994, pp 425–427

The cutting constant of the rock and the influence of the secondary fracturing of the rock on its value : V. Miklusova, E. lazarova & M. Labas, in: Geomechanics 93. Proc. conference, Ostrava, 1993, ed Z. Rakowski, (Balkema), 1994, pp 425–427

Hydrocarbon Trap Types and Deformation Styles Modeled Using Quantified Rates of Salt Movement, Louisiana Margin

ABSTRACT Definitions are proposed for the various salt features along the Louisiana continental margin. The nine general features include salt wedges, salt tongues, pillows, diapirs, wings, canopies, sheets, sills, and dikes. The definitions are derived from the interpreted salt geometry and contain an implied genesis. These definitions are here proposed with the hope of promoting a dialogue to result in a suite of accepted definitions. Movement of salt appears to result from the confining pressure of sediment deposition. Deposition is controlled by sealevel oscillations, subsidence, and source. Salt movements include lateral flow, buoyancy, and isostacy. Lateral flow may occur under the Louisiana shelf-break, terminating basinward at the present Sigsbee Escarpment; isostacy may occur under the upper- middle slope and outer shelf. Buoyancy appears to be dominant under the middle and inner shelf, coastal plain, and South Louisiana and Northern Louisiana-Arkansas interior basins. Calculated rates of salt motion apparently may range from as high as 17cm/yr (salt wedge migrating during Plio-Pleistocene only during lowstands) to as low as cms/1000 yrs (salt pillows under the inner shelf, coastal plain, and interior basins). Given these calculations, it has been possible to prepare computer-derived plots of fracturing models around the various salt features. Salt movements from 10-100 m/m y up to 10-200 km/m y have been modeled, using Mohr's criterion for failure, in order to examine the domains of primary and secondary fracturing around salt bodies. The results of modeling show that (1) the higher the rate of salt motion, the larger the range of fracturing; (2) the domain of fracturing increases with larger size and with faster migration. For large, high speed salt features, the fracturing domain apparently extends as far as 2-4 km in the vertical and 3-5 km in the lateral directions.

Modeling Salt Tectonic Evolution and Sedimentary Fracturing along Northern Gulf of Mexico

Massive hydrocarbon reservoirs associated with salt features along the northern Gulf of Mexico prompts interest into the synergistic and dynamic evolution of the two features. Use of geologic models from the evolution of salt movement in the northern gulf provides constraints on movement rates. Salt originally deposited some 150 Ma on continental crust, i.e., South Louisiana basin, migrating to present position along Sigsbee Escarpment, would, on average, move at 0.3 cm/yr; or, if movement occurred only during lowstand, at 0.5 cm/yr. If extruded salt lay along present shelf-break during Upper Miocene-Pliocene (10 to 2.5 Ma), then the extrusion rate would be, on average, 7 cm/yr and, during lowstand only, 17 cm/yr. Under the slope, salt is suggested to respond isostatically to new deposition, yielding a vertical migration rate of cms/yr, given sedimentation rates of a kilometer per single fourth order (Milankovitch) lowstand. Under the shelf, buried salt apparently moves by buoyancy at cms/1000 yrs. Given this model for salt tectonic rates, it is possible to examine domains of primary and secondary fracturing, using Mohr's criteria for failure, surrounding salt features of the Louisiana offshore. It is shown that (1) the higher the rate of salt motion, the larger the range more » of fracturing; (2) the domain of fracturing around salt features depends on salt size, sediment properties, and salt speed. Generally the fracturing domain extends 2-4 km in the vertical direction, and 3-5 km in the lateral direction. « less

Modeling Fracturing and Faulting Within Sediments Around Rapidly Moving Lateral Salt Sheets (1)

Numerical modeling of high-speed motion of salt sheets at various rates (50-150 km/m.y.), depths, and salt sheet thicknesses through sediments is illustrated in this study. Mohr's criterion for failure is used to examine the domains of primary and secondary fracturing in formations surrounding the salt body. Deformations of sediments are also displayed. It is shown that (1) the higher the rate of salt motion, the larger the distance range of fracturing away from the advancing tip, and (2) the domain of fracturing around the salt body depends on the salt size and sediment properties as well as on the salt speed. Generally the fracturing domain extends as far as 2-4 km in the vertical, and to about 3-5 km in the lateral directions from the tip. Several inferences from the m deling results may be of significance in attempts to evaluate hydrocarbon proneness of formations in association with salt sheets.

Sedimentary Fracturing around Rapidly Moving Salt Sheets

High-speed motion through sediments of salt sheets at various rates (10-20 km/Ma), depths, and salt sheet thicknesses are presented in this study. Mohr's criterion for failure is used to examine the domains of primary and secondary fracturing surrounding the mobile salt sheets. Sediment structures are also examined. Three sets of numerical examples of initiation of fracturing and/or faulting in formations around a rapidly moving salt sheet are illustrated. It is shown that (1) the higher the rate of salt motion, the larger the range of fracturing; and (2) the domain of fracturing around the sale body depends on the salt size and sediment properties as well as on the salt speed. Generally, the fracturing domain extends to as far as 2-4 km in the vertical and to about 3-5 km in the lateral direction around the forward edge' of the salt sheet.

Explorationist's Model of a Patch-Reef Trap: ABSTRACT

A geologic model of a carbonate patch-reef complex is proposed. When applied to subsurface control where wells have been cored, the model is designed to actually position a well location with respect to a stratigraphic trap formed by the patch reef. The model is lithofacies governed and is based upon those facies relationships observed among several Gulf Cost patch-reef examples. The relationship between variations in effective porosity and permeability and the variations in lithofacies appears to be direct, judging from the examples studies. This direct relationship points to the formation of a permeability barrier stratigraphic trap where certain updip and lateral facies changes occur. Only primary porosity and permeability are considered by the model. The effects of secondary porosity, fracturing, dolomitization, and/or secondary cementation can be considered by a second model applied as an overprint to the first. Application of the model as an additional exploratory tool in carbonates where reservoir quality is a function of lithofacies distribution can lead to the drilling and discovery of numerous subtle stratigraphic traps. End_of_Article - Last_Page 525------------