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Borehole direct current (dc) resistivity problems are solved using finite difference method (FDM) primarily to extend the log interpretation in the two-dimensional (2D) domain. Transitional invaded zone, flushed zone, uncontaminated zone, shoulder beds and borehole filled with mud are simulated simultaneously in the model. Linear variation of resistivity is assumed for transitional invaded zone. Normal, lateral and modified laterolog7 ( LL7 4 ) electrode configuration (discussed in the text) responses were computed using expanding rectangular grid system. LL7 4 responses with variable geometric factor did show some superiority over the conventional normal and lateral for conductive target beds sandwiched between the resistive beds. For relatively small borehole diameter, all three probes can detect resistive target bed satisfactorily. However, for highly conductive mud both LL7 4 and normal electrode configuration fail to detect any signal from the target bed. Bypassing effect of the current for LL7 4 is more than that in normal log. Lateral log shows a signal of conductive invaded zone generated by the conductive mud. Bucking current ratio peaks and the geometric factor peaks in LL7 4 can detect the bed boundaries.

Direct current resistivity responses for two-dimensional (2-D) borehole model with cylindrical symmetry are computed using the finite difference method (FDM). The effects of shoulder beds, borehole mud, flushed zone, invaded zone, and uncontaminated zone are taken together to compute the normal and lateral log responses. The invasion zone is assumed to be a transitional zone. The effects of target bed thickness, resistivity of the invaded zone, radius of invasion and resistivity of the shoulder beds on the 2-D resistivity departure curves (RDCs) are demonstrated. Sensitivities in the 2-D RDCs remains up to the electrode separation (L) equal to the target bed thickness. For L ³ 5H, the target bed thickness), the entire information comes from the shoulder beds. This 2-D forward problem can be used for 2-D inversion to determine the resistivity of the uncontaminated zone and radius of invasion.

Combined and interactive inversion of D.C. resistivity and time domain induced polarisation sounding data (i) appears to be an effective approach for quantitative interpretation of induced polarisation sounding data, (ii) shows better resolution of the subsurface for a certain class of earth models, (iii) provides more information about the subsurface, (iv) is one of the approaches for studying saline water and fresh water problems. Field observations, taken in a sedimentary terrain near Federicton Junction, New Brunswick, Canada, were analysed. Weighted and non weighted ridge regression estiamtors were used to solve the inverse problems. Two tier inversion algorithm is chosen where resistivities and thicknesses are modified in the first stage. Apparent chargeability sounding curves with negative values for certain elecrode spreads, in between, revealed the presence of negligibly polarisable layers. These zones, which also show low resistivities, are demarcated as saline water pockets. Induced polarisation forward model curves are resistivity model dependent. Vertical resolution in an IP sounding curve is the best in Q type and worst in A type resistivity models for K type (m1 ma) layer chargeabilities.

The CTU based stimulation jobs practiced for the horizontal oil wells completed in the, LIII limestone reservoirs of giant Mumbai High field in the Western Offshore basin of India was gradually losing its desired efficacy due to the falling reservoir pressures. As an alternative, the back surging technology used for post gravel pack perforation tunnel cleaning operations in sandstone reservoir was recasted with minor modifications and applied for stimulating the horizontal wells in the saturated, multilayered LIII reservoir Wells IQ-9H and S16-P1H located in the downdipside and crestal part and of the field respectively, were identified for pilot tests so that the field trials are subjected to widely differing geological and petrophysical environment. While Well IQ-9H is placed in the thin uppermost layer of LIII limestone overlain by a huge shale sequence, well S16-P1H is placed within an intermediate layer of the same limestone formation overlain by a large gas cap in the upper eight layers and underlain by a shale band. Encouraging results in both these wells prompted more extensive use of the technology elsewhere in the heart of the field, and results of around twenty 8–1/2" horizontal sections and forty-one 6" drain holes completed in the last three years have been deemed to be successful without exceptions, both in terms of effective stimulation and rig-time savings to US $ 30,000 /well , thereby proving to be a cost effective substitute over conventional bulldozing and CTU based jobs. The surge tool is tailormade for instantaneous drawdown across the formation face thereby removing all post-perforation fines. The assembly is run on a lock mandrel to an appropriate landing nipple profile. The tool is surface dressed with shear screws, number and type chosen to suit the pressure likely to be created across it. After the horizontal well is completed with packer, the dressed back surge assembly is installed over the landing nipple through slick line. The tubing volume above the back surge tool is knocked out for creating the differential pressure. The preinstalled shear pins gives way to the formation fluids. The back surge tool is ultimately retrieved once the stabilised flow is attained. The paper details the design of the surge tool, its preference over other existing systems in practice and the performance enhancement profile alongwith the financial indices in the 61 wells it has been tested in the LIII reservoir of Mumbai High field.

Combined resistivity and induced polarisation (time domain) soundings were carried out near Fredericton Junction, New Brunswick, Canada in order to delineate saline water and fresh water zones. IP sounding takes about three times as long as resistivity sounding for the same maximum electrode spread. Simultaneous decrease of resistivity and IP is a diagonostic feature for locating saline water zones. Combined resistivity and IP survey can give better resolution of the subsurface than do resistivity soundings alone. Apparent chargeability can change sign with changes in electrode separations. Apparent inverse relation between ρ a — m a curves can be used as a qualitative diagnostic for locating fresh water aquifers. Borehole lithologs, geophysical logs and water analysis data indicate that deeper aquifers are more saline.

This note gives a suitable explanation for the widely divergent views expressed by different authors regarding the depth of exploration in direct current electrical methods. The diversity in opinion is due to the variable nature of a certain depth factor D L defined in the text. All views are however correct within a specified domain of resistivity, layer thickness and electrode separation. There exists a definition that is specific and quantitative and which remains invariant for a particular electrode configuration. Limiting depth of exploration can explain the wall effect correction in d.c. resistivity model experiments.

An infinitely resistive semi-infinite horizontal bed of finite thickness is assumed to be present in a sedimentary column of finite thickness and resistivity. The sedimentary column overlies a basement of infinite resistivity. An attempt has been made in this paper to theoretically compute the resistivity profiles perpendicular to the strike of the bed for line sources with Schlumberger, three-electrode and unipole configurations and to quantitatively assess the basement effect on the profiles when the depth of the basement is 1.5–3 times that of the resistive bed. The Schwarz-Christoffel method of conformai transformation and a numerical method of solving non-linear differential equations are used to solve the boundary value problem. The data analyses show that: 1. (1) the percentage reduction of the resistivity or unipole anomaly (defined in the text) increases very rapidly with the gradual decrease in the depth of the basement. For a ratio of 0.5 of the depth to the top of the body to the depth of the basement, the resistivity anomalies decrease by 25 to 50% for different electrode systems with respect to their respective anomalies for infinite basement depth; 2. (2) the three-electrode system gives a sharper anomaly; 3. (3) electrode spacing has very little effect on the percentage reduction of the resistivity anomaly; and 4. (4) the unipole anomaly is most sensitive to the depth of the basement.

Based on published results, an attempt has been made to correlate the electrical conductivities and seismic velocities obtained from magnetotelluric (MT) sounding, geomagnetic depth sounding (GDS), earthquakes, deep nuclear explosions and dispersion of surface waves. The correlation profile extends from the Scandinavian Shield to Mariana Islands, and passes through parts of Europe, the Atlantic Ocean, Canada, the U.S.A., the Pacific Ocean and Hawaiian Islands. Inverted MT, GDS and seismic data are plotted against depth on a regional scale to obtain geoelectric and seismic sections. Regional variation of the depth of the Moho is shown. Large scale lateral crust-mantle heterogeneities along with the tectonically active zones are broadly reflected in these sections. HCLs (high conductivity layers) and LVZs (low velocity zones) are delineated. Phase transition of silicates may explain the electrical conductivity and seismic velocity changes at a depth of about 400 km. The western U.S.A. (Basin and Range province, Colorado Plateau) near the East Pacific Rise reveals significant lateral variations along with a few partial melt pockets. Heterogeneities extend up to 1000 km. A tentative temperature section has been constructed based on the electrical conductivities of the probable components.