An EPT Interpretation Procedure and Application in Freshwater, Shaly Oil Sands

Abstract

Summary This paper reviews the basic electromagnetic field theory of microwave propagation and the techniques used to measure, process, and interpret the propagation and the techniques used to measure, process, and interpret the data obtained with the electromagnetic propagation tool (EPT). The theoretical basis for an interpretation procedure for the EPT log is developed and the application of EPT-derived porosity and apparent fluid resistivity to the dual water model is reviewed. The application of this interpretation model using Schlumberger's Cyber Service Unit (CSU) for processing is discussed and the formats of the CYBERLOOK wellsite processing is discussed and the formats of the CYBERLOOK wellsite processing are reviewed. processing are reviewed. The application of this interpretation technique to freshwater, shaly, low-gravity oil sands is discussed. Several examples are reviewed showing the comparisons between log computations and conventional core data. Introduction The EPT provides geophysical measurements that previously have not been available to the logging industry. The EPT uses two receivers to measure, the phase shift and signal amplitude levels of a 1.1 × 10 9 cycles/sec [1.1 GHz] signal propagating through the formation. The differential response yields the propagation travel time, and signal attenuation, which are the basic measurements that are displayed on the log. The theoretical basis (Appendix A) of these measurements, as well as interpretation techniques, have been presented in the literature since about 1977. The intent of this paper is to expand on the interpretation techniques that have been presented previously. Wellsite CSU computations of water saturation in shallow, low-gravity, oil sands with unknown water resistivities specifically are covered. This difficult problem is compounded by EOR techniques using steam or waterfloods. The freshwater steam will condense and commingle with the connate water, resulting in an admixture of "new formation water." The CYBERLOOK wellsite EPT computation in such a sand sequence is discussed and the results are compared to conventional core analysis. Further complications to accurate water saturation calculations occur when the well penetrates an active steamdrive sand zone. The CYBERLOOK EPT computation technique has been modified, for processing at a central computing center, to permit calculation of residual oil saturation (ROS) within the steamdrive phase. This technique allows the effectiveness of the steam drive to be evaluated. The interpretation procedure is corroborated by comparison to conventional core data. Measurement Hardware The EPT's sensors are four microwave antennas mounted on the mandrel of the sonde. These antennas (Fig. 1) are pressed against the borehole wall by the backup arm of the powered caliper tool (PCD), which has a standard MICROLOG pad on the backup arm. The antenna configuration consists of two transmitters and two receivers operating in a borehole compensated (BHC) mode of operation-i.e., the outside antennas alternatively radiate 1.1 × 10 9 cycles/sec [1.1 GHz] energy, which is received by the two interior antennas that, in turn, measure the differential phase and signal levels. The BHC method helps cancel errors associated with pad tilt and minor instrumentation imbalances. The EPT generally is run in combination with other telemetry- compatible tools such as the Litho-Density-tool, Compensated Neutron (CNL), Formation Density Compensated (FDC), and gamma ray tools (Fig. 2). The combination logging speed is 1,800 ft/hr [549 m/h]. The tool is designed to operate under these conditions:R resistivities greater than 0.3 m,formation pressures less than 20,000 psi [137 895 kPa],formation temperatures less than 350F [177C], andborehole diameter range of 6 1/4 to 22 in. [16 to 59 cm]. The hole size has no significant effect on the EPT measurement as long as the hole diameter is within the tool's operational limits. However, hole rugosity can result in erratic and erroneously high, measurements. This occurs whenever drilling fluids get between the antenna block and the formation. Mudcakes with a thickness less than 3/8 in. [0.7 cm] cause no EPT measurement problems. As mudcake thicknesses increase above 1/8 in. [0.7 cm], the, will begin responding to the properties of the mudcake until the total tool signal reflects the mudcake properties at mudcake thicknesses of about 3/4 in. [2 cm]. The tool will not measure the formation characteristics accurately in holes with air- or oil-based mud systems because of the short propagation times (low dielectric constants) of both air and oil. Thin layers of either of these fluids between the antenna block and the formation can cause the tool to respond only to the fluid, not to the formation. JPT p. 1763