Fractured Crystalline Rock Research Articles

Hydrology of fractured crystalline rocks, henderson mine, Colorado : Robinson, C S Min Engng, V30, N8, Aug 1978, P1185–1194

Hydrology of fractured crystalline rocks, henderson mine, Colorado : Robinson, C S Min Engng, V30, N8, Aug 1978, P1185–1194

Siting criteria for heat extraction from hot dry rock; Application to Switzerland

For selecting possible hot dry rock extraction sites for geothermal energy applications, the following criteria have been considered: (i) depth to the crystalline basement, (ii) temperatures at depth, (iii) pattern of regional stress field and (iv) natural permeability (=degree of fracturing) of basement rocks. A contour map of the basement topography is presented. From outcrops at the nothern border of Switzerland (crystalline rocks of the Black Forest massif, mainly granites and gneisses of Hercynian age) the basement dips gently toward the SE under the Mesozoic and Tertiary sediments of the Molasse Basin and reaches its maximum depth (∼7 km) underneath the front of the Alps. Some 30 km further SE the basement rocks appear at the surface (Aar- and Gotthard-massif, Penninic units), where they are deformed and fractured to a great extent. Temperature-depth profiles have been obtained by model calculations. Locally increased heat product on (in granite batholiths) at the base of the Molasse Basin, combined with the blanketing effect of the overlying sediments, could raise the temperatures to 150–170°C at a depth of 5 km. According to earthquake fault-plane solutions (P-axes) the regional stress field in the area of the Swiss Alps and in its northern Foreland is characterized by the maximum horizontal compression oriented N(150±20)°E in the upper crust.In situ stress determinations (overcoring experiments) show that considerable excess horizontal compressive stress is present in the Alpine crust (up to 200 bar). The deep Alpine tunnels exhibited considerable fracturing of crystalline rocks at depths greater than 1–2 km. Information about the degree of fracturing has also been obtained by refraction profiles. The velocitydepth functions show lower than normal velocities in the uppermost 1.5 km, indicating that the rocks there are fractured. A 30–40 km wide region, running along the axis of the Molasse Basin (which coincides with the majority of the population and most of the industry of Switzerland) would provide the best hot dry rock sites.

Water level fluctuations due to earth tides in a well pumping from slightly fractured crystalline rock

At the Savannah River plant of the Atomic Energy Commission near Aiken, South Carolina, there are three distinct groundwater systems: the coastal plain sediments, the crystalline metamorphic rocks, and a buried Triassic basin. The coastal plain sediments include several Cretaceous and Tertiary granular aquifers and aquicludes, the total thickness being about 305 m. Below these sediments, water occurs in small fractures in crystalline metamorphic rock (hornblende schist and gneiss with lesser amounts of quartzite). Water level fluctuations due to earth tides are recorded in the crystalline metamorphic rock system and in the coastal plain sediments. No water level fluctuations due to earth tides have been observed in wells in the Triassic rock because of the very low permeability. The water level fluctuations due to earth tides in the crystalline rock are about 10 cm, and those in the sediments are about 1.8 cm. The use of water level fluctuations due to earth tides to calculate porosity appears to present practical difficulties both in the crystalline metamorphic rock system and in the coastal plain sediments. In a 1‐yr pumping test on a well in the crystalline metamorphic rock the flow was controlled to within 0.1% of the total discharge, which was 0.94 l/s. The water level fluctuations due to earth tides in the pumping well were 10 cm, the same as when this well was not being pumped.

On the permeability of some fractured crystalline rocks : MUNDI, EK WALLACE, JR 4F, 20R. BULL. ASSOC. ENGNG. GEOL. V10, N4, 1973, P299–312

On the permeability of some fractured crystalline rocks : MUNDI, EK WALLACE, JR 4F, 20R. BULL. ASSOC. ENGNG. GEOL. V10, N4, 1973, P299–312

TECHNIQUES FOR LOCATING WATER WELLS IN FRACTURED CRYSTALLINE ROCKSa

Abstract. Since 1964, this geologist has used surficial geology to locate hundreds of water wells in fractured crystalline rocks of the Colorado Rockies. The techniques, procedures and priorities that have been established during that period have been incorporated into a training program that is used to teach new personnel how to determine the best water well location on a given tract.In summary, new geologists are taught how to determine the water needs of the client, “read” the surficial geology, determine the best or controlling water associated features of the area, determine the depths of the wells to take maximum advantage of geohydrologic conditions, and prepare a report in layman's terms so that the client fully understands the ground‐water conditions and what can be expected from wells.By using these methods, our staff is able to maintain a 95%+ success ratio and provide these services at a reasonable fee. This paper will present our program which should be applicable to any area that has similar geology.

Specific Capacities of Wells in Crystalline Rocks

ABSTRACTIn the Rothschild area, Wisconsin, most water wells are completed in glacial deposits. When glacial deposits are unsaturated or thin, wells are completed in fractured crystalline rocks of Pre‐Cambrian age. Specific capacity data from 56 wells tapping these fractured rocks reveal: Yields and specific capacity are generally small, with a median specific capacity of log 0.17 gpm/ft and a mean value of log 0.10 gpm/ft. Wells which are close together may have substantially different specific capacities. The occurrence of saturated sand and gravel above the crystalline rocks has no obvious influence on yield or specific capacity. Specific capacities of wells seem to be inversely proportional to (a) thickness of rocks penetrated below the water table, (b) depth of the wells below the water table, and (c) depth of the wells below the land surface. Assuming that specific capacities of wells are related to the number of contributing fractures penetrated and that the number of fractures diminishes linearly with depth, the following relation is derived: image where Sc= specific capacity, Q = discharge, s = drawdown, B = a constant, b = fracture frequency at the land surface, z = depth, and E = maximum depth of fracturing. In the Rothschild area, E = 100 feet.In exploring for ground water in a fractured rock, the problem is to find the area of maximum b. If sound scientific methods are not available, then exploration will consist of drilling wells at random until a satisfactory well is completed. However, because wells achieve 60 to 80 percent of their yield in the first 20 feet below the water table, test wells should be pump tested when they have reached a depth of about 20 feet below the water table. Unsuccessful wells need not be drilled to completion.

Waste Disposal and Earthquakes at the Rocky Mountain Arsenal, Derby, Colorado

The seismic activity triggered by injection into the RMA well at Derby, and the earthquakes that continued after injection was stopped should serve as a warning to the waste disposal engineer that no underground disposal project is routine. project is routine. Introduction From 1962 until 1966, waste fluids were injected into the pre-Cambrian formations northwest of Denver, Colo. Since injection started into the 12,000-ft well, earthquakes have been observed in the area. This seismic activity, although different in character, continued when injection was stopped. Consideration has been given to removal of the waste fluids from the well to reduce earthquake magnitude and frequency. This article is a case history of the well, its operation, the observed earthquakes, and a discussion of possible mechanisms. Considerable information is available in various reports. See the references for more detailed information than can be given here. Drilling and Completion The Rocky Mountain Arsenal (RMA) disposed of waste waters by various means until 1961, when the deep waste disposal well was completed. Before the well was completed eight geologic formations were considered as possible injection reservoirs: the Hygiene sandstone, the Codell sandstone, the "J" sandstone, the Dakota sandstone, the Morrison formation, the Lyons formation and the Fountain formation. The hole was to be drilled to a total depth to penetrate the pre-Cambrian rocks. pre-Cambrian rocks. During drilling it was found that the potential reservoirs were tightly cemented with silica and that Cambrian was then considered as a disposal prospect. Waste disposal through wells is practiced in many states. The RMA disposal well is unique in that it is by far the deepest (12,000 ft) and in that the reservoir consists of fractures in crystalline rock as opposed to sedimentary rocks in other projects. Operating conditions of the well fall within the range of all wells. The maximum wellhead injection pressure for the RMA well was 1,100 psi during an injection rate of 300 gal/min. The greatest depth of other waste disposal wells is 7,650 ft; the maximum wellhead pressure is 400 psi, and the maximum injection rate is 700 gal/ min. During the drilling of the well, lost circulation was observed in the Lyons formation (9,582 to 9,772 ft). Eight instances of mud losses were noted. These ranged from a 10- to a 320-bbl loss. Cores indicated the formation to be fractured. The Fountain formation (9,772 to 11,880 ft) appeared to be highly fractured to a depth of 10,400 ft, with some fracturing at greater depths. Lost circulation was frequently experienced while drilling through the Fountain, mostly between 9,960 and 10,250 ft. The lowest mud loss took place at 11,080 and was the most serious one in the well, with some 1,350 bbl lost. The well was cased to 11,171 ft and subsequent drilling did not encounter lost circulation until after a core was cut in the pre-Cambrian (11,950 ft total depth). P. 983