Model Study On Through Casing Time Domain Electromagnetic (TEM) Probing

Summary

Analytical and numerical solutions of time domain electromagnetic responses of earth model with axis symmetry have been derived and model responses have been calculated with a pulse source stimulating inside metal casing. Anomalous pattern of TEM responses varying with casing parameters (thickness, conductivity and magnetic permeability) and sensitivity to resistivity changes of formation outside of casing were analyzed. Model results have shown that through casing TEM sounding is feasible and has advantages of multi-components, multi-parameters, deep and untouched probing.

Introduction

Electrical and electromagnetic well logging methods have been the most direct and reliable methods for quantitative evaluation of oil and gas reservoirs. Studies on through casing resistivity logging (TCRL) have been initialized since 1980s by several well logging service companies for purpose of residual oil evaluation of reservoir in producing field. Baker Atlas and Schlumberger have, early or late, developed TCRL apparatus and carried out technical services world wide with promising application results.

Some difficulties still remain and need to be solved properly in data processing and interpretation due to the casing effect. For instance, cementing zone correction, correction of casing effects, resolution of thin beds, limitations for evaluating residuals by using the attenuation index, and the sensitivity to saturated water bearing layers, etc.. Some of these problems caused by environmental factors, while others are caused by the imperfect of processing and interpretation methods, and the limitation of the observation system itself is the key factor. The limitation of the observation method is stated as: this system delivers DC current through casing wall and then diffusing to the formation. Formation resistivity probing at various depth can be achieved only by varying the electrode spacing since the DC fields have no sounding capability. Moreover, the down-hole current supply is limited to be <6A by the logging cable.

The time domain electromagnetic method (TEM) has advantages of high resolution, high data quality, high efficiency of cost and productivity among numerous electromagnetic methods. It specially suitable for structure mapping in exploration stage and dynamic monitoring of reservoir for production stage due to its high sensitivity to resistive layer. The TEM method which is mature on theory and has been widely used for prospecting on surface is going to be employed for down-hole through casing resitivity imaging with high resolution and large penetration depth. The key factor for bringing this idea to practice is to develop down-hole tool for large power pulse source and highly sensitive receivers. To emit large power pulse with low frequency in cased well can break through the shielding of the casing. To observe data in time domain in the cased well, or between wells, or on surface, or combine of well and surface observations can gave better vertical resolution and larger probing distances than conventional electrical logging. Dynamic reservoir monitoring can be easily achieved if time lapse observations are carried out.

Axis Symmetry Earth Model with Casing

As shown in Figure 1, a two dimensional model in cylindrical coordinates ( r,z) with axis symmetry in ϕ is set up to simulate a pulse source in cased well.