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1、GEOPHYSICAL WELL LOGGING Introduction to Well Logging Purpose of Logging 1) Lack of Core (older wells) 2) Drillers logs - poor and subjective 3) Core/geol. logs - limited to well bore 4) See behind casing 5) Qualitative measurement in-situ 6) Correlation (SP, natural gamma) - objective record (?) 4
2、Basic Types - electrical (resistivity, conductivity, SP) - nuclear (density, porosity, natural radioactivity) - sonic (acoustic-porosity, fractures, correlation with seismic section) -others (Caliper, magnetic, gravity, fluid column, temperature, flow meter, fluid conductivity logs, etc.) “Geophysic
3、al“, “wire line“, “E-logs“ generally refer to the same thing - they are a part of the process of well production - mud logs, DSTs and production tests may also be run with geophysical logs. Properties sought - porosity- composition (rock and fluid) - degree of saturation - permeability - metal conte
4、nt - heat flux - rock quality - in-situ stress Important Dates 1927-1931: Doll, Schlumberger brothers - France, 1stresistivity, SP 1935 Dipmeter log late 1940s - Gamma-ray, neutron 1950s - Focused resistivity tools, induction tools 1960s - Digital recording, full waveform sonic (3D velocity) 1970s -
5、 Borehole televiewer, VSP, Slimline sondes, integrated circuitry 1980s - Borehole radar, EM propagation, cross-hole tomography, deep logs Costs of logging (1986) Basic suite: -$5-10 per foot plus mobilization charge -Municipal water well $8,000 for 700 ft. running of SP, gamma, neutron, induction, s
6、onic, and density Drill hole environment very important for logging preparation and Logging Results interpretation Drilling Methods 1) Cable tool2) Rotary Fluid Circulation System - water/air for shallow holes Drilling mud - water base with colloidal and silt particles additives: (1) bentonite - vis
7、cosity control (2) barite - density control (3) salt - retards clay swelling sandstone; limestone and dolomite. Igneous and metamorphic rock are less common sources of groundwater that usually store and transport water through fractures. The fractures in these crystalline rocks are hard to find and
8、may not be connected to a source of freshwater recharge. Fractures can produce large volumes of water but they may not remove unwanted substances during transport from source to point of use. Porosity One of the main terms to discuss is porosity. Porosity is the empty space within rock. This would i
9、nclude gas pockets formed in the volcanic material called pumice. Porosity is the void volume divided by the total volume. = void volume/total volume When it is said that a sand has 40% porosity it means that a given volume will have 60% rock materials and 40% void space that could be filled with wa
10、ter. Permeability Permeability is the ability of fluids to move through connected pore space under unequal pressure. The more uniform the particles, larger the pore spaces and the more interconnected the greater the permeability. The unit of measurement for permeability is the “Darcy.” One Darcy is
11、when 1 sq. cm. of rock releases1 cc of unit viscosity in 1 second under 1 atm/cm of pressure. The SP Curve that is part of a Standard Electric Log is a function of permeability but not a direct measurement of it. Hydraulic Conductivity Another related term is hydraulic conductivity. Hydraulic conduc
12、tivity is the term used when the properties of the water are also considered along with the materials through which the water moves. Viscosity of the water increases as the water temperature decreases, making it thicker and slower moving through formations. Hydraulic conductivity is a flow rate in g
13、allons per day (gpd) that moves through a cross section of an aquifer one foot thick and one mile wide under a hydraulic gradient of 1 foot per mile, with units of gpd/ft2 Formation Factor Formation Factor (F) is the ratio of the electrical resistivity of a rock 100% saturated with water (Ro) to the
14、 resistivity of the water with which it is saturated (Rw), Archie (1942). F = Ro/Rw As the porosity and permeability of a formation increase, Ro will decrease because more space is taken up by the formation water and less space by rock particles that resist an electrical current. Formation Factor is
15、 also related to porosity by the formula below where f is porosity and m is the cementation exponent. F = 1/fm Another modified version of this formula replaces the value 1, used for clean, shale free rocks with the coefficient a, that can vary with clay content. So the new formula would be as shown
16、 below. F = a/fm Borehole Conditions The followings are some borehole factors that can affect logs and some notes about log responses. +Salinity: of drilling fluid +Mud Cake: layer of mud particle +Permeability in water wells they are normally run to insure that the production pumping equipment will be properly aligned. Boreholes drilled in harder fractured material can be quite deviated depending on the dip and strike of the formations. In mining and min
