Cernent Resistivity and Implications for Measurement of Formation Resistivity Through Casing

Abstract

Measurement of formation resistivity through casing is an emerging logging technology, with prototype logging tools currently under development. Modeling studies show that the presence of cement could reduce the sensitivity of the TCRT [TM] (Through Casing Resistivity Tool [TM]). Physically this tool has some similarity to a laterolog. Measurements would be seriously affected if the cement was much more resistive than the formation. We have carried out a laboratory study of cement resistivity with the funding from the Gas Research Institute. Our main objective was to provide hard data to use in assessing the impact of cement resistivity on measurements made with the TCRT [TM]. Our results show that cement resistivity is generally low, varying from less than 1 to 8 -m at 120 degrees F. Light-weight cements were generally the least resistive, with Class A, G and H cements more resistive. Resistivity was essentially independent of either confining or pore pressure. It was also independent of either current density or voltage. Cement resistivity appears to vary with temperature according to Arp's equation, as would be expected for nonconductive material saturated with water. Thin sections and mercury injection capillary pressure data show that porosity in cement is dominantly microporosity, with pore throat diameter less than 0.1 microns. In addition helium porosity is high, ranging from 35 to 40%. The low observed resistivity is due to large amounts of water-filled microporosity. The results of this study signify that the presence of cement will not seriously degrade measurements of resistivity through casing in many environments. Our work concentrated on cements normally used on the North Slope of Alaska, as well as Class H cement, a common Gulf Coast cement. Potential applications of the TCRT [TM] include water flood monitoring at Prudhoe Bay and elsewhere, and detection of "missed pay" behind casing. Introduction Recent technological advances indicate the feasibility of measurement of formation resistivity through casing. Patents on the basic technology are held by Gard et al, Kaufman, and Vail. The concept has been demonstrated by successful measurement of formation resistivity in a cased well. Vail is currently developing a prototype logging tool. The basic concepts for the measurement are described by Kaufman. Early in the development of the technology it was recognized that a high resistivity cement annulus or high casing contact resistance could affect the measurement of formation resistivity. The tool proposed by Vail operates similarly to a laterolog, and is therefore sensitive to the presence of large resistances between the casing and the formation. P. 365^

Field Test Results of a Slim Through Casing Resistivity Array Instrument Prototype

“…Because no other cement information was available at the time of the data processing, the cement correction was performed using as the cement thickness half the difference between the bit size and casing size and using as the cement resistivity 5 Ohm-m, the average value from a laboratory cement study. 28 We conclude that the cement effect is usually small and can be removed.…”

“…[22][23][24][25][26][27][28] From an historical perspective, there is an additional major point to stress. When the first measurement techniques were proposed, there was considerable concern that the voltage measurements inside the casing would not necessarily be representative of quantities directly related to the formation resistivity.…”

Looking Behind Casing: Evaluation and Application of Cased-Hole Resistivity in Saudi Arabia

“…Since no in-situ R Cmt measurement can be made today, assumptions must be made on cement resistivity. Klein (1993) studied the cement electrical properties in the laboratory and showed that its resistivity is in the range of 1 to 10 ohmm.…”